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Hygroscopic and CCN properties of aerosol particles in boreal forests
Abstract
The measurements of the hygroscopic and cloud condensation nuclei (CCN) properties of sub-micrometer atmospheric aerosol particles were performed with two tandem differential mobility analysers (TDMA) and a CCN counter at the Hyytiälä forest field station in south-central Finland during the BIOFOR campaign. The TDMAs were used to measure hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10–365 nm when taken from the dry state (relative humidity RH <5%) to RH=90%. The CCN counter was used to study the activation of aerosol particles when exposed to supersaturated conditions. The measurements show clear diurnal pattern of particle solubility. The pattern was strongest for particles in nucleation and Aitken modes. The lowest growth factor (soluble fraction) values were detected during late evening and early morning and the maximum was observed during noon-afternoon. The highest soluble fractions were determined for nucleation mode particles. The response of hygroscopic growth to changes of relative humidity suggests that the soluble compounds are either fully soluble or deliquescent well before 70% RH. The hygroscopic growth was investigated additionally by a detailed model using the size-resolved composition from the impactor samples. The comparison between different instruments shows good consistency. We found good agreement for the 20 nm growth factors measured with two TDMAs, not only on average but also regarding the temporal variation. The similar conclusion was drawn for comparison of TDMA with CCNC for Aitken mode particles with dry sizes 50 and 73 nm. Differences between wet and dry spectra measured using APS and CSASP spectrometer probes were used to derive growth factors for coarse mode particles. Growth factors for coarse mode particles (Dp ca. 2 μm) ranged between 1.0 and 1.6. Agreement between the evolution of growth factors with time for both accumulation and coarse modes was observed regularly. However, similar portions of the data set also indicated clear differences and consequently different chemical compositions between both modes. When the differences between both modes were observed, the coarse mode always behaved in a less hygroscopic manner, with growth factors near one.
... Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | environment due to the diverse properties of these compounds and their temporal variability (Hämeri et al., 2001; Ehn et al., 2007; Raatikainen et al., 2010; Hong et al., 2014). Measurements of the hygroscopicity of newly-formed particles are critical to obtain information on the chemical species involved in the particle formation. ...
... (DMPS) since January 1996 (Aalto et al., 2001 ). The non-refractory chemical composition and O : C of submicron aerosols were measured using a High Resolution Aerosol Mass Spectrometer (HR-AMS, Aerodyne Research Inc., Billerica, USA). ...
... If the Kelvin effect is taken into account, the increase in the HGF of 15 and 20 nm particles was comparable with that of 100 nm-sized particles. A similar behavior has also been observed in previous studies at our site (Ehn et al., 2007; Hämeri et al., 2001). ...
Measurements of the hygroscopicity of 15–145 nm particles in
a boreal forest environment were conducted using two Hygroscopicity
Tandem Differential Mobility Analyzer (HTDMA) systems during the
Pan-European Gas-AeroSOIs-climate interaction Study (PEGASOS) campaign
in spring 2013. Measurements of the chemical composition of non-size
segregated particles were also performed using a High-Resolution
Aerosol Mass Spectrometer (HR-AMS) in parallel with hygroscopicity
measurements. On average, the hygroscopic growth factor (HGF) of
particles was observed to increase from the morning until
afternoon. In case of accumulation mode particles, the main reasons
for this behavior were increases in the ratio of sulfate to organic
matter and oxidation level (O : C ratio) of the organic matter in
the particle phase. Using an O : C dependent hygroscopic growth
factor of organic matter (HGForg), fitted using the
inverse Zdanovskii–Stokes–Robinson (ZSR) mixing rule, clearly
improved the agreement between measured HGF and that predicted based
on HR-AMS composition data. Besides organic oxidation level, the
influence of inorganic species was tested when using the ZSR mixing
rule to estimate the hygroscopic growth factor of organics in the
aerosols. While accumulation and Aitken mode particles were predicted
fairly well by the bulk aerosol composition data, the hygroscopicity
of nucleation mode particles showed little correlation. However, we
observed them to be more sensitive to the gas phase concentration of
condensable vapors: the more there was sulfuric acid in the gas phase,
the more hygroscopic the nucleation mode particles were. No clear
dependence was found between the extremely low-volatility organics
(ELVOCs) concentration and the HGF of particles of any size.
... Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | environment due to the diverse properties of these compounds and their temporal variability (Hämeri et al., 2001; Ehn et al., 2007; Raatikainen et al., 2010; Hong et al., 2014). Measurements of the hygroscopicity of newly-formed particles are critical to obtain information on the chemical species involved in the particle formation. ...
... (DMPS) since January 1996 (Aalto et al., 2001 ). The non-refractory chemical composition and O : C of submicron aerosols were measured using a High Resolution Aerosol Mass Spectrometer (HR-AMS, Aerodyne Research Inc., Billerica, USA). ...
... If the Kelvin effect is taken into account, the increase in the HGF of 15 and 20 nm particles was comparable with that of 100 nm-sized particles. A similar behavior has also been observed in previous studies at our site (Ehn et al., 2007; Hämeri et al., 2001). ...
Measurements of the hygroscopicity of 15–145 nm particles in
a boreal forest environment were conducted using two Hygroscopicity
Tandem Differential Mobility Analyzer (HTDMA) systems during the
Pan-European Gas-AeroSOIs-climate interaction Study (PEGASOS) campaign
in spring 2013. Measurements of the chemical composition of non-size
segregated particles were also performed using a High-Resolution
Aerosol Mass Spectrometer (HR-AMS) in parallel with hygroscopicity
measurements. On average, the hygroscopic growth factor (HGF) of
particles was observed to increase from the morning until
afternoon. In case of accumulation mode particles, the main reasons
for this behavior were increases in the ratio of sulfate to organic
matter and oxidation level (O : C ratio) of the organic matter in
the particle phase. Using an O : C dependent hygroscopic growth
factor of organic matter (HGForg), fitted using the
inverse Zdanovskii–Stokes–Robinson (ZSR) mixing rule, clearly
improved the agreement between measured HGF and that predicted based
on HR-AMS composition data. Besides organic oxidation level, the
influence of inorganic species was tested when using the ZSR mixing
rule to estimate the hygroscopic growth factor of organics in the
aerosols. While accumulation and Aitken mode particles were predicted
fairly well by the bulk aerosol composition data, the hygroscopicity
of nucleation mode particles showed little correlation. However, we
observed them to be more sensitive to the gas phase concentration of
condensable vapors: the more there was sulfuric acid in the gas phase,
the more hygroscopic the nucleation mode particles were. No clear
dependence was found between the extremely low-volatility organics
(ELVOCs) concentration and the HGF of particles of any size.
... The frequent occurrence of nucleation events in boreal forests influences the aerosol properties of different size modes (Hämeri et al., 2001 ). CCN studies for this type of environment have investigated the diurnal variation of critical activation diameter, hygroscopicity, solubility , and the properties of characteristic particle sizes and modes (e.g., Aalto and Kulmala, 2000; Hämeri et al., 2001; Sihto et al., 2010). It is expected that the influence of particle formation events and secondary organic aerosol (SOA) production give rise to CCN rich in carbonaceous material, but with a small fraction of inorganic salts that further augments their hygroscopicity (Sihto et al., 2010). ...
... Diurnal trends in CFSTGC-derived and HTDMA-derived κ values are seen for all particle sizes sampled with minimum κ values occurring in the early morning (near sunrise) and maximum κ values occurring in the afternoon (Fig. 7a and c). This diurnal trend corresponds well with those found by previous studies at the Hyytiälä site (e.g., Hämeri et al., 2001; Väkevä et al., 2002; Boy et al., 2004; Ehn et al., 2007). CFSTGC-derived κ values covary with AMS-derived inorganic volume fraction, daily temperature trends, and AMS al., 2008) and organics in the atmosphere typically become more oxidized as they age; this suggests that aerosol hygroscopicity is also determined by the aging of those organics. ...
... This suggests that the trend in organic composition and oxidation state of both small CCNrelevant size particles and larger particles sampled in bulk measurements is the same and also that more aged (oxidized) particles have higher hygroscopicities than less oxidized or- ganics. The diurnal trend in κ displays maximum values in the afternoon and minimum values in the early morning as has been seen in previous studies by Hämeri et al. (2001 Hämeri et al. ( ), Väkevä et al. (2002 Hämeri et al. ( ), Boy et al. (2004, and Ehn et al. (2007). The high variability of afternoon κ values and the general increase in κ observed for larger particles confirm the results of these earlier studies. ...
Measurements of size-resolved cloud condensation nuclei (CCN), subsaturated hygroscopic growth, size distribution, and chemical composition were collected from March through May, 2007, in the remote Boreal forests of Hyytiälä, Finland, as part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaign. Hygroscopicity parameter, kappa, distributions were derived independently from Continuous Flow-Streamwise Thermal Gradient CCN Chamber (CFSTGC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) measurements. CFSTGC-derived kappa values for 40, 60, and 80 nm particles range mostly between 0.10 and 0.40 with an average of 0.20 ± 0.10; this is characteristic of highly oxidized organics and reflect their dominant influence in this environment. HTDMA-derived kappa were generally 30 % lower. Diurnal trends of kappa show a minimum at sunrise and a maximum in the late afternoon; this trend covaries with inorganic mass fraction and the m/z 44 organic mass fraction given by a quadrupole aerosol mass spectrometer, further illustrating the importance of ageing on aerosol hygroscopicity. The chemical dispersion inferred from the observed kappa distributions indicates that while 60 and 80 nm dispersion increases around midday, 40 nm dispersion remains constant. Additionally, 80 nm particles show a markedly higher level of chemical dispersion than both 40 and 60 nm particles. An analysis of droplet activation kinetics for the sizes considered indicates that the CCN activate as rapidly as (NH4)2SO4 calibration aerosol.
... A significant amount of external mixing for particles at a forested site was found by Hämeri et al. (2001). Our observations resemble more the hygroscopic behavior of pristine Amazon forest (Zhou et al. 1999). ...
... Hygroscopic modes were monomodal; bimodal distributions were sporadic occurrences, comprising 5% of the measurements performed at relative humidity greater than 75%. However, growth factors observed at Golden Ears were similar to the less hygroscopic particles found by Hämeri et al. (2001). Growth factors for less hygroscopic particles at boreal forest (Hämeri et al. 2001) ranged between 1.12 and 1.17 and between 1.2 and 1.4 for the Amazon; these measurements were preformed at 90%. ...
... However, growth factors observed at Golden Ears were similar to the less hygroscopic particles found by Hämeri et al. (2001). Growth factors for less hygroscopic particles at boreal forest (Hämeri et al. 2001) ranged between 1.12 and 1.17 and between 1.2 and 1.4 for the Amazon; these measurements were preformed at 90%. Using Equation (7) and the b = 0.066 growth factor for Golden Ears, particle growth factor at 90% relative humidity was calculated to be 1.18. ...
A tandem differential mobility analyzer (TDMA) system was developed to measure particle growth factors over a range of relative humidities from 50–85%. This system was deployed in the Lower Fraser Valley, and data was collected at Golden Ears Provincial Park and Eagle Ridge Mountain during the Pacific 2001 field study. Following humidification, particles either had a single hygroscopic mode or a bimodal distribution with more hygroscopic and less hygroscopic modes. Growth factors for the less hygroscopic mode were found to be consistent throughout the study. Monomodal distributions were the more common result. Their growth factor ranged between that of the less and the more hygroscopic modes. Using observed and calculated growth factors, relative organic and inorganic fractions were estimated. Particles at these sites were largely organic. Organic fractions near unity were found at Golden Ears forest and during rainy days at Eagle Ridge.
... Aerosol particles can shrink or grow in size by exchanging water vapour with the surrounding air as relative humidity (RH) changes, thus having a direct effect on the radiation balance of the atmosphere through the change of scattering and absorption of light (Tang et al. 1981 ). Furthermore, hygroscopicity has a strong influence on the ability of particles to act as cloud condensation nuclei (Hämeri et al. 2001; Petters and Kreidenweis 2007; Reutter et al. 2009). Consequently, it also affects global climate indirectly. ...
... To conclude, the oxidation of organic soot is also known as the main ageing process which makes particles become more hydrophilic (Martin et al. 2013). Petäjä et al. 2007; Sakurai et al. 2005; Väkevä et al., 2002b), in boreal forests (Hämeri et al. 2001; Park et al. 2009) and in coastal regions (Buzorius et al. 2004; Väkevä et al., 2002a). Sakurai et al. (2005) measured G f in an urban background of Atlanta and reported that the 10 nm nucleation particles were more hygroscopic , showing a growth factor value of ∼1.4 that was equivalent to 10 nm ammonium sulphate particles. ...
... Comparison between measured and extrapolated N CCN constitutes a so-called hygroscopicity-CCN closure. In the last few decades, intensive efforts have been made in achieving hygroscopicity-CCN closure (Cai et al., 2018;Cerully et al., 2011;Hansen et al., 2015;Hämeri et al., 2001;Hong et al., 2014;Jurányi et al., 2013;Kim et al., 2011). For example, Kawana et al. (2016) found that in urban areas of Tokyo, the aerosol organic matter played a vital role in CCN activity, whereby the N CCN was greatly underestimated if organic matter was assumed to be completely insoluble. ...
We present an aerosol cloud condensation nuclei (CCN)
closure study over the north-east Atlantic Ocean using six approximating
methods. The CCN number concentrations (NCCN) were measured at four
discrete supersaturations (SSs; 0.25 %, 0.5 %, 0.75 % and 1.0 %). Concurrently,
aerosol number size distribution, sub-saturation hygroscopic growth factor
and bulk PM1 chemical composition were obtained at matching time
resolution and after a careful data validation exercise. Method A used a
constant bulk hygroscopicity parameter κ of 0.3; method B used bulk
PM1 chemical composition measured by an aerosol mass spectrometer
(AMS); method C utilised a single growth factor (GF) size (165 nm) measured
by a humidified tandem differential mobility analyser (HTDMA); method D
utilised size-dependent GFs measured at 35, 50, 75, 110 and 165 nm; method E
divided the aerosol population into three hygroscopicity modes
(near-hydrophobic, more-hygroscopic and sea-salt modes), and the total CCN
number in each mode was cumulatively added up; method F used the full-size-scale GF probability density function (GF–PDF) in the most complex approach.
The studied periods included high-biological-activity and low-biological-activity seasons in clean marine and polluted continental air masses to
represent and discuss the most contrasting aerosol populations.
Overall, a good agreement was found between estimated and measured NCCN
with linear regression slopes ranging from 0.64 to 1.6. The temporal
variability was captured very well, with Pearson's R value ranging from 0.76
to 0.98 depending on the method and air mass type. We further compared the
results of using different methods to quantify the impact of size-dependent
hygroscopicity and mixing state and found that ignoring size-dependent
hygroscopicity induced overestimation of NCCN by up to 12 %, and
ignoring a mixing state induced overestimation of NCCN by up to 15 %. The error induced by assuming an internal mixing in highly polluted
cases was largely eliminated by dividing the full GF–PDF into three
conventional hygroscopic modes, while assuming an internal mixing in clean
marine aerosol did not induce significant error.
... The hygroscopic properties of aerosol particles in the northern European boreal forests under sub-and supersaturated conditions have been studied extensively using the Hygroscopicity Tandem Differential Mobility Analyzer (HT-DMA) and size-resolved CCN counter (Hämeri et al., 2001;Birmili et al., 2009;Sihto et al., 2011;Cerully et al., 2011;Paramonov et al., 2013;Jaatinen at al., 2014). These results specifically show that in summer the aerosol particles are enriched in organic species produced by biomass burning and biogenic emissions, which overall decrease their hygroscopicity and CCN activity when compared to other locations in Europe. ...
In this study we describe the hygroscopic properties of accumulation- and
coarse-mode aerosol particles sampled at the Zotino Tall Tower Observatory
(ZOTTO) in central Siberia (61° N, 89° E) from 16 to 21
June 2013. The hygroscopic growth measurements were supplemented with
chemical analyses of the samples, including inorganic ions and
organic/elemental carbon. In addition, the microstructure and chemical
compositions of aerosol particles were analyzed by x-ray micro-spectroscopy
(STXM-NEXAFS) and transmission electron microscopy (TEM). A mass closure
analysis indicates that organic carbon accounted for 61 and 38 % of
particulate matter (PM) in the accumulation mode and coarse mode,
respectively. The water-soluble fraction of organic matter was estimated to
be 52 and 8 % of PM in these modes. Sulfate, predominantly in the form of
ammoniated sulfate, was the dominant inorganic component in both size modes:
~ 34 % in the accumulation mode vs. ~ 47 % in the coarse
mode.
The hygroscopic growth measurements were conducted with a filter-based
differential hygroscopicity analyzer (FDHA) over the range of 5–99.4 %
RH in the hydration and dehydration operation modes. The FDHA study indicates
that both accumulation and coarse modes exhibit pronounced water uptake
approximately at the same relative humidity (RH), starting at
~ 70 %, while efflorescence occurred at different humidities, i.e.,
at ~ 35 % RH for submicron particles vs. ~ 50 % RH for
supermicron particles. This ~ 15 % RH difference was attributed to
higher content of organic material in the submicron particles, which
suppresses water release in the dehydration experiments.
The kappa mass interaction model (KIM) was applied to characterize and
parameterize non-ideal solution behavior and concentration-dependent water
uptake by atmospheric aerosol samples in the 5–99.4 % RH range. Based on
KIM, the volume-based hygroscopicity parameter, κv, was
calculated. The κv,ws value related to the water-soluble
(ws) fraction was estimated to be ~ 0.15 for the accumulation mode
and ~ 0.36 for the coarse mode, respectively. The obtained
κv,ws for the accumulation mode is in good agreement with
earlier data reported for remote sites in the Amazon rain forest (κv ≈ 0.15) and a Colorado mountain forest
(κv ≈ 0.16 ).
We used the Zdanovskii–Stokes–Robinson (ZSR) mixing rule to predict the
chemical composition dependent hygroscopicity, κv,p. The
obtained κv,p values overestimate the experimental
FDHA-KIM-derived κv,ws by factors of 1.8 and 1.5 for the
accumulation and coarse modes, respectively. This divergence can be explained
by incomplete dissolution of the hygroscopic inorganic compounds resulting
from kinetic limitations due to a sparingly soluble organic coating. The TEM
and STXM-NEXAFS results indicate that aged submicron (> 300 nm)
and supermicron aerosol particles possess core–shell structures with an
inorganic core, and are enriched in organic carbon at the mixed particle
surface. The direct FDHA kinetic studies provide a bulk diffusion coefficient
of water of ~ 10−12 cm2 s−1 indicating a semi-solid
state of the organic-rich phase leading to kinetic limitations of water
uptake and release during hydration and dehydration cycles.
Overall, the present ZOTTO data set, obtained in the growing season, has
revealed a strong influence of organic carbon on the hygroscopic properties
of the ambient aerosols. The sparingly soluble organic coating controls
hygroscopic growth, phase transitions, and microstructural rearrangement
processes. The observed kinetic limitations can strongly influence the
outcome of experiments performed on multi-second timescales, such as the
commonly applied HTDMA (Hygroscopicity Tandem Differential Mobility Analyzer)
and CCNC (Cloud Condensation Nuclei Counter) measurements.
... However, hygroscopicity data for ambient aerosols is still far from sufficient (Swietlicki et al., 2008). For S c measurement of size-selected ambient particles only a few studies are available (Hudson and Da, 1996; Hämeri et al., 2001; Dusek et al., 2006; Hudson, 2007; Cerully et al., 2011). In this study, we try to characterize hygroscopicity and S c of aerosols measured in Asian continental outflow. ...
Aerosol size distribution, total concentration (i.e. condensation nuclei (CN) concentration, N <sub>CN</sub>), cloud condensation nuclei (CCN) concentration ( N <sub>CCN</sub>), hygroscopicity at ~90% relative humidity (RH) were measured at a background monitoring site at Gosan, Jeju Island, south of the Korean Peninsula in August 2006, April to May 2007 and August to October 2008. Similar measurements took place in August 2009 at another background site (Baengnyeongdo Comprehensive Monitoring Observatory, BCMO) on the island of Baengnyeongdo, off the west coast of the Korean Peninsula. Both islands were found to be influenced by continental sources regardless of season and year. Average values for all of the measured N <sub>CCN</sub> at 0.2, 0.6 and 1.0% supersaturations ( S ), N <sub>CN</sub>, and geometric mean diameter ( D <sub>g</sub>) from both islands were in the range of 1043–3051 cm<sup>−3</sup>, 2076–4360 cm<sup>−3</sup>, 2713–4694 cm<sup>−3</sup>, 3890–5117 cm<sup>−3</sup> and 81–98 nm, respectively. Although the differences in D <sub>g</sub> and N <sub>CN</sub> were small between Gosan and BCMO, N <sub>CCN</sub> at various S was much higher at the latter, which is closer to China.
Most of the aerosols were internally mixed and no notable differences in hygroscopicity were found between the days of strong pollution influence and the non-pollution days for both islands. During the 2008 and 2009 campaigns, critical supersaturation for CCN nucleation ( S <sub>c</sub>) for selected particle sizes was measured. Particles of 100 nm diameters had mean S <sub>c</sub> of 0.19 ± 0.02% during 2008 and those of 81 and 110 nm diameters had mean S <sub>c</sub> of 0.26 ± 0.07% and 0.17 ± 0.04%, respectively, during 2009. The values of the hygroscopicity parameter (κ), estimated from measured S <sub>c</sub>, were mostly higher than the κ values obtained from the measured hygroscopic growth at ~90% RH.
For the 2008 campaign, N <sub>CCN</sub> at 0.2, 0.6 and 1.0% S were predicted based on measured dry particle size distributions and various ways of representing particle hygroscopicity. The best closure was obtained when temporally varying and size-resolved hygroscopicity information from the HTDMA was used, for which the average relative deviations from the measured values were 28 ± 20% for 0.2% S (mostly under-prediction), 25 ± 52% for 0.6% (balanced between over- and under-prediction) and 19 ± 15% for 1.0% S (balanced). Prescribing a constant hygroscopicity parameter suggested in the literature (κ = 0.3) for all sizes and times resulted in average relative deviations of 28–41% where over-prediction was dominant. When constant hygroscopicity was assumed, the relative deviation tended to increase with decreasing N <sub>CCN</sub>, which was accompanied by an increase of the sub-100 nm fraction. These results suggest that hygroscopicity information for particles of diameters smaller than 100 nm is crucial for more accurate predictions of N <sub>CCN</sub>. For confirmation when κ = 0.17, the average κ for sub-100 nm particles in this study, was applied for sub-100 nm and κ = 0.3 for all other sizes, the CCN closure became significantly better than that with κ = 0.3 for all sizes.
... Titos et al., 2014b;Zieger et al., 2012;Fierz-Schmidhauser et al., 2010b). In boreal environments, the aerosol particles are typically less hygroscopic (Swietlicki et al., 2008;Ehn et al., 2007;Petäjä et al., 2005;Hämeri et al., 2001) due to a large contribution of organics (Allan et al., 2006). So far, the f (RH) of particles representative for boreal regions has not been characterized in great detail. ...
Ambient aerosol particles can take up water and thus change their optical
properties depending on the hygroscopicity and the relative humidity (RH) of
the surrounding air. Knowledge of the hygroscopicity effect is of crucial
importance for radiative forcing calculations and is also needed for the
comparison or validation of remote sensing or model results with in-situ
measurements. Specifically, particle light scattering depends on RH and can
be described by the scattering enhancement factor f(RH), which is defined
as the particle light scattering coefficient at defined RH divided by its dry
value (RH
... Measurements of aerosol hygroscopic properties have been performed extensively around the world both in laboratory and field studies using different techniques, e.g., an Electrodynamic Balance (EB, Tang and Munkelwitz, 1994) and Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA, Liu et al., 1978; Swietlicki et al., 2008). With these techniques hygroscopic properties of many inorganic and some organic compounds have been targeted specifically (Hämeri et al., 2001; Wise et al., 2003; Massoli et al., 2010). Inorganic salts, e.g., ammonium sulfate and sodium chloride, show hysteresis when exposed to an increasing and decreasing relative humidity environment, while organic aerosols can often take up water at lower RH than the Deliquescence Relative Humidity (DRH) of inorganic salts (Sjogren et al., 2008 ). ...
... The hygroscopic properties of aerosol particles in the northern European boreal forests under sub-and supersaturated conditions have been studied extensively using the Hygroscopicity Tandem Differential Mobility Analyzer (HT-DMA) and size-resolved CCN counter (Hämeri et al., 2001;Birmili et al., 2009;Sihto et al., 2011;Cerully et al., 2011;Paramonov et al., 2013;Jaatinen at al., 2014). These results specifically show that in summer the aerosol particles are enriched in organic species produced by biomass burning and biogenic emissions, which overall decrease their hygroscopicity and CCN activity when compared to other locations in Europe. ...
In this study we describe the hygroscopic properties of accumulation- and coarse-mode aerosol particles sampled at the Zotino Tall Tower Observatory (ZOTTO) in central Siberia (61° N, 89° E) from 16 to 21 June 2013. The hygroscopic growth measurements were supplemented with chemical analyses of the samples, including inorganic ions and organic/elemental carbon. In addition, the microstructure and chemical compositions of aerosol particles were analyzed by x-ray micro-spectroscopy (STXM-NEXAFS) and transmission electron microscopy (TEM). A mass closure analysis indicates that organic carbon accounted for 61 and 38 % of particulate matter (PM) in the accumulation mode and coarse mode, respectively. The water-soluble fraction of organic matter was estimated to be 52 and 8 % of PM in these modes. Sulfate, predominantly in the form of ammoniated sulfate, was the dominant inorganic component in both size modes: ~ 34 % in the accumulation mode vs. ~ 47 % in the coarse mode.
The hygroscopic growth measurements were conducted with a filter-based differential hygroscopicity analyzer (FDHA) over the range of 5–99.4 % RH in the hydration and dehydration operation modes. The FDHA study indicates that both accumulation and coarse modes exhibit pronounced water uptake approximately at the same relative humidity (RH), starting at ~ 70 %, while efflorescence occurred at different humidities, i.e., at ~ 35 % RH for submicron particles vs. ~ 50 % RH for supermicron particles. This ~ 15 % RH difference was attributed to higher content of organic material in the submicron particles, which suppresses water release in the dehydration experiments.
The kappa mass interaction model (KIM) was applied to characterize and parameterize non-ideal solution behavior and concentration-dependent water uptake by atmospheric aerosol samples in the 5–99.4 % RH range. Based on KIM, the volume-based hygroscopicity parameter, κv, was calculated. The κv,ws value related to the water-soluble (ws) fraction was estimated to be ~ 0.15 for the accumulation mode and ~ 0.36 for the coarse mode, respectively. The obtained κv,ws for the accumulation mode is in good agreement with earlier data reported for remote sites in the Amazon rain forest (κv ≈ 0.15) and a Colorado mountain forest (κv ≈ 0.16 ).
We used the Zdanovskii–Stokes–Robinson (ZSR) mixing rule to predict the chemical composition dependent hygroscopicity, κv,p. The obtained κv,p values overestimate the experimental FDHA-KIM-derived κv,ws by factors of 1.8 and 1.5 for the accumulation and coarse modes, respectively. This divergence can be explained by incomplete dissolution of the hygroscopic inorganic compounds resulting from kinetic limitations due to a sparingly soluble organic coating. The TEM and STXM-NEXAFS results indicate that aged submicron (> 300 nm) and supermicron aerosol particles possess core–shell structures with an inorganic core, and are enriched in organic carbon at the mixed particle surface. The direct FDHA kinetic studies provide a bulk diffusion coefficient of water of ~ 10−12 cm2 s−1 indicating a semi-solid state of the organic-rich phase leading to kinetic limitations of water uptake and release during hydration and dehydration cycles.
Overall, the present ZOTTO data set, obtained in the growing season, has revealed a strong influence of organic carbon on the hygroscopic properties of the ambient aerosols. The sparingly soluble organic coating controls hygroscopic growth, phase transitions, and microstructural rearrangement processes. The observed kinetic limitations can strongly influence the outcome of experiments performed on multi-second timescales, such as the commonly applied HTDMA (Hygroscopicity Tandem Differential Mobility Analyzer) and CCNC (Cloud Condensation Nuclei Counter) measurements.
... Titos et al., 2014b;Zieger et al., 2012;Fierz-Schmidhauser et al., 2010b). In boreal environments, the aerosol particles are typically less hygroscopic (Swietlicki et al., 2008;Ehn et al., 2007;Petäjä et al., 2005;Hämeri et al., 2001) due to a large contribution of organics (Allan et al., 2006). So far, the f (RH) of particles representative for boreal regions has not been characterized in great detail. ...
Ambient aerosol particles can take up water and thus change their optical
properties depending on the hygroscopicity and the relative humidity (RH) of
the surrounding air. Knowledge of the hygroscopicity effect is of crucial
importance for radiative forcing calculations and is also needed for the
comparison or validation of remote sensing or model results with in situ
measurements. Specifically, particle light scattering depends on RH and can
be described by the scattering enhancement factor f(RH), which is defined
as the particle light scattering coefficient at defined RH divided by its dry
value (RH
... These results suggest that, during the early stages of an event under these conditions, the lower end of the size distribution undergoes the greatest relative change in hygroscopicity. Other studies have observed a slight increase in hygroscopicity during new particle formation events and have attributed this to elevated sulfuric acid concentrations on event days, which may directly contribute to the nucleation reactions and influence particle hygroscopicity through condensation (Hä meri et al., 2001; Ehn et al., 2007). The combination of size distribution and CCN activation measurements during nanoparticle events demonstrates the importance of particle growth in converting nascent aerosols into CCN. ...
During autumn, 2006, variation in the frequency of aerosol nucleation events, as inferred from nanoparticle growth events, and associated hygroscopicity were investigated as a function of air mass transport history at a mixed deciduous forest in central Virginia, U.S. Above-canopy size distributions of aerosols between 0.012 and 0.700 μm diameter, size-resolved particle hygroscopicity at eight dry diameters between 0.012 and 0.400 μm, and cloud condensation nuclei (CCN) activity were characterized. Air mass back trajectories were clustered to identify source regions. Growth events were most frequent in fast-moving air masses (mean = 9 m s−1) that originated over the north central U.S. Under these flow regimes, mean values for preexisting sub-μm aerosol number concentrations (4700 cm−3), corresponding surface area (142 μm2 cm−3), air temperature (6.2 °C), and relative humidity (RH, 49.4%) were relatively low compared to other regimes. Under stagnant flow conditions (mean = 3 m s−1), mean number concentrations were higher (>6000 cm−3) and size fractions
The chemical composition of aerosol particles is crucial to understanding their formation and evolution in the atmosphere. However, very limited information is available for the chemical composition of ultrafine particles, particularly for nanoclusters and newly formed particles, due to the lack of valid analytical methods. This work reviews the online measurement techniques for characterizing the chemical composition of atmospheric clusters and sub-20 nm particles. The commonly used instrumentations are divided into two categories: direct techniques based on mass spectrometry and indirect measurement techniques mainly relying on the physical properties (e.g., hygroscopicity and volatility). The advantages of these techniques are compared and their limitations in the lab and field application are summarized. The combination of direct and indirect techniques is discussed, and this may provide more comprehensive understanding of chemical information of atmospheric clusters and particles. We propose that the newly developing instrumentations are needed to improve the collection efficiency for direct techniques, or the chemical resolution for indirect techniques. Future development should focus on obtaining simultaneous measurements of particle physical and chemical properties, which can be helpful in improving the accuracy of modeling and the understanding of particle formation and evolution.
The concentration of cloud condensation nuclei (CCN) is an essential
parameter affecting aerosol–cloud interactions within warm clouds. Long-term
CCN number concentration (NCCN) data are scarce; there are a lot more
data on aerosol optical properties (AOPs). It is therefore valuable to
derive parameterizations for estimating NCCN from AOP measurements. Such
parameterizations have already been made, and in the present work a new parameterization
is presented. The relationships between NCCN, AOPs, and size
distributions were investigated based on in situ measurement data from six
stations in very different environments around the world. The relationships
were used for deriving a parameterization that depends on the scattering
Ångström exponent (SAE), backscatter fraction (BSF), and total
scattering coefficient (σsp) of PM10 particles. The analysis
first showed that the dependence of NCCN on supersaturation (SS) can be
described by a logarithmic fit in the range SS <1.1 %, without
any theoretical reasoning. The relationship between NCCN and AOPs was
parameterized as NCCN≈((286±46)SAE ln(SS/(0.093±0.006))(BSF − BSFmin) + (5.2±3.3))σsp, where BSFmin is the minimum BSF, in practice
the 1st percentile of BSF data at a site to be analyzed. At the lowest
supersaturations of each site (SS ≈0.1 %), the average bias,
defined as the ratio of the AOP-derived and measured NCCN, varied from
∼0.7 to ∼1.9 at most sites except at a
Himalayan site where the bias was >4. At SS >0.4 % the average bias ranged from ∼0.7 to ∼1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was
higher, ∼1.4–1.9. In other words, at SS >0.4 % NCCN was estimated with an average uncertainty of approximately
30 % by using nephelometer data. The biases were mainly due to the biases
in the parameterization related to the scattering Ångström exponent
(SAE). The squared correlation coefficients between the AOP-derived and
measured NCCN varied from ∼0.5 to ∼0.8. To
study the physical explanation of the relationships between NCCN and
AOPs, lognormal unimodal particle size distributions were generated and
NCCN and AOPs were calculated. The simulation showed that the
relationships of NCCN and AOPs are affected by the geometric mean
diameter and width of the size distribution and the activation diameter. The
relationships of NCCN and AOPs were similar to those of the observed
ones.
To better understand the sources, formation, and the transport of air pollutants over North China Plain (NCP), a four-week intensive campaign during summertime in 2014 was conducted in a central NCP rural site. In this study, particle hygroscopicity and volatility measurements were focused to characterize the thermodynamic properties of nanoparticles and gain insight into chemical composition of nanoparticles during the new particle formation (NPF) events. The water-soluble fractions of 30 and 50 nm newly formed particles were respectively 0.64 ± 0.06 and 0.61 ± 0.06, indicating that the water-soluble chemical compounds, most likely ammonium sulfate, dominated the condensational growth of newly formed particles over the NCP. Due to containing higher water-soluble fraction, nanoparticles can be activated as cloud condensation nuclei (CCN) at lower supersaturation in the atmosphere of NCP in contrast to cleaner environments, such as Melpitz (Central European background) and Hyytiälä (boreal forest) during the NPF events. Our observations showed that the NPF and subsequent growth significantly resulted in an enhancement in CCN number concentration. The ranges of enhancement factors of CCN number concentration for supersaturation (SS) = 0.2, 0.4, 0.8% were respectively 1.9–7.0, 2.7–8.4, and 3.6–10.1. After being heated to 300 °C, the shrink factors for 30 and 50 nm particles were respectively 0.35 and 0.38. This indicated that non-volatile compounds could be produced during the growth process of newly formed particles.
As a part of EUCAARI activities, the annual cycle of cloud condensation nuclei (CCN) concentrations and critical diameter for cloud droplet activation as a function of supersaturation were measured using a CCN counter and a HTDMA (hygroscopicity tandem differential mobility analyzer) at SMEAR II station, Hyytiälä, Finland. The critical diameters for CCN activation were estimated from (i) the measured CCN concentration and particle size distribution data, and (ii) the hygroscopic growth factors by applying κ-Köhler theory, in both cases assuming an internally mixed aerosol. The critical diameters derived by these two methods were in good agreement with each other. The effect of new particle formation on the diurnal variation of CCN concentration and critical diameters was studied. New particle formation was observed to increase the CCN concentrations by 70–110%, depending on the supersaturation level. The average value for the κ-parameter determined from hygroscopicity measurements was κ = 0.18 and it predicted well the CCN activation in boreal forest conditions in Hyytiälä. The derived critical diameters and κ-parameter confirm earlier findings with other methods, that aerosol particles at CCN sizes in Hyytiälä are mostly organic, but contain also more hygrosopic, probably inorganic salts like ammonium sulphate, making the particles more CCN active than pure secondary organic aerosol.
This study examines the hygroscopicity of newly formed particles (smaller than 50 nm in particle mobility diameter) during two atmospheric new particle formation events with and without clear growth process at mid-level mountain range in Central Germany based on HCCT field campaign. Particle hygroscopicity measurements show that the particle soluble fractions at the end of event for two events are, respectively 60% (45 nm particles for the event with clear growth) and 20% (30 nm particles for the event without clear growth), stressing that non-soluble organic compounds may play a key role in particle growth during new particle formation event. Such significant difference in particle hygroscopicity also suggests that the chemical species responsible for nucleation particle growth are considerably different between the two selected NPF events. During both events, the hygroscopicity of newly formed particles decreased with particle growth, indicating that more less-hygroscopic compounds contribute to the subsequent condensation in contrast to the earlier stage. Sulfuric acid was considered to be responsible of the NPF event and represent the highly hygroscopic compounds. However, calculation demonstrated that sulfuric acid condensation failed to fully explain the observed soluble fraction in the nucleation mode particles. Therefore, we hypothesize that some water-soluble matters may explain the missing soluble fraction.
A Volatility-Hygroscopicity Tandem Differential Mobility Analyzer (VH-TDMA)
was applied to study the hygroscopicity and volatility properties of
submicron atmospheric aerosol particles in a boreal forest environment in
Hyytiälä, Finland during the summer of 2010. Aitken and accumulation
mode internally mixed particles (50 nm, 75 nm and 110 nm in diameter) were
investigated. Hygroscopicity was found to increase with particle size. The
relative mass fraction of organics and SO42- is probably the major
contributor to the fluctuation of the hygroscopicity for all particle sizes.
The Cloud Condensation Nuclei Counter (CCNC)-derived hygroscopicity
parameter κ was observed to be slightly higher than
κ calculated from VH-TDMA data under sub-saturated conditions,
potential reasons for this behavior are discussed shortly. Also, the
size-resolved volatility properties of particles were investigated. Upon
heating, more small particles evaporated compared to large particles. There
was a significant amount of aerosol volume (non-volatile material) left, even
at heating temperatures of 280 °C. Using size resolved
volatility-hygroscopicity analysis, we concluded that there was always
hygroscopic material remaining in the particles at different heating
temperatures, even at 280 °C. This indicates that the
observed non-volatile aerosol material did not consist solely of black
carbon.
Observations of aerosol hygroscopic growth and CCN activation spectra for submicron particles are reported for the T1 ground site outside of Mexico City during the MIRAGE 2006 campaign. κ-Köhler theory is used to evaluate the characteristic hygroscopicity parameter, κ*, for the CCN active aerosol population using both size-resolved HTMDA and size-resolved CCNc measurements. Organic mass fractions (forg) are evaluated from size-resolved aerosol mass spectrometer (AMS) measurements, from which predictions of the hygroscopicity parameter are compared against κ*. Strong diurnal changes in aerosol water uptake parameters and aerosol composition are observed. We find that new particle formation (NPF) events are correlated with an increased κ* and CCN-active fraction during the daytime, with greater impact on smaller particles. During NPF events, the number concentration of 40 nm particles acting as CCN at 0.51% ± 0.06% supersaturation can surpass by more than a factor of two the corresponding concentrations of 100 nm particles. We also find that at 06:00–08:00 LT throughout the campaign, fresh traffic emissions result in substantial changes to the chemical distribution of the aerosol, with on average 65% externally mixed fraction for 40 nm particles and 30% externally mixed fraction for 100 nm particles, whereas at midday nearly all particles of both sizes can be described as "internally mixed". Average activation spectra and growth factor distributions are analyzed for different time periods characterizing the daytime (with and without NPF events), the early morning "rush hour" and the entire campaign. We show that κ* derived from CCNc measurements decreases as a function of size during all time periods, while the CCN-active fraction increases as a function of size. Size-resolved AMS measurements do not predict the observed trend for κ* versus particle size, which can be attributed to unresolved mixing state and the presence of refractory material not measured by the AMS. Measured κ* typically ranges from 0.2 to 0.35, and organics typically make up 60–85 % of the aerosol mass in the size range studied. We show that κAMS is able to describe CCN concentrations reasonably well, provided mixing-state information is available, especially at the highest CCN concentrations. This is consistent with other CCN studies carried out in urban environments, and is partly due to the fact that the highest CCN concentrations occur during the daytime when the aerosol is internally mixed. During the early morning rush hour, however, failing to account for the aerosol mixing state results in systematic overestimation of CCN concentrations by as much as 50–100% on average.
Measurements of size-resolved cloud condensation nuclei (CCN) concentrations, subsaturated hygroscopic growth, size distribution, and chemical composition were collected from March through May, 2007, in the remote Boreal forests of Hyytiälä, Finland, as part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaign. Hygroscopicity parameter, κ , distributions were derived independently from Continuous Flow-Streamwise Thermal Gradient CCN Chamber (CFSTGC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) measurements. CFSTGC-derived κ values for 40, 60, and 80 nm particles range mostly between 0.10 and 0.40 with an average characteristic of highly oxidized organics of 0.20 ± 0.10, indicating that organics play a dominant role for this environment. HTDMA-derived κ were generally 30% lower. Diurnal trends of κ show a minimum at sunrise and a maximum in the late afternoon; this trend covaries with inorganic mass fraction and the m/z 44 organic mass fraction given by a quadrupole aerosol mass spectrometer, further illustrating the importance of organics in aerosol hygroscopicity. The chemical dispersion inferred from the observed κ distributions indicates that while 60 and 80 nm dispersion increases around midday, 40 nm dispersion remains constant. Additionally, 80 nm particles show a markedly higher level of chemical dispersion than both 40 and 60 nm particles. An analysis of droplet activation kinetics for the sizes considered indicates that most of the CCN activate as rapidly as (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> calibration aerosol.
New particle formation (NPF) and growth is an important source of cloud
condensation nuclei (CCN). In this study, we investigated the chemical
species driving new particle growth to the CCN sizes on the basis of particle
hygroscopicity measurements carried out at the research station Melpitz,
Germany. Three consecutive NPF events occurred during summertime were chosen
as examples to perform the study. Hygroscopicity measurements showed that the
(NH4)2SO4-equivalent water-soluble fraction accounts for 20 and
16 % of 50 and 75 nm particles, respectively, during the NPF events.
Numerical analysis showed that the ratios of H2SO4 condensational
growth to the observed particle growth were 20 and 13 % for 50 and 75 nm
newly formed particles, respectively. Aerosol mass spectrometer measurements
showed that an enhanced mass fraction of sulfate and ammonium in the newly
formed particles was observed when new particles grew to the sizes larger
than 30 nm shortly after the particle formation period. At a later time, the
secondary organic species played a key role in the particle growth. Both
hygroscopicity and aerosol mass spectrometer (AMS) measurements and numerical
analysis confirmed that organic compounds were major contributors driving
particle growth to CCN sizes. The critical diameters at different
supersaturations estimated using AMS data and κ-Köhler theory
increased significantly during the later course of NPF events. This indicated
that the enhanced organic mass fraction caused a reduction in CCN efficiency
of newly formed particles. Our results implied that the CCN production
associated with atmospheric nucleation may be overestimated if assuming that
newly formed particles can serve as CCN once
they grow to a fixed particle size, an assumption made in some previous
studies, especially for organic-rich
environments. In our study, the enhancement in CCN number concentration
associated with individual NPF events were 63, 66, and 69 % for 0.1, 0.4,
and 0.6 % supersaturation, respectively.
Hygroscopic properties are an important parameter in determining the atmospheric behaviour of aerosols and their optical properties, influencing the direct and indirect effect of aerosols upon climate. As a result, particle hygroscopicity has received much attention with a rapid increase of publications in recent years. Likewise, hygroscopicity is an important characteristic influencing the deposition efficiency of particles in the human respiratory tract by affecting the particle size. The object of this study is to review the existing knowledge on the hygroscopic growth factor (Gf) of atmospheric submicron particles and its influence on the lung deposition calculation. The study briefly reviews first the Gf values of particles generated from various sources, including nucleation, traffic emissions and biomass burning, discussing the spatial and temporal variations. It then summarises Gf values of submicron particles and number fraction of each hygroscopic group measured in different ambient environments. These include marine, roadside, urban background and rural environments. The study concludes by estimating the lung deposition efficiency of ambient particles using a modified version of the International Commission on Radiological Protection (ICRP) model for hygroscopic particles. Furthermore, the effect of hygroscopicity on lung deposition efficiency of ambient particles has been estimated. The ICRP model seems to predict well the deposition efficiency (DE) values for small ambient particles in the extra-thoracic and tracheo-bronchial region, but not the alveolar region, where they are overestimated. However, for larger particles (D
p
> 200 nm), the ICRP model underestimates the DE values, with the extra-thoracic region the most affected of the three. Hygroscopic atmospheric particles with a small diameter (D
p
< 200 nm) showed a lower total lung deposition than hydrophobic particles of the same initial size due to their hygroscopic properties. On the other hand, larger hygroscopic particles (D
p
> 200 nm) showed much higher lung deposition than hydrophobic particles.
We measured the 222Rn content of the air by continuously collecting particle-bound daughter nuclides 214Pb and 214Bi onto glass-fibre filters and counting their beta particle emissions with Geiger-Müller tubes. A scintillation gamma spectrometer system measured external radiation, which main components are gamma and cosmic radiation. Our purpose was to detect long-term, seasonal and diurnal variations in 222Rn activity concentration and external radiation dose rate in a Finnish boreal forest during the years 2000-2006. The long-term variations in activity concentration and dose rate were small, whereas the annual variations were more pronounced. In late summer and autumn, the diurnal cycle of 222Rn activity concentration was strongest, whereas the diurnal cycle of external radiation dose rate was practically non-existing throughout the year. We utilised the 222Rn and external radiation measurements also when calculating air ion production rate in the lower boundary layer. Based on our results, the total ion production rate varied in the range 4.2-17.6 ion pairs cm-3 s-1. The fraction of 222Rn contribution in the ion production varied in the range 0-0.43, with average fraction 0.11 ± 0.07. These results indicate that ion production was typically dominated by the external radiation on our measurement site.
New particle formation (NPF) and growth is an important source of cloud condensation nuclei (CCN). In this study, we investigated the potential chemical species driving new particle growth to the CCN sizes on the basis of particle hygroscopicity measurements carried out at the research station Melpitz, Germany. The predicted CCN number concentrations using κ-Köhler theory were analyzed to assess the contribution of NPF to possible CCN. Three consecutive NPF events occurred during summertime were chosen as examples to perform the study. Hygroscopicity measurements showed that the (NH4)2SO4-equivalent water-soluble fraction respectively accounts for 20 and 16% of 50 and 75 nm particles during the NPF events. Numerical analysis showed the ratios of H2SO4 condensational growth to the observed particle growth were 20 and 13% for 50 and 75 nm newly formed particles, respectively. Both hygroscopicity measurements and numerical analysis confirmed that organic compounds were major contributors driving particle growth to CCN sizes. The critical diameters at different supersaturations estimated using AMS data and κ-Köhler theory increased significantly during the later course of NPF events. This indicated that the enhanced organic mass fraction caused a reduction in CCN efficiency of newly formed particles. Our results implied that the CCN production associated with atmospheric nucleation may be overestimated if assuming that newly formed particles can serve as CCN in case they grow to a fixed particle size, which was used in some previous studies, especially for organic-rich environments. In our study, the enhancement in CCN number concentration associated with individual NPF events have been 63, 66, 69% for supersaturation 0.1, 0.4, and 0.6%, respectively.
[1] The formation and growth of new particles has been evaluated using a revised version of a simple, but novel, theoretical tool. The concentration of condensable vapors and their source rates has been estimated using the aerosol condensation sink together with the measured particle growth rate. Also, by adding the coagulation sink and the measured formation rate of 3 nm particles, the formation rate of 1 nm particles and their concentration can be estimated. Condensation and coagulation sinks can be obtained from ambient aerosol size distribution data. The method has been applied to analyze the particle formation and growth rates observed during coastal and boreal forest nucleation events. The condensation sinks are typically 4–7 × 10−3 s−1 in the forest and 2 × 10−3 s−1 under coastal conditions, while the coagulation sinks for 1, 2, and 3 nm particles are typically smaller by factors 1.5–2, 5–7, and 11–15, respectively. The measured growth rates are 2–10 nm/h for the boreal forest and range from 15 to 180 nm/h at the coast, corresponding to a vapor concentration of 2–13 × 107 cm−3 and 108 cm−3 to 109 cm−3, respectively. The vapor source rate was 1–2 × 105 cm−3s−1 in the boreal forest and 2–5 × 106 cm−3s−1 in the coastal environment. The estimated formation rate of 1 nm particles in the forest environment was 8–20 cm−3s−1 and 300–10,000 cm−3s−1 at the coast. The concentration of 1 nm particles was estimated to be 2000–5000 and 4 × 104–7 × 106 particles cm−3 in forest and at coast, respectively.
During May of 2003, smoke from fires in the Yucatan Peninsula was transported across the Gulf of Mexico and into Texas where it caused significant enhancement in measured aerosol concentrations and reduced visibility. During this event, the formation and growth of aerosol particles has been observed by a differential mobility analyzer (DMA) / tandem differential mobility analyzer (TDMA) system to characterize the size distribution and size-resolved hygroscopicity of the aerosol. The most number concentration is by the particles smaller than 100 nm, but the integrated number concentrations for over 100 nm increased due to the aerosol growth. Hygroscopic growth factor increase from 1.2 to 1.4 for 25, 50, and 100 nm particles during the nucleating period. This distribution and the aerosol properties derived from the TDMA data were used to calculate the growth rate. Particle growth rates were in the range 1-12 nm/hr.
The Volatility-Hygroscopicity Tandem Differential Mobility Analyzer
(VH-TDMA) was applied to study the hygroscopicity and volatility
properties of submicron atmospheric aerosol in a boreal forest
environment in Hyytiälä, Finland during the summer of 2010.
Aitken and accumulation mode particles (50 nm, 75 nm and 110 nm) were
investigated. The results suggest that the particles were internally
mixed at all sizes. Hygroscopicity was found to increase with size. The
relative mass fraction of organics and SO42- is
probably the major contributor to the fluctuation of the hygroscopicity
for all particle sizes. The Cloud Condensation Nuclei counter
(CCNc)-derived hygroscopicity parameter κ was slightly higher than
κ calculated from VH-TDMA data under sub-saturated conditions,
which can be explained by the fact that particulate organics have a
different degree of dissolution in sub- and supersaturated conditions.
Also, the size-resolved volatility properties of particles were
investigated. Upon heating, small particles evaporated more compared to
large particles. There was a significant amount of aerosol volume
(non-volatile material) left even at heating temperatures above 280
°C. Using size resolved volatility-hygroscopicity analysis, we
concluded that there was always hygroscopic material remaining in the
particles of different sizes at all different heating temperatures, even
above 280 °C. This indicates that the observed non-volatile aerosol
material was not consisting solely of black carbon.
Cluster particles (0.8-1.9 nm) are key entities involved in nucleation
and new particle formation processes in the atmosphere. Cluster ions
were characterized in clear sky conditions at the Puy de Dôme
station (1465 m a.s.l.). The studied data set spread over five years
(February 2007-February 2012), which provided a unique chance to observe
seasonal variations of cluster ion properties at high altitude.
Statistical values of the cluster ion concentrations and diameters are
reported for both positive and negative polarities. Cluster ions were
found to be ubiquitous at the Puy de Dôme and displayed an annual
variation with lower concentrations in spring. Positive cluster ions
were less numerous than negative, but were larger in diameter. Negative
cluster ion properties were not sensitive to the occurrence of a new
particle formation (NPF) event, while positive cluster ions appeared to
be significantly more numerous and larger on event days. The parameters
of the balance equation for the positive cluster concentration are
reported separately for the different seasons and for the NPF event days
and non-event days. The steady-state assumption suggests that the
ionization rate is balanced with two sinks: the ion recombination and
the attachment onto background aerosol particles, referred to as
"aerosol ion sink". The aerosol ion sink was predominant compared to the
recombination sink. The positive ionization rates derived from the
balance equation (Qcalc) were well correlated with the
ionization rates obtained from radon measurement (Qmeas).
When ignoring the gamma radiation contribution to the ion production,
Qcalc is on average higher than Qmeas during the
warm season. In contrast, when a seasonal gamma contribution is taken
into account, Qmeas always exceeds Qcalc. We found
that neither the aerosol ion sink nor the ionization rate (calculated or
measured, with or without the gamma contribution) were significantly
different on event days compared to non-event days, and thus, they were
not able to explain the different positive cluster concentrations
between event and non-event days. Hence, the excess of positive small
ions on event days may derive from an additional constant source of ions
leading to a non-steady state.
Organic compounds are known to comprise a significant fraction of the
atmospheric aerosol population and have been found to contribute to the
concentration of cloud condensation nuclei (CCN). Much of this organic
material is secondary in nature; secondary organic aerosol (SOA) is
formed when volatile organic compounds are oxidized to form less
volatile products, which then condense into the aerosol phase. Many
organic compounds found in the atmosphere, of both anthropogenic and
biogenic origin, have been found to produce SOA. Such reactions
typically result in complex mixtures of products, only a fraction of
which have been identified. Thus while there have been several studies
exploring the potential for organic particles to act as CCN (including
some of the compounds identified in SOA products), there have been
almost no direct investigation of the potential CCN activity of SOA.
This paper presents the results of a series of experiments measuring
directly the CCN activity of SOA produced by the ozonolysis of several
common biogenic compounds. Six compounds were studied: five
monoterpenes (α -pinene, β -pinene, Δ
3-carene, limonene, terpinolene) and one terpinoid alcohol
(terpinen-4-ol). The chosen monoterpenes represent an estimated 87% of
global monoterpene emissions, while the terpenoid alcohols make up
approximately 25% of the other biogenic compounds capable of forming
SOA. In each experiment, SOA was generated under controlled conditions
at the Caltech indoor facility. Over several hours, CCN concentrations
were measured at supersaturations ranging from 0.27% to 0.80%. These
data are compared to simultaneous particle concentration and size
distribution observations to determine the relationship between particle
diameter and CCN activity. The analysis indicates considerable
variation in CCN activity among the experiments; possible causes for
such variability are explored.
Cluster particles (0.8-1.9 nm) are key entities involved in nucleation
and new particle formation processes in the atmosphere. Cluster ions
were characterized in clear sky conditions at the Puy de Dôme
station (1465 m a.s.l). The studied dataset spread over five years
(February 2007-February 2012), which provided a unique chance to catch
seasonal variations of cluster ion properties at high altitude.
Statistical values of the cluster ion concentration and diameter are
reported for both positive and negative polarities. Cluster ions were
found to be ubiquitous at the Puy de Dôme and displayed an annual
variation with lower concentrations in spring. Positive cluster ions
were less numerous than negative ones but were larger in diameters.
Negative cluster ion properties seemed insensitive to the occurrence of
a new particle formation (NPF) event while positive cluster ions
appeared to be significantly more numerous and larger on event days. The
parameters of the balance equation for the positive cluster
concentration are reported, separately for the different seasons and for
the NPF event days and non-event days. The steady state assumption
suggests that the ionization rate is balanced with two sinks which are
the ion recombination and the attachment on aerosol particles, referred
as "aerosol ion sink". The aerosol ion sink was found to be higher
during the warm season and dominated the loss of ions. The positive
ionization rates derived from the balance equation were well correlated
with the ionization rates obtained from radon measurement, and they were
on average higher in summer and fall compared to winter and spring.
Neither the aerosol ion sink nor the ionization rate were found to be
significantly different on event days compared to non-event days, and
thus they were not able to explain the different positive cluster
concentrations between event and non-event days. Hence, the excess of
positive small ions on event days may derive from an additional source
of ions coupled with the fact that the steady state was not verified on
event days.
Observations of aerosol hygroscopic growth and CCN activation spectra for submicron particles are reported for the T1 ground site outside of Mexico City during the MIRAGE 2006 campaign. κ -Köhler theory is used to evaluate the characteristic water uptake coefficient, κ *, for the CCN active aerosol population using both size-resolved HTDMA and size-resolved CCNc measurements. Organic mass fractions, f <sub>org</sub>, are evaluated from size-resolved aerosol mass spectrometer (AMS) measurements, from which κ <sub>AMS</sub> is inferred and compared against κ *.
Strong diurnal profiles of aerosol water uptake parameters and aerosol composition are observed. We find that new particle formation (NPF) events are correlated with an increased κ * and CCN-active fraction during the daytime, with greater impact on smaller particles. During NPF events, the number concentration of 40 nm particles acting as CCN can surpass by more than a factor of two the concentrations of 100 nm particles acting as CCN, at supersaturations of 0.51% ± 0.06%. We also find that at 06:00–08:00 in the morning throughout the campaign, fresh traffic emissions result in substantial changes to the chemical distribution of the aerosol, with on average 65% externally-mixed fraction for 40 nm particles and 30% externally-mixed fraction for 100 nm particles, whereas at midday nearly all particles of both sizes can be described as internally-mixed.
Average activation spectra and growth factor distributions are analyzed for different time periods characterizing the daytime (with and without NPF events), the early morning "rush hour", and the entire campaign. We show that κ * derived from CCNc measurements decreases as a function of size during all time periods, while the CCN-active fraction increases as a function of size. Size-resolved AMS measurements do not predict the observed trend for κ * versus particle size, which can be attributed to unresolved mixing-state and the presence of refractory material not measured by the AMS. Measured κ * typically ranges from 0.2 to 0.35, and organics typically make up 60–85% of the aerosol mass in the size range studied. Despite some disagreement between κ <sub>AMS</sub> and κ <sub>CCNc</sub>, we show that κ <sub>AMS</sub> is able to describe CCN concentrations reasonably well, especially at the highest CCN concentrations. This is consistent with other CCN studies carried out in urban environments, and is partly due to the fact that the highest CCN concentrations occur during the daytime when the aerosol is internally-mixed and the organic fraction is relatively low. During the early morning rush hour, however, failing to account for the aerosol mixing-state often results in systematic overestimation of CCN concentrations by 50–100%.
The hygroscopic growth of the atmospheric aerosol is a critical
parameter for quantifying the anthropogenic radiative forcing. Until
now, there has been a lack of long term measurements due to limitations
in instrumental techniques. In this work, for the first time the
seasonal variation of the hygroscopic properties of a continental
background aerosol has been described, based on more than two years of
continuous measurements. In addition to this, the diurnal variation of
the hygroscopic growth has been investigated, as well as the seasonal
variation in CCN concentration. These physical properties of the aerosol
have been measured with a Hygroscopic Tandem Differential Mobility
Analyzer (H-TDMA), a Differential Mobility Particle Sizer (DMPS), and a
Cloud Condensation Nuclei Counter (CCNC). The results show that smaller
particles are generally less hygroscopic than larger ones, and that
there is a clear difference in the hygroscopic properties between the
Aitken and the accumulation mode. A seasonal cycle was found for all
particle sizes. In general, the average hygroscopic growth is lower
during wintertime, due to an increase in the relative abundance of less
hygroscopic or hydrophobic particles. Monthly averages showed that the
hygroscopic growth factors of the two dominating hygroscopic modes (one
barely hygroscopic and one more hygroscopic) were relatively stable. The
hygroscopic growth additionally showed a diurnal cycle, with higher
growth factors during day time. CCN predictions based on H-TDMA data
underpredicted the activated CCN concentration with 7% for 1% water
supersaturation (s). The underprediction increases with decreasing s,
most likely due to a combination of measurement and modeling
uncertainties. It was found that although the aerosol is often
externally mixed, recalculating to an internal mixture with respect to
hygroscopicity did not change CCN parameterizations significantly.
The importance of the particle mixing state and hygroscopicity to cloud
droplet activation was investigated using an adiabatic cloud parcel
model. The particle mixing state was described in terms of the
hygroscopic growth properties. Various aerosol types were considered,
and the input parameters describing the particle size distribution and
water uptake properties were taken from the field measurement data
reported in the literature. A series of sensitivity studies were
conducted where the treatment of the particle mixing state was
simplified as compared to the reference model configuration that
resolved particle populations both as a function of the size and
hygroscopicity. The results show that describing the hygroscopicity and
mixing state of an aerosol population by a single parameter induced
differences of up to 12% compared to the reference case in the cloud
droplet concentrations (CDNCs) in marine and continental background
areas. In urban and rural environments where particles display a high
degree of external mixing, the relative differences in CDNC were up to
35% if the particles were assumed to be internally mixed. It was also
found that a computationally efficient and relatively accurate approach
is to treat less and more hygroscopic particles separately using mean
properties of each population. In this case, the relative differences in
CDNC were generally below 20%. The model calculations also suggest that
the mixing state of a particle distribution is reflected in the
size-resolved activation efficiency (fraction of particles activated
into cloud droplets versus particle dry diameter).
We measured air ion size distributions with an air ion spectrometer in the size range of 0.34-40.3 nm both indoors (in July) and outdoors (in August) in Helsinki, Finland in 2004. At the same time we measured particle number concentrations and size distributions with two condensation particle counters (indoors) and differential mobility particle sizer (outdoors). Our main focus was to study new-particle formation in an urban site. We observed new-particle formation indoors almost every day, even many times a day, and four times outdoors. Indoors, the observed growth rates were 2.3-4.9 nm h-' for 1.3-3-nm ions, 6.5-8.7 nm h(-1) for 3-7-nm ions and 5.1-8.7 nm h-' for 7-20-nm ions. Outdoor ions (3-7 nm) grew at rates as large as 15.4 nm h-'. Outdoor ion and particle number concentrations were dependent on the wind direction, whereas indoor concentrations were dependent on ventilation conditions. Secondary particle formation and growth affected concentrations both indoors and outdoors. We concluded, based on our measurement results and simulated penetration of outdoor particles through the ventilation system, that we had indoor sources for secondary particles.
This study examines the hygroscopicity of newly formed particles
(smaller than 50 nm in particle mobility diameter) during two
atmospheric new particle formation events with and without clear growth
process at mid-level mountain range in Central Germany based on HCCT
field campaign. Particle hygroscopicity measurements show that the
particle soluble fractions at the end of event for two events are,
respectively 60% (45 nm particles for the event with clear growth) and
20% (30 nm particles for the event without clear growth), stressing that
non-soluble organic compounds may play a key role in particle growth
during new particle formation event. Such significant difference in
particle hygroscopicity also suggests that the chemical species
responsible for nucleation particle growth are considerably different
between the two selected NPF events. During both events, the
hygroscopicity of newly formed particles decreased with particle growth,
indicating that more less-hygroscopic compounds contribute to the
subsequent condensation in contrast to the earlier stage. Sulfuric acid
was considered to be responsible of the NPF event and represent the
highly hygroscopic compounds. However, calculation demonstrated that
sulfuric acid condensation failed to fully explain the observed soluble
fraction in the nucleation mode particles. Therefore, we hypothesize
that some water-soluble matters may explain the missing soluble
fraction.
During May of 2003, smoke from fires on the Yucatan Peninsula was transported across the Gulf of Mexico and into Texas where it caused a significant enhancement in measured aerosol concentrations. The 24-hour average PM2.5 concentration measured in Austin on 10 May was 50.1 mug/m3, which was more than twice that of the highest daily average concentration measured during any other month in 2003. During this event, a differential mobility analyzer/tandem differential mobility analyzer (DMA/TDMA) system was used to characterize the size distribution and size-resolved hygroscopicity and volatility of the aerosol. The hygroscopicity data were used to isolate the less hygroscopic biomass burning particles from other aerosol types. Biomass burning aerosol-only size distributions were then constructed by coupling the size-resolved fraction of particles attributed to the fires with the overall size distribution. These distributions, and the aerosol properties derived from the TDMA data, were used to examine the impact of the smoke on predicted cloud condensation nuclei (CCN) spectra. The influence of the smoke on cloud droplet concentrations and the influence of other aerosol types present on the activation efficiency of the smoke were evaluated using a cloud parcel model. For a subset of the updraft speeds considered, the model predicted that the cloud droplet concentration would sometimes be lower when both smoke and pollution aerosols entered cloud relative to that when only smoke was present. Whereas these cases in which an increased aerosol concentration resulted in a decreased cloud droplet concentration were rare, the inclusion of the pollution aerosol in the model always reduced the activation efficiency of the smoke aerosol, which would influence both its evolution during transport and its atmospheric removal rate.
Aerosols, both natural and anthropogenic, are important factors with respect to the radiation budget of the atmosphere. One important property is the ability of an aerosol particle to take up water which can have an impact on aerosol optical properties and cloud formation. To date little is known about how aerosol water uptake depends on the chemical composition of the aerosol. In this study an in-situ measurement setup to determine the chemical composition of atmospheric aerosols as a function of hygroscopicity is presented. This has been done by connecting a custom-built Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) and an Aerosol Time-of-Flight Mass Spectrometers (ATOFMS), commercially available from TSI (Model 3800). Thus, single particle bipolar mass spectra from aerosols leaving the HTDMA could be obtained as a function of hygroscopic growth factor. For these studies the HTDMA was deployed at a relative humidity of 82% and particles with a dry diameter of 260 nm were selected. This novel setup was laboratory-tested with mixed hydrophobic and hygroscopic aerosols. Subsequently, several sets of field experiments were performed during the last year. Two datasets were obtained during wintertime 2007 in Switzerland: One in the urban Zurich environment and the other at the remote high alpine station Jungfraujoch (JFJ). Further data was obtained in July 2007 in Abisko, Sweden. Located 200km north of the Arctic Circle, air masses from different directions were distinguished and examined. In Zurich a large data set was obtained within less than two days of measurements due to a high aerosol loading. At the JFJ, due to low particle concentrations, a longer sampling period was required. At the Abisko station particle concentration (i.e., data acquisition rate) strongly depended on the airmass origin. Both in Zurich and at the JFJ two different growth factor modes were observed. First results from these two locations show that most aerosols were generally internally mixed. A large contribution of organics and biomass combustion was found in the non-hygroscopic growth mode particles for both locations. Refractory material (e.g. metals, mineral dust, fly ash elements) was also highly enhanced in the non-hygroscopic particles. Sulfate was found to be a constituent in almost all particles independent of their growth factor. The Abisko dataset is currently being evaluated.
The formation of aerosols, and subsequent cloud condensation nuclei, remains one of the least understood atmospheric processes upon which global climate change critically depends. Under atmospheric conditions, the process of homogeneous nucleation (formation of stable clusters ~ 1 nm in size), and their subsequent growth into new particles (>3 nm), determines the aerosol and cloud nuclei population, yet, hitherto, no theory has elucidated the new particle formation phenomenon in detail. In this study, we present a new theory which provides a mechanistic explanation for new particle formation via activation of stable inorganic clusters by organic vapors. The new nano-particle activation theory is analogous to Köhler theory which describes cloud formation in a supersaturated water vapor field but differs in that it describes the activation of inorganic stable nano-clusters into aerosol particles in a supersaturated organic vapor which initiates spontaneous and rapid growth of clusters. Inclusion of the new theory into aerosol formation models predicts that increases in organic vapor densities lead to even greater increases in particle production, which, in turn, will influence the global radiative cooling effect of atmospheric aerosols.
Aerosol size distribution, total concentration (i.e., condensation nuclei (CN) concentration, N <sub>CN</sub>), cloud condensation nuclei (CCN) concentration ( N <sub>CCN</sub>), hygroscopicity at ~90 % relative humidity (RH) were measured at a background monitoring site at Gosan, Jeju Island, south of the Korea Peninsula in August 2006, April to May 2007 and August to October 2008. Similar measurement took place in August 2009 at another background site (Baengnyeongdo Comprehensive Monitoring Observatory, BCMO) on the island of Baengnyeongdo, off the west coast of the Korean Peninsula. Both islands were found to be influenced by continental sources regardless of season and year. Average values for all of the measured N <sub>CCN</sub> at 0.2, 0.6 and 1.0 % supersaturations ( S ), N <sub>CN</sub>, and geometric mean diameter ( D <sub>g</sub>) from both islands were in the range of 1043–3051 cm<sup>−3</sup>, 2076–4360 cm<sup>−3</sup>, 2713–4694 cm<sup>−3</sup>, 3890–5117 cm<sup>−3</sup> and 81–98 nm, respectively. Although the differences in D <sub>g</sub> and N <sub>CN</sub> were small between Gosan and BCMO, N <sub>CCN</sub> at various S was much higher at the latter, which is closer to China.
Most of the aerosols were internally mixed and no notable differences in hygroscopicity were found between the days of strong pollution influence and the non-pollution days for both islands. During the 2008 and 2009 campaigns, critical supersaturation for cloud nucleation ( S <sub>c</sub>) for selected particle sizes was measured. Particles of 100 nm diameters had mean S <sub>c</sub> of 0.19 ± 0.02 % during 2008 and those of 81 and 110 nm diameters had mean S <sub>c</sub> of 0.26 ± 0.07 % and 0.17 ± 0.04 %, respectively, during 2009. Hygroscopicity parameters estimated from the measured S <sub>c</sub> were mostly higher than the ones from the measured hygroscopic growth at ~90 % RH.
For the 2008 campaign, N <sub>CCN</sub> at 0.2, 0.6 and 1.0 % S were predicted based on the measured dry particle size distribution and various ways of representing aerosol hygroscopicity. The best closure was obtained when temporally varying and size-resolved hygroscopicity information from HTDMA was used, for which the average relative deviations from the measured values were 19 % for 1.0 % S and 28 % for 0.2 % S . Prescribing a constant hygroscopicity parameter suggested in literature (κ = 0.3) for all sizes and time resulted in the average relative deviations, 25–40 %. When constant hygroscopicity was assumed, the relative deviation tended to increase with decreasing N <sub>CCN</sub>, which was accompanied by increase of sub-100 nm fraction. These results suggest that hygroscopicity information for aerosols of diameters smaller than 100 nm is crucial for more accurate prediction of N <sub>CCN</sub>.
A novel analytical method is presented to estimate the total condensable vapor concentration in the ambient atmosphere. The method requires the aerosol size distribution as its only input and is thus straightforward enough to apply to atmospheric data. In order to estimate the condensable vapor concentrations over boreal forest regions, the method was applied to four years of new particle formation days at SMEAR II site in southern Finland. The growth rate of newly formed particles provided additional information of the concentration of low-volatile vapors. Hence the contribution of low-volatile and semivolatile vapors to particle growth can be separated. The analysis indicated that the median of the condensable vapor concentrations during the nucleation events was approximately 7.6 × 107 cm−3, with values varying between 2.0 × 107 and 1.8 × 108 cm−3. Of the total concentration, the contribution of low-volatile compounds was 70% on average but ranged from 22% to over 100%. Sensitivity tests indicated that the obtained percentage of low volatiles is only slightly sensitive to chosen model parameters. When applying the method to ambient aerosol distribution measurements, the largest uncertainty is related to visually undetectable inhomogeneities in air mass and the boundary layer dynamics. These uncertainty factors together with the use of measured dry aerosol sizes instead of ambient sizes can lead the analysis to somewhat overestimate the importance of the low-volatile compounds to total aerosol growth. Even so, our results suggest that identification of submicron aerosol composition during new particle formation can provide information on the compounds that grow nucleation mode particles to larger sizes.
Continuous measurements of hygroscopicity and volatility of atmospheric
aerosol particles of 4-10 nm diameter were conducted with a nanometer
tandem differential mobility analyzer (Nano TDMA) during the Aerosol
Nucleation and Real-time Characterization Experiment (ANARChE), which
took place in Atlanta in July and August 2002. In the Nano TDMA
measurements, particles were exposed to either a high humidity
(˜90% RH) or an elevated temperature (˜100°C) downstream
of the first differential mobility analyzer (DMA) and were then resized
by the second DMA to determine the change in size due to water uptake or
evaporation. There were several days when nucleation occurred and high
concentrations of sub-20 nm particles were observed, and during those
events, particles of 4-10 nm diameter were very hygroscopic and
nonvolatile. These observations, together with parallel Thermal
Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements
of sub-20 nm particle composition, suggest that the particles were
mostly composed of ammoniated sulfates. This finding strongly supports
the hypothesis that the nucleation and subsequent growth of
nanoparticles were driven by reactions involving sulfuric acid and
ammonia during this study.
Air quality models used as a scientific basis for air pollution strategy development need physically sound and computationally efficient schemes to describe aerosol formation, interaction, and evolution. In this work, we present and evaluate an aerosol dynamics module MONO32, which is designed for use in regional air pollution models. The module presents aerosol size distribution with four monodisperse modes and characterizes aerosol chemical composition with seven components, so 32 prognostic equations are needed to describe particle mass and number concentration. MONO32 accounts for nucleation, condensation, and coagulation processes. Two different time integration schemes, a FORTRAN NAG-library routine and a two-step scheme, were tested. Both integration methods showed the same accuracy in calculating particle number and size evolution, while the two-step scheme was computationally much more efficient. A mode-merging method was implemented in MONO32 to account for the transfer of aerosol mass and number to a larger mode as particles grow by condensation and coagulation. MONO32 compared reasonably well with the sectional model AEROFOR2 with 54 sections; for example, difference in the total number concentration after 24-hour simulation was less than 15-25%. MONO32 was also verified against measurements available from the Biogenic Aerosol Formation in the Boreal Forest 3 (BIOFOR3) campaign. Two typical nucleation episodes were chosen for testing, and by using the nucleation rate and the condensable vapor source rate calculated from the measurements, MONO32 was able to predict the evolution of the total number concentration and the growth rate of the nucleation mode quantitatively in good agreement with the observations. Deviations in the resulting nucleation mode diameter were 11-16%. However, the results appeared to be quite sensitive to the hygroscopic properties of the condensable organic vapor. MONO32 showed an acceptable accuracy for long-range transport modeling in describing aerosol dynamics while being computationally efficient. Therefore it is recommended for implementation and further testing in the regional three-dimensional Eulerian model.
In this paper we summarize recent experimental, theoretical and observational results on the formation and growth of atmospheric nanoparticles. During the last years significant progress has occurred to explain atmospheric nucleation and initial steps of the growth. Due to climatic and health effects of fine and ultrafine particles the formation and growth of new aerosol particles is of growing interest. The question “How and under which conditions does the formation of new atmospheric aerosol particles take place?” has exercised the minds of scientists since the time of John Aitken, who in the late 1880s built the first apparatus to measure the number of dust and fog particles. However, only during the last 15–20 years has the measurement technology developed to such a level that size distributions of nanometer-size particles and concentrations of gases participating in particle formation can be measured in the atmosphere. Also from a theoretical point of view atmospheric nucleation mechanisms have not been resolved: several mechanisms such as ion-induced (or ion mediated) nucleation, ternary and kinetic (barrier-less) nucleation have been suggested. In the most recent theory, the activation of existing neutral and/or ion clusters has been suggested.
The hypothesis is tested that the nucleating material responsible for the production of new particle formation over boreal forests, particularly in spring, is a result of the release of gases of decomposition of organic material in soil and leaf litter generated by micro-organisms. It is shown that the supply of moisture to the soil by melting snow in spring or by rain in summer greatly enhances the frequency of nucleation events. While low air temperatures and low pre-existing particle surface areas also increase the probability of new particle formation, freezing of the soil and leaf litter reduce it. The observations are consistent with the hypothesis and the seasonal and interannual variability are difficult to explain without it.
The hygroscopic growth of the atmospheric aerosol is a critical parameter for quantifying the anthropogenic radiative forcing. Until now, there has been a lack of long term measurements due to limitations in instrumental techniques. In this work, for the first time the seasonal variation of the hygroscopic properties of a continental background aerosol has been described, based on more than two years of continuous measurements. In addition to this, the diurnal variation of the hygroscopic growth has been investigated, as well as the seasonal variation in CCN concentration. These physical properties of the aerosol have been measured with a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA), a Differential Mobility Particle Sizer (DMPS), and a Cloud Condensation Nuclei Counter (CCNC). The results show that smaller particles are generally less hygroscopic than larger ones, and that there is a clear difference in the hygroscopic properties between the Aitken and the accumulation mode. A seasonal cycle was found for all particle sizes. In general, the average hygroscopic growth is lower during wintertime, due to an increase in the relative abundance of less hygroscopic or barely hygroscopic particles. Monthly averages showed that the hygroscopic growth factors of the two dominating hygroscopic modes (one barely hygroscopic and one more hygroscopic) were relatively stable. The hygroscopic growth additionally showed a diurnal cycle, with higher growth factors during day time. CCN predictions based on H-TDMA data underpredicted the activated CCN number concentration with 7% for a 1% water supersaturation ratio. The underprediction increases with decreasing s, most likely due to a combination of measurement and modeling uncertainties. It was found that although the aerosol is often externally mixed, recalculating to an internal mixture with respect to hygroscopicity did not change the CCN concentration as a function of supersaturation significantly.
Within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thüringer Wald (Germany) during October 2001 and 2002, the dry number size distribution and hygroscopic growth of aerosol particles upwind cloud and the dry number size distributions of interstitial particles and cloud droplet residuals inside cloud were measured at connected flow conditions. The connected flow between the upwind and in-cloud summit site was meteorologically predicted and experimentally confirmed for three selected cloud events. For these events, it could be verified that entrainment and droplet deposition had only a minor influence on the evolution of the particle size distribution between the two sites. Hence, the size resolved soluble volume fraction of the cloud input aerosol particles determined from the hygroscopic growth measurements could be related to the particle activation inferred from the particle size distributions observed inside cloud. The shape and steepness of the scavenging fraction as a function of particle diameter was found to correlate with the increase of soluble volume fraction with size, which had implications for the droplet activation diameter of the cloud condensation nuclei (CCN) that ranged between 110 and 180nm. The minimum soluble volume fraction εmin that was required to serve as CCN was determined for three different dry diameters from the relation of the particle volume fraction and scavenging fraction. From the comparison with εmin predictions from classical Köhler theory it is inferred that aerosol particles remained in the interstitial phase although they should have been activated. A discussion of different processes which have the general ability to explain this finding favoured the hypothesis of organic surface films retarding the uptake of water molecules.
We have estimated the contributions of mass concentrations of major aerosol species (sulfate, carbonaceous material, and sea salt) to cloud condensation nuclei (CCN) number concentrations from simultaneous measurements of aerosol number size distributions, impactor-derived mass size distributions of aerosol species, and CCN number concentrations (measured at 0.5% supersaturation). Our approach involves the derivation of number size distributions from the impactor data by numerical data inversion from which the mass contributions of sulfate, organic material and sea salt to CCN number concentrations are estimated. The utility of this approach is demonstrated by the results obtained at Point Reyes, California, where we compared the derived number size distributions with directly measured number size distributions. Direct measurements of the CCN concentrations for 0.5% supersaturation showed agreement of +/-25% with CCN derived from impactor data. The contribution of sulfate to CCN concentrations was found to be between 20 and 65% versus contribution of organic particles between 4 and 80%. Thus sometimes sulfate can contribute most of the CCN, while at other times organic aerosols dominate the CCN concentrations.
Simultaneous measurements of aerosol particles and their expected gas phase precursors were made at Idaho Hill, Colorado, a remote continental site. This study used apparatus and techniques similar to those employed in an earlier study at the Mauna Loa Observatory, Hawaii [Weber et al., 1995]. New particle formation, identified by the presence of ultrafine particles (nominally 3 to 4 nm diameter), was commonly observed in downslope (westerly) air and was correlated with high sulfuric acid (H2SO4) concentrations, low relative humidity and low particle surface area concentrations. The data point to H2SO4 as a principle nucleation precursor species with typical daytime concentrations between 106 and 107 molecules cm-3. Particle production was observed at H2SO4 concentrations that are well below predicted values for binary nucleation of H2O and H2SO4, suggesting that another species participated. Particle growth rates were estimated from the data with two independent approaches and in both cases were ~5 to 10 times higher than can be explained by condensation of H2SO4 and its associated water. This suggests that species in addition to H2SO4 were also making large contributions to ultrafine particle growth. Finally, calculated steady-state H2SO4 concentrations were found to be in good agreement with measured values if the mass accommodation coefficient for H2SO4 on aerosol surfaces was assumed equal to ~1.
The hygroscopic properties of aerosol formed by oxidation of three monoterpenes, limonene, α-pinene, and β-pinene, were measured using a tandem differential mobility analyzer (TDMA). The experiments were performed in the European Photoreactor (EUPHORE) in Valencia, Spain. The experiments included ozonolysis and photooxidation with and without ammonium sulfate seed aerosol. Pure organic particles, formed by oxidation of the terpenes in the absence of the seed aerosol, proved to be slightly hygroscopic. The hygroscopic growth factor (G) was close to 1.10 at relative humidity 84% ± 1%, which is often observed as the G of the less hygroscopic mode of atmospheric aerosol in field measurements. In the experiments with ammonium sulfate seed aerosol G decreased from approximately 1.5 before the start of terpene oxidation to approximately 1.1 as the oxidation products condensed on the particles. G was not proportional to the organic layer thickness but decreased with increasing organic volume fraction. Our analysis shows that in the internally mixed particles, ammonium sulfate and the organic products take up water independently of one another.
Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composition of newly formed ultrafine atmospheric aerosols is not known with certainty; new particles have variously been conjectured to be sulfates, organic compounds, and sulfate/organic mixtures. The 1993 Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, provided an opportunity to examine the question of which class of compounds, i.e., sulfates or organics, make the major contribution to new particle formation in the unpolluted troposphere. This study compared the production rates of sulfuric acid (from the oxidation of sulfur dioxide) and oxidized organic compounds to gauge their relative contributions to the formation of ultrafine particles. Potential organic precursor species examined in this study were the naturally occurring terpenes alpha- and beta pinene, and the anthropogenic hydrocarbons toluene, m-xylene, ethyl benzene, 1,2,4 trimethyl benzene, and methylcyclohexane. The calculated production of oxidized organics appeared well correlated with total particle surface area and volume, suggesting that at least some of the organic compounds formed in gas phase reactions condensed upon the preexisting aerosol. New particle formation was found to be more highly associated with elevated production of gas phase sulfuric acid (via the SO2-OH reaction) than with production of oxidized organic products, although data from one day, during which sulfuric acid production and total aerosol surface area were both lower than usual, provided evidence for the involvement of terpene species in new particle formation. The results suggest that for this continental site, sulfuric acid was probably responsible for most of the observed new ultrafine particle formation. Low-volatility organic compounds may have caused particle formation under the right conditions, but were more likely to condense upon preexisting particles.
A 3 week measurement campaign was undertaken to study the effect of local weather parameters, transportation from an urban area, structure of boundary layer, and precipitation on submicron (8-450 nm in mobility diameter) aerosol particles in urban background area in Finland. Also, the concentrations of NOx, O3, and SO2 were monitored. The most important meteorological factor affecting aerosol particles was shown to be local wind direction. It was also seen that the diurnal behavior of boundary layer plays an important role for aerosol particle concentration and size distribution and gas phase chemistry at the ground level. Even the few occurrences of new particle formation that were observed seem to be connected with changes in the boundary layer. Clear indications of the possible effect of precipitation (rain or snow) on aerosol size distributions could not be detected in this study. The effect is obviously small compared to the influences of other meteorological processes.
Particle concentrations and size distributions have been measured from different heights inside and above a borcal forest during three BIOFOR campaigns (14 April-22 May 1998, 27 July-21 August 1998 and 20 March-24 April 1999) in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm-3 and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm-3. A coarse mode was also present, extending up to sizes of 20 μm having a number concentration of 1.2 cm-3, volume mean diameter of 2.0 μ and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle production, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410-45 000 cm-3, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encountered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentration increases.
Aerosol and trace gas measurements were made at Kejimkujik National
Park, Nova Scotia, Canada, during the summer of 1996. A case study from
July 7-8 provides evidence of nucleation and condensation of products
related to the oxidation of different biogenic emissions. Particles from
5 nm to 50 nm in diameter evolved during the afternoon and early evening
associated with variations in isoprene. Late in the evening the α-
and β-pinene mixing ratios and the aerosol particle volume
increased. Soon after, there was a sharp increase in
RO2H/H2O2 that persisted until about
0100 LT. The initial increases in the pinenes and aerosols were strong
and influenced by changes in winds. After 2200 LT, and into the early
morning, the winds were relatively steady, and the α- and
β-pinene mixing ratios continually decreased. The decay of
α-pinene is explained through reaction with O3.
However, the addition of OH radicals from the reaction of terpenes with
O3 is necessary to explain the observed rate of decay of
β-pinene. During the same time, the aerosol volume increased with
the decrease in α- and β-pinene. The volume increase was
distributed 40∶60 between particles in a mode centered at 80-90 nm
and particles > 150 nm. The fine particle mass concentrations of the
measured inorganic ions (sulfate, nitrate, chloride, ammonium, sodium,
and calcium) and organic ions (oxalate, formate, acetate, pyruvate,
propionate) account for 25-30% of the total aerosol volume during the
period (2.7 μm3 cm-3) indicating that the
aerosol volume increase was due to unidentified species. Assuming that
the increase in the aerosol was the result of products from the
oxidation of α- and β-pinene, an aerosol mass yield of 13% is
estimated. The concentrations of cloud condensation nuclei active at
0.2% supersaturation were enhanced by the appearance of the 80-90 nm
mode pointing to at least some of these forest-generated particles as
being able to serve as nuclei for cloud droplets at common atmospheric
supersaturations.
Nucleation of new, ultra-fine, aerosol particles has been observed in the clean marine coastal atmosphere under a variety of conditions. These nucleation events were observed to occur frequently over spatial scales of 10's-100's of metres and temporal scales of seconds to minutes. Two conditions appeared to be necessary for nucleation event to occur: low tide and solar irradiation. The requirement of low tide conditions suggests that the exposed shore area provides the source of new particle precursors. It is speculated that VOC and/or alkyl halide derivatives contribute to nucleation under these conditions. Nucleation rates were calculated to be ~103-104cm-3s-1, suggesting that the coastal zone is an important source of atmospheric nuclei.
Air parcels in the marine boundary layer (MBL) are mixed up through nonprecipitating clouds at the top of the MBL many times (on average) before they can be removed by precipitation scavenging. The equivalent dry size of the particles (cloud condensation nuclei, CCN) upon which droplets are formed increases because of liquid phase oxidation of soluble trace gases during the cloud processing. The observed separation of the submicron size distribution into an interstitial mode and cloud droplet residue mode makes it possible to infer the effective MBL cloud supersaturation and cloud droplet concentrations from surface measurements of the aerosol size distribution during periods when nonprecipitating MBL clouds are present in the back trajectory and the MBL is well mixed. The effect of particle composition on the accuracy of the inferred cloud supersaturations is evaluated. A large database of hundreds of size distributions taken on an Atlantic and a Pacific cruise and an airship flight off the Oregon coast are used to calculate the range of effective MBL cloud supersaturations and droplet concentrations encountered during these expeditions. The inferred droplet concentrations on the Pacific cruise were mostly in the 25 to 150 cm-3 range, whereas they were mostly in the 50 to 500 cm-3 range for the Atlantic cruise. The inferred effective supersaturation in the tropical MBL clouds was typically in the 0.15% to 0.25% range. Recent work of Tang and Munkelwitz [1994] would indicate that particles consisting of mixtures of ammonium sulfate and sulfuric acid would not have recrystalized in the differential mobility analyzer (DMA) within the range of relative humidities (45% to 60%) at which the DMA was operated. At these humidities the hydrated size can be as much as 20% greater than the dry size. Corrections for the hydrated size within the DMA at the time of measurement are included and are also used to correct previous measurements of the relationship between dry size and critical supersaturation made using the Naval Research Laboratory (NRL) DMA and NRL thermal gradient CCN counter.
The chapter addresses the problems in aerosol measuring techniques and selection of sampling and measuring methods. Theoretic evaluation of mass, size, mean density, and mean refractive index as functions of the relative humidity is discussed. The chapter also discusses techniques for measuring the mass as a function of the relative humidity; determination of the mean density; and measuring the mean complex index of refraction. Results of the measurements, discussion of the results, coefficients of mass increase, mean densities and real parts of the mean complex index of refractions, and applicability of the results are also described. Finally, the chapter then reviews the model computations and approximation formulas based upon measured properties.
Cloud condensation nucleus concentration (CCN) was measured directly at a supersaturation of 0.5% with a thermal gradient diffusion cloud chamber at Cape Grim, Tasmania, during the First Aerosol Characterization Experiment (ACE 1) field study in November and December of 1995. Number-size distributions N(Dp) from 3 to 800 nm diameter and the hygroscopic properties of the aerosol in the 30 to 300 nm diameter range (which contains most of the CCN active at 0.5%) were measured concurrently at the same location. This data set provides a basis to compare measured and modeled CCN concentrations. A critical particle diameter that would form cloud droplets at 0.5% supersaturation was derived from the hygroscopic growth data including consideration of the hydration of the size distribution measurement. This empirically derived diameter incorporates the effects of soluble and insoluble mass as well as an effective van't Hoff factor and surface tension as described by Köhler theory for heterogeneous nucleation of cloud droplets. The size distributions were integrated for diameters greater than the critical value and compared to the directly measured CCN concentrations. The modeled CCN concentration was 95 cm-3 during baseline sector periods and 128 cm-3 overall. This was about 20% greater than the directly measured CCN concentration and well correlated (R2=0.7) with measured CCN. Two thirds of the CCN at 0.5% supersaturation derive from an accumulation mode (80nm
In situ measurements of size-dependent water uptake by atmospheric particles made with a tandem differential mobility analyzer (TDMA) and size-resolved chemical composition of aerosol samples collected with cascade impactors in the Smoky Mountains have been examined in order to ascertain the influence of organic carbon compounds on aerosol hygroscopicity. Particles were dried to ~5% relative humidity (RH) before entering the TDMA, leading us to believe that salts of ammonium and sulfate were in crystalline states for relative humidities below their expected deliquescent points. TDMA-measured water content was found to be in excess of the sulfate-associated water modeled using laboratory data for binary aqueous solutions and the method of Zdanovskii-Stokes-Robinson for multicomponent solutions over a wide range of humidities (RH=5-85%). Furthermore, excess water was observed to increase in proportion to the organic fraction of mass associated with each examined size in the range 0.05 to 0.4 mum. These data are used to obtain an empirical relationship between the amount of water associated with particulate organics and relative humidity. This analysis shows that organic-associated water content is considerably less than that of sulfate compounds, on a volume basis, for high RH, but comparable or greater for low RH. These results are consistent with laboratory data for water absorption by a range of organics vis-à-vis ammonium salts of sulfate.
An ultrafine tandem differential mobility analyzer has been developed for measurements of the hygroscopicity of ultrafine aerosol particles, between 8 and 50 nm in mobility diameter. In this paper, the main operation features of the device are presented along with a detailed evaluation for the limits of its operation. The instrument is suitable for both laboratory-generated and atmospheric aerosol measurements. Hygroscopic growth data are presented for ammonium sulphate particles in the ultrafine size range, and comparisons are made with both experimental literature data and with theory. The data include determination of hygroscopic growth curves, deliquescence behavior, and hysteresis. These data will find applications in studies of the formation and growth of atmospheric aerosols.
Classical theory of binary homogeneous nucleation is extended to the ternary system H2SO4-NH3-H2O. For NH3 mixing ratios exceeding about 1 ppt, the presence of NH3 enhances the binary H2SO4-H2O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H2SO4-NH3-H2O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H2SO4-NH3-H2O ternary nucleation rate is independent of relative humidity over a large range of H2SO4 concentrations has wide atmospheric consequences. The limiting component for ternary H2SO4-NH3-H2O nucleation is, as in the binary H2SO4-H2O case, H2SO4; however, the H2SO4 concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.
Water activities, densities, and refractive indices over extended concentration ranges at 25°C are reported for solution droplets containing a single salt of either (NHâ)âSOâ, NHâHSOâ, (NHâ)âH(SOâ)â, NaâSOâ, NaHSOâ, or NaNOâ, which are common constituents of atmospheric aerosols. The extensive data reported are obtained from experiments using the single-particle levitation technique recently developed for measuring the thermodynamic and optical properties of microdroplets. These data should find application in mathematical models predicting the dynamic behavior, visibility reduction, and radiative effects of atmospheric sulfate and nitrate aerosols. 32 refs., 13 figs., 3 tabs.
Observations of the nature and the size distribution of the atmospheric aerosol can be used to predict the proportion that should be active in cloud droplet formation at various supersaturations. A much smaller proportion is often found to be active in static diffusion cloud chambers except when the air is very clean. Studies of droplet formation on the aerosol in certain supersaturated organic vapours and the variable delays in cloud formation in diffusion chambers both suggest the presence on the aerosol of adsorbed or condensed surface-active organic components that delay or suppress cloud formation.RésuméOn peut utiliser des observations sur la nature et la distribution dimensionnelle de l'aérosol atmosphérique pour prédire la proportion de cet aérosol qui interviendra dans la formation des gouttlettes de nuage à différentes sursaturations. On trouve en général qu'une proportion plus faible est active dans les chambres à nuage à diffusion statique, sauf lorsque l'air est très pur. Des études de la formation de gouttelettes sur l'aérosol dans certaines vapeurs organiques sursaturées et les temps variables de formation de nuage observés dans des chambres à diffusion suggèrent la présence sur l'aérosol de substances organiques à surface active adsorbées ou condensées qui retardent ou empêchent la formation de nuage.
Anthropogenic emissions leading to atmospheric aerosols have increased dramatically over the past century. Airborne particles have been implicated in human health effects, visibility reduction in urban and regional areas, acidic deposition, and altering the earth's radiation balance. The atmosphere subjects aerosol particles to an array of transport and transformation processes that alter their size, number, and composition; the transformation processes include condensation and evaporation. homogeneous nucleation, coagulation, and chemical reactions. A major goal of our research has been to use first principles to gain a predictive understanding of the physical and chemical processes that govern the dynamics, size. and chemical composition of atmospheric aerosols. We review here the current state of our ability to model this atmospheric aerosol behavior.
A Cloud Condensation Nuclei Counter (CCNC) has been constructed, calibrated and used in field conditions. The CCNC was used in a rural/boreal forest site (Hyytiälä), in marine/coastal sites (Tenerife and Mace Head), and in an urban site (Helsinki). Expres- sions for determining the soluble volume fraction of particles were derived. The soluble fraction of Aitken mode particles was determined and the CCNC number concentra- tions were observed. In the marine air masses the soluble fraction was seen to be around 0.8. In the rural site the soluble fraction was around 0.3 and some diurnal variation was seen. In Helsinki the soluble fraction was somewhat higher than in the boreal forest site.
Number size distribution of ambient submicron and ultrafine aerosol particles have been measured on a continuous basis (every 10 minutes) for three quarters of the year 1996, at a forest site in Southern Finland. Continuous monitoring offers additional insight over the diurnal dynamics of the submicron size distribution, including existence of clearly separate size modes as well as events of new particle formation. Selected examples of the measured size distributions are presented, including the particle formation events observed at the measurement site. Typical characteristics of days with particle formation events versus days of no events are discussed.
The optical and chemical properties of atmospheric particles and their
ability to act as cloud condensation nuclei (CCN) depend strongly upon
their affinity for water. Laboratory experiments have shown that water
soluble substances such as ammonium sulfate, ammonium nitrate, and
sodium chloride, which are major inorganic components of atmospheric
particles, absorb water in an amount proportional to water vapor
pressure. Analogous information about the interactions between water and
organics, which are another major component of atmospheric particles, is
lacking. Here we analyze concurrent observations of particle chemical
composition and water content from a continental nonurban (Grand Canyon)
and an urban (Los Angeles) location to determine whether the water
content of atmospheric particles is influenced by the presence of
organics. By comparing the observed water content with the water content
expected to be associated with the inorganic fraction, we find that the
aggregate hygroscopic properties of inorganic particles are altered
substantially when organics are also present. Furthermore, the
alterations can be positive or negative. For the nonurban location,
organics enhance water absorption by inorganics. In the relative
humidity (RH) range of 80-88% organics account for 25-40% of the total
water uptake, on average. For the urban location, on the other hand, the
net effect of organics is to diminish water absorption of the inorganics
by 25-35% in the RH range of 83-93%.
All aspects of aerosol physics important in the oformation, evolution and removal of particulate material in the atmosphere are presented, and the influence of such particles on the climate and weather are outlined. The book opens with a discussion of the physics of aerosols and derives some f the more important relationships in the physics of single aerosol particles. These are then used as a basis for subsequent examination of interactions between particles and the dynamics of populations of particles relative to the evolution amd maintenance of particle size distributions in the atmosphere and for the production - modification and coagulation-removal cycle. The balance between production and removal is then reviewed and the regions of the size spectrum where the various formative and removal processes are most effective are identified. The last five chapters are devoted to the influence of atmospheric particles on weather, atmospheric optics and radiative transfer, atmospheric electricity and atmospheric energetics and climate.
Data on the submicron aerosol size distribution, the supersaturation
spectrum, and the relationship between dry size and critical
supersaturation taken during a cruise of the USNS Hayes from Norfolk,
Virginia, to Piraeus, Greece, are presented. Measured values of the
critical supersaturation required to nucleate natural aerosols of known
size are given and used to convert the size distribution to the
supersaturation spectrum. The supersaturation spectrum derived in this
manner shows good agreement with the supersaturation spectrum measured
directly. The observed decrease in particles smaller than 0.06 µm
in an air mass which moved 100 mi out to sea could be explained by
coagulation and mixing with marine air. The absence of particles below
about 0.02 µm on most occasions in marine air was consistent with
the finding that the number of CCN active at 0.8% was nearly equal to
the total CN count.
Aerosol formation in the atmosphere is an important process to understand, in that such particles may act as the cloud condensation nuclei responsible for the 'cloud-climate' effect, and could locally be hazardous to health. The number-concentration of total atmospheric aerosols and cloud condensation nuclei is largely contributed by organic aerosols. Much of the organic aerosol is formed from atmospheric gas-to-particle conversion, and the common and widespread non-methane hydrocarbons emitted by vegetation have been investigated as possible precursors. But strong evidence for a quantitative link between biogenic hydrocarbon emission and organic aerosol formation has so far been lacking. Here we present measurements of gaseous and particulate atmospheric species from a forested area to show that some hydrocarbons (for example, terpenes) emitted by vegetation are photo-oxidized to organic acids (for example, pinonic acids), which condense to form organic aerosols. Thus the forests, through their production of large quantities of organic aerosols, could be of considerable significance both for climate (through cloud-condensation-nuclei formation) and for heterogeneous atmospheric chemical processes.
ABSTRACTA Tandem Differential Mobility Analyser (TDMA) was used to study the hygroscopic growth of individual ambient aerosol particles in the Po Valley, Italy. The measurements were made during the GCE fog experiment in November 1989. During fog, the interstitial aerosol (Dp(at ambient relative humidity) < 5 µm) was sampled. Two modes of particles with different hygroscopic growth were found for 0.030 µm < Dp(dry) < 0.20 µm. On average, the fraction of particles in the two modes were almost equal. The mean growth factor at 85% r.h. was 1.44 ± 0.14 for the more-hygroscopic mode and 1.1 ± 0.07 for the less-hygroscopic mode. The growth factors and the proportion of the particles that were less hygroscopic varied considerably from day to day, but no significant size dependence was seen. Comparison of growth factors for pure salt particles and the measured growth factors indicates that both hygroscopic modes contain a major insoluble part. The effect of the external mixing of hygroscopic properties on the activation of particles to fog droplets is discussed and the fraction of particles that were activated as a function of particle size is predicted. Comparison with the measured scavenging fraction as a function of particle size shows that the hygroscopic properties of the individual particle are as important as the particle size in determining if it will be activated in a fog.
Size-segregated chemical aerosol analysis of a total 5 integrated samples has been performed for the atmospheric aerosol during events of new particle formation. The experiments were conducted during the BIOFOR 3 measurement campaign at a boreal forest site in southern Finland in spring 1999. Aerosol samples collected by a cascade low-pressure impactor were taken selectively to distinguish particle formation event aerosol from non-event aerosol. The division into “event” and “non-event” cases was done “in situ” at field, based on the on-line submicron number size distribution. The results on the chemical ionic composition of the particles show only small differences between the event and non-event sample sets. The event samples show lower concentrations of total sulfate and ammonium as well as light dicarboxylic acids such as oxalate, malonate and succinate. In the event samples, nucleation mode particle MSA (methanesulphonic acid) was found to be present exceeding the concentrations found in the non-event samples, but at larger particle sizes the sample sets contained rather similar concentrations of MSA. The most significant difference between the event and non-event sets was found for dimethylammonium, ionic component of dimethylamine ((CH3)2NH), which seems to be present in the particle phase during the particle formation periods and/or during the subsequent particle growth. The absolute event sample dimethylamine concentrations were more than 30-fold greater than the non-event concentrations in the accumulation mode size range. On the other hand, the non-event back-up filter stage for sub-30 nm particles contained more dimethylamine than the event samples. This fractionation is probably a condensation artifact of the impactor sampling. A simple mass balance estimate is performed to evaluate the quality and consistency of the results for the overall mass concentration.
The effects of particle hygroscopicity and the availability of condensable material (other than water) in the gas phase on cloud droplet formation and the radiative properties of clouds have been studied using an adiabatic air parcel model with detailed multicomponent condensation. The pre-existing log-normal particle distribution used is bimodal in size and bimodal in hygroscopicity. To simulate this, four log-normal distributions were used and in each mode particles were assumed to be internally mixed, i.e. they are composed partly of salt and partly of an insoluble substance. The mean diameters, standard deviations, total number of pre-existing particles, the mass fraction of the soluble salt and initial concentration of condensable vapour were varied in the simulations. There is a clear effect of hygroscopicity on the activated fraction of aerosol particles in our simulations. Thus hygroscopicity of pre-existing aerosol particles and concentrations of condensable gases can also influence the optical thickness and reflectance of clouds. The change in optical thickness varies as a function of the number concentration of pre-existing particles, having a maximum (Δ/τ = 0.2) near a concentration of 1000/cc.
Particle concentrations and size distributions have been measured from different heights inside and above a boreal forest during three BIOFOR in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm−3 and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm−3. A coarse mode was also present, extending up to sizes of 20 mm having a number concentration of 1.2 cm−3, volume mean diameter of 2.0 mm and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle produc-tion, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410–45 000 cm−3, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encoun-tered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentra-tion increases.
Measurements of the hygroscopic properties of sub-micrometer atmospheric aerosol particles were performed with hygroscopic tandem differential mobility analysers (H-TDMA) at 5 sites in the subtropical north-eastern Atlantic during the second Aerosol Characterization Experiment (ACE-2) from 16 June to 25 July 1997. Four of the sites were in the marine boundary layer and one was, at least occasionally, in the lower free troposphere. The hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10–440 nm were generally measured for changes in relative humidity (RH) from
Measurements of ambient monoterpenes, sulphur dioxide, nitric acid, ammonia and particulate organic carbon were made in a Scots pine forest in southern Finland as part of the BIOFOR (Biogenic aerosol formation in the boreal forest) project in the summer of 1998 and spring of 1999. Scots pine branch emission measurements were made with the chamber technique for selected days. Steady state −OH and NO·3 concentrations were calculated and source terms for the production of secondary and condensable gases from the oxidation of terpenes and of SO2 were determined. The purpose of the project was to investigate the source of new particles (nucleation events) observed at the site. Forest emission rates of monoterpenes were not found to be exceptionally high prior to or during the occurrence of events. Neither the relative composition of the monoterpene emission nor that of the forest concentrations showed significant deviations prior to or during event periods. Source terms for secondary organic compounds were only slightly higher (weakly significant) on event days as compared to non-event days and did not correlate to maxima in ultra-fine particle concentrations on event days. Nucleation events were not observed during nighttime when the production of secondary organic compounds, and probably of secondary organic aerosol was greatest. Thus, we conclude that the oxidation products of the terpenes were not the nucleating species observed at Hyytiälä. Correlations between nocturnal increases in particulate organic carbon and the source term for secondary organic compounds indicate that the increase could have been due to condensation processes if the aerosol yield was at least 10–15%. Sulphur dioxide and NH3 concentrations, as well as the H2SO4 source term were significantly higher during event periods, and the H2SO4 source term together with the NH3 concentration correlated well with the daily maxima in ultra-fine particle number concentrations. The results indicate that SO2 and NH3 were involved in the mechanism for nucleation events at Hyytiälä. The H2SO4 source term was not high enough to account for the entirety of the observed growth rate of the new particles. A substantial part of the growth ought to have been due to condensation of secondary organic material from the oxidation of terpenes. The data indicate that a secondary organic aerosol yield on the order of 10% would suffice.
Nucleation of near nm sized aerosol particles and subsequent growth to ∼100 nm in 1–2 days has in recent years been frequently observed in the continental boundary layer at several European locations. In 1998–99, this was the focus of the BIOFOR experiment in Hyytil in the boreal Finnish forest. Nucleation occurred in arctic and to some extent in polar air masses, with a preference for maritime air in transition to continental air masses, and never in sub-tropical air. The air masses originated north of the BIOFOR experiment by paths from the southwest to northeast sector. The nucleation was also associated with cold air advection behind cold fronts, never warm air advection. This may relate to low pre-existing aerosol concentration, low cloudiness and large diurnal amplitudes in the continental boundary layer associated with cold air advection and clear skies. Arctic and polar air together with cold air advection did not always lead to nucleation. The most important limiting meteorological factors were cold front passages and high cloudiness, probably through reduced photochemistry and wet scavenging of precursor gases and new aerosol particles. The preference for nucleation to occur in arctic air masses, which seldom form in the summer, suggests a meteorological explanation for the annual cycle of nucleation, which has a minimum in summer. The connection to cold-air outbreaks suggests that the maximum in nucleation events during spring and autumn may be explained by the larger latitudinal temperature gradients and higher cyclone activity at that time of the year. Nucleation was observed on the same days over large parts (1000-km distance) of the same air mass. This suggests that the aerosol nucleation spans from the microphysical scale to the synoptic scale, perhaps connected through boundary layer and mesoscale processes.
Taking advantage of only the measured aerosol particles spectral evolution as a function of time, a new analytical tool is developed to derive formation and growth properties of nucleation mode aerosols. This method, when used with hygroscopic growth-factors, can also estimate basic composition properties of these recently-formed particles. From size spectra the diameter growth-rate can be obtained, and aerosol condensation and coagulation sinks can be calculated. Using this growth-rate and condensation sink, the concentration of condensable vapours and their source rate can be estimated. Then, combining the coagulation sink together with measured number concentrations and apparent source rates of 3 nm particles, 1 nm particle nucleation rates and concentration can be estimated. To estimate nucleation rates and vapour concentration source rates producing new particle bursts over the Boreal forest regions, three cases from the BIOFOR project were examined using this analytical tool. In this environment, the nucleation mode growth-rate was observed to be 2–3 nm hour−1, which required a condensable vapour concentration of 2.5–4×107 cm−3 and a source rate of approximately 7.5–11×104 cm−3 s−1 to be sustained. The formation rate of 3 nm particles was ≈1 particle cm−3 s−1 in all three cases. The estimated formation rate of 1 nm particles was 10–100 particles cm−3 s−1, while their concentration was estimated to be between 10,000 and 100,000 particles cm−3. Using hygroscopicity data and mass flux expressions, the mass flux of insoluble vapour is estimated to be of the same order of magnitude as that of soluble vapour, with a soluble to insoluble vapour flux ratio ranging from 0.7 to 1.4 during these nucleation events.
In this study a cloud model describing an ascending, adiabatically closed parcel was used to show the influence of the quality of data about the chemical composition of aerosol particles on the model results. The input data for the model were based on measured aerosol chemical compositions which were compiled from literature data. Model results influenced by incomplete chemical characterization of the particles are the radii of the large droplets, which depend on the activation parameter, and the pH-values of the droplets, which depend on the ionic balance of the particles.
The hygroscopic growth of individual aerosol particles has been measured with a Tandem Differential Mobility Analyser. The hygroscopic growth spectra were analysed in terms of diameter change with increasing RH from ≤ 20% to 85%. The measurements were carried out during the GCE cloud experiment at Kleiner Feldberg, Taunus, Germany in October and November 1990.
Two groups of particles with different hygroscopic growth were observed. The less-hygroscopic group had average growth factors of 1.11, 1.04 and 1.02 for particle diameters of 50, 150 and 300 nm, respectively. The more-hygroscopic group had average growth factors of 1.34, 1.34, and 1.37 for the same particle diameters. The average fraction of less-hygroscopic particles was about 50%. Estimates of the soluble fractions of the particles belonging to the two groups are reported.
Hygroscopic growth spectra for total aerosol, interstitial aerosol and cloud drop residuals were measured. A comparison of these hygroscopic growths of individual aerosol particles provides clear evidence for the importance of hygroscopic growth in nucleation scavenging. The measured scavenged fraction of particles as a function of diameter can be explained by the hygroscopic growth spectra.
Although organic compounds typically constitute a substantial fraction of the fine particulate matter (PM) in the atmosphere, their molecular composition remains poorly characterized. This is largely because atmospheric particles contain a myriad of diverse organic compounds, not all of which extract in a single solvent or elute through a gas chromatograph; therefore, a substantial portion typically remains unanalyzed. Most often the chemical analysis is performed on a fraction that extracts in organic solvents such as benzene, ether or hexane; consequently, information on the molecular composition of the water-soluble fraction is particularly sparse and incomplete.
This paper investigates theoretically the characteristics of the water-soluble fraction by splicing together various strands of information from the literature. We identify specific compounds that are likely to contribute to the water-soluble fraction by juxtaposing observations regarding the extraction characteristics and the molecular composition of atmospheric particulate organics with compound-specific solubility and condensibility for a wide variety of organics. The results show that water-soluble organics, which constitute a substantial fraction of the total organic mass, include C2 to C7 multifunctional compounds (e.g., diacids, polyols, amino acids). The importance of diacids is already recognized; our results provide an impetus for new experiments to establish the atmospheric concentrations and sources of polyols, amino acids and other oxygenated multifunctional compounds.
Measurements of aerosol formation during thephotooxidation of
\textNOx {\text{NO}}_x
and dark experiments withelevated ozone concentrations were performed. Theevolution of the aerosol was simulated by theapplication of a gas/particle absorption model inconnection with a chemical reaction mechanism. Thefractional aerosol yield is shown to be a function ofthe organic aerosol mass concentration andtemperature. Ozone and, for selected hydrocarbons, theNO3 reaction of the compounds were found torepresent efficient routes to the formation ofcondensable products. For initial hydrocarbon mixingratios of about 100 ppb, the fractional aerosol yieldsfrom daylight runs have been estimated to be 5%for open-chain hydrocarbons, such as ocimene andlinalool, 5–25% for monounsaturated cyclicmonoterpenes, such as -pinene, d-3-carene, orterpinene-4-ol, and 40% for a cyclic monoterpenewith two double bonds like d-limonene. For the onlysesquiterpene investigated, trans-caryophyllene, afractional aerosol yield of close to 100% wasobserved. The majority of the compounds studied showedan even higher aerosol yield during dark experimentsin the presence of ozone.
Organic compounds in the atmosphere can influence the activation, growth and lifetimes of haze, fog and cloud droplets by changing the condensation and evaporation rates of liquid water by these aqueous aerosol particles. Depending on the nature and properties of the organic compounds, the change can be to enhance or reduce these rates. In this paper we used a tandem differential mobility analyzer (TDMA) to examine the effect of tetracosane, octanoic acid, and lauric acid on the hygroscopic properties of NaCl aerosol particles at relative humidities (RH) between 30 and 95%. These organic compounds have been identified in ambient aerosol particle samples. A slight lowering of the deliquescence relative humidity (DRH) and suppression of hygroscopic growth for the NaCl-organic compound mixtures were observed when compared to pure NaCl particles. The growth of pure NaCl particles was 2.25 in diameter at 85% RH while the growth of the mixed particles was 1.3 to 1.7 in particle diameter at 85% RH with organic mass fraction of 30–50%. This shows that these organic compounds have to be present in rather large mass fractions to effect the hygroscopic behavior to a similar degree observed for ambient aerosol during field measurements. Despite the mixing of the organic material with NaCl, hysteresis was observed for decreasing RH histories, suggesting the formation of metastable droplets. These laboratory results are strikingly similar to ambient field results. For example, if the total organic mass fraction of the particles is between 0.30 and 0.50, the particle growth at 85% RH is about a factor of 1.4 for the laboratory and field measurements. Such reduction in growth compared to the pure inorganic salt is in contradiction to speculations concerning significant effects by organic compounds on cloud condensation nuclei and thus formation on clouds.
Submicron aerosol particles with detectable sizes of 7 nm diameter and larger were measured in the mid-latitude free troposphere up to the lower stratosphere over Western Europe in summer 1994. A statistical analysis of the occurrence and variability of condensation nuclei larger than 18 rm diameter (N18) and nuclei mode particles (UCN, 7 nm < Dp < 18 nm) showed the following results: The median Na18 concentrations were found to be around 1500 cm−3 throughout the middle and upper free troposphere and dropped down to a few hundred per cm 3 1–2 km inside the stratosphere. Nuclei mode particles were always present above about 5 km altitude and showed the highest median concentrations in the upper free troposphere. The median UCN concentrations increased from about 50 cm−3 in the lower to about 300 cm−3 in the upper free troposphere and decreased to about 150 cm−3 above the tropopause. At the same time the number ratio of UCN to the rest of the sub-micron spectrum steadily increased with altitude from 0.05 to almost 0.4. During the passage of a cold front tropospheric UCN concentrations doubled and occasionally ‘burst’ events with several thousand UCN cm−3 were observed. Above the tropopause nucleation bursts occurred more frequently and were typically extended over 10–30 km horizontal distance. These burst regions were also characterised by high mean wind speeds and horizontal wind shears. Our measurements suggest frequently occurring particle production processes in the free troposphere that do not necessary require low values of pre-existing aerosol surface area. Nucleation events may primarily be attributed to dynamically- induced mixing processes and their strength might depend on the amplitude of temperature- and RH-fluctuations.
Size-resolved aerosol growth measurements (growth ≡ moist particle diameter/dry particle diameter) and chemical composition monitoring were conducted during a 3 month period in the winter of 1990 at the South Rim of Grand Canyon National Park, AZ as part of the Navajo Generating Station Visibility Study. Particle growth data are from a Tandem Differential Mobility Analyzer (TDMA). Typically for relative humidities above 75%, the TDMA-measured moist particle distribution is distinctly bimodal, indicating two aerosol fractions based on growth and providing direct evidence of an external mixture of soluble and insoluble constituents. In this study both particle fractions grew in size, thus the terms “more hygroscopic” and “less hygroscopic” were used to distinguish them. Micro-Orifice Uniform Deposit Impactors (MOUDI) collected size-segregated 24 h duration samples for subsequent analysis by XRF, ion chromatography, and by thermo-optical analysis (i.e. for carbon).A model that synthesizes growth and compositional information was developed to partition the overall volume fraction of the soluble material as determined from the MOUDI composition data, ε, to the two growth fractions obtained from the TDMA data (i.e. ε is partitioned between εm and ε1). The model calculates εm and ε1 for each TDMA measurement as well as growth capacity of the soluble material at the measurement relative humidity, Gs, which is assumed to be the same for both growth fractions. Model results indicate that on average, the more hygroscopic particles are composed of equal volumes of soluble and insoluble materials, while the less hygroscopic fraction is dominated by insoluble material (about 85%).
A TDMA system (Tandem Differential Mobility Analyzer; Rader D.J. and McMurry P.H. J. Aerosol Sci. 17, 771–787, 1986) was used to measure the sensitivity of particle size to relative humidity for monodisperse Los Angeles aerosols. Measurements were made at Claremont, CA on 13 days between 19 June and 3 September 1987, in conjunction with the Southern California Air Quality Study (SCAQS). The particle sizes that were studied ranged from 0.05 μm to 0.5 μm diameter at ambient relative humidity (typically 45–65%).The data provide clear evidence that these atmospheric aerosols were externally mixed. Monodisperse ambient aerosols were often found to split into nonhygroscopic (no water uptake) and hygroscopic portions when humidified. An average of 30% of the particles in the 0.2–0.5 μm range were nonhygroscopic. However, the proportion of the particles that was nonhygroscopic varied considerably from day to day and was, on occasion, as high as 70–80% of the particles. There was no clear evidence for nonhygroscopic 0.05 μm particles, but the data are not definitive on this point.The data also show that for the hydrophilic aerosol fraction, the larger particles (0.4–0.5 μm) grew more when humidified than did smaller particles (0.05–0.2 μm). As relative humidities were increased from 50% to 90%, particle diameters grew by average factors of 1.46 ±0.02 (for 0.5 μm particles), 1.49 ± 0.08 (0.4 μm), 1.19 ± 0.08 (0.2 μm) and 1.12 ± 0.05 (0.05 μm). Similarly, when particles were dried from 50% RH to 6–10% RH, particle diameters changed by factors ranging from 0.94 ± 0.03 (0.5 μm) to 0.98 ± 0.01 (0.05 μm).
The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.
In this study the possible conditions for new aerosol particle formation in a background area of Helsinki have been analysed. The measurements of aerosol particle size distribution, main gaseous pollutant compounds, UV spectra and meteorological parameters were performed during April–May 1993. The main interest was concentrated on the investigations of photochemical OH radical formation, the oxidation of gas phase SO2 to H2SO4 and the formation of H2SO4H2O aerosol particles. The measurements were analysed using a model for OH radical formation and aerosol dynamics. The analysis of aerosol size distributions was carried out using positive matrix factorization. The main conclusion is that based on our model analysis no evidence of new particle formation in the vicinity of the measurement station was found. However, the high concentrations of aerosol particles in the ultrafine size range indicate that some other particle formation pathways are to be considered.
Data that were acquired in Claremont, CA, during the Southern California Air Quality Study (SCAQS, summer 1987) and at the Grand Canyon, AZ, during the 1990 Navajo Generating Station Visibility study are examined for information on variabilities in composition among particles of a given size and as a function of size. At both sites a tandem differential mobility analyser (TDMA, McMurry and Stolzenburg, Atmospheric Environment 23: 497–507, 1989) was used to measure hygroscopic properties for particles in the 0.05–0.5 μm diameter range, and the size-resolved composition for major ionic species, carbon and elements was obtained which cascade impactors. At the Grand Canyon the elemental composition, including carbon, was measured by microscopy for 1100 individual submicrometer particles.
A case study of gas-Tellus 53B
- P Saxena
- L M Hildemann
- P H Mcmurry
- F Hopper
- F Brechtel
Saxena, P., Hildemann, L. M., McMurry, P. H. and Hopper, F. and Brechtel, F. 1999. A case study of gas-Tellus 53B (2001), 4 379