No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Enhanced formation of Isoprene-derived Organic Aerosol in Sulfur-rich Power Plant Plumes during Southeast Nexus (SENEX).: Sulfate affects isoprene SOA formation
Abstract
We investigate the effects of anthropogenic sulfate on secondary organic aerosol (SOA) formation from biogenic isoprene through airborne measurements in the southeastern United States as part of the Southeast Nexus (SENEX) field campaign. In a flight over Georgia, organic aerosol (OA) is enhanced downwind of the Harllee Branch power plant, but not the Scherer power plant. We find that the OA enhancement is likely caused by the rapid reactive uptake of isoprene epoxydiols (IEPOX) in the sulfate-rich plume of Harllee Branch, which was emitting at least three times more sulfur dioxide (SO2) than Scherer and more aerosol sulfate was produced downwind. The contrast in the evolution of isoprene-derived OA concentration between two power plants with different SO2 emissions provides an opportunity to investigate the magnitude and mechanisms of particle sulfate on isoprene-derived OA formation. We estimate that 1 µg sm-3 reduction of sulfate would decrease the isoprene-derived OA by 0.23 ± 0.08 µg sm-3. Based on a parameterization of the IEPOX heterogeneous reactions, we find that the effects of sulfate on isoprene-derived OA formation in the power plant plume arises from enhanced particle surface area and particle acidity, which increases both IEPOX uptake to particles and subsequent aqueous-phase reactions, respectively. The observed relationships between isoprene-OA, sulfate, particle pH, and particle water in previous field studies are explained using these findings.
... Long-term field measurements show a decreasing trend of OA in the southeast US (Attwood et al., 2014;Hidy et al., 2014;Kim et al., 2015), which is likely linked to reductions in anthropogenic POA and SOA (Blanchard et al., 2016;Ridley et al., 2018), sulfate (Blanchard et al., 2016;Malm et al., 2017;Marais et al., 2017;Xu et al., 2015aXu et al., , 2016, and NO x Pye et al., 2010Pye et al., , 2019Xu et al., 2015a). The influence of sulfate on OA is thought to be mainly due to its influence on the uptake of isoprene gas-phase oxidation products, which are often small molecules that cannot directly condense due to high vapor pressure but may undergo aqueous-phase reactive uptake onto wet sulfate particles to form aqueous SOA, as suggested by extensive laboratory and field studies Hu et al., 2015;Li et al., 2016;Liggio et al., 2005;McNeill et al., 2012;Riedel et al., 2016;Shrivastava et al., 2017;Surratt et al., 2010;Tan et al., 2012;Xu et al., 2016Xu et al., , 2015a. ...
... Long-term field measurements show a decreasing trend of OA in the southeast US (Attwood et al., 2014;Hidy et al., 2014;Kim et al., 2015), which is likely linked to reductions in anthropogenic POA and SOA (Blanchard et al., 2016;Ridley et al., 2018), sulfate (Blanchard et al., 2016;Malm et al., 2017;Marais et al., 2017;Xu et al., 2015aXu et al., , 2016, and NO x Pye et al., 2010Pye et al., , 2019Xu et al., 2015a). The influence of sulfate on OA is thought to be mainly due to its influence on the uptake of isoprene gas-phase oxidation products, which are often small molecules that cannot directly condense due to high vapor pressure but may undergo aqueous-phase reactive uptake onto wet sulfate particles to form aqueous SOA, as suggested by extensive laboratory and field studies Hu et al., 2015;Li et al., 2016;Liggio et al., 2005;McNeill et al., 2012;Riedel et al., 2016;Shrivastava et al., 2017;Surratt et al., 2010;Tan et al., 2012;Xu et al., 2016Xu et al., , 2015a. NO x plays a complex role in regulating oxidation capacity, different oxidation pathways and aerosol water content through aerosol nitrate Kroll et al., 2005Kroll et al., , 2006Li et al., 2018;Ng et al., 2017;Presto et al., 2005;Shrivastava et al., 2019;Zheng et al., 2015;Ziemann and Atkinson, 2012). ...
... The strong dependence of IEPOX SOA on sulfate is well established by laboratory and field work: wet sulfate particles provide the surface and volume of liquid media for IEPOX reactive uptake Eddingsaas et al., 2010;Riva et al., 2016;Xu et al., 2015aXu et al., , 2016 and serve as nucleophiles for nucleophilic addition to form organosulfates (Nguyen et al., 2014;Surratt et al., 2007a). Sulfate (SO 2− 4 ), together with ammonium (NH + 4 ), nitrate (NO − 3 ) and other ions, regulates proton (H + ) activity (a H + ) that can catalyze the ring opening of epoxide group leading to the formation of IEPOX SOA (Gaston et al., 2014;Pye et al., 2013;Surratt et al., 2007b). ...
Organic aerosol (OA), with a large biogenic fraction in the summertime southeast US, adversely impacts air quality and human health. Stringent air quality controls have recently reduced anthropogenic pollutants including sulfate, whose impact on OA remains unclear. Three filter measurement networks provide long-term constraints on the sensitivity of OA to changes in inorganic species, including sulfate and ammonia. The 2000–2013 summertime OA decreases by 1.7 % yr−1–1.9 % yr−1 with little month-to-month variability, while sulfate declines rapidly with significant monthly difference in the early 2000s. In contrast, modeled OA from a chemical-transport model (GEOS-Chem) decreases by 4.9 % yr−1 with much larger monthly variability, largely due to the predominant role of acid-catalyzed reactive uptake of epoxydiols (IEPOX) onto sulfate. The overestimated modeled OA dependence on sulfate can be improved by implementing a coating effect and assuming constant aerosol acidity, suggesting the needs to revisit IEPOX reactive uptake in current models. Our work highlights the importance of secondary OA formation pathways that are weakly dependent on inorganic aerosol in a region that is heavily influenced by both biogenic and anthropogenic emissions.
... Increasing NO x shifts isoprene oxidation from low-NO x conditions to high-NO x conditions and enhances nighttime SOA formation via nitrate radical oxidation of monoterpenes (Xu et al., 2015). High SO 2 emission leads to abundant sulfate and acidic particles, which accelerate BSOA production due to the "salting-in" effect and acid-catalyzed reactions (Offenberg et al., 2009;Xu et al., 2016). In polluted regions, the increase in O 3 levels due to high emissions of NO x and VOCs likely results in significant SOA formation via the ozonolysis of BVOCs (Sipilä et al., 2014;Riva et al., 2017). ...
... Moreover, sulfate could influence SOA formation. Sulfate is a key species in particles that determines the aerosol liquid water amount, aerosol acidity, and particle surface area (Xu et al., 2015(Xu et al., , 2016. Thus, an increase in sulfate could promote aqueous and heterogeneous reactions. ...
... As anthropogenic emissions can enhance BSOA formation (Hoyle et al., 2011), the reduction of anthropogenic emissions indeed lowers BSOA production (Carlton et al., 2018). As the oxidation product of SO 2 , sulfate is a key species in particles and determines aerosol acidity and surface area (Xu et al., 2015(Xu et al., , 2016; thus, sulfate could promote BSOA formation via acid-catalyzed heterogeneous reactions. A recent study found that SO 2 could directly react with organic peroxides from monoterpene ozonolysis and form substantial organosulfates (Ye et al., 2018). ...
Secondary organic aerosol (SOA) formation from biogenic precursors
is affected by anthropogenic emissions, which are not well understood in
polluted areas. In this study, we accomplished a year-round campaign at nine
sites in polluted areas located in the Pearl River Delta (PRD) region during
2015. We measured typical biogenic SOA (BSOA) tracers from isoprene,
monoterpenes, and β-caryophyllene, as well as major gaseous and
particulate pollutants and investigated the impact of anthropogenic
pollutants on BSOA formation. The concentrations of BSOA tracers were in the
range of 45.4 to 109 ng m−3 with the majority composed of products from
monoterpenes (SOAM, 47.2±9.29 ng m−3),
isoprene (SOAI, 23.1±10.8 ng m−3), and β-caryophyllene (SOAC, 3.85±1.75 ng m−3). We found that
atmospheric oxidants, Ox (O3 plus NO2), and sulfate
correlated well with later-generation SOAM tracers, but this was not the case for
first-generation SOAM products. This suggested that high Ox and
sulfate levels could promote the formation of later-generation SOAM products,
which probably led to the relatively aged SOAM that we observed in the PRD. For
the SOAI tracers, both 2-methylglyceric acid (NO/NO2-channel
product) and the ratio of 2-methylglyceric acid to 2-methyltetrols
(HO2-channel products) exhibit NOx dependence, indicating the
significant impact of NOx on SOAI formation pathways. The
SOAC tracer was elevated in winter at all sites and was positively correlated
with levoglucosan, Ox, and sulfate. Thus, the unexpected increase in
SOAC in wintertime might be highly associated with the enhancement of
biomass burning, O3 chemistry, and the sulfate component in the PRD. The
BSOAs that were estimated using the SOA tracer approach showed the highest
concentration in fall and the lowest concentration in spring with an annual
average concentration of 1.68±0.40 µg m−3. SOAM
dominated the BSOA mass all year round. We also found that BSOA correlated
well with sulfate and Ox. This implied a significant effect from
anthropogenic pollutants on BSOA formation and highlighted that we could
reduce BSOA by controlling the anthropogenic emissions of
sulfate and Ox precursors in polluted regions.
... As shown in Fig. 3, the τ g−p /τ g−w ratio decreased from 0.82 to 0.71 and 0.61 when the SO 2 concentration increased from 0 to 33 and 56 ppbv. It suggests that the sulfate formed via SO 2 oxidation could serve as seed particles (Jaoui et al., 2008) and increase the surface areas of particles (Xu et al., 2016). These roles are favorable to partitioning more SOAforming vapors into the particle phase , consequently enhancing SOA yields. ...
... This is well supported by the time-series variations in the fraction of organic ion groups (CH + , CHO + , and CHO + gt1 -containing more than one oxygen atom) (Fig. S12a), which shows the higher fraction of CHO + gt1 and lower fraction of CH + obtained at higher SO 2 concentration, consequently resulting in a higher OS C of SOA. Previous studies mostly reported that the enhancement of SOA yield in the presence of SO 2 was ascribed to the functionalization and oligomerization reactions (Cao and Jang, 2007;Jaoui et al., 2008;Liu et al., 2016b;Xu et al., 2016). If the oligomerization reaction plays a predominant role in the presence of SO 2 which will lead to particle-phase H 2 SO 4 , the carbon number of oligomers will increase but their net O/C or H/C values will show little change, consequently resulting in little change in the oxidation state of SOA (Chen et al., 2011). ...
... The similar results were also observed in Fig. S13b when increasing SO 2 concentration. In other words, SO 2 played a more important role in the formation of organic S-containing compounds and the formation or uptake of low-MW species, compared to the formation of high-MW species (i.e., oligomers) that should be reasonably produced via the acid-catalyzed heterogeneous reactions (Cao and Jang, 2007;Jaoui et al., 2008;Liu et al., 2016b;Xu et al., 2016). ...
2-Methoxyphenol (guaiacol) is derived from the lignin pyrolysis
and taken as a potential tracer for wood smoke emissions. In this work, the
effect of SO2 at atmospheric levels (0–56 ppbv) on secondary organic
aerosol (SOA) formation and its oxidation state during guaiacol
photooxidation was investigated in the presence of various inorganic seed
particles (i.e., NaCl and (NH4)2SO4). Without SO2 and seed
particles, SOA yields ranged from (9.46±1.71) % to (26.37±2.83) % and could be well expressed by a one-product model. According to
the ratio of the average gas-particle partitioning timescale (τ‾g-p) over the course of the experiment to the vapor wall deposition
timescale (τg−w), the determined SOA yields were
underestimated by a factor of ∼2. The presence of
SO2 resulted in enhancing SOA yield by 14.04 %–23.65 %. With
(NH4)2SO4 and NaCl seed particles, SOA yield was enhanced by
23.07 % and 29.57 %, respectively, which further increased
significantly to 29.78 %–53.43 % in the presence of SO2,
suggesting that SO2 and seed particles have a synergetic contribution to
SOA formation. The decreasing trend of the τ‾g-p/τg-w ratio in the presence of seed particles and SO2
suggested that more SOA-forming vapors partitioned into the particle phase,
consequently increasing SOA yields. It should be noted that SO2 was
found to be in favor of increasing the carbon oxidation state (OSC) of
SOA, indicating that the functionalization or the partitioning of
highly oxidized products into particles should be more dominant than
the oligomerization. In addition, the average N∕C ratio of SOA was
0.037, which revealed that NOx participated in the photooxidation
process, consequently leading to the formation of organic N-containing
compounds. The experimental results demonstrate the importance of SO2 on
the formation processes of SOA and organic S-containing compounds and are also
helpful to further understand SOA formation from the atmospheric
photooxidation of guaiacol and its subsequent impacts on air quality and
climate.
... Similar results concerning the effect of particle acidity on SOA yields were reported in other studies (Kleindienst et al., 2006;T. Liu et al., 2016;Jaoui et al., 2008;Xu et al., 2016). However, Ng et al. (2007b) found that particle acidity had a negligible effect on SOA yields from the photooxidation of aromatics, possibly due to the low RH (∼ 5 %) used in their work. ...
... Under acidic conditions, the gas-phase oxidation products of eugenol partitioned onto the particle phase would be further oxidized into low-volatility products or produce oligomers by acid-catalyzed heterogeneous reactions, subsequently enhancing SOA yields (Cao and Jang, 2007;Jaoui et al., 2008;T. Liu et al., 2016;Xu et al., 2016). In addition, the formed sulfate not only serves as the substrate for product condensation and likely participates in new particle formation (NPF) (Jaoui et al., 2008;Wang et al., 2016), but also enhances the surface areas of particles to facilitate heterogeneous reactions on aerosols (Xu et al., 2016). ...
... Liu et al., 2016;Xu et al., 2016). In addition, the formed sulfate not only serves as the substrate for product condensation and likely participates in new particle formation (NPF) (Jaoui et al., 2008;Wang et al., 2016), but also enhances the surface areas of particles to facilitate heterogeneous reactions on aerosols (Xu et al., 2016). These roles of sulfate are also favorable for increasing SOA yields. ...
Methoxyphenols are an important organic component of wood-burning emissions
and considered to be potential precursors of secondary organic aerosol
(SOA). In this work, the rate constant and SOA formation potential for the
OH-initiated reaction of 4-allyl-2-methoxyphenol (eugenol) were investigated
for the first time in an oxidation flow reactor (OFR). The rate constant was
8.01±0.40×10-11 cm3 molecule−1 s−1,
determined by the relative rate method. The SOA yield first increased and
then decreased as a function of OH exposure and was also dependent on
eugenol concentration. The maximum SOA yields (0.11–0.31) obtained at
different eugenol concentrations could be expressed well by a one-product
model. The carbon oxidation state (OSC) increased linearly and
significantly as OH exposure rose, indicating that a high oxidation degree
was achieved for SOA. In addition, the presence of SO2 (0–198 ppbv)
and NO2 (0–109 ppbv) was conducive to increasing SOA yield, for which
the maximum enhancement values were 38.6 % and 19.2 %, respectively. The
N∕C ratio (0.032–0.043) indicated that NO2 participated in the
OH-initiated reaction, subsequently forming organic nitrates. The results
could be helpful for further understanding the SOA formation potential from
the atmospheric oxidation of methoxyphenols and the atmospheric aging process
of smoke plumes from biomass burning emissions.
... It was proposed that isoprene-derived BSOA was enhanced by the solubility of IEPOX in the aqueous phase of particles due to the salt-in effect and the subsequent nucleophilic addition of sulfate to the IEPOX ring. Later on, the nucleophilic effect of SO 4 2À was considered small (Xu et al., 2016), consistent Journal of Geophysical Research: Atmospheres with a previous study (Lin, Knipping, et al., 2013). Xu et al. (2016) pointed out that sulfate drives both aerosol acidity and surface area, with enhancement of aerosol surface area primarily responsible for enhanced IEPOX reactive uptake (thus more IEPOX-derived SOA) in sulfur-rich power plant plumes. ...
... Later on, the nucleophilic effect of SO 4 2À was considered small (Xu et al., 2016), consistent Journal of Geophysical Research: Atmospheres with a previous study (Lin, Knipping, et al., 2013). Xu et al. (2016) pointed out that sulfate drives both aerosol acidity and surface area, with enhancement of aerosol surface area primarily responsible for enhanced IEPOX reactive uptake (thus more IEPOX-derived SOA) in sulfur-rich power plant plumes. Similar interpretation for a linear correlation between sulfate and isoprene-formed BSOA was also given by Marais et al. (2016Marais et al. ( , 2017. ...
... As discussed in section 3.3, the spatial patterns of isoprene-formed BSOA were more associated with the particle surface concentration in the simulation, which is driven by sulfate levels. The influence of sulfate on aerosol acidity could also be one of the causes of the correlation (Marais et al., 2016(Marais et al., , 2017Xu et al., 2016). ...
Biogenic secondary organic aerosol (BSOA) contributes significantly to summertime organic particulate matter. In this study, a simulation of BSOA for summer 2012 over China is performed using the Community Multiscale Air Quality model with an explicit representation of BSOA formation via reactive uptake of isoprene-derived intermediates, species-specific parameters for monoterpene-derived BSOA production, and a multigenerational oxidation scheme. Simulated BSOA tracks the measurements well (normalized mean bias of 1% and r² of 0.59). Elevated BSOA occurs in the Sichuan Basin and southeastern China (except the coastal areas) where concentrations are mostly within the range of 5–7.5 μg/m³. Nitrate oxidation dominates fresh BSOA originating from monoterpenes. Simulated monoterpene-formed BSOA increases by a factor of 3–11 with multigenerational oxidation. Isoprene-formed BSOA shows high concentrations in southwestern China, formed primarily by two methyltetrols (2-MT) and organosulfates (62%–83%). High concentrations in this region are driven by the abundance of IEPOX and high particle surface area of sulfate-laden aerosols. NOx emissions influence the formation pathways of isoprene-formed BSOA, as a high ratio of two-methylglyceric acid (2-MG)/2-MT is found in the North China Plain and part of the Yangtze River Delta Region. The simulated isoprene-formed BSOA is more strongly correlated with sulfate than with particle water mass concentration or particle acidity, in part because sulfate increases the surface area increasing uptake for heterogeneous reactions. Decreases in isoprene-formed BSOA were simulated with a pH increase of 2, implying that particle acidity mediates the BSOA formation.
... Previous field observations showed strong correlation between isoprene OA and sulfate (Xu et al., 2015a(Xu et al., , 2016aBudisulistiorini et al., 2015). Moreover, airborne measurements over power plant plumes in Georgia, US, observed enhanced isoprene OA formation in the sulfate-rich power plant plume (Xu et al., 2016a). ...
... Previous field observations showed strong correlation between isoprene OA and sulfate (Xu et al., 2015a(Xu et al., , 2016aBudisulistiorini et al., 2015). Moreover, airborne measurements over power plant plumes in Georgia, US, observed enhanced isoprene OA formation in the sulfate-rich power plant plume (Xu et al., 2016a). To probe the relationship between isoprene OA and sulfate, we conducted perturbation experiments in August 2015 by injecting acidic sulfate particles (i.e., a mixture of H 2 SO 4 and MgSO 4 ) into the 2 m 3 Teflon chamber. ...
... To probe the relationship between isoprene OA and sulfate, we conducted perturbation experiments in August 2015 by injecting acidic sulfate particles (i.e., a mixture of H 2 SO 4 and MgSO 4 ) into the 2 m 3 Teflon chamber. This mimics the airborne measurements over power plants, which introduce sulfate into the atmosphere (Xu et al., 2016a). ...
Atmospheric organic aerosol (OA) has important impacts on climate and human health but its sources remain poorly understood. Biogenic monoterpenes and sesquiterpenes are important precursors of secondary organic aerosol (SOA), but the amounts and pathways of SOA generation from these precursors are not well constrained by observations. We propose that the less-oxidized oxygenated organic aerosol (LO-OOA) factor resolved from positive matrix factorization (PMF) analysis on aerosol mass spectrometry (AMS) data can be used as a surrogate for fresh SOA from monoterpenes and sesquiterpenes in the southeastern US. This hypothesis is supported by multiple lines of evidence, including lab-in-the-field perturbation experiments, extensive ambient ground-level measurements, and state-of-the-art modeling. We performed lab-in-the-field experiments in which the ambient air is perturbed by the injection of selected monoterpenes and sesquiterpenes, and the subsequent SOA formation is investigated. PMF analysis on the perturbation experiments provides an objective link between LO-OOA and fresh SOA from monoterpenes and sesquiterpenes as well as insights into the sources of other OA factors. Further, we use an upgraded atmospheric model and show that modeled SOA concentrations from monoterpenes and sesquiterpenes could reproduce both the magnitude and diurnal variation of LO-OOA at multiple sites in the southeastern US, building confidence in our hypothesis. We estimate the annual average concentration of SOA from monoterpenes and sesquiterpenes in the southeastern US to be roughly 2µgm⁻³.
... At a forested site in California, 2-MTOOS exhibited no correlation with aerosol acidity (Worton et al., 2013). In the SEUS, observational studies that investigated the roles of aerosol acidity and sulfate loading at iSOA production found that IEPOX-derived SOA showed a weak correlation with aerosol acidity but exhibited strong correlation with aerosol sulfate (Budisulistiorini et al., 2013Lin, Knipping, et al., 2013;Xu et al., 2015Xu et al., , 2016. More recently, modeling and observational studies demonstrated that aerosols in the SEUS are acidic enough to form IEPOX-derived SOA and isoprene oxidation products selectively uptake only on sulfate-containing particles (Budisulistiorini et al., 2017;Liu et al., 2017). ...
... Xu et al. (2015) pointed out that sulfate can control IEPOX-SOA formation by the concerted nucleophilic addition to the IEPOX ring and/or via the salting-in effect that influences the solubility of IEPOX. Recent studies (Budisulistiorini et al., 2017;Riedel et al., 2016;Xu et al., 2016;Zhang et al., 2018) have demonstrated that the pseudo first-order heterogeneous reaction rate constant for IEPOX reactive uptake linearly correlates with particle surface area. Higher sulfate in the aerosol increases the number of active surface sites for reactive uptake process. ...
... suggesting that sulfate promotes HMML SOA formation. Similar to IEPOX, sulfate also increases the solubility of HMML by salting-in effect, enhances the ring-opening reactions by providing abundant nucleophilic reagents , and promotes the reactive uptake process by providing more surface area (Budisulistiorini et al., 2017;Riedel et al., 2016;Xu et al., 2016;Zhang et al., 2018). This direct influence of sulfate on HMML SOA formation is important in the PRD due to high sulfate levels. ...
Atmospheric photooxidation of isoprene forms isoprene epoxydiols (IEPOX) and hydroxymethel-methyl-α-lactone (HMML) via hydroperoxyl radical (HO2) channel and NO/NO2 channel, respectively. Reactive uptake of these epoxides onto particles produces isoprene secondary organic aerosols (iSOA). Currently, there is little information regarding these two epoxides during iSOA formation in polluted regions. In this study, iSOA tracers from IEPOX and HMML were measured from summer to fall in the heavily polluted Pearl River Delta (PRD) region. The total concentration of the iSOA tracers ranged from 5.77 to 466 ng m⁻³. Isoprene SOA tracers correlated well with sulfate (p < 0.01), indicating that the abundant sulfate in the PRD plays an important role in iSOA formation. A kinetic model of IEPOX loss showed that 58% of IEPOX could undergo ring-opening reactions under the polluted PRD conditions in summer. This leads to high levels of IEPOX-derived SOA tracers in the PRD. High temperature in the PRD (>22 °C) suppresses the production of HMML, likely as a result of fast decomposition of HMML's precursor under high temperatures. Thus, the HMML-derived tracers had lower levels than the IEPOX-derived SOA tracers during the whole campaign. The ratios of the IEPOX-derived tracers to the HMML-derived SOA tracers in summer were ~3 times higher than those in fall. This seasonal trend may be explained by the relative high isoprene/NOx ratio, temperature, and fast heterogeneous reaction of IEPOX in summer. Our study shows that in highly polluted regions like PRD, reduction in SO2 emission can significantly reduce iSOA formation.
... 7−9 Both field and laboratory studies have demonstrated that the increases in SO 2 and sulfate greatly stimulate iSOA formation. 10,11 NO x is generally associated with reduced SOA yield from isoprene. 12,13 The yield of iSOA could increase with high OA mass loadings in a pollution plume. ...
... Detailed information about the estimation of aerosol acidity and water content as well as the influence of organic acid on aerosol acidity can be found in Text S2. The uptake coefficient (γ) of an epoxide can be estimated using eqs 2−4 10,27 γ α ...
... Biogenically-derived SOA in the Southeast US can comprise up to half of the total organic mass in the summertime and up to 30% year-around [3,7,11,12]. Terpene species such as α-pinene and β-pinene, are significant contributors to the mass of SOA, since they react very fast with atmospheric oxidants with high yields [13]. ...
... In the Southeastern US, variability in IEPOX OA formation is not correlated with inorganic aerosol acidity [2,12]. However, this is the case when water and sulfate availability are high and, therefore, future reductions in SO2 emissions which will inevitably reduce the total amount of available sulfate and liquid water can lead to different behavior, potential forcing IEPOX OA formation to a regime where acidity plays a larger role. ...
Formation of aerosol from biogenic hydrocarbons relies heavily on anthropogenic emissions since they control the availability of species such as sulfate and nitrate, and through them, aerosol acidity (pH). To elucidate the role that acidity and emissions play in regulating Secondary Organic Aerosol (SOA), we utilize the 2013 Southern Oxidant and Aerosol Study (SOAS) dataset to enhance the extensive mechanism of isoprene epoxydiol (IEPOX)-mediated SOA formation implemented in the Community Multiscale Air Quality (CMAQ) model (Pye et al., 2013), which was then used to investigate the impact of potential future emission controls on IEPOX OA. We found that the Henry’s law coefficient for IEPOX was the most impactful parameter that controls aqueous isoprene OA products, and a value of 1.9 × 107 M atm−1 provides the best agreement with measurements. Non-volatile cations (NVCs) were found in higher-than-expected quantities in CMAQ and exerted a significant influence on IEPOX OA by reducing its production by as much as 30% when present. Consistent with previous literature, a strong correlation of isoprene OA with sulfate, and little correlation with acidity or liquid water content, was found. Future reductions in SO2 emissions are found to not affect this correlation and generally act to increase the sensitivity of IEPOX OA to sulfate, even in extreme cases.
... a previous laboratory study which showed that the SOA production was promoted by photochemical oxidation of toluene in the presence of sulfate seed particles (i.e., MnSO 4 and ZnSO 4 ) (Chu et al., 2013). Previous studies also found that aerosol acidity and sulfate concentration could affect SOA formation (Eddingsaas et al., 2012;Nah et al., 2019;L. Xu et al., 2016). An excellent correlation between SOA and P het (R = 0.93) was found, which implied that sulfate formed by heterogeneous reactions could change the aerosol acidity and concurrently served as seeds in the SOA formation. Additionally, the transportation emissions could also contribute to the total SOA concentration to some extent in a wa ...
A field campaign (29 May to 14 June 2018) was conducted at a mountain site in central China. The chemical composition of non‐refractory submicron particulate matter (NR‐PM1) and the particle number size distribution (PNSD) were measured, respectively. The mean NR‐PM1 mass concentration was 20.94 ± 10.14 μg m⁻³, among which organics (47%) was the most abundant component, followed by sulfate (37%), ammonium (11%), nitrate (4%), and chloride (1%). Notably, sulfate accounted for more than 70% of the secondary inorganic aerosols. Positive matrix factorization (PMF) analysis of the composition data resulted in three organic aerosol (OA) factors: hydrocarbon‐like OA (HOA), oxygenated OA I (OOA‐I), and oxygenated OA II (OOA‐II). The secondary organic aerosol (SOA) composed of the latter two factors (SOA: OOA‐I + OOA‐II) was dominant in OA (80.7%). The PMF analysis of the PNSD data yielded three factors: new particle formation related mode, growth mode, and accumulation mode, among which the last factor dominated both number and volume ratios. The sulfate formation was characterized by the sulfur oxidation ratio (SOR), heterogeneous sulfate production rate (Phet), and gaseous sulfuric acid concentration, representing secondary sulfate transformation, heterogeneous reactions, and gas phase reactions, respectively. The results showed that Phet was well correlated with both sulfate concentrations and SOR, especially during the polluted periods. Our study demonstrates that photochemically‐driven heterogeneous reactions contribute dominantly to the sulfate formation and SOA is formed predominantly by photochemical oxidation of volatile organic compounds under high temperatures and ultraviolet (UV) intensities, and NOX concentrations on Mt. Wudang during the campaign period.
... IEPOX entering aqueous phase on the surfaces of particles will undergo free radical oxidation reactions, nucleophilic reactions and polymerization (Riva et al., 2016;Otto et al., 2019). Many environmental factors affect heterogeneous reactions of IEPOX, such as NOx (de Sá et al., 2017;Zhang et al., 2017), SO 2 Marais et al., 2016;Xu et al., 2016), sulfate Riva et al., 2019), organic coatings, humidity and aerosol acidity (Riva et al., 2016;Zhang et al., 2018). Under normal atmospheric conditions (i.e., [OH·] gas = 1 × 10 6 molecule cm −3 , [SO 4 ...
Isoprene is the most abundant non-methane hydrocarbon (NMHC) emitted by vegetation, and the precursor that makes the greatest contribution to secondary organic aerosols (SOA) in the troposphere. Atmospheric oxidation of isoprene produces a series of reactive intermediates, isoprene epoxydiols (IEPOX). The reactive uptake of IEPOX is the significant formation pathway of atmospheric SOA. In this work, four isomers of IEPOX were synthesized, derivatized by silylation reagents, and measured by gas chromatography/mass spectrometry (GC/MS). The electron-impact (EI) and methane chemical ionization (CI) sources mass spectra of trimethylsilyl ester (TMS) derivatives of IEPOX isomers were obtained and the fragmentation behaviors of the derivatives were examined. Moreover, the hydrogen nuclear magnetic resonance (H-NMR) spectra of IEPOX isomers were also obtained and the peak intensities in the H-NMR spectra were analyzed. Based on the standard spectra, IEPOX isomers were identified in ambient PM2.5 aerosols in the Gongga Mountain (China). The peak sequence of TMS derivatives of IEPOX isomers in GC/MS chromatogram was δ4-IEPOX, δ1-IEPOX, cis-β-IEPOX and trans-β-IEPOX. The isomers with the highest concentrations were δ1-IEPOX (threo- and erythro-). The mass ratios of IEPOX to 2-methyltetrols were 0.02-6.0 and the concentrations of IEPOX were 0.8-41.6 ng/m3 in the PM2.5 aerosols. The current study verified the core roles of IEPOX as active intermediates in photo oxidation of isoprene in ambient atmosphere.
... Did the isoprene removal in these plumes lead to formation of SOA? Interestingly, we only saw isoprene SOA formation in plumes that were rich in sulfur, such as the Harllee Branch plant near Atlanta (Figure 4). The analysis of these observations by Sally Ng's group showed that the enhanced particle surface and acidity of the sulfate-rich aerosol promoted isoprene SOA formation (Xu et al., 2016). This was another vivid demonstration of how anthropogenic emissions can enhance biogenic SOA formation. ...
Plain Language Summary
Fine particles in the atmosphere are an important air pollutant as they can penetrate deep into the lungs of people. They also scatter sunlight, which makes the atmosphere look hazy and which has a cooling effect on climate. A large fraction of fine particles consists of organic molecules, but it has been a scientific challenge to find out where these come from. This article describes the results from field studies in New England, Los Angeles, and the Southeast U.S. These regions are very different in their emissions of organic compounds to the atmosphere: man‐made emissions are high in Los Angeles, while natural emissions from forests in the Southeast U.S. dominate over other sources. The contrast between the observations in the different regions gave insight into which organic molecules are chemically transformed and end up as fine particles in the atmosphere.
... Overall, increased acidity levels are associated with enhanced SOA yields due to acid-catalyzed heterogeneous reactions, such as epoxide ring-opening reactions (Eddingsaas et al., 2010;Mael et al., 2015) or reactions of carbonyls (Jang et al., 2002). This phenomenon has been observed for a wide range of SOA precursors, including isoprene (Edney et al., 2005;Surratt et al., 2007a;Jaoui et al., 2010;Kuwata et al., 2015;Xu et al., 2016), specifically its photooxidation products methacroelin (Zhang et al., 2012b), methyl vinyl ketone (Chan et al., 2013), and isoprene epoxydiols (IEPOX) (Riva et al., 2016d), monoterpenes (such as α-pinene) (Czoschke and Jang, 2006;Northcross and Jang, 2007;Offenberg et al., 2009;Lal et al., 2012;Han et al., 2016), sesquiterpenes (such as βcaryophyllene) (Offenberg et al., 2009;Chan et al., 2011), 1,3-butadiene (Lewandowski et al., 2015), 2-methyl-3-buten-2-ol (MBO) (Zhang et al., 2012a, aromatics (such as toluene) (Cao and Jang, 2007), and aldehydes (Jang et al., 2003a). In addition to increased kinetics of acid-catalyzed reactions, SOA yield can be enhanced in terms of mass due to the formation of larger oligomers (Gao et al., 2004;Chan et al., 2013;Kuwata et al., 2015). ...
Atmospheric aerosol particles impact climate by scattering or absorbing solar radiation and acting as cloud condensation and ice nuclei, but there is high uncertainty regarding the magnitude of these climate effects. The physicochemical properties of aerosol particles dictate their climate impacts, yet are challenging to accurately and quantitatively measure, as aerosols are highly complex in terms of chemical composition, size, phase, and morphology (i.e. mixing state). Methods enabling more detailed and quantitative investigations of aerosol particle properties are necessary in order to improve mechanistic understanding of multiphase aerosol processes occurring in the atmosphere. In this dissertation, novel Raman microspectroscopic techniques for improved analysis of the chemical composition of both laboratory-generated and ambient aerosol particles were developed to further understanding of aerosol mixing state, which will enable better predictions of the climate impacts associated with aerosols. Computer-controlled Raman mapping (CC-Raman) and surface enhanced Raman microspectroscoy (SERS) were applied to the study of aerosol particles to improve characterization of chemical composition. CC-Raman used automated mapping to increase throughput and particle statistics by analyzing up to 100 particles per hour, in comparison to much slower manual characterization. CC-Raman analysis provides detailed information regarding functional groups present, size, morphology, and the mixing of secondary chemical species with primary components. SERS enables detection of trace organic and/or inorganic species present within particles, observation of complex inter- and intraparticle variability, and characterization of smaller, more atmospherically-relevant sized aerosol particles with diameters smaller than the diffraction limit (as small as 150 nm). In comparison to traditional vibrational spectroscopy techniques, these advances greatly increase the potential for characterization of aerosol particle physicochemical properties with Raman analysis. A Raman microspectroscopic method for measuring single particle pH was developed. Aerosol pH plays an important role in many multiphase processes, such as secondary organic aerosol (SOA) formation, but is difficult to measure due to the minute volumes of aerosol particles and the non-conservative nature of the hydronium ion. Traditional indirect measurement methods and predictions of aerosol pH via thermodynamic modeling often disagree and are associated with limitations regarding measurement inputs and equilibrium assumptions. In contrast, this method coupling Raman microspectroscopy with extended Debye-Hückel activity calculations allows for direct determination of acidity of individual particles based on measurements of acid and conjugate base vibrational modes. Several atmospherically relevant inorganic and organic acid-base equilibria systems are compatible with this method, including HNO3/NO3-, HSO4-/SO42-, HC2O4-/C2O42-/ CH3COOH/CH3COO-/ and HCO3-/CO32-, covering a broad pH range (-1 to 10). A second complementary method for direct measurement of size-resolved bulk aerosol pH using quantitative colorimetric image processing of aerosol particles collected on pH indicator paper was also developed. In addition to aerosol pH measurements, these methods were used to investigate the effect of RH on particle acidity, gas-particle partitioning of acidic chemical species, the relationship between ionic strength and H+ activity coefficients, and other aspects of ion behavior under non-ideal conditions. Direct measurement of aerosol pH through these methods will improve fundamental mechanistic understanding of critical pH-dependent aerosol processes. The methods developed in this dissertation and their application to the study of atmospheric aerosol particles will yield more detailed measurements of particle physicochemical properties, providing new insights into the mechanisms of multiphase atmospheric processes and improving understanding of the impact of aerosols on climate.
... PNA and PA concentrations had no correlations with H þ insitu (P > 0.05), while HGA, AGA, HDMGA and MBTCA concentrations had positive correlations with H þ i nsitu (P < 0.05). The limited impact of H þ insitu on PNA and PA was due to (1) high aerosol acidity could accelerate SOA M production via the salting-in effect and acid-catalyzed reactions (Xu et al., 2016), (2) high acidity could promote the release of semi-volatile PNA and PA from particle phase into gas phase to balance the vapor pressure, and (3) there was relatively low aerosol acidity and its constant status at the monitoring site (Yu et al., 1999;Ding et al., 2011). Most tracers of SOA M in Fuzhou had positive correlations with T (0.69 > r > 0.47, P < 0.05) and negative correlations with RH (P > 0.05). ...
Secondary organic aerosol (SOA) plays an important role in global climate change and air quality, and SOA tracers most directly characterize the sources and formation mechanisms of SOA. Four seasons of observation of SOA tracers was carried out in a coastal city of southeastern China. Fourteen p.m.2.5-bound SOA tracers, including isoprene (SOAI), α/β-pinene (SOAM), β-caryophyllene (SOAC), and toluene (SOAA), were measured using the GC-MS method. The concentrations of SOA tracers in Fuzhou were 25.9 ± 19.9 ng m⁻³ (SOAM), 7.45 ± 8.53 ng m⁻³ (SOAI), 3.15 ± 1.99 ng m⁻³ (SOAC), and 2.63 ± 1.54 ng m⁻³ (SOAA). The elevated SOAI concentration in summer was mainly controlled by high biogenic isoprene emission and strong oxidation, and biomass burning contributed strongly to DHOPA (SOAA) in fall. SO4²⁻ and H + insitu had an increased impact on later-generation SOAI products and low-NOx SOAM products. Based on the ratio of MGA/MTL and MBTCA/(PA + PNA), atmospheric oxidation capacity (Ox, =NO2+O3) had a significant impact on the aerosol aging of SOA tracers. The increased proportions of low-NOx SOAI products were 3.23–7.21 times higher than those of high-NOx products from non-haze to haze periods, suggesting the influence of high SO4²⁻ concentration and RH on the reaction channel for SOAI formation. The percentages of later-generation SOAM products during RT were 2.46 times higher than those of first-generation products, suggesting the influence of aerosol aging during the regional transport. Both continental Asian outflow and biomass-burning plumes promoted precursor emissions and the formation process of SOA during the haze pollution events. These related findings help to understand the occurrence, sources and formation of SOA in coastal areas.
... The enhanced radical production from HONO is further discussed in the Supplementary Information and was found to be small. Finally, aerosol formation downwind from the Scherer and Harllee Branch power plants was studied and presented elsewhere (Xu et al., 2016). Emissions of SO 2 from Harllee Branch were much higher and, consequently, more sulfate aerosol as well as organic aerosol derived from isoprene were formed in that plume. ...
... Finally, aerosol formation downwind from the Scherer and Harllee Branch power plants was studied and presented elsewhere (Xu et al., 2016). Emissions of SO 2 from Harllee Branch were much higher and, consequently, more sulfate aerosol as well as organic aerosol derived from isoprene were formed in that plume. ...
Plain Language Summary
Hydroxyl radicals are the main chemical species that removes trace gases from the atmosphere. They determine the atmospheric lifetime of some greenhouse gases and chemicals involved with the destruction of the stratospheric ozone layer. Hydroxyl reactions also play an important role in air pollution chemistry. Measuring hydroxyl radicals is very challenging because of their high reactivity and low concentrations. Some recent measurements have shown unexpectedly high concentrations in relatively clean conditions. In this work, we indirectly estimated the dependence of hydroxyl radicals on the concentration of nitrogen oxides downwind from power plants in the Southeast United States. We observed that mixing ratios of isoprene, a reactive hydrocarbon released from deciduous trees to the atmosphere, were systematically lower in power plant plumes, caused by higher hydroxyl radical concentrations at the elevated nitrogen oxide concentrations. These findings can be explained by known chemical reactions but are not consistent with some studies that found unexpectedly high hydroxyl concentrations in relatively clean conditions.
... These oxidation processes produce HCHO and oxidized organic compounds with low volatility that condense to form SOA (Robinson et al., 2007;Ziemann and Atkinson, 2012). The yield of HCHO and SOA from hydrocarbon oxidation thus depends on the VOC precursors, oxidants (OH, O 3 , and NO 3 ), RO 2 reaction pathway (e.g., NO levels), and pre-existing aerosol abundance and properties Pye et al., 2010;Marais et al., 2016Marais et al., , 2017Xu et al., 2016). Moreover, although the lifetime of HCHO (1-3 h) is shorter than OA (1 week), HCHO continues to form from slower-reacting VOCs, as well as from the oxidation of latergeneration products. ...
Organic aerosol (OA) is one of the main components of the global particulate burden and intimately links natural and anthropogenic emissions with air quality and climate. It is challenging to accurately represent OA in global models. Direct quantification of global OA abundance is not possible with current remote sensing technology; however, it may be possible to exploit correlations of OA with remotely observable quantities to infer OA spatiotemporal distributions. In particular, formaldehyde (HCHO) and OA share common sources via both primary emissions and secondary production from oxidation of volatile organic compounds (VOCs). Here, we examine OA–HCHO correlations using data from summertime airborne campaigns investigating biogenic (NASA SEAC⁴RS and DC3), biomass burning (NASA SEAC⁴RS), and anthropogenic conditions (NOAA CalNex and NASA KORUS-AQ). In situ OA correlates well with HCHO (r=0.59–0.97), and the slope and intercept of this relationship depend on the chemical regime. For biogenic and anthropogenic regions, the OA–HCHO slopes are higher in low NOx conditions, because HCHO yields are lower and aerosol yields are likely higher. The OA–HCHO slope of wildfires is over 9 times higher than that for biogenic and anthropogenic sources. The OA–HCHO slope is higher for highly polluted anthropogenic sources (e.g., KORUS-AQ) than less polluted (e.g., CalNex) anthropogenic sources. Near-surface OAs over the continental US are estimated by combining the observed in situ relationships with HCHO column retrievals from NASA's Ozone Monitoring Instrument (OMI). HCHO vertical profiles used in OA estimates are from climatology a priori profiles in the OMI HCHO retrieval or output of specific period from a newer version of GEOS-Chem. Our OA estimates compare well with US EPA IMPROVE data obtained over summer months (e.g., slope =0.60–0.62, r=0.56 for August 2013), with correlation performance comparable to intensively validated GEOS-Chem (e.g., slope =0.57, r=0.56) with IMPROVE OA and superior to the satellite-derived total aerosol extinction (r=0.41) with IMPROVE OA. This indicates that OA estimates are not very sensitive to these HCHO vertical profiles and that a priori profiles from OMI HCHO retrieval have a similar performance to that of the newer model version in estimating OA. Improving the detection limit of satellite HCHO and expanding in situ airborne HCHO and OA coverage in future missions will improve the quality and spatiotemporal coverage of our OA estimates, potentially enabling constraints on global OA distribution.
... For example, some field studies have shown that a 1 μg/m 3 decrease in sulfate can lead to a 0.2-0.42 μg/m 3 decrease in isoprene SOA (Blanchard et al., 2016;Budisulistiorini et al., 2017;Pye et al., 2013;Shrivastava et al., 2017;Xu et al., , 2016. Similarly, decreases in NO x have been shown to decrease biogenic SOA formation but there are also studies that have shown increases of biogenic SOA in some regimes with decreased NO x (Table 1; de Sa et al., 2017;Edwards et al., 2017;Kroll et al., 2006;Lane et al., 2008;Liu et al., 2016;Matsui et al., 2014;Ng et al., 2007;Pye et al., 2010Pye et al., , 2013Pye et al., , 2015Rollins et al., 2012;Wildt et al., 2014;Xu et al., 2014Zhang et al., 2017;Zheng et al., 2015). ...
During the 2013 Southern Oxidant and Aerosol Study, Fourier transform infrared spectroscopy (FTIR) and aerosol mass spectrometer (AMS) measurements of submicron mass were collected at Look Rock (LRK), Tennessee, and Centreville (CTR), Alabama. Carbon monoxide and submicron sulfate and organic mass concentrations were 15–60% higher at CTR than at LRK, but their time series had moderate correlations (r ~ 0.5). However, NOx had no correlation (r = 0.08) between the two sites with nighttime-to-early-morning peaks 3–10 times higher at CTR than at LRK. Organic mass (OM) sources identified by FTIR Positive Matrix Factorization (PMF) had three very similar factors at both sites: fossil fuel combustion-related organic aerosols, mixed organic aerosols, and biogenic organic aerosols (BOA). The BOA spectrum from FTIR is similar (cosine similarity > 0.6) to that of lab-generated particle mass from the photochemical oxidation of both isoprene and monoterpenes under high NOx conditions from chamber experiments. The BOA mass fraction was highest during the night at CTR but in the afternoon at LRK. AMS PMF resulted in two similar pairs of factors at both sites and a third nighttime NOx-related factor (33% of OM) at CTR but a daytime nitrate-related factor (28% of OM) at LRK. NOx was correlated with BOA and LO-OOA for NOx concentrations higher than 1 ppb at both sites, producing 0.5 ± 0.1 μg/m³ for CTR-LO-OOA and 1.0 ± 0.3 μg/m³ for CTR-BOA additional biogenic OM for each 1 ppb increase of NOx.
To quantify the volatility of organic aerosols (OA), a comprehensive campaign was conducted in the Chinese megacity. Volatility distributions of OA and particle‐phase organic nitrate (pON) were estimated based on five methods: (a) empirical method and (b) kinetic model based on the measurement of a thermodenuder (TD) coupled with an aerosol mass spectrometer; (c) Formula‐based SIMPOL model‐driven method; (d) Element‐based estimations using molecular formula measurements of OA; and (e) gas/particle partitioning. Our results demonstrate that the ambient OA volatility distribution shows good agreement between the two heating methods and the formula‐based method when assuming ambient OA was mainly composed of organic nitrate (pON), organic sulfate and acid groups using the SIMPOL model. However, the element‐based method tends to overestimate the volatility of OA compared to the above three methods, suggesting large uncertainties in the parameterizations or in the representativeness of the molecular measurements that need further refinement. The volatility of ambient OA is generally lower than that of the laboratory‐derived secondary OA, emphasizing the impact of aging. A large fraction at the higher and lower volatility ranges (approximately log C* ≤ −9 and ≥2 μg m⁻³) was found for pON, implying the importance of both extremely low volatile and semi‐volatile species. Overall, this study evaluates different methods for volatility estimation and gives new insight into the volatility of OA and pON in urban areas.
Secondary organic aerosol (SOA) exerts a considerable influence on atmospheric chemistry. However, little information about the vertical distribution of SOA in the alpine setting is available, which limited the simulation of SOA using chemical transport models. Here, a total of 15 biogenic and anthropogenic SOA tracers were measured in PM2.5 aerosols at both the summit (1840 m a.s.l.) and foot (480 m a.s.l.) of Mt. Huang during the winter of 2020 to explore their vertical distribution and formation mechanism. Most of the determined chemical species (e.g., BSOA and ASOA tracers, carbonaceous components, major inorganic ions) and gaseous pollutants at the foot of Mt. Huang were 1.7-3.2 times higher concentrations than those at the summit, suggesting the relatively more significant effect of anthropogenic emissions at the ground level. The ISORROPIA-II model showed that aerosol acidity increases as altitude decreases. Air mass trajectories, potential source contribution function (PSCF), and correlation analysis of BSOA tracers with temperature revealed that SOA at the foot of Mt. Huang was mostly derived from the local oxidation of volatile organic compounds (VOCs), while SOA at the summit was mainly influenced by long-distance transport. The robust correlations of BSOA tracers with anthropogenic pollutants (e.g., NH3, NO2, and SO2) (r = 0.54-0.91, p < 0.05) indicated that anthropogenic emissions could promote BSOA productions in the mountainous background atmosphere. Moreover, most of SOA tracers (r = 0.63-0.96, p < 0.01) and carbonaceous species (r = 0.58-0.81, p < 0.01) were correlated well with levoglucosan in all samples, suggesting that biomass burning played an important role in the mountain troposphere. This work demonstrated that daytime SOA at the summit of Mt. Huang was significantly influenced by the valley breeze in winter. Our results provide new insights into the vertical distributions and provenance of SOA in the free troposphere over East China.
Observed surface organic aerosols (OA) concentrations slightly increased in the western US (WUS) but significantly decreased in the eastern US (EUS) in summer, and continuously decreased in winter over the US region. To understand the driving factors for the long‐term surface OA trend, we apply a revised version of the Community Atmosphere Model version 6 with comprehensive tropospheric and stratospheric chemistry representation, considering the heterogeneous formation of isoprene‐epoxydiol‐derived secondary organic aerosols (SOAIE) and fast photolysis rate of monoterpene‐derived secondary organic aerosols (MTSOA) to diagnose the OA evolution in 1988–2019. Compared to older versions, the revised model better reproduces the climatology, seasonal cycle, and long‐term trend of surface OA as evaluated against the Interagency Monitoring of Protected Visual Environments measurements. We find the decrease in EUS summertime OA is likely attributed to the interplay between SOAIE and MTSOA. With anthropogenic emissions reduction, primary organic aerosols (POA) declined, SOAIE decreased along with sulfate, while MTSOA increased along with biogenic emissions driven by a warming climate. POA from wildfires with a significant trend of 2.9% yr⁻¹ and considerable interannual variation of 62.8% drive the statistically insignificant but increasing WUS summertime OA, while anthropogenic POA dominates the decreasing wintertime OA in the US. Through sensitivity experiments, we find MTSOA show linear responses to the increasing monoterpenes emissions and negligible responses to NOx emissions reduction due to the mutual offsets between MTSOA components from different oxidation pathways. This study reveals the increasingly important role of MTSOA in summertime OA under a warming climate.
To understand the source, formation, and seasonality of biogenic secondary organic aerosol (BSOA), a nine-stage cascade impactor was utilized to collect size-segregated particulate samples from April 2017 to January 2018 in Beijing, China. BSOA tracers derived from isoprene, monoterpene, and sesquiterpene were measured with gas chromatography-mass spectrometry. Isoprene and monoterpene SOA tracers exhibited significant seasonal variations, with a summer maximum and a winter minimum. Dominance of 2-methyltetrols (an isoprene SOA tracer) with a good correlation with levoglucosan (a biomass burning tracer), which when combined with the detection of methyltartaric acids (possible indicators for aged isoprene) in summer, implies possible biomass burning and long-range transport. In contrast, sesquiterpene SOA tracer (β-caryophyllinic acid) was dominant in winter and was probably associated with the local burning of biomass. Bimodal size distributions were observed for most isoprene SOA tracers, consistent with previous laboratory experiments and field studies showing that they can be formed not only in the aerosol phase but also in the gas phase. Monoterpene SOA tracers cis-pinonic acid and pinic acid showed a coarse-mode peak (5.8-9.0 μm) in four seasons due to their volatile nature. Sesquiterpene SOA tracer β-caryophyllinic acid showed a unimodal pattern with a major fine-mode peak (1.1-2.1 μm), which is linked to local biomass burning. The tracer-yield method was used to quantify the contributions of isoprene, monoterpene, and sesquiterpene to secondary organic carbon (SOC) and SOA. The highest isoprene SOC and SOA concentrations occurred in summer (2.00 μgC m-3 and 4.93 μg m-3, respectively), contributing to 1.61% of OC and 5.22% of PM2.5, respectively. These results suggest that BSOA tracers are promising tracers for understanding the source, formation, and seasonality of BSOA.
Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.
To better understand the formation process of biogenic and anthropogenic secondary organic aerosols (BSOA and ASOA) in the marine atmosphere under the background of global warming, aerosol samples were collected over three summers (i.e., 2014, 2016 and 2018) from the Bering Sea (BS) to the western North Pacific (WNP). The results showed that temporally, atmospheric concentrations of isoprene-derived SOA (SOAI) tracers were the lowest in 2014 regardless of the marine region, while atmospheric concentrations of monoterpenes-derived SOA (SOAM) tracers in this year were the highest and the aerosols were more aged than those in the other two years. In comparison, the concentrations of β-caryophyllene-derived and toluene-derived SOA (SOAC and SOAA) tracers were relatively low overall. Spatially, the concentrations of SOA tracers were significantly higher over the WNP than over the BS, with SOA tracers over the BS mainly coming from marine sources, while the WNP was strongly influenced by terrestrial inputs. In particular, for land-influenced samples from the WNP, NOx-channel products of SOAI were more dependent on O3 and SO2 relative to HO2-channel product, and the high atmospheric oxidation capacity and SO2 could promote the formation of later-generation SOAM products. The extent of terrestrial influence was further quantified using a principal component analysis (PCA)-generalized additive model (GAM), which showed that terrestrial emissions explained more than half of the BSOA tracers' concentrations and contributed almost all of the ASOA tracer. In addition, the assessment of secondary organic carbon (SOC) highlighted the key role of anthropogenic activities in organic carbon levels in offshore areas. Our study revealed significant contributions of terrestrial natural and anthropogenic sources to different SOA over the WNP, and these relevant findings help improve knowledge about SOA in the marine atmosphere.
A combination of online and offline mass spectrometric techniques was used to characterize the chemical composition of secondary organic aerosol (SOA) generated from the photooxidation of α-pinene in an atmospheric simulation chamber. The filter inlet for gases and aerosols (FIGAERO) coupled with a high-resolution time-of-flight iodide chemical ionization mass spectrometer (I−-ToF-CIMS) was employed to track the evolution of gaseous and particulate components. Extracts of aerosol particles sampled onto a filter at the end of each experiment were analysed using ultra-performance liquid chromatography ultra-high-resolution tandem mass spectrometry (LC-Orbitrap MS). Each technique was used to investigate the major SOA elemental group contributions in each system. The online CIMS particle-phase measurements show that organic species containing exclusively carbon, hydrogen, and oxygen (CHO group) dominate the contribution to the ion signals from the SOA products, broadly consistent with the LC-Orbitrap MS negative mode analysis, which was better able to identify the sulfur-containing fraction. An increased abundance of high-carbon-number (nC≥16) compounds additionally containing nitrogen (CHON group) was detected in the LC-Orbitrap MS positive ionization mode, indicating a fraction missed by the negative-mode and CIMS measurements. Time series of gas-phase and particle-phase oxidation products provided by online measurements allowed investigation of the gas-phase chemistry of those products by hierarchical clustering analysis to assess the phase partitioning of individual molecular compositions. The particle-phase clustering was used to inform the selection of components for targeted structural analysis of the offline samples. Saturation concentrations derived from nearly simultaneous gaseous and particulate measurements of the same ions by FIGAERO-CIMS were compared with those estimated from the molecular structure based on the LC-Orbitrap MS measurements to interpret the component partitioning behaviour. This paper explores the insight brought to the interpretation of SOA chemical composition by the combined application of online FIGAERO-CIMS and offline LC-Orbitrap MS analytical techniques.
A combination of online and offline mass spectrometric techniques was used to characterize the chemical composition of secondary organic aerosol (SOA) generated from the photooxidation of α-pinene in an atmospheric simulation chamber. The filter inlet for gases and aerosols (FIGAERO) coupled with a high-resolution time-of-flight iodide chemical ionization mass spectrometer (I–ToF-CIMS) was employed to track the evolution of gaseous and particulate components. Extracts of aerosol particles sampled onto a filter at the end of each experiment were analyzed using ultra-performance liquid chromatography ultra-high-resolution tandem mass spectrometry (LC-Orbitrap MS). Each technique was used to investigate the major SOA elemental group contributions in each system. The online CIMS particle-phase measurements show that organic species containing exclusively carbon, hydrogen and oxygen (CHO group) dominate the contribution to the ion signals from the SOA products, broadly consistent with the LC-Orbitrap MS negative mode analysis which was better able to identify the sulphur-containing fraction. An increased abundance of high carbon number (nC ≥ 16) compounds additionally containing nitrogen (CHON group) was detected in the LC-Orbitrap MS positive ionisation mode, indicating a fraction missed by the negative mode and CIMS measurements. Time series of gas-phase and particle-phase oxidation products provided by online measurements allowed investigation of the gas-phase chemistry of those products by hierarchical clustering analysis to assess the phase partitioning of individual molecular compositions. The particle-phase clustering was used to inform the selection of components for targeted structural analysis of the offline samples. Saturation concentrations derived from near-simultaneous gaseous and particulate measurements of the same ions by FIGAERO-CIMS were compared with those estimated from the molecular structure based on the LC-Orbitrap MS measurements to interpret the component partitioning behaviour. This paper explores the insight brought to the interpretation of SOA chemical composition by the combined application of online FIGAERO-CIMS and offline LC-Orbitrap MS analytical techniques.
Atmospheric aerosols are a significant public health hazard and have substantial impacts on the climate. Secondary organic aerosols (SOAs) have been shown to phase separate into a highly viscous organic outer layer surrounding an aqueous core. This phase separation can decrease the partitioning of semi-volatile and low-volatile species to the organic phase and alter the extent of acid-catalyzed reactions in the aqueous core. A new algorithm that can determine SOA phase separation based on their glass transition temperature (Tg), oxygen to carbon (O:C) ratio and organic mass to sulfate ratio, and meteorological conditions was implemented into the Community Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 and was used to simulate the conditions in the continental United States for the summer of 2013. SOA formed at the ground/surface level was predicted to be phase separated with core–shell morphology, i.e., aqueous inorganic core surrounded by organic coating 65.4 % of the time during the 2013 Southern Oxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeastern United States. Our estimate is in proximity to the previously reported ∼70 % in literature. The phase states of organic coatings switched between semi-solid and liquid states, depending on the environmental conditions. The semi-solid shell occurring with lower aerosol liquid water content (western United States and at higher altitudes) has a viscosity that was predicted to be 102–1012 Pa s, which resulted in organic mass being decreased due to diffusion limitation. Organic aerosol was primarily liquid where aerosol liquid water was dominant (eastern United States and at the surface), with a viscosity
Organic aerosol (OA), with a large biogenic fraction in summertime southeast US, adversely impacts on air quality and human health. Stringent air quality controls have recently reduced anthropogenic pollutants including sulfate, whose impact on OA remains unclear. Three filter measurement networks provide long-term constraints on the sensitivity of OA to changes in inorganic species, including sulfate and ammonia. The 2000–2013 summertime OA decreases by 1.7~1.9 %/year with little month-to-month variability, while sulfate declines rapidly with significant monthly difference in early 2000s. In contrast, modeled OA from a chemical-transport model (GEOS-Chem) decreases by 4.9 %/year with much larger month-to-month variability, largely due to the predominant role of acid-catalyzed reactive uptake of epoxydiols (IEPOX) onto sulfate. The overestimated modeled OA dependence on sulfate can be improved by implementing a coating effect and assuming constant aerosol acidity, suggesting the needs to revisit IEPOX reactive uptake in current models. Our work highlights the importance of secondary OA formation pathways that are weakly dependent on inorganic aerosol in a region that is heavily influenced by both biogenic and anthropogenic emissions.
Multiple observation data sets – Interagency Monitoring of Protected Visual
Environments (IMPROVE) network data, the Automated Smoke Detection and Tracking
Algorithm (ASDTA), Hazard Mapping System (HMS) smoke plume shapefiles and
aircraft acetonitrile (CH3CN) measurements from the NOAA Southeast
Nexus (SENEX) field campaign – are used to evaluate the HMS–BlueSky–SMOKE (Sparse Matrix Operator Kernel Emission)–CMAQ (Community Multi-scale Air Quality Model)
fire emissions and smoke plume prediction system. A similar configuration is
used in the US National Air Quality Forecasting Capability (NAQFC). The
system was found to capture most of the observed fire signals. Usage of
HMS-detected fire hotspots and smoke plume information was valuable for
deriving both fire emissions and forecast evaluation. This study also
identified that the operational NAQFC did not include fire contributions
through lateral boundary conditions, resulting in significant simulation
uncertainties. In this study we focused both on system evaluation and
evaluation methods. We discussed how to use observational data correctly to
retrieve fire signals and synergistically use multiple data sets. We also
addressed the limitations of each of the observation data sets and
evaluation methods.
Acidity, defined as pH, is a central component of aqueous
chemistry. In the atmosphere, the acidity of condensed phases (aerosol
particles, cloud water, and fog droplets) governs the phase partitioning of
semivolatile gases such as HNO3, NH3, HCl, and organic acids and
bases as well as chemical reaction rates. It has implications for the
atmospheric lifetime of pollutants, deposition, and human health. Despite
its fundamental role in atmospheric processes, only recently has this field
seen a growth in the number of studies on particle acidity. Even with this
growth, many fine-particle pH estimates must be based on thermodynamic model
calculations since no operational techniques exist for direct measurements.
Current information indicates acidic fine particles are ubiquitous, but
observationally constrained pH estimates are limited in spatial and temporal
coverage. Clouds and fogs are also generally acidic, but to a lesser degree
than particles, and have a range of pH that is quite sensitive to
anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient
ammonia. Historical measurements indicate that cloud and fog droplet pH has
changed in recent decades in response to controls on anthropogenic
emissions, while the limited trend data for aerosol particles indicate
acidity may be relatively constant due to the semivolatile nature of the
key acids and bases and buffering in particles. This paper reviews and
synthesizes the current state of knowledge on the acidity of atmospheric
condensed phases, specifically particles and cloud droplets. It includes
recommendations for estimating acidity and pH, standard nomenclature, a
synthesis of current pH estimates based on observations, and new model
calculations on the local and global scale.
Anthropogenic emissions alter secondary organic aerosol (SOA) formation chemistry from naturally emitted isoprene. We use correlations of tracers and tracer ratios to provide new perspectives on sulfate, NOx, and particle acidity influencing isoprene-derived SOA in two isoprene-rich forested environments representing clean to polluted conditions-wet and dry seasons in central Amazonia and Southeastern U.S. summer. We used a semivolatile thermal desorption aerosol gas chromatograph (SV-TAG) and filter samplers to measure SOA tracers indicative of isoprene/HO2 (2-methyltetrols, C5-alkene triols, 2-methyltetrol organosulfates) and isoprene/NO
x
(2-methylglyceric acid, 2-methylglyceric acid organosulfate) pathways. Summed concentrations of these tracers correlated with particulate sulfate spanning three orders of magnitude, suggesting that 1 μg m-3 reduction in sulfate corresponds with at least ∼0.5 μg m-3 reduction in isoprene-derived SOA. We also find that isoprene/NO
x
pathway SOA mass primarily comprises organosulfates, ∼97% in the Amazon and ∼55% in Southeastern United States. We infer under natural conditions in high isoprene emission regions that preindustrial aerosol sulfate was almost exclusively isoprene-derived organosulfates, which are traditionally thought of as representative of an anthropogenic influence. We further report the first field observations showing that particle acidity correlates positively with 2-methylglyceric acid partitioning to the gas phase and negatively with the ratio of 2-methyltetrols to C5-alkene triols.
The reactive uptake of isoprene epoxydiols (IEPOX) is a significant source of isoprene-derived secondary organic aerosols (SOA). Multiple field studies have reported that summertime isoprene-derived SOA in the Southeastern U.S. correlated strongly with sulfate mass concentration. However, previous laboratory studies have focused largely on the effect of aerosol acidity on the reactive uptake of IEPOX. In this study, we investigated the role of inorganic sulfate aerosols in SOA formation arising from the reactive uptake of trans-β-IEPOX (the predominant IEPOX isomer) at 50–56% RH in laboratory chamber experiments. Our measurements showed that the SOA mass concentration increased with the sulfate mass for both highly acidic and less acidic seed aerosols. This was due to the roles that sulfate played in SOA formation as a particle-phase reactant and as a contributor to aerosol surface area and volume. Higher concentrations of SOA were formed when highly acidic seed aerosols were used, consistent with previous laboratory studies. SOA mass concentration and composition were also observed to be dependent on the injection order of IEPOX and sulfate seed aerosols (i.e., injection of IEPOX first vs. Injection of seed aerosols first) in the chamber experiments. Higher SOA mass concentrations were measured in experiments where sulfate seed aerosols were introduced into the chamber first, followed by IEPOX. Volatility measurements showed that the SOA formed in the “seed aerosols first” experiments likely contained larger quantities of low volatility organic matter compared to SOA formed in the “IEPOX first” experiments. These results showed that the mass concentration and composition of IEPOX-derived SOA formed in chamber experiments can be sensitive to mixing conditions in the chamber brought about by slight differences in experimental methodology (in this case injection procedure). The sensitivity of SOA formation to the amount of seed aerosols and injection procedure used in chamber experiments indicated that caution should be exercised when extrapolating laboratory data to ambient conditions.
Guaiacol (2-methoxyphenol) has a high emission rate from wood burning and mainly exists in the gas phase, but the formation potential of secondary organic aerosol (SOA) from the atmospheric oxidation of guaiacol has not been well determined yet. In this work, SOA formation from the gas-phase reaction of guaiacol with OH radicals was investigated using an oxidation flow reactor (OFR) under different experimental conditions. The results showed that SOA yield was dependent on guaiacol concentration, OH exposure, and the presence of SO 2 and NO 2 . SOA yield firstly increased and then decreased as a function of OH exposure. The maximum SOA yield (0.28–0.54) obtained at different guaiacol concentrations could be well-expressed by a one-product model. The SOA oxidation degree was represented by the carbon oxidation state (OS C ) and f 44 /f 43 (the ratio of organic mass fractions of m/z 44 to m/z 43), which both increased linearly and significantly with the increase of OH exposure. In addition, SO 2 and NO 2 promoted SOA formation, for which the maximum yield enhancements were 13.38% and 10.69%, respectively. The N/C ratio (0.034–0.045) indicated that NO 2 participated in the OH-initiated reaction of guaiacol, consequently resulting in the formation of organic nitrates. The experimental results would be helpful to further the understanding of SOA formation from the atmospheric oxidation of guaiacol and its subsequent impacts on air quality and climate.
Multiple observation data sets: Interagency Monitoring of Protected Visual Environments (IMPROVE) network data, Automated Smoke Detection and Tracking Algorithm (ASDTA), Hazard Mapping System (HMS) smoke plume shapefiles and aircraft acetonitrile (CH3CN) measurements from the NOAA Southeast Nexus (SENEX) field campaign are used to evaluate the HMS-BlueSky-SMOKE CMAQ fire emissions and smoke plume prediction system. A similar configuration is used in the US National Air Quality Forecasting Capability (NAQFC). The system was found to capture most of the observed fire signals. Usage of HMS-detected fire hotspots and smoke plume information were valuable for both deriving fire emissions and forecast evaluation. This study also helped identified that the operational NAQFC did not include fire contributions through lateral boundary conditions resulting in significant simulation uncertainties. In this study we focused both on system evaluation and evaluation methods. We discussed how to use observational data correctly to filter out fire signals and synergistically use multiple data sets. We also addressed the limitations of each of the observation data sets and evaluation methods.
Acidity is a critical chemical property of atmospheric aerosols that governs key atmospheric processes, such as secondary organic aerosol formation. However, aerosol acidity is challenging to measure due to the non-conservative nature of H + ion concentration, which changes with relative humidity and aerosol water content. Current methods used to determine acidity include ion balance, phase partitioning, and thermodynamic modeling, which are all indirect and have limitations. Herein, we discuss the recent development of a direct method to measure aerosol acidity using vibrational spectroscopy. For this spectroscopic method, the vibrational modes of several organic and inorganic acids and their conjugate bases are distinct (often ∼ 50 cm -1 apart) and measurable, despite structurally differing by only a proton. Using Raman microspectroscopy, the pH of individual particles (> 1 μm) has been determined for a range of chemical equilibrium systems at pH values near the pK a, including bisulfate-sulfate, bicarbonate-carbonate, nitric acid-nitrate, bioxalate-oxalate, and acetic acid-acetate, as well as a mixture of bisulfate-sulfate and bioxalate-oxalate. The integrated peak areas for the acid and conjugate base vibrational modes are calibrated for concentration and combined with thermodynamic calculations of activity coefficients to determine H + activity, and, ultimately, pH. The development of direct methods for measuring aerosol acidity is needed to mechanistically understand multiphase chemistry in a range of important atmospheric systems.
Organic aerosol (OA) is one of the main components of the global particulate burden and intimately links natural and anthropogenic emissions with air quality and climate. It is challenging to accurately represent OA in global models. Direct quantification of global OA abundance is not possible with current remote sensing technology; however, it may be possible to exploit correlations of OA with remotely observable quantities to infer OA spatiotemporal variability. In particular, formaldehyde (HCHO) and OA share common sources via both primary emissions and secondary production from oxidation of volatile organic compounds (VOCs). We examine OA-HCHO correlations using data from summer-time airborne campaigns investigating biogenic (NASA SEAC4RS and DC3), biomass burning (NASA SEAC4RS) and anthropogenic conditions (NOAA CalNex and NASA KORUS-AQ). In situ OA correlates well with HCHO (r = 0.59–0.97) but the slope and intercept of this relationship vary with chemical regime. For biogenic and anthropogenic regions, the OA-vs-HCHO slope is higher in low NOx conditions, where HCHO yields are lower and aerosol yields are likely higher. The OA-vs-HCHO slope of wild fires is more than 9 times higher than that associated with biogenic and anthropogenic sources. An estimate of near-surface OA is derived by combining observed in situ relationships with HCHO column retrievals from NASA’s Ozone Monitoring Instrument (OMI). We evaluate this OA estimate against OA observations from the US EPA IMPROVE network and simulated OA from the GEOS-Chem global chemical transport model. The OA estimate compares well with IMPROVE data obtained over summer months (e.g. slope = 0.62, r = 0.56 for August 2013), comparable to intensively validated GEOS-Chem performance (e.g. slope = 0.57, r = 0.56) and superior to the correlation with satellite-derived total aerosol extinction (r = 0.41). Improving the detection limit of satellite HCHO and expanding in situ airborne HCHO and OA coverage in future missions will improve the quality and spatiotemporal coverage of this OA estimate, potentially enabling constraints on the global OA distribution.
The implementation of stringent emission regulations has resulted in the decline of anthropogenic pollutants including sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon monoxide (CO). In contrast, ammonia (NH3) emissions are largely unregulated, with emissions projected to increase in the future. We present real-time aerosol and gas measurements from a field study conducted in an agriculturally intensive region in the southeastern US during the fall of 2016 to investigate how NH3 affects particle acidity and secondary organic aerosol (SOA) formation via the gas–particle partitioning of semi-volatile organic acids. Particle water and pH were determined using the ISORROPIA II thermodynamic model and validated by comparing predicted inorganic HNO3-NO3⁻ and NH3-NH4⁺ gas–particle partitioning ratios with measured values. Our results showed that despite the high NH3 concentrations (average 8.1±5.2ppb), PM1 was highly acidic with pH values ranging from 0.9 to 3.8, and an average pH of 2.2±0.6. PM1 pH varied by approximately 1.4 units diurnally. Formic and acetic acids were the most abundant gas-phase organic acids, and oxalate was the most abundant particle-phase water-soluble organic acid anion. Measured particle-phase water-soluble organic acids were on average 6% of the total non-refractory PM1 organic aerosol mass. The measured molar fraction of oxalic acid in the particle phase (i.e., particle-phase oxalic acid molar concentration divided by the total oxalic acid molar concentration) ranged between 47% and 90% for a PM1 pH of 1.2 to 3.4. The measured oxalic acid gas–particle partitioning ratios were in good agreement with their corresponding thermodynamic predictions, calculated based on oxalic acid's physicochemical properties, ambient temperature, particle water, and pH. In contrast, gas–particle partitioning ratios of formic and acetic acids were not well predicted for reasons currently unknown. For this study, higher NH3 concentrations relative to what has been measured in the region in previous studies had minor effects on PM1 organic acids and their influence on the overall organic aerosol and PM1 mass concentrations.
We present measurements of highly oxidized multifunctional molecules (HOMs) detected in the gas phase using a high-resolution time-of-flight chemical ionization mass spectrometer with nitrate reagent ion (NO3- CIMS). The measurements took place during the 2013 Southern Oxidant and Aerosol Study (SOAS 2013) at a forest site in Alabama, where emissions were dominated by biogenic volatile organic compounds (BVOC). Primary BVOC emissions were represented by isoprene mixed with various terpenes, making it a unique sampling location compared to previous NO3- CIMS deployments in monoterpene-dominated environments. During SOAS 2013, the NO3- CIMS detected HOMs with oxygen-to-carbon (O:C) ratios between 0.5 and 1.4 originating from both isoprene (C5) and monoterpenes (C10), as well as hundreds of additional HOMs with carbon number between C3 and C20. We used Positive Matrix Factorization (PMF) to deconvolve the complex dataset and extract information about classes of HOMs with similar temporal trends. This analysis revealed three isoprene-dominated and three monoterpene-dominated PMF factors. We observed significant amounts of isoprene- and monoterpene-derived organic nitrates (ON) in most factors. The abundant presence of ON was consistent with previous studies that have highlighted the importance of NOx-driven chemistry at the site. One of the isoprene-dominated factors had strong correlation with SO2 plumes likely advected from nearby coal-fired power plants, and was dominated by an isoprene-derived ON (C5H10N2O8). These results indicate that anthropogenic emissions played a significant role in the formation of low volatility compounds from BVOC emissions in the region.
The volatility distribution of the
organic aerosol (OA) and its sources during the Southern Oxidant and Aerosol
Study (SOAS; Centreville, Alabama) was constrained using measurements from an
Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS) and a thermodenuder (TD). Positive matrix factorization (PMF)
analysis was applied on both the ambient and thermodenuded high-resolution
mass spectra, leading to four factors: more oxidized oxygenated OA (MO-OOA),
less oxidized oxygenated OA (LO-OOA), an isoprene epoxydiol (IEPOX)-related
factor (isoprene-OA) and biomass burning OA (BBOA). BBOA had the highest mass
fraction remaining (MFR) at 100 °C, followed by the isoprene-OA, and
the LO-OOA. Surprisingly the MO-OOA evaporated the most in the TD. The
estimated effective vaporization enthalpies assuming an evaporation
coefficient equal to unity were 58 ± 13 kJ mol−1 for the LO-OOA,
89 ± 10 kJ mol−1 for the MO-OOA, 55 ± 11 kJ mol−1
for the BBOA, and 63 ± 15 kJ mol−1 for the isoprene-OA. The
estimated volatility distribution of all factors covered a wide range
including both semi-volatile and low-volatility components. BBOA had the
lowest average volatility of all factors, even though it had the lowest
O : C ratio among all factors. LO-OOA was the more volatile factor and
its high MFR was due to its low enthalpy of vaporization according to the
model. The isoprene-OA factor had intermediate volatility, quite higher than
suggested by a few other studies. The analysis suggests that deducing the
volatility of a factor only from its MFR could lead to erroneous conclusions.
The oxygen content of the factors can be combined with their estimated
volatility and hygroscopicity to provide a better view of their physical
properties.
Concentrations of atmospheric trace species in the United States have changed dramatically over the past several decades in response to pollution control strategies, shifts in domestic energy policy and economics, and economic development (and resulting emission changes) elsewhere in the world. Reliable projections of the future atmosphere require models to not only accurately describe current atmospheric concentrations, but to do so by representing chemical, physical and biological processes with conceptual and quantitative fidelity. Only through incorporation of the processes controlling emissions and chemical mechanisms that represent the key transformations among reactive molecules can models reliably project the impacts of future policy, energy and climate scenarios. Efforts to properly identify and implement the fundamental and controlling mechanisms in atmospheric models benefit from intensive observation periods, during which collocated measurements of diverse, speciated chemicals in both the gas and condensed phases are obtained. The Southeast Atmosphere Studies (SAS, including SENEX, SOAS, NOMADSS and SEAC4RS) conducted during the summer of 2013 provided an unprecedented opportunity for the atmospheric modeling community to come together to evaluate, diagnose and improve the representation of fundamental climate and air quality processes in models of varying temporal and spatial scales.
This paper is aimed at discussing progress in evaluating, diagnosing and improving air quality and climate modeling using comparisons to SAS observations as a guide to thinking about improvements to mechanisms and parameterizations in models. The effort focused primarily on model representation of fundamental atmospheric processes that are essential to the formation of ozone, secondary organic aerosol (SOA) and other trace species in the troposphere, with the ultimate goal of understanding the radiative impacts of these species in the southeast and elsewhere. Here we address questions surrounding four key themes: gas-phase chemistry, aerosol chemistry, regional climate and chemistry interactions, and natural and anthropogenic emissions. We expect this review to serve as a guidance for future modeling efforts.
As part of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa
(DACCIWA) project, an airborne campaign was designed to measure a large range
of atmospheric constituents, focusing on the effect of anthropogenic
emissions on regional climate. The presented study details results of the
French ATR42 research aircraft, which aimed to characterize gas-phase,
aerosol and cloud properties in the region during the field campaign carried
out in June/July 2016 in combination with the German Falcon 20 and the
British Twin Otter aircraft. The aircraft flight paths covered large areas of
Benin, Togo, Ghana and Côte d'Ivoire, focusing on emissions from large urban
conurbations such as Abidjan, Accra and Lomé, as well as remote
continental areas and the Gulf of Guinea. This paper focuses on aerosol
particle measurements within the boundary layer (< 2000 m), in
particular their sources and chemical composition in view of the complex mix
of both biogenic and anthropogenic emissions, based on measurements from a
compact time-of-flight aerosol mass spectrometer (C-ToF-AMS) and ancillary
instrumentation.
Background concentrations (i.e. outside urban plumes) observed from the ATR42
indicate a fairly polluted region during the time of the campaign, with
average concentrations of carbon monoxide of 131 ppb, ozone of 32 ppb, and
aerosol particle number concentration ( > 15 nm) of 735 cm−3 stp.
Regarding submicron aerosol composition (considering non-refractory species
and black carbon, BC), organic aerosol (OA) is the most abundant species
contributing 53 %, followed by SO4 (27 %), NH4 (11 %),
BC (6 %), NO3 (2 %) and minor contribution of Cl
(< 0.5 %). Average background PM1 in the region was
5.9 µg m−3 stp. During measurements of urban pollution
plumes, mainly focusing on the outflow of Abidjan, Accra and Lomé,
pollutants are significantly enhanced (e.g. average concentration of CO of
176 ppb, and aerosol particle number concentration of 6500 cm−3 stp),
as well as PM1 concentration (11.9 µg m−3 stp).
Two classes of organic aerosols were estimated based on C-ToF-AMS:
particulate organic nitrates (pONs) and isoprene epoxydiols secondary
organic aerosols (IEPOX–SOA). Both classes are usually associated with the
formation of particulate matter through complex interactions of anthropogenic
and biogenic sources. During DACCIWA, pONs have a fairly small contribution
to OA (around 5 %) and are more associated with long-range transport from
central Africa than local formation. Conversely, IEPOX–SOA provides a
significant contribution to OA (around 24 and 28 % under background and
in-plume conditions). Furthermore, the fractional contribution of IEPOX–SOA
is largely unaffected by changes in the aerosol composition (particularly the
SO4 concentration), which suggests that IEPOX–SOA concentration is
mainly driven by pre-existing aerosol surface, instead of aerosol chemical
properties. At times of large in-plume SO4 enhancements (above
5 µg m−3), the fractional contribution of IEPOX–SOA to OA
increases above 50 %, suggesting only then a change in the IEPOX–SOA-controlling mechanism. It is important to note that IEPOX–SOA constitutes a
lower limit to the contribution of biogenic OA, given that other processes
(e.g. non-IEPOX isoprene, monoterpene SOA) are likely in the region. Given
the significant contribution to aerosol concentration, it is crucial that
such complex biogenic–anthropogenic interactions are taken into account in
both present-day and future scenario models of this fast-changing, highly
sensitive region.
Atmospheric organic aerosol (OA) has important impacts on climate and human health but its sources remain poorly understood. Biogenic monoterpenes and sesquiterpenes are critical precursors of OA. The OA generation from these precursors predicted by models has considerable uncertainty owing to a lack of appropriate observations as constraints. Here, we perform novel lab-in-the-field experiments, which allow us to study OA formation under realistic atmospheric conditions and offer a connection between laboratory and field studies. Based on the lab-in-the-field experimental approach and positive matrix factorization analysis on aerosol mass spectrometry data, we provide a measure of OA from monoterpenes and sesquiterpenes in the southeastern U.S. Further, we use an upgraded atmospheric model and reproduce the measured OA concentration from monoterpenes and sesquiterpenes at multiple sites in the southeastern U.S., building confidence in the observed attribution of monoterpene SOA. We show that the annual average concentration of OA from monoterpenes and sesquiterpenes in the southeastern U.S. is ~ 2.1 µg m−3. This amount is substantially higher than represented in current regional models and accounts for 21 % of World Health Organization PM2.5 standard, indicating a significant contributor of environmental risk to the 77 million habitants in the southeastern U.S.
Multiple observation data sets, including Interagency Monitoring of Protected Visual Environments (IMPROVE) network data, Automated Smoke Detection and Tracking Algorithm (ASDTA), Hazard Mapping System (HMS) smoke plume shapefiles and aircraft acetonitrile (CH3CN) measurements from the NOAA Southeast Nexus (SENEX) field campaign are used to evaluate the HMS-BlueSky-SMOKE-CMAQ fire emissions and smoke plume prediction system. A similar configuration is used in the National Air Quality Forecasting Capability (NAQFC). The system was found to capture signatures of most of the observed fire signals. Use of HMS-detected fire hotspots and smoke plume information are valuable for both initiating fire emissions and evaluating model simulations. However, we also found that the current system does not include fire contributions through lateral boundary condition and missed fires that are not associated with visible smoke plumes resulting in significant simulation uncertainties. In this study we focused not only on model evaluation but also on evaluation methods. We discuss how to use observational data correctly to filter out fire signals and synergistic use of multiple data sets together. We also address the limitations of each of the observation data sets and of the evaluation methods.
Recent modeling and field studies have highlighted a relationship between sulfate concentrations and secondarily formed organic aerosols related to isoprene and other volatile biogenic gaseous emissions. The relationship between these biogenic emissions and sulfate is thought to be primarily associated with the effect of sulfate on aerosol acidity, increased aerosol water at high relative humidities, and aerosol volume. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program provides aerosol concentration levels of sulfate (SO4) and organic carbon (OC) at 136 monitoring sites in rural and remote areas of the United States over time periods of between 15 and 28 years. This dataset allows for an examination of relationships between these variables over time and space. The relative decreases in SO4 and OC were similar over most of the eastern United States, even though concentrations varied dramatically from one region to another. The analysis implied that for every unit decrease in SO4 there was about a 0.29 decrease in organic aerosol mass (OA = 1.8 × OC). This translated to a 2 μg/m3 decrease in biogenically derived secondary organic aerosol over 15 years in the southeastern United States. The analysis further implied that 35% and 27% in 2001 and 2015, respectively, of average total OA may be biogenically derived secondary organic aerosols and that there was a small but significant decrease in OA not linked to changes in SO4 concentrations. The analysis yields a constraint on ambient SO4–OC relationships that should help to refine and improve regional-scale chemical transport models.
Particle acidity affects aerosol concentrations, chemical composition, toxicity and nutrient bioavailability. We present a summary of thermodynamic analysis of comprehensive observations of ambient aerosol collected over the US and E.Mediterranean to understand the levels and drivers of aerosol pH. We find that acidic aerosol in the fine mode is ubiquitous, with levels that range between 0 and 2 throughout most of the data examined. The strong acidity is largely from the large difference in volatility between sulfate (the main acidic compound, which resides completely in the aerosol phase) and ammonia (the main neutralizing agent, which partitions between aerosol and gas-phase). This counterintuitive, but thermodynamically consistent finding explains why aerosol acidity in the southeastern United States has not decreased over the last decades, despite a 70% reduction in sulfates and a constant ammonia background. We then demonstrate that evaluation of model-predicted pH is critical for model predictions of semi-volatile species, e.g., nitrate.
As part of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project, an airborne campaign was designed to measure a large range of atmospheric constituents, focusing on the effect of anthropogenic emissions on regional climate. The presented study details results of the French ATR42 research aircraft, which aimed to characterize gas-phase, aerosol and cloud properties in the region during the field campaign carried out in June/July 2016 in combination with the German Falcon 20 and the British Twin Otter aircraft. The aircraft flight paths covered large areas of Benin, Togo, Ghana and Ivory Coast, focusing on emissions from large urban conurbations such as Abidjan, Accra and Lomé, as well as remote continental areas and the Gulf of Guinea. This manuscript focuses on aerosol particle measurements within the boundary layer ( 15 nm) of 735 cm−3 stp. Regarding submicron aerosol composition (considering non-refractory species and Black Carbon, BC), organic aerosol (OA) is the most abundant species contributing 53 %, followed by SO4 (27 %), NH4 (11 %), BC (6 %), NO3 (2 %) and minor contribution of Cl (
The dihydroxycarbonyls 3,4-dihydroxy-2-butanone (DHBO) and 2,3-dihydroxy-2-methylpropanal (DHMP) formed from isoprene oxidation products in the atmospheric gas phase under low-NO conditions can be expected to form aqSOA in the tropospheric aqueous phase because of their solubility. In the present study, DHBO and DHMP were investigated concerning their radical-driven aqueous-phase oxidation reaction kinetics. For DHBO and DHMP the following rate constants at 298 K are reported: k (OH + DHBO) = (1.0 ± 0.1) × 10(9) L mol(-1) s(-1), k (NO3 + DHBO) = (2.6 ± 1.6) × 10(6) L mol(-1) s(-1), k (SO4(-) + DHBO) = (2.3 ± 0.2) × 10(7) L mol(-1) s(-1), k (OH + DHMP) = (1.2 ± 0.1) × 10(9) L mol(-1) s(-1), k (NO3 + DHMP) = (7.9 ± 0.7) × 10(6) L mol(-1) s(-1), k (SO4(-) + DHMP) = (3.3 ± 0.2) × 10(7) L mol(-1) s(-1) were determined together with their respective temperature dependences. The product studies of both DHBO and DHMP revealed hydroxydicarbonyls, short chain carbonyls and carboxylic acids, such as hydroxyacetone, methylglyoxal, lactic and pyruvic acid as oxidation products with single yields up to 25%. The achieved carbon balance was 75% for DHBO and 67% for DHMP. An aqueous-phase oxidation scheme for both DHBO and DHMP was developed based on the experimental findings to show their potential to contribute to the aqSOA formation. It can be expected that the main contribution to aqSOA occurs via acid formation while other short-chain oxidation products are expected to back-partition into the gas phase to undergo further oxidation there.
Isomeric epoxydiols from isoprene photooxidation (IEPOX) have been shown to
produce substantial amounts of secondary organic aerosol (SOA) mass and are
therefore considered a major isoprene-derived SOA precursor. Heterogeneous
reactions of IEPOX on atmospheric aerosols form various aerosol-phase
components or "tracers" that contribute to the SOA mass burden. A limited
number of the reaction rate constants for these acid-catalyzed aqueous-phase
tracer formation reactions have been constrained through bulk laboratory
measurements. We have designed a chemical box model with multiple
experimental constraints to explicitly simulate gas- and aqueous-phase
reactions during chamber experiments of SOA growth from IEPOX uptake onto
acidic sulfate aerosol. The model is constrained by measurements of the
IEPOX reactive uptake coefficient, IEPOX and aerosol chamber wall losses,
chamber-measured aerosol mass and surface area concentrations, aerosol
thermodynamic model calculations, and offline filter-based measurements of
SOA tracers. By requiring the model output to match the SOA growth and
offline filter measurements collected during the chamber experiments, we
derive estimates of the tracer formation reaction rate constants that have
not yet been measured or estimated for bulk solutions.
Given significant challenges with available measurements of aerosol acidity, proxy methods are
frequently used to estimate the acidity of atmospheric particles. In this study, four of the most
common aerosol acidity proxies are evaluated and compared: (1) the ion balance method, (2) the
molar ratio method, (3) thermodynamic equilibrium models, and (4) the phase partitioning of
ammonia. All methods are evaluated against predictions of thermodynamic models and against direct
observations of aerosol-gas equilibrium partitioning acquired in Mexico City during the MILAGRO
study. The ion balance and molar ratio methods assume that any deficit in inorganic cations
relative to anions is due to the presence of H+; and that a higher H+ loading and
lower cation/anion ratio both correspond to increasingly acidic particles (i.e., lower pH). Based
on the MILAGRO measurements, no correlation is observed between H+ levels inferred with
the ion balance and aerosol pH predicted by the thermodynamic models and ammonia–ammonium
(NH3–NH4+) partitioning. Similarly, no relationship is observed between the
cation / anion molar ratio and predicted aerosol pH. Using only measured aerosol chemical
composition as inputs without any constraint for the gas phase, the Extended Aerosol Inorganics
Model (E-AIM) and ISORROPIA-II thermodynamic equilibrium models tend to predict aerosol pH levels
that are inconsistent with the observed NH3–NH4+ partitioning. The modeled pH
values from both models run with gas + aerosol inputs agreed well with the aerosol pH
predicted by the phase partitioning of ammonia. It appears that (1) thermodynamic models
constrained by gas + aerosol measurements, and (2) the phase partitioning of ammonia provide
the best available predictions of aerosol pH. Furthermore, neither the ion balance nor the molar
ratio can be used as surrogates for aerosol pH, and published studies to date with conclusions
based on such acidity proxies may need to be reevaluated. Given the significance of acidity for
chemical processes in the atmosphere, the implications of this study are important and far
reaching.
The reactive partitioning of cis and trans β-IEPOX
was investigated on hydrated inorganic seed particles, without the addition
of acids. No organic aerosol (OA) formation was observed on dry ammonium
sulfate (AS); however, prompt and efficient OA growth was observed for the
cis and trans β-IEPOX on AS seeds at liquid water
contents of 40–75% of the total particle mass. OA formation from IEPOX is
a kinetically limited process, thus the OA growth continues if there is a
reservoir of gas-phase IEPOX. There appears to be no differences, within
error, in the OA growth or composition attributable to the
cis / trans isomeric structures. Reactive uptake of IEPOX
onto hydrated AS seeds with added base (NaOH) also produced high OA loadings,
suggesting the pH dependence for OA formation from IEPOX is weak for AS
particles. No OA formation, after particle drying, was observed on seed
particles where Na+ was substituted for NH4+. The Henry's Law
partitioning of IEPOX was measured on NaCl particles (ionic strength
~9 M) to be 3 × 107 M atm−1 (−50 / +100%).
A small quantity of OA was produced when NH4+ was present in the
particles, but the chloride (Cl-) anion was substituted for sulfate
(SO42-), possibly suggesting differences in nucleophilic strength of
the anions. Online time-of-flight aerosol mass spectrometry and offline
filter analysis provide evidence of oxygenated hydrocarbons, organosulfates,
and amines in the particle organic composition. The results are consistent
with weak correlations between IEPOX-derived OA and particle acidity or
liquid water observed in field studies, as the chemical system is
nucleophile-limited and not limited in water or catalyst activity.
Isoprene, the most abundant non-methane volatile organic compound
(VOC) emitted into the atmosphere, is known to undergo gas phase
oxidation to form eight different hydroxynitrate isomers in "high
NOx" environments. These hydroxynitrates are known to
affect the global and regional formation of ozone and secondary
organic aerosol (SOA), as well as affect the distribution of
nitrogen. In the present study, we have synthesized three of the
eight possible hydroxynitrates: 4-hydroxy-3-nitroxy isoprene
(4,3-HNI) and E/Z-1-hydroxy-4-nitroxy isoprene (1,4-HNI). Oxidation
of the 4,3-HNI isomer by the OH radical was monitored using a flow tube
chemical ionization mass spectrometer (FT-CIMS), and its OH rate
constant was determined to be
(3.64 ± 0.41) × 10−11 cm3
molecule−1 s−1. The
products of 4,3-HNI oxidation were monitored, and a mechanism to
explain the products was developed. An isoprene epoxide (IEPOX) –
a species important in SOA chemistry and thought to originate only
from "low NOx" isoprene oxidation – was found as a minor,
but significant product. Additionally, hydrolysis kinetics of the
three synthesized isomers were monitored with NMR. The bulk, neutral
solution hydrolysis rate constants for 4,3-HNI and the 1,4-HNI
isomers were (1.59±0.03 × 10−5 s−1 and
(6.76 ± 0.09) × 10−3 s−1, respectively. The
hydrolysis reactions of each isomer were found to be general
acid-catalyzed. The reaction pathways, product yields and
atmospheric implications for both the gas phase and aerosol-phase
reactions are discussed.
Isoprene is the most abundant non-methane biogenic volatile organic compound (BVOC), but the processes governing secondary organic aerosol (SOA) formation from isoprene oxidation are only beginning to become understood and selective quantification of the atmospheric particulate burden remains difficult. Organic aerosol above a tropical rainforest located in Danum Valley, Borneo, Malaysia, a high isoprene emission region, was studied during Summer 2008 using Aerosol Mass Spectrometry and offline detailed characterisation using comprehensive two dimensional gas chromatography. Observations indicate that a substantial fraction (up to 15% by mass) of atmospheric sub-micron organic aerosol was observed as methylfuran (MF) after thermal desorption. This observation was associated with the simultaneous measurements of established gas-phase isoprene oxidation products methylvinylketone (MVK) and methacrolein (MACR). Observations of MF were also made during experimental chamber oxidation of isoprene. Positive matrix factorisation of the AMS organic mass spectral time series produced a robust factor which accounts for an average of 23% (0.18 μg m<sup>−3</sup>), reaching as much as 53% (0.50 μg m<sup>−3</sup>) of the total oraganic loading, identified by (and highly correlated with) a strong MF signal. Assuming that this factor is generally representative of isoprene SOA, isoprene derived aerosol plays a significant role in the region. Comparisons with measurements from other studies suggest this type of isoprene SOA plays a role in other isoprene dominated environments, albeit with varying significance.
We deployed a High-Resolution Time-of-Flight Aerosol Mass Spectrometer
(HR-ToF-AMS) and an Aerosol Chemical Speciation Monitor (ACSM) to
characterize the chemical composition of submicron non-refractory particulate matter
(NR-PM$_{1}$) in the southeastern USA. Measurements were performed in both
rural and urban sites in the greater Atlanta area, Georgia (GA), and
Centreville, Alabama (AL), for approximately 1 year as part of
Southeastern Center for Air Pollution and Epidemiology study (SCAPE) and
Southern Oxidant and Aerosol Study (SOAS). Organic aerosol (OA) accounts for
more than half of NR-PM1 mass concentration regardless of sampling
sites and seasons. Positive matrix factorization (PMF) analysis of
HR-ToF-AMS measurements identified various OA sources, depending on location
and season. Hydrocarbon-like OA (HOA) and cooking OA (COA) have important,
but not dominant, contributions to total OA in urban sites (i.e., 21–38 %
of total OA depending on site and season). Biomass burning OA (BBOA)
concentration shows a distinct seasonal variation with a larger enhancement
in winter than summer. We find a good correlation between BBOA and brown
carbon, indicating biomass burning is an important source for brown carbon,
although an additional, unidentified brown carbon source is likely present
at the rural Yorkville site. Isoprene-derived OA factor (isoprene-OA) is only
deconvolved in warmer months and contributes 18–36 % of total OA. The
presence of isoprene-OA factor in urban sites is more likely from local
production in the presence of NOx than transport from rural sites.
More-oxidized and less-oxidized oxygenated organic aerosol (MO-OOA and
LO-OOA, respectively) are dominant fractions (47–79 %) of OA in all sites.
MO-OOA correlates well with ozone in summer but not in winter, indicating
MO-OOA sources may vary with seasons. LO-OOA, which reaches a daily maximum
at night, correlates better with estimated nitrate functionality from
organic nitrates than total nitrates.
Based on the HR-ToF-AMS measurements, we estimate that the nitrate
functionality from organic nitrates contributes 63–100 % to the total
measured nitrates in summer. Furthermore, the contribution of organic nitrates
to total OA is estimated to be 5–12 % in summer, suggesting that organic
nitrates are important components in the ambient aerosol in the southeastern
USA. The spatial distribution of OA is investigated by comparing simultaneous
HR-ToF-AMS measurements with ACSM measurements at two different sampling
sites. OA is found to be spatially homogeneous in summer due possibly to
stagnant air mass and a dominant amount of regional secondary organic
aerosol (SOA) in the southeastern USA. The homogeneity is less in winter,
which is likely due to spatial variation of primary emissions.
We observe that the seasonality of OA concentration shows a clear
urban/rural contrast. While OA exhibits weak seasonal variation in the urban
sites, its concentration is higher in summer than winter for rural sites.
This observation from our year-long measurements is consistent with 14 years
of organic carbon (OC) data from the SouthEastern Aerosol Research and
Characterization (SEARCH) network. The comparison between short-term
measurements with advanced instruments and long-term measurements of basic
air quality indicators not only tests the robustness of the short-term
measurements but also provides insights in interpreting long-term
measurements. We find that OA factors resolved from PMF analysis on
HR-ToF-AMS measurements have distinctly different diurnal variations. The
compensation of OA factors with different diurnal trends is one possible
reason for the repeatedly observed, relatively flat OA diurnal profile in
the southeastern USA. In addition, analysis of long-term measurements shows
that the correlation between OC and sulfate is substantially stronger in
summer than winter. This seasonality could be partly due to the effects of
sulfate on isoprene SOA formation as revealed by the short-term intensive
measurements.
The budget of atmospheric secondary organic aerosol (SOA) is very uncertain, with recent estimates suggesting a global source of between 12 and 1820 Tg (SOA) a<sup>−1</sup>. We used a dataset of aerosol mass spectrometer (AMS) observations from 34 different surface locations to evaluate the GLOMAP global chemical transport model. The standard model simulation (which included SOA from monoterpenes only) underpredicted organic aerosol (OA) observed by the AMS and had little skill reproducing the variability in the dataset. We simulated SOA formation from biogenic (monoterpenes and isoprene), lumped anthropogenic and lumped biomass burning volatile organic compounds (VOCs) and varied the SOA yield from each precursor source to produce the best overall match between model and observations. We assumed that SOA is essentially non-volatile and condenses irreversibly onto existing aerosol. Our best estimate of the SOA source is 140 Tg (SOA) a<sup>−1</sup> but with a large uncertainty range which we estimate to be 50–380 Tg (SOA) a<sup>−1</sup>. We found the minimum in normalised mean error (NME) between model and the AMS dataset when we assumed a large SOA source (100 Tg (SOA) a<sup>−1</sup>) from sources that spatially matched anthropogenic pollution (which we term antropogenically controlled SOA). We used organic carbon observations compiled by Bahadur et al. (2009) to evaluate our estimated SOA sources. We found that the model with a large anthropogenic SOA source was the most consistent with these observations, however improvement over the model with a large biogenic SOA source (250 Tg (SOA) a<sup>−1</sup>) was small. We used a dataset of <sup>14</sup>C observations from rural locations to evaluate our estimated SOA sources. We estimated a maximum of 10 Tg (SOA) a<sup>−1</sup> (10 %) of the anthropogenically controlled SOA source could be from fossil (urban/industrial) sources. We suggest that an additional anthropogenic source is most likely due to an anthropogenic pollution enhancement of SOA formation from biogenic VOCs. Such an anthropogenically controlled SOA source would result in substantial climate forcing. We estimated a global mean aerosol direct effect of −0.26 ± 0.15 Wm<sup>−2</sup> and indirect (cloud albedo) effect of −0.6<sup>+0.24</sup><sub>−0.14</sub> Wm<sup>−2</sup> from anthropogenically controlled SOA. The biogenic and biomass SOA sources are not well constrained with this analysis due to the limited number of OA observations in regions and periods strongly impacted by these sources. To further improve the constraints by this method, additional OA observations are needed in the tropics and the Southern Hemisphere.
In this paper we report chemically resolved measurements of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In situ measurements of particle phase molecular markers were dominated by secondary compounds and could be categorized into three factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidation products of temperature-driven local biogenic emissions and (3) local light-driven emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g., temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher due to more substantial emissions of isoprene and enhanced photochemistry. Methyl chavicol oxidation contributed similarly to OA during both identified meteorological periods. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during cooler conditions, even though emissions of the precursors were lower. Following the first precipitation event of the fall the abundances of the monoterpene oxidation products increased dramatically, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (>75 %). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction (< 1 %) of the total OA mass and suggest that other mechanisms, e.g., NO<sub>x</sub> enhancement of oxidant levels, are more likely to be responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site.
A suite of offline and real-time gas- and particle-phase measurements was
deployed at Look Rock, Tennessee (TN), during the 2013 Southern Oxidant and
Aerosol Study (SOAS) to examine the effects of anthropogenic emissions on
isoprene-derived secondary organic aerosol (SOA) formation. High- and
low-time-resolution PM2.5 samples were collected for analysis of known
tracer compounds in isoprene-derived SOA by gas chromatography/electron
ionization-mass spectrometry (GC/EI-MS) and ultra performance liquid
chromatography/diode array detection-electrospray ionization-high-resolution
quadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QTOFMS). Source
apportionment of the organic aerosol (OA) was determined by positive matrix
factorization (PMF) analysis of mass spectrometric data acquired on an
Aerodyne Aerosol Chemical Speciation Monitor (ACSM). Campaign average mass
concentrations of the sum of quantified isoprene-derived SOA tracers
contributed to ~ 9 % (up to 28 %) of the total OA mass,
with isoprene-epoxydiol (IEPOX) chemistry accounting for ~ 97 % of the quantified tracers. PMF analysis resolved a factor with a
profile similar to the IEPOX-OA factor resolved in an Atlanta study and was
therefore designated IEPOX-OA. This factor was strongly correlated
(r2 > 0.7) with 2-methyltetrols, C5-alkene triols,
IEPOX-derived organosulfates, and dimers of organosulfates, confirming the
role of IEPOX chemistry as the source. On average, IEPOX-derived SOA tracer
mass was ~ 26 % (up to 49 %) of the IEPOX-OA factor mass,
which accounted for 32 % of the total OA. A low-volatility oxygenated
organic aerosol (LV-OOA) and an oxidized factor with a profile similar to
91Fac observed in areas where emissions are biogenic-dominated were also
resolved by PMF analysis, whereas no primary organic aerosol (POA) sources
could be resolved. These findings were consistent with low levels of primary
pollutants, such as nitric oxide (NO ~ 0.03 ppb), carbon
monoxide (CO ~ 116 ppb), and black carbon (BC ~ 0.2 μg m−3). Particle-phase sulfate is fairly correlated
(r2 ~ 0.3) with both methacrylic acid epoxide
(MAE)/hydroxymethyl-methyl-α-lactone (HMML)- (henceforth called
methacrolein (MACR)-derived SOA tracers) and IEPOX-derived SOA tracers, and
more strongly correlated (r2 ~ 0.6) with the IEPOX-OA
factor, in sum suggesting an important role of sulfate in isoprene SOA
formation. Moderate correlation between the MACR-derived SOA tracer
2-methylglyceric acid with sum of reactive and reservoir nitrogen oxides
(NOy; r2 = 0.38) and nitrate (r2 = 0.45) indicates the
potential influence of anthropogenic emissions through long-range transport.
Despite the lack of a clear association of IEPOX-OA with locally estimated
aerosol acidity and liquid water content (LWC), box model calculations of
IEPOX uptake using the simpleGAMMA model, accounting for the role of acidity
and aerosol water, predicted the abundance of the IEPOX-derived SOA tracers
2-methyltetrols and the corresponding sulfates with good accuracy (r2
~ 0.5 and ~ 0.7, respectively). The modeling and
data combined suggest an anthropogenic influence on isoprene-derived SOA
formation through acid-catalyzed heterogeneous chemistry of IEPOX in the
southeastern US. However, it appears that this process was not limited by
aerosol acidity or LWC at Look Rock during SOAS. Future studies should
further explore the extent to which acidity and LWC as well as aerosol
viscosity and morphology becomes a limiting factor of IEPOX-derived SOA, and
their modulation by anthropogenic emissions.
Gas-phase water-soluble organic matter (WSOMg) is ubiquitous in the troposphere. In the summertime, the potential for these gases to partition to particle-phase liquid water (H2Optcl) where they can form secondary organic aerosol (SOAAQ) is high in the Eastern US and low elsewhere, with the exception of an area near Los Angeles, CA. This spatial pattern is driven by mass concentrations of H2Optcl, not WSOMg. H2Optcl mass concentrations are predicted to be high in the Eastern US, largely due to sulfate. The ability of sulfate to increase H2Optcl is well established and routinely included in atmospheric models; however WSOMg partitioning to this water and subsequent SOA formation is not. The high mass concentrations of H2Optcl in the southeast (SE) US but not the Amazon may help explain why biogenic SOA mass concentrations are high in the SE US but low in the Amazon. Furthermore, during the summertime in the Eastern US, the potential for organic gases to partition into liquid water is greater than their potential to partition into organic matter (OM) because concentrations of WSOMg and H2Optcl are higher than semi-volatile gases and OM. Thus, unless condensed phase yields are substantially different (> ~ order of magnitude), we expect that SOA formed through aqueous-phase pathways (SOAAQ) will dominate in the Eastern US. These findings also suggest that H2Optcl is largely anthropogenic and provide a previously unrecognized mechanism by which anthropogenic pollutants impact the amount of SOA mass formed from biogenic organic emissions. The previously reported estimate of the controllable fraction of biogenic SOA in the Eastern US (50%) is likely too low.
Substantial amounts of secondary organic aerosol (SOA) can be formed from
isoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainly
under low-NO conditions. Total IEPOX-SOA, which may include SOA formed from
other parallel isoprene oxidation pathways, was quantified by applying
positive matrix factorization (PMF) to aerosol mass spectrometer (AMS)
measurements. The IEPOX-SOA fractions of organic aerosol (OA) in multiple field studies across
several continents are summarized here and show consistent patterns with the
concentration of gas-phase IEPOX simulated by the GEOS-Chem chemical
transport model. During the Southern Oxidant and Aerosol Study (SOAS),
78 % of PMF-resolved IEPOX-SOA is accounted by the measured IEPOX-SOA
molecular tracers (2-methyltetrols, C5-Triols, and IEPOX-derived organosulfate
and its dimers), making it the highest level of molecular identification of
an ambient SOA component to our knowledge. An enhanced signal at
C5H6O+ (m/z 82) is found in PMF-resolved IEPOX-SOA spectra.
To investigate the suitability of this ion as a tracer for IEPOX-SOA, we
examine fC5H6O
(fC5H6O=
C5H6O+/OA) across multiple field,
chamber, and source data sets. A background of
~ 1.7 ± 0.1 ‰ (‰ = parts per thousand) is
observed in studies strongly influenced by urban, biomass-burning, and other
anthropogenic primary organic aerosol (POA). Higher background values of
3.1 ± 0.6 ‰ are found in studies strongly influenced by
monoterpene emissions. The average laboratory monoterpene SOA value
(5.5 ± 2.0 ‰) is 4 times lower than the average for IEPOX-SOA
(22 ± 7 ‰), which leaves some room to separate both
contributions to OA. Locations strongly influenced by isoprene emissions
under low-NO levels had higher fC5H6O
(~ 6.5 ± 2.2 ‰ on average) than other sites, consistent
with the expected IEPOX-SOA formation in those studies.
fC5H6O in IEPOX-SOA is always elevated
(12–40 ‰) but varies substantially between locations, which is
shown to reflect large variations in its detailed molecular composition. The
low fC5H6O (< 3 ‰) reported in
non-IEPOX-derived isoprene-SOA from chamber studies indicates that this
tracer ion is specifically enhanced from IEPOX-SOA, and is not a tracer for
all SOA from isoprene. We introduce a graphical diagnostic to study the
presence and aging of IEPOX-SOA as a triangle plot of fCO2
vs. fC5H6O. Finally, we develop a simplified
method to estimate ambient IEPOX-SOA mass concentrations, which is shown to
perform well compared to the full PMF method. The uncertainty of the tracer
method is up to a factor of ~ 2, if the
fC5H6O of the local IEPOX-SOA is not
available. When only unit mass-resolution data are available, as with the
aerosol chemical speciation monitor (ACSM), all methods may perform less well
because of increased interferences from other ions at m/z 82. This study
clarifies the strengths and limitations of the different AMS methods for
detection of IEPOX-SOA and will enable improved characterization of this OA
component.
Oxidation of isoprene is an important source of secondary organic material
(SOM) in atmospheric particles, especially in areas such as the Amazon Basin.
Information on the viscosities, diffusion rates, and mixing times within
isoprene-derived SOM is needed for accurate predictions of air quality,
visibility, and climate. Currently, however, this information is not
available. Using a bead-mobility technique and a poke-flow technique combined
with fluid simulations, the relative humidity (RH)-dependent viscosities of
SOM produced from isoprene photo-oxidation were quantified for
20–60 μm particles at 295 ± 1 K. From 84.5 to 0% RH,
the viscosities for isoprene-derived SOM varied from
~ 2 × 10−1 to ~ 3 × 105 Pa s,
implying that isoprene-derived SOM ranges from a liquid to a semisolid over
this RH range. These viscosities correspond to diffusion coefficients of
~ 2 × 10−8 to
~ 2 × 10−14 cm2 s−1 for large organic
molecules that follow the Stokes–Einstein relation. Based on the diffusion
coefficients, the mixing time of large organic molecules within 200 nm
isoprene-derived SOM particles ranges from approximately 0.1 h to less than
1 s. To illustrate the atmospheric implications of this study's results, the
Amazon Basin is used as a case study for an isoprene-dominant forest.
Considering the RH and temperature range observed in the Amazon Basin and
with some assumptions about the dominant chemical compositions of SOM
particles in the region, it is likely that SOM particles in this area are
liquid and reach equilibrium with large gas-phase organic molecules on short
time scales, less than or equal to approximately 0.1 h.
Smog chamber experiments were conducted to investigate chemical and physical transformations of organic aerosol (OA) during photo-oxidation of open biomass burning emissions. The experiments were carried out at the US Forest Service's Fire Science Laboratory as part of the third Fire Lab at Missoula Experiment (FLAME III). We investigated 12 different fuels commonly burned in North American wildfires. The experiments feature atmospheric and plume aerosol and oxidant concentrations; aging times ranged from 3–4.5 h. OA production, expressed as a mass enhancement ratio (ratio of OA to primary OA (POA) mass), was highly variable. OA mass enhancement ratios ranged from 2.9 in experiments where secondary OA (SOA) production nearly tripled the POA concentration, to 0.7 in experiments where photo-oxidation resulted in a 30% loss of the OA mass. The campaign-average OA mass enhancement ratio was 1.7 ± 0.7 (mean ± 1 σ); therefore, on average, there was substantial SOA production. In every experiment, the OA was chemically transformed. Even in experiments with net loss of OA mass, the OA became increasingly oxygenated and less volatile with aging, indicating that photo-oxidation transformed the POA emissions. Levoglucosan concentrations were also substantially reduced with photo-oxidation. The transformations of POA were extensive; using levoglucosan as a tracer for POA, unreacted POA only contributed 17% of the campaign-average OA mass after 3.5 h of exposure to typical atmospheric hydroxyl radical (OH) levels. Heterogeneous reactions with OH could account for less than half of this transformation, implying that the coupled gas-particle partitioning and reaction of semi-volatile vapors is an important and potentially dominant mechanism for POA processing. Overall, the results illustrate that biomass burning emissions are subject to extensive chemical processing in the atmosphere, and the timescale for these transformations is rapid.
Long-term (1999 to 2013) data from the Southeastern Aerosol Research and
Characterization (SEARCH) network are used to show that anthropogenic
emission reductions led to important decreases in fine-particle organic
aerosol (OA) concentrations in the southeastern US On average, 45 %
(range 25 to 63 %) of the 1999 to 2013 mean organic carbon (OC)
concentrations are attributed to combustion processes, including fossil fuel
use and biomass burning, through associations of measured OC with combustion
products such as elemental carbon (EC), carbon monoxide (CO), and nitrogen oxides (NOx). The 2013 mean combustion-derived OC concentrations were
0.5 to 1.4 µg m−3 at the five sites operating in that year. Mean
annual combustion-derived OC concentrations declined from 3.8 ± 0.2 µg m−3 (68 % of total OC) to 1.4 ± 0.1 µg m−3
(60 % of total OC) between 1999 and 2013 at the urban Atlanta, Georgia,
site (JST) and from 2.9 ± 0.4 µg m−3 (39 % of total OC) to
0.7 ± 0.1 µg m−3 (30 % of total OC) between 2001 and 2013
at the urban Birmingham, Alabama (BHM), site. The urban OC declines coincide
with reductions of motor vehicle emissions between 2006 and 2010, which may
have decreased mean OC concentrations at the urban SEARCH sites by
> 2 µg m−3. BHM additionally exhibits a decline in OC
associated with SO2 from 0.4 ± 0.04 µg m−3 in 2001 to
0.2 ± 0.03 µg m−3 in 2013, interpreted as the result of
reduced emissions from industrial sources within the city. Analyses using
non-soil potassium as a biomass burning tracer indicate that biomass burning
OC occurs throughout the year at all sites. All eight SEARCH sites show an
association of OC with sulfate (SO4) ranging from 0.3 to 1.0 µg m−3 on average, representing ∼ 25 % of the 1999 to 2013
mean OC concentrations. Because the mass of OC identified with SO4
averages 20 to 30 % of the SO4 concentrations, the mean
SO4-associated OC declined by ∼ 0.5 to 1 µg m−3
as SO4 concentrations decreased throughout the SEARCH region. The 2013
mean SO4 concentrations of 1.7 to 2.0 µg m−3 imply that
future decreases in mean SO4-associated OC concentrations would not
exceed ∼ 0.3 to 0.5 µg m−3. Seasonal OC
concentrations, largely identified with ozone (O3), vary from 0.3 to
1.4 µg m−3 ( ∼ 20 % of the total OC
concentrations).
A year-long near-real-time characterization of non-refractory submicron
aerosol (NR-PM1) was conducted at an urban (Atlanta, Georgia, in 2012)
and rural (Look Rock, Tennessee, in 2013) site in the southeastern US using
the Aerodyne Aerosol Chemical Speciation Monitor (ACSM) collocated with
established air-monitoring network measurements. Seasonal variations in
organic aerosol (OA) and inorganic aerosol species are attributed to
meteorological conditions as well as anthropogenic and biogenic emissions in
this region. The highest concentrations of NR-PM1 were observed during
winter and fall seasons at the urban site and during spring and summer at
the rural site. Across all seasons and at both sites, NR-PM1 was
composed largely of OA (up to 76 %) and sulfate (up to 31 %). Six
distinct OA sources were resolved by positive matrix factorization applied
to the ACSM organic mass spectral data collected from the two sites over the
1 year of near-continuous measurements at each site: hydrocarbon-like OA
(HOA), biomass burning OA (BBOA), semi-volatile oxygenated OA (SV-OOA),
low-volatility oxygenated OA (LV-OOA), isoprene-derived epoxydiols (IEPOX) OA
(IEPOX-OA) and 91Fac (a factor dominated by a distinct ion at m∕z 91 fragment
ion previously observed in biogenic influenced areas). LV-OOA was observed
throughout the year at both sites and contributed up to 66 % of total OA
mass. HOA was observed during the entire year only at the urban site (on
average 21 % of OA mass). BBOA (15–33 % of OA mass) was observed during
winter and fall, likely dominated by local residential wood burning
emission. Although SV-OOA contributes quite significantly ( ∼ 27 %), it was observed only at the urban site during colder seasons.
IEPOX-OA was a major component (27–41 %) of OA at both sites, particularly
in spring and summer. An ion fragment at m∕z 75 is well correlated with the
m∕z 82 ion associated with the aerosol mass spectrum of IEPOX-derived secondary
organic aerosol (SOA). The contribution of 91Fac to the total OA mass was
significant (on average 22 % of OA mass) at the rural site only during
warmer months. Comparison of 91Fac OA time series with SOA tracers measured
from filter samples collected at Look Rock suggests that isoprene oxidation
through a pathway other than IEPOX SOA chemistry may contribute to its
formation. Other biogenic sources could also contribute to 91Fac, but there
remains a need to resolve the exact source of this factor based on its
significant contribution to rural OA mass.
Aircraft observations of meteorological, trace gas, and aerosol properties were made during May-September 2013 in the southeastern United States (US) under fair-weather, afternoon conditions with well-defined planetary boundary layer structure. Optical extinction at 532-nm was directly measured at relative humidities (RHs) of ∼15, ∼70, and ∼90-% and compared with extinction calculated from measurements of aerosol composition and size distribution using the κ-Köhler approximation for hygroscopic growth. The calculated enhancement in hydrated aerosol extinction with relative humidity, f(RH), calculated by this method agreed well with the observed f(RH) at ∼90-% RH. The dominance of organic aerosol, which comprised 65-±-10-% of particulate matter with aerodynamic diameter < -1-μm in the planetary boundary layer, resulted in relatively low f(RH) values of 1.43-±-0.67 at 70-% RH and 2.28-±-1.05 at 90-% RH. The subsaturated F-Köhler hygroscopicity parameter F for the organic fraction of the aerosol must have been < -0.10 to be consistent with 75-% of the observations within uncertainties, with a best estimate of F- Combining double low line -0.05. This subsaturated F value for the organic aerosol in the southeastern US is broadly consistent with field studies in rural environments. A new, physically based, single-parameter representation was developed that better described f(RH) than did the widely used gamma power-law approximation.
In the southeastern US, substantial emissions of isoprene from deciduous trees undergo atmospheric oxidation to form secondary organic aerosol (SOA) that contributes to fine particulate matter (PM2.5). Laboratory studies have revealed that anthropogenic pollutants, such as sulfur dioxide (SO2), oxides of nitrogen (NO
x
), and aerosol acidity, can enhance SOA formation from the hydroxyl radical (OH)-initiated oxidation of isoprene; however, the mechanisms by which specific pollutants enhance isoprene SOA in ambient PM2.5 remain unclear. As one aspect of an investigation to examine how anthropogenic pollutants influence isoprene-derived SOA formation, high-volume PM2.5 filter samples were collected at the Birmingham, Alabama (BHM), ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). Sample extracts were analyzed by gas chromatography-electron ionization-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) to identify known isoprene SOA tracers. Tracers quantified using both surrogate and authentic standards were compared with collocated gas- and particle-phase data as well as meteorological data provided by the Southeastern Aerosol Research and Characterization (SEARCH) network to assess the impact of anthropogenic pollution on isoprene-derived SOA formation. Results of this study reveal that isoprene-derived SOA tracers contribute a substantial mass fraction of organic matter (OM) (~ 7 to ~ 20 %). Isoprene-derived SOA tracers correlated with sulfate (
SO
4
2
-
) (r2 = 0.34, n = 117) but not with NO
x
. Moderate correlations between methacrylic acid epoxide and hydroxymethyl-methyl-α-lactone (together abbreviated MAE/HMML)-derived SOA tracers with nitrate radical production (P[NO3]) (r2 = 0.57, n = 40) were observed during nighttime, suggesting a potential role of the NO3 radical in forming this SOA type. However, the nighttime correlation of these tracers with nitrogen dioxide (NO2) (r2 = 0.26, n = 40) was weaker. Ozone (O3) correlated strongly with MAE/HMML-derived tracers (r2 = 0.72, n = 30) and moderately with 2-methyltetrols (r2 = 0.34, n = 15) during daytime only, suggesting that a fraction of SOA formation could occur from isoprene ozonolysis in urban areas. No correlation was observed between aerosol pH and isoprene-derived SOA. Lack of correlation between aerosol acidity and isoprene-derived SOA is consistent with the observation that acidity is not a limiting factor for isoprene SOA formation at the BHM site as aerosols were acidic enough to promote multiphase chemistry of isoprene-derived epoxides throughout the duration of the study. All in all, these results confirm previous studies suggesting that anthropogenic pollutants enhance isoprene-derived SOA formation.
The Southeast United States (US) might not have warmed as much as the rest of the country over the past 50-100 years. Providing an improved understanding of this potential anomaly, and specifically the roles played by aerosols, was one of the main goals for the Southeast Atmosphere Study (SAS). Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant inthe Southeast US and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the SAS studyand was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were almost all within the stated uncertainties. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions.
Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO_2) and hydroperoxyl (HO_2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F_(MAE formation)). The strong temperature dependence of F_(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS;~1 ng m^(–3)) and MAE-derived organosulfates (MAE-OS;~1 ng m^(–3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m^(–3)) relative to MAE-OS (<0.0005 ng m^(–3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10−100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NO_x, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.
The chemical link between isoprene and formaldehyde (HCHO) is a strong, non-linear function of NOx (= NO + NO2). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the Southeast US, we quantify HCHO production across the urban-rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly-emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NOx values (roughly 0.1–2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv−1), while background HCHO increases by more than a factor of 2 (from 1.5 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D chemical box model. Both models reproduce the NOx dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models under-estimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or under-estimated hydroxyl radical concentrations. Moreover, we find that the total organic peroxy radical production rate is essentially independent of NOx, as the increase in oxidizing capacity with NOx is largely balanced by a decrease in VOC reactivity. Thus, the observed NOx dependence of HCHO mainly reflects the changing fate of organic peroxy radicals.
Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake probabilities (γ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC4RS) and ground-based (SOAS) observations over the Southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NOx ≡ NO + NO2) over the Southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO2) react significantly with both NO (high-NOx pathway) and HO2 (low-NOx pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from isoprene oxidation, consistent with the observed relationship of OA and formaldehyde (a product of isoprene oxidation). The yield is mainly contributed by two immediate gas-phase precursors, isoprene epoxydiols (IEPOX, 58 % of isoprene SOA) from the low-NOx pathway and glyoxal (28 %) from both low- and high-NOx pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC4RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the indirect effect of sulfate on aerosol acidity and volume, rather than a direct mechanistic role for sulfate. Isoprene SOA concentrations increase as NOx emissions decrease (favoring the low-NOx pathway for isoprene oxidation), but decrease as SO2 emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US EPA projects 2013–2025 decreases in anthropogenic emissions of 34 % for NOx (leading to 7 % increase in isoprene SOA) and 48 % for SO2 (35 % decrease in isoprene SOA). The combined projected decreases in NOx and SO2 emissions reduce isoprene SOA yields from 3.3 to 2.3 %. Reducing SO2 emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM2.5 from SO2 emission controls.
The formation of secondary organic aerosols (SOAs) combined with the partitioning of semivolatile organic components can impact numerous aerosol properties including cloud condensation nuclei (CCN) activity, hygro-scopicity, and volatility. During the summer 2013 Southern Oxidant and Aerosol Study (SOAS) field campaign in a rural site in the southeastern United States, a suite of instruments including a CCN counter, a thermodenuder (TD), and a high-resolution time-of-flight aerosol mass spectrometer (AMS) were used to measure CCN activity, aerosol volatility, composition, and oxidation state. Particles were either sampled directly from ambient or through a particle-into-liquid sampler (PILS), allowing the investigation of the water-soluble aerosol component. Ambient aerosols exhibited size-dependent composition with larger particles being more hygroscopic. The hygroscopicity of thermally denuded aerosols was similar between ambient and PILS-generated aerosols and showed limited dependence on volatilization. Results of AMS three-factor positive matrix factorization (PMF) analysis for the PILS-generated aerosols showed that the most hygroscopic components are most likely the most and the least volatile features of the aerosols. No clear relationship was found between organic hygroscopicity and the oxygen-to-carbon ratio; in fact, isoprene-derived organic aerosols (isoprene-OAs) were found to be the most hygro-scopic factor, while at the same time being the least oxidized and likely most volatile of all PMF factors. Considering the diurnal variation of each PMF factor and its associated hygroscopicity, isoprene-OA and more-oxidized oxy-genated organic aerosols are the prime contributors to hy-groscopicity and co-vary with less-oxidized oxygenated organic aerosols in a way that induces the observed diurnal invariance in total organic hygroscopicity. Biomass burning organic aerosols contributed little to aerosol hygroscopicity, which is expected since there was little biomass burning activity during the sampling period examined.
Vertical profiles of submicron aerosol from in situ aircraft-based
measurements were used to construct aggregate profiles of chemical,
microphysical, and optical properties. These vertical profiles were collected
over the southeastern United States (SEUS) during the summer of 2013 as part
of two separate field studies: the Southeast Nexus (SENEX) study and the
Study of Emissions and Atmospheric Composition, Clouds, and Climate Coupling
by Regional Surveys (SEAC4RS). Shallow cumulus convection was observed
during many profiles. These conditions enhance vertical transport of trace
gases and aerosol and create a cloudy transition layer on top of the
sub-cloud mixed layer. The trace gas and aerosol concentrations in the
transition layer were modeled as a mixture with contributions from the mixed
layer below and the free troposphere above. The amount of vertical mixing, or
entrainment of air from the free troposphere, was quantified using the
observed mixing ratio of carbon monoxide (CO). Although the median aerosol
mass, extinction, and volume decreased with altitude in the transition layer,
they were ~10 % larger than expected from vertical mixing alone.
This enhancement was likely due to secondary aerosol formation in the
transition layer. Although the transition layer enhancements of the
particulate sulfate and organic aerosol (OA) were both similar in magnitude,
only the enhancement of sulfate was statistically significant. The column
integrated extinction, or aerosol optical depth (AOD), was calculated for
each individual profile, and the transition layer enhancement of extinction
typically contributed less than 10 % to the total AOD. Our measurements
and analysis were motivated by two recent studies that have hypothesized an
enhanced layer of secondary aerosol aloft to explain the summertime
enhancement of AOD (2–3 times greater than winter) over the southeastern
United States. The first study attributes the layer aloft to secondary organic
aerosol (SOA) while the second study speculates that the layer aloft could be
SOA or secondary particulate sulfate. In contrast to these hypotheses, the
modest enhancement we observed in the transition layer was not dominated by
OA and was not a large fraction of the summertime AOD.
Long-term (1999 to 2013) data from the Southeastern Aerosol Research and Characterization (SEARCH) network are used to characterize the effects of anthropogenic emission reductions on fine particle organic aerosol (OA) concentrations in the southeastern US. On average, 45 % (range 25 to 63 %) of the 1999 to 2013 mean organic carbon (OC) concentrations are attributed to combustion processes, including fossil-fuel use and biomass burning, through associations of measured OC with combustion products such as elemental carbon (EC), carbon monoxide (CO), and nitrogen oxides (NOx). The 2013 mean combustion-derived OC concentrations were 0.5 to 1.4 μg m−3 at the five sites operating in that year. Mean annual combustion-derived OC concentrations declined from 3.8 ± 0.2 μg m−3 (68 % of total OC) to 1.4 ± 0.1 μg m−3 (60 % of total OC) between 1999 and 2013 at the urban Atlanta, Georgia, site (JST) and from 2.9 ± 0.4 μg m−3 (39 % of total OC) to 0.7 ± 0.1 μg m−3 (30 % of total OC) between 2001 and 2013 at the urban Birmingham, Alabama, site (BHM). The urban OC declines coincide with reductions of motor-vehicle emissions between 2006 and 2010, which may have decreased mean OC concentrations at the urban SEARCH sites by > 2 μg m−3. BHM additionally exhibits a decline in OC associated with SO2 from 0.4 ± 0.04 μg m−3 in 2001 to 0.2 ± 0.03 μg m−3 in 2013, interpreted as the result of reduced emissions from industrial sources within the city. Analyses using non-soil potassium as a biomass-burning tracer indicate that biomass-burning OC occurs throughout the year at all sites. All eight SEARCH sites show an association of OC with sulfate (SO4) ranging from 0.3 to 1.0 μg m−3 on average, representing ~ 25 % of the 1999 to 2013 mean OC concentrations. Because the mass of OC associated with SO4 averages 20 to 30 % of the SO4 concentrations, the mean SO4-associated OC declined by ~ 0.5 to 1 μg m−3 as SO4 decreased throughout the SEARCH region. The 2013 mean SO4 concentrations of 1.7 to 2.0 μg m−3 imply that future decreases in mean SO4-associated OC concentrations would not exceed ~ 0.3 to 0.5 μg m−3. Seasonal OC concentrations, largely associated with ozone (O3), vary from 0.3 to 1.4 μg m−3 (~ 20 % of the total OC concentrations).
Particle water and pH are predicted using meteorological observations (relative humidity (RH), temperature (T)), gas/particle composition, and thermodynamic modeling (ISORROPIA-II). A comprehensive uncertainty analysis is included, and the model is validated. We investigate mass concentrations of particle water and related particle pH for ambient fine-mode aerosols sampled in a relatively remote Alabama forest during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various sites in the southeastern US during different seasons, as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study. Particle water and pH are closely linked; pH is a measure of the particle H+ aqueous concentration and depends on both the presence of ions and amount of particle liquid water. Levels of particle water, in turn, are determined through water uptake by both the ionic species and organic compounds. Thermodynamic calculations based on measured ion concentrations can predict both pH and liquid water but may be biased since contributions of organic species to liquid water are not considered. In this study, contributions of both the inorganic and organic fractions to aerosol liquid water were considered, and predictions were in good agreement with measured liquid water based on differences in ambient and dry light scattering coefficients (prediction vs. measurement: slope = 0.91, intercept = 0.5 μg m−3, R2 = 0.75). ISORROPIA-II predictions were confirmed by good agreement between predicted and measured ammonia concentrations (slope = 1.07, intercept = −0.12 μg m−3, R2 = 0.76). Based on this study, organic species on average contributed 35% to the total water, with a substantially higher contribution (50%) at night. However, not including contributions of organic water had a minor effect on pH (changes pH by 0.15 to 0.23 units), suggesting that predicted pH without consideration of organic water could be sufficient for the purposes of aqueous secondary organic aerosol (SOA) chemistry. The mean pH predicted in the Alabama forest (SOAS) was 0.94 ± 0.59 (median 0.93). pH diurnal trends followed liquid water and were driven mainly by variability in RH; during SOAS nighttime pH was near 1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1 to 3 in the winter at other sites. The systematically low pH levels in the southeast may have important ramifications, such as significantly influencing acid-catalyzed reactions, gas–aerosol partitioning, and mobilization of redox metals and minerals. Particle ion balances or molar ratios, often used to infer pH, do not consider the dissociation state of individual ions or particle liquid water levels and do not correlate with particle pH.
Real-time mass spectra of the non-refractory species in submicron aerosol
particles were recorded in a tropical rainforest in the central Amazon Basin
during the wet season from February to March 2008, as a part of the
Amazonian Aerosol Characterization Experiment (AMAZE-08). Organic material
accounted on average for more than 80% of the non-refractory submicron
particle mass concentrations during the period of measurements. There was
insufficient ammonium to neutralize sulfate. In this acidic, isoprene-rich,
HO2-dominant environment, positive-matrix factorization of the time
series of particle mass spectra identified four statistical factors to
account for the 99% of the variance in the signal intensities of the organic
constituents. The first factor was identified as associated with regional
and local pollution and labeled "HOA" for its hydrocarbon-like
characteristics. A second factor was associated with long-range transport
and labeled "OOA-1" for its oxygenated characteristics. A third factor,
labeled "OOA-2," was implicated as associated with the reactive uptake of
isoprene oxidation products, especially of epoxydiols to acidic haze, fog, or
cloud droplets. A fourth factor, labeled "OOA-3," was consistent with
an association with the fresh production of secondary organic material (SOM)
by the mechanism of gas-phase oxidation of biogenic volatile organic
precursors followed by gas-to-particle conversion of the oxidation products.
The suffixes 1, 2, and 3 on the OOA labels signify ordinal ranking with
respect to the extent of oxidation represented by the factor. The process of
aqueous-phase oxidation of water-soluble products of gas-phase
photochemistry might also have been associated to some extent with the OOA-2
factor. The campaign-average factor loadings had a ratio of 1.4:1 for
OOA-2 : OOA-3, suggesting the comparable importance of particle-phase compared
to gas-phase pathways for the production of SOM during the study period.
Isoprene is a potentially highly significant but currently poorly quantified
source of secondary organic aerosols (SOA). This is especially important in
the tropics, where large rainforests act as significant sources of isoprene.
Methylfuran, produced through thermal decomposition during analysis, has
recently been suggested as a marker for isoprene SOA formation through the
isoprene epoxydiol (IEPOX) route, which mostly occurs under low NOx
conditions. This is manifested as a peak at m/z=82 in Aerodyne Aerosol Mass
Spectrometer (AMS) data. Here we present a study of this marker measured
during five flights over the Amazon rainforest on board the UK Facility for
Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft during the
South American Biomass Burning Analysis (SAMBBA) campaign. Cases where this
marker is and is not present are contrasted and linked to the presence of
acidic seed particles, lower NOx concentrations and higher humidities.
There are also data to suggest a role of organic nitrogen in the particulate
composition. Furthermore, an inspection of the vertical trends of the marker
indicates that concentrations are highest at the top of the boundary layer
(possibly due to semivolatile repartitioning) and that upwards through the free
troposphere, the mass spectral profile evolves towards that of low
volatility oxygenated aerosol. These observations offer insights into the
behaviour of IEPOX-derived SOA formation above the Amazon rainforest and the
suitability of methylfuran as a marker for this process.
A series of experiments (the Southern Oxidant and Aerosol Study-SOAS) took place in central Alabama in June–July 2013 as part of the broader Southern Atmosphere Study (SAS). These projects were aimed at studying oxidant photochemistry and formation and impacts of aerosols at a detailed process level in a location where high biogenic organic vapor emissions interact with anthropogenic emissions, and the atmospheric chemistry occurs in a subtropical climate in North America. The majority of the ground-based experiments were located at the Southeastern Aerosol Research and Characterization (SEARCH) Centreville (CTR) site near Brent, Alabama, where extensive, unique aerometric measurements of meteorology, trace gases and particles have been made from the early 1990s through 2013. The SEARCH network data permits a characterization of temporal and spatial context of the SOAS findings. The long-term measurements show that the SOAS experiments took place during the second wettest and coolest year in the 2000–2013 period, with lower than average solar radiation. The pollution levels at CTR and other SEARCH sites were the lowest since full measurements began in 1999. This dataset provides a perspective for the SOAS program in terms of long-term average chemistry (chemical climatology) and short-term comparisons of summer average spatial variability across the Southeast at high temporal (hourly) resolution. Changes in anthropogenic gas and particle emissions between 1999 and 2013, account for the decline in pollutant concentrations at the monitoring sites in the region. The long-term and short-term data provide an opportunity to contrast SOAS results with temporally and spatially variable conditions in support for the development of tests for the robustness of SOAS findings.
Given significant challenges with available measurements of aerosol acidity,
proxy methods are frequently used to estimate the acidity of atmospheric
particles. In this study, four of the most common aerosol acidity proxies are
evaluated and compared: (1) the ion balance method, (2) the molar ratio
method, (3) thermodynamic equilibrium models, and (4) the phase partitioning
of ammonia. All methods are evaluated against predictions of thermodynamic
models and against direct observations of aerosol–gas equilibrium
partitioning acquired in Mexico City during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) study. The ion balance and molar ratio
methods assume that any deficit in inorganic cations relative to anions is
due to the presence of H+ and that a higher H+ loading and lower
cation / anion ratio both correspond to increasingly acidic particles
(i.e., lower pH). Based on the MILAGRO measurements, no correlation is
observed between H+ levels inferred with the ion balance and aerosol pH
predicted by the thermodynamic models and NH3–NH4+
partitioning. Similarly, no relationship is observed between the
cation / anion molar ratio and predicted aerosol pH. Using only measured
aerosol chemical composition as inputs without any constraint for the gas
phase, the E-AIM (Extended Aerosol Inorganics Model) and ISORROPIA-II
thermodynamic equilibrium models tend to predict aerosol pH levels that are
inconsistent with the observed NH3–NH4+ partitioning. The
modeled pH values from both E-AIM and ISORROPIA-II run with gas + aerosol
inputs agreed well with the aerosol pH predicted by the phase partitioning of
ammonia. It appears that (1) thermodynamic models constrained by
gas + aerosol measurements and (2) the phase partitioning of ammonia
provide the best available predictions of aerosol pH. Furthermore, neither
the ion balance nor the molar ratio can be used as surrogates for aerosol pH,
and previously published studies with conclusions based on such acidity
proxies may need to be reevaluated. Given the significance of acidity for
chemical processes in the atmosphere, the implications of this study are
important and far reaching.
A suite of offline and real-time gas- and particle-phase measurements was deployed at Look Rock, Tennessee (TN), during the 2013 Southern Oxidant and Aerosol Study (SOAS) to examine the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol (SOA) formation. High- and low-time resolution PM2.5 samples were collected for analysis of known tracer compounds in isoprene-derived SOA by gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography/diode array detection-electrospray ionization-high-resolution quadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QTOFMS). Source apportionment of the organic aerosol (OA) was determined by positive matrix factorization (PMF) analysis of mass spectrometric data acquired on an Aerodyne Aerosol Chemical Speciation Monitor (ACSM). Campaign average mass concentrations of the sum of quantified isoprene-derived SOA tracers contributed to ~9% (up to 26%) of the total OA mass, with isoprene-epoxydiol (IEPOX) chemistry accounting for ~97% of the quantified tracers. PMF analysis resolved a factor with a profile similar to the IEPOX-OA factor resolved in an Atlanta study and was therefore designated IEPOX-OA. This factor was strongly correlated (r2>0.7) with 2-methyltetrols, C5-alkene triols, IEPOX-derived organosulfates, and dimers of organosulfates, confirming the role of IEPOX chemistry as the source. On average, IEPOX-derived SOA tracer mass was ~25% (up to 47%) of the IEPOX-OA factor mass, which accounted for 32% of the total OA. A low-volatility oxygenated organic aerosol (LV-OOA) and an oxidized factor with a profile similar to 91Fac observed in areas where emissions are biogenic-dominated were also resolved by PMF analysis, whereas no primary organic aerosol (POA) sources could be resolved. These findings were consistent with low levels of primary pollutants, such as nitric oxide (NO~0.03ppb), carbon monoxide (CO~116 ppb), and black carbon (BC~0.2 μg m−3). Particle-phase sulfate is fairly correlated (r2~0.3) with both MAE- and IEPOX-derived SOA tracers, and more strongly correlated (r2~0.6) with the IEPOX-OA factor, in sum suggesting an important role of sulfate in isoprene SOA formation. Moderate correlation between the methacrylic acid epoxide (MAE)-derived SOA tracer 2-methylglyceric acid with sum of reactive and reservoir nitrogen oxides (NOy; r2=0.38) and nitrate (r2=0.45) indicates the potential influence of anthropogenic emissions through long-range transport. Despite the lack of a~clear association of IEPOX-OA with locally estimated aerosol acidity and liquid water content (LWC), box model calculations of IEPOX uptake using the simpleGAMMA model, accounting for the role of acidity and aerosol water, predicted the abundance of the IEPOX-derived SOA tracers 2-methyltetrols and the corresponding sulfates with good accuracy (r2~0.5 and ~0.7, respectively). The modeling and data combined suggest an anthropogenic influence on isoprene-derived SOA formation through acid-catalyzed heterogeneous chemistry of IEPOX in the southeastern US. However, it appears that this process was not limited by aerosol acidity or LWC at Look Rock during SOAS. Future studies should further explore the extent to which acidity and LWC becomes a limiting factor of IEPOX-derived SOA, and their modulation by anthropogenic emissions.
Organosulfates are important secondary organic aerosol (SOA) components and good tracers for aerosol heterogeneous reactions. However, the knowledge of their spatial distribution, formation conditions, and environmental impact is limited. In this study, we report two organosulfates, an isoprene‐derived isoprene epoxydiols (IEPOX) (2,3‐epoxy‐2‐methyl‐1,4‐butanediol) sulfate and a glycolic acid (GA) sulfate, measured using the NOAA Particle Analysis Laser Mass Spectrometer (PALMS) on board the NASA DC8 aircraft over the continental U.S. during the Deep Convective Clouds and Chemistry Experiment (DC3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS). During these campaigns, IEPOX sulfate was estimated to account for 1.4% of submicron aerosol mass (or 2.2% of organic aerosol mass) on average near the ground in the southeast U.S., with lower concentrations in the western U.S. (0.2–0.4%) and at high altitudes (<0.2%). Compared to IEPOX sulfate, GA sulfate was more uniformly distributed, accounting for about 0.5% aerosol mass on average, and may be more abundant globally. A number of other organosulfates were detected; none were as abundant as these two. Ambient measurements confirmed that IEPOX sulfate is formed from isoprene oxidation and is a tracer for isoprene SOA formation. The organic precursors of GA sulfate may include glycolic acid and likely have both biogenic and anthropogenic sources. Higher aerosol acidity as measured by PALMS and relative humidity tend to promote IEPOX sulfate formation, and aerosol acidity largely drives in situ GA sulfate formation at high altitudes. This study suggests that the formation of aerosol organosulfates depends not only on the appropriate organic precursors but also on emissions of anthropogenic sulfur dioxide (SO 2 ), which contributes to aerosol acidity.
Isoprene, the most abundant non-methane volatile organic compound (VOC)
emitted into the atmosphere, is known to undergo gas phase oxidation to form
eight different hydroxynitrate isomers in "high-NOx"
environments. These hydroxynitrates are known to affect the global and
regional formation of ozone and secondary organic aerosol (SOA), as well as
affect the distribution of nitrogen. In the present study, we have
synthesized three of the eight possible hydroxynitrates: 4-hydroxy-3-nitroxy
isoprene (4,3-HNI) and E / Z-1-hydroxy-4-nitroxy isoprene (1,4-HNI).
Oxidation of the 4,3-HNI isomer by the OH radical was monitored using a flow
tube chemical ionization mass spectrometer (FT-CIMS), and its OH rate
constant was determined to be
(3.64 ± 0.41) × 10−11 cm3 molecule−1 s−1.
The products of 4,3-HNI oxidation were monitored, and a mechanism to explain
the products was developed. An isoprene epoxide (IEPOX) – a species
important in SOA chemistry and thought to originate only from
"low-NOx" isoprene oxidation – was found as a minor, but
significant, product. Additionally, hydrolysis kinetics of the three synthesized isomers
were monitored with nuclear magnetic resonance (NMR). The bulk, neutral solution hydrolysis rate constants
for 4,3-HNI and the 1,4-HNI isomers were
(1.59 ± 0.03) × 10−5 s−1 and
(6.76 ± 0.09) × 10−3 s−1, respectively. The
hydrolysis reactions of each isomer were found to be general acid-catalyzed.
The reaction pathways, product yields and atmospheric implications for both
the gas phase and aerosol phase reactions are discussed.
Elemental compositions of organic aerosol (OA) particles provide useful constraints on OA sources, chemical evolution, and effects. The Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is widely used to measure OA elemental composition. This study evaluates AMS measurements of atomic oxygen-to-carbon (O : C), hydrogen-to-carbon (H : C), and organic mass-to-organic carbon (OM : OC) ratios, and of carbon oxidation state ([bar over OS][subscript C]) for a vastly expanded laboratory data set of multifunctional oxidized OA standards. For the expanded standard data set, the method introduced by Aiken et al. (2008), which uses experimentally measured ion intensities at all ions to determine elemental ratios (referred to here as "Aiken-Explicit"), reproduces known O : C and H : C ratio values within 20% (average absolute value of relative errors) and 12%, respectively. The more commonly used method, which uses empirically estimated H[subscript 2]O[superscript +] and CO[superscript +] ion intensities to avoid gas phase air interferences at these ions (referred to here as "Aiken-Ambient"), reproduces O : C and H : C of multifunctional oxidized species within 28 and 14% of known values. The values from the latter method are systematically biased low, however, with larger biases observed for alcohols and simple diacids. A detailed examination of the H[subscript 2]O[superscript +], CO[superscript +], and CO[subscript 2][superscript +] fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO[superscript +] and especially H[subscript 2]O[superscript +] produced from many oxidized species. Combined AMS–vacuum ultraviolet (VUV) ionization measurements indicate that these ions are produced by dehydration and decarboxylation on the AMS vaporizer (usually operated at 600 °C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 °C. These results are used together to develop an "Improved-Ambient" elemental analysis method for AMS spectra measured in air. The Improved-Ambient method uses specific ion fragments as markers to correct for molecular functionality-dependent systematic biases and reproduces known O : C (H : C) ratios of individual oxidized standards within 28% (13%) of the known molecular values. The error in Improved-Ambient O : C (H : C) values is smaller for theoretical standard mixtures of the oxidized organic standards, which are more representative of the complex mix of species present in ambient OA. For ambient OA, the Improved-Ambient method produces O : C (H : C) values that are 27% (11%) larger than previously published Aiken-Ambient values; a corresponding increase of 9% is observed for OM : OC values. These results imply that ambient OA has a higher relative oxygen content than previously estimated. The [bar over OS][subscript C] values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 [bar over OS][subscript C] units). This indicates that [bar over OS][subscript C] is a more robust metric of oxidation than O : C, likely since [bar over OS][subscript C] is not affected by hydration or dehydration, either in the atmosphere or during analysis.
Significance
Atmospheric secondary organic aerosol has substantial impacts on climate, air quality, and human health. However, the formation mechanisms of secondary organic aerosol remain uncertain, especially on how anthropogenic pollutants (from human activities) control aerosol formation from biogenic volatile organic compounds (emitted by vegetation) and the magnitude of anthropogenic influences. Although possible mechanisms have been proposed based on laboratories studies, a coherent understanding of anthropogenic−biogenic interactions in ambient environments has not emerged. Here, we provide direct observational evidence that secondary organic aerosol formed from biogenic isoprene and monoterpenes is greatly mediated by anthropogenic SO 2 and NO x emissions based on integrated ambient measurements and laboratory studies.
The reactive partitioning of cis and trans β-IEPOX was investigated
on hydrated inorganic seed particles, without the addition of acids. No
organic aerosol (OA) formation was observed on dry ammonium sulfate
(AS); however, prompt and efficient OA growth was observed for the cis
and trans β-IEPOX on AS seeds with liquid water contents of 40-75%
of the total particle mass. OA formation from IEPOX is a
kinetically-limited process; thus the OA growth continues if there is a
reservoir of gas-phase IEPOX. There appears to be no differences, within
error, in the OA growth or composition attributable to the cis/trans
isomeric structures. Reactive uptake of IEPOX onto hydrated AS seeds
with added base (NaOH) also produced high OA loadings, suggesting the
pH-dependence for OA formation from IEPOX is weak for AS particles. No
OA formation, after particle drying, was observed on seed particles
where Na+ was substituted for NH4+. The
Henry's Law partitioning of IEPOX was measured on NaCl particles (ionic
strength ~9 M) to be 3 × 107 M atm-1. A
small quantity of OA was produced when NH4+ was
present in the particles, but the chloride (Cl-) anion was
substituted for sulfate (SO42-), suggesting
differences in nucleophilic strength of the anions. Online
time-of-flight aerosol mass spectrometry and offline filter analysis
provide evidence of oxygenated hydrocarbons, organosulfates and,
notably, amines in the particle organic composition. The results help
explain the substantial quantities of ambient IEPOX-derived OA observed
under neutralized conditions. Experiments and models aimed at
understanding OA production from IEPOX, or other epoxides, should
consider the NH4+ activity, in conjunction with
H+ activity (i.e., particle acidity) and nucleophile
activity.
The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1) is a modeling framework for estimating fluxes of biogenic compounds between terrestrial ecosystems and the atmosphere using simple mechanistic algorithms to account for the major known processes controlling biogenic emissions. It is available as an offline code and has also been coupled into land surface and atmospheric chemistry models. MEGAN2.1 is an update from the previous versions including MEGAN2.0, which was described for isoprene emissions by Guenther et al. (2006) and MEGAN2.02, which was described for monoterpene and sesquiterpene emissions by Sakulyanontvittaya et al. (2008). Isoprene comprises about half of the total global biogenic volatile organic compound (BVOC) emission of 1 Pg (1000 Tg or 1015 g) estimated using MEGAN2.1. Methanol, ethanol, acetaldehyde, acetone, α-pinene, β-pinene, t-β-ocimene, limonene, ethene, and propene together contribute another 30% of the MEGAN2.1 estimated emission. An additional 20 compounds (mostly terpenoids) are associated with the MEGAN2.1 estimates of another 17% of the total emission with the remaining 3% distributed among >100 compounds. Emissions of 41 monoterpenes and 32 sesquiterpenes together comprise about 15% and 3%, respectively, of the estimated total global BVOC emission. Tropical trees cover about 18% of the global land surface and are estimated to be responsible for ~80% of terpenoid emissions and ~50% of other VOC emissions. Other trees cover about the same area but are estimated to contribute only about 10% of total emissions. The magnitude of the emissions estimated with MEGAN2.1 are within the range of estimates reported using other approaches and much of the differences between reported values can be attributed to land cover and meteorological driving variables. The offline version of MEGAN2.1 source code and driving variables is available from
Filter-based PM2.5 samples were chemically analyzed to
investigate secondary organic aerosol (SOA) formation from isoprene in a
rural atmosphere of the southeastern US influenced by both anthropogenic
sulfur dioxide (SO2) and ammonia (NH3) emissions.
Daytime PM2.5 samples were collected during summer 2010 using
conditional sampling approaches based on pre-defined high and low
SO2 or NH3 thresholds. Known molecular-level
tracers for isoprene SOA formation, including 2-methylglyceric acid,
3-methyltetrahydrofuran-3,4-diols, 2-methyltetrols, C5-alkene
triols, dimers, and organosulfate derivatives, were identified and
quantified by gas chromatography coupled to electron ionization mass
spectrometry (GC/EI-MS) and ultra performance liquid chromatography
coupled to electrospray ionization high-resolution quadrupole
time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). Mass
concentrations of six isoprene low-NOx SOA tracers
contributed to 12-19% of total organic matter (OM) in PM2.5
samples collected during the sampling period, indicating the importance
of the hydroxyl radical (OH)-initiated oxidation (so-called
photooxidation) of isoprene under low-NOx conditions that
leads to SOA formation through reactive uptake of gaseous isoprene
epoxydiols (IEPOX) in this region. IEPOX-derived SOA tracers were
enhanced under high-SO2 sampling scenarios, suggesting that
SO2 oxidation increases aerosol acidity of sulfate aerosols
needed for enhancing heterogeneous oxirane ring-opening reactions of
IEPOX. No clear associations between isoprene SOA formation and high and
low NH3 conditional samples were found. Furthermore, weak
correlations between aerosol acidity and mass of IEPOX SOA tracers
suggests that IEPOX-derived SOA formation might be modulated by other
factors as well in addition to aerosol acidity. Positive correlations
between sulfate aerosol loadings and IEPOX-derived SOA tracers for
samples collected under all conditions indicates that sulfate aerosol
could be a surrogate for surface area in the uptake of IEPOX onto
preexisting aerosols.
Multiphase chemistry of isomeric isoprene epoxydiols (IEPOX) has been shown to be the dominant source of isoprene-derived secondary organic aerosol (SOA). Recent studies have reported particles composed of ammonium bisulfate (ABS) mixed with model organics exhibit slower rates of IEPOX uptake. In the present study, we investigate the effect of atmospherically-relevant organic coatings of α-pinene (AP) SOA on the reactive uptake of trans-β-IEPOX onto ABS particles under different conditions and coating thicknesses. Single particle mass spectrometry was used to characterize in real-time particle size, shape, density, and quantitative composition before and after reaction with IEPOX. We find that IEPOX uptake by pure sulfate particles is a volume-controlled process, which results in particles with uniform concentration of IEPOX-derived SOA across a wide range of sizes. Aerosol acidity was shown to enhance IEPOX-derived SOA formation, consistent with recent studies. The presence of water has a weaker impact on IEPOX-derived SOA yield, but significantly enhanced formation of 2-methyltetrols, consistent with offline filter analysis. In contrast, IEPOX uptake by ABS particles coated with AP-derived SOA is lower compared to that of pure ABS particles, strongly dependent on particle composition, and therefore on particle size.
Organic nitrates are an important aerosol constituent in locations where biogenic hydrocarbon emissions mix with anthropogenic NOx sources. While regional and global chemical transport models may include a representation of organic aerosol from monoterpene reactions with nitrate radicals (the primary source of particle-phase organic nitrates in the Southeast United States), secondary organic aerosol (SOA) models can underestimate yields. Furthermore, SOA parameterizations do not explicitly take into account organic nitrate compounds produced in the gas phase. In this work, we developed a coupled gas and aerosol system to describe the formation and subsequent aerosol-phase partitioning of organic nitrates from isoprene and monoterpenes with a focus on the Southeast United States. The concentrations of organic aerosol and gas-phase organic nitrates were improved when particulate organic nitrates were assumed to undergo rapid (t = 3 h) pseudo-hydrolysis resulting in nitric acid and nonvolatile secondary organic aerosol. In addition, up to 60% of less oxidized-oxygenated organic aerosol (LO-OOA) could be accounted for via organic nitrate mediated chemistry during the Southern Oxidants and Aerosol Study (SOAS). A 25% reduction in nitrogen oxide (NO + NO2) emissions was predicted to cause a 9% reduction in organic aerosol for June 2013 SOAS conditions at Centreville, Alabama.
The atmospheric oxidation of isoprene by the OH radical leads to the formation of several isomers of an unsaturated hydroxy hydroperoxide, ISOPOOH. Oxidation of ISOPOOH by OH produces epoxydiols, IEPOX, which have been shown to contribute mass to secondary organic aerosol (SOA). We present kinetic rate constant measurements for OH + ISOPOOH using synthetic standards of the two major isomers of ISOPOOH: (1,2)- and (4,3)-ISOPOOH. At 297 K, the total OH rate constant is 7.5±1.2×10(-11) cm(3) molecule(-1) s(-1) for (1,2)-ISOPOOH and 1.18±0.19 ×10(-10) cm(3) molecule(-1) s(-1) for (4,3)-ISOPOOH. Abstraction of the hydroperoxy hydrogen accounts for approximately 12% and 4% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The sum of all H-abstractions account for approximately 15% and 7% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The major product observed from both ISOPOOH isomers was IEPOX (cis-β and trans-β isomers), with a ~2:1 preference for trans-β IEPOX and similar total yields from each ISOPOOH isomer (~70-80%). An IEPOX global production rate of more than 100 Tg C each year is estimated from this chemistry using a global 3D chemical transport model, similar to earlier estimates. Finally, approximately 13% of the reactivity proceeds via addition of O2 following the addition of OH at 297 K and 745 Torr. In the presence of NO, these peroxy radicals lead to formation of small carbonyl compounds. Under HO2 dominated chemistry, no products are observed from these channels. We suggest that the major products, multifunctional peroxides, are lost to the chamber walls. In the atmosphere, formation of these compounds may contribute to organic aerosol mass.
We describe the products of the reaction of the hydroperoxy radical (HO2) with the alkylperoxy radical formed following addition of the nitrate radical (NO3) and O2 to isoprene. NO3 adds preferentially to the C1 position of isoprene (>6 times more favorably than addition to C4), followed by the addition of O2 to produce a suite of nitrooxy alkylperoxy radicals (RO2). At an RO2 lifetime of ~30 s, δ-nitrooxy and β-nitrooxy alkylperoxy radicals are present in similar amounts. Gas-phase product yields from the RO2 + HO2 pathway are identified as 0.75-0.78 isoprene nitrooxy hydroperoxide (INP), 0.22 methyl vinyl ketone (MVK) + formaldehyde (CH2O) + hydroxyl radical (OH) + nitrogen dioxide (NO2), and 0-0.03 methacrolein (MACR) + CH2O + OH + NO2. We further examined the photochemistry of INP and identified propanone nitrate (PROPNN) and isoprene nitrooxy hydroxyepoxide (INHE) as the main products. INHE undergoes similar heterogeneous chemistry as isoprene dihydroxy epoxide (IEPOX), likely contributing to atmospheric secondary organic aerosol formation.
A combination of flow reactor studies and chamber modeling is used to constrain two uncertain parameters central to the formation of secondary organic aerosol (SOA) from isoprene-derived epoxides: (1) the rate of heterogeneous uptake of epoxide to the particle phase and (2) the molar fraction of epoxide reactively taken up that contributes to SOA, the SOA yield (phi(SOA)). Flow reactor measurements of the trans-beta-isoprene epoxydiol (trans-beta-IEPOX) and methacrylic acid epoxide (MAE) aerosol reaction probability (gamma) were performed on atomized aerosols with compositions similar to those used in chamber studies. Observed gamma ranges for trans-beta-IEPOX and MAE were 6.5 x 10(-4)-0.021 and 4.9-5.2 X 10(-4), respectively. Through the use of a time-dependent chemical box model initialized with chamber conditions and gamma measurements, phi(SOA) values for trans-beta-IEPOX and MAE on different aerosol compositions were estimated between 0.03-021 and 0.07-025, respectively, with the MAE phi(SOA) showing more uncertainty.
Over the past decade, it has become clear that aqueous chemical processes occurring in cloud droplets and wet atmospheric particles are an important source of organic atmospheric particulate matter. Reactions of water-soluble volatile (or semivolatile) organic gases (VOCs or SVOCs) in these aqueous media lead to the formation of highly oxidized organic particulate matter (secondary organic aerosol; SOA) and key tracer species, such as organosulfates. These processes are often driven by a combination of anthropogenic and biogenic emissions, and therefore their accurate representation in models is important for effective air quality management. Despite considerable progress, mechanistic understanding of some key aqueous processes is still lacking, and these pathways are incompletely represented in 3D atmospheric chemistry and air quality models. In this article, the concepts, historical context, and current state of the science of aqueous pathways of SOA formation are discussed.
The oxidation of isoprene is a globally significant source of secondary organic material (SOM). The relative importance of different parallel pathways, however, remains inadequately understood and quantified. SOM production from isoprene photooxidation was studied under hydroperoxyl-dominant conditions for <5% relative humidity and at 20 °C in the presence of highly acidic to completely neutralized sulfate particles. Isoprene photooxidation was separated from SOM production by using two continuously mixed flow reactors connected in series and operated at steady state. Two on-line mass spectrometers separately sampled the gas and particle phases in the reactor outflow. The loss of specific gas-phase species as contributors to the production of SOM was thereby quantified. The produced SOM mass concentration was directly proportional to the loss of isoprene epoxydiol (IEPOX) isomers from the gas phase. IEPOX isomers lost from the gas phase accounted for (46 ± 11)% of the produced SOM mass concentration. The IEPOX isomers comprised (59 ± 21)% (molecular count) of the loss of monitored gas-phase species. The implication is that for the investigated reaction conditions the SOM production pathways tied to IEPOX isomers accounted for half of the SOM mass concentration.
The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γIEPOX) and its dependence upon particle composition remain poorly constrained. We report measurements of γIEPOX for trans-β-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γIEPOX is more than 500 times greater on ammonium bisulfate (γ ~ 0.05) than on ammonium sulfate (γ ≤ 1 x 10(-4)). We could accurately predict γIEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry's law coefficient (1.7 x 10(8) M/atm). Suppression of γIEPOX was observed on particles containing both ammonium bisulfate and polyethylene glycol (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH < 0), but is greater than 25 hours on less acidic particles (pH > 3). This work highlights the importance of aerosol acidity for accurately predicting the fate of IEPOX and anthropogenically influenced biogenic SOA formation.
A High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (< 20 ppm) and mass-resolving power (R > 5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity towards more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (> 1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly-evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign.
Isoprene epoxydiols (IEPOX) form in high yields from the OH-initiated oxidation of isoprene under low-NO conditions. These compounds contribute significantly to secondary organic aerosol formation. Their gas-phase chemistry has, however, remained largely unexplored. In this study, we characterize the formation of IEPOX isomers from the oxidation of isoprene by OH. We find that cis- and trans-β-IEPOX are the dominant isomers produced, accounting respectively for 31 ± 5 % and 66 ± 4 % of the IEPOX yield from low-NO oxidation of isoprene. Three isomers of IEPOX, including cis- and trans-β, were synthesized and oxidized by OH in environmental chambers under high- and low-NO conditions. We find that IEPOX reacts with OH at 299 K with rate coefficients of (0.84 ± 0.07)×10-11, (1.52 ± 0.07)×10-11, and (0.98 ± 0.05)×10-11 cm3 molecule-1 s-1 for the δ1, cis-β, and trans-β isomers. Finally, yields of the first-generation products of IEPOX + OH oxidation were measured, and a new mechanism of IEPOX oxidation is proposed here to account for the observed products. The substantial yield of glyoxal and methylglyoxal from IEPOX oxidation may help explain elevated levels of those compounds observed in low-NO environments with high isoprene emissions.
Recent laboratory and field work has suggested that isoprene-derived epoxides (IEPOX) are crucial intermediates that can explain the existence of a variety of compounds found in ambient secondary organic aerosol (SOA). However, IEPOX species are also able to undergo gas phase oxidation, which competes with the aerosol phase processing of IEPOX. In order to better quantify the atmospheric fate of IEPOX, the gas phase OH reaction rate constants and product formation mechanisms have been determined using a flow tube chemical ionization mass spectrometry technique. The new OH rate constants are generally larger than previous estimations and some features of the product mechanism are well predicted by the Master Chemical Mechanism Version 3.2 (MCM v3.2), while other features are at odds with MCM v3.2. Using a previously proposed kinetic model for the quantitative prediction of the atmospheric fate of IEPOX, it is found that gas phase OH reaction is an even more dominant fate for chemical processing of IEPOX than previously suggested. The present results suggest that aerosol phase processing of IEPOX will be competitive with gas phase OH oxidation only under conditions of high liquid water content and low pH SOA conditions.
Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.