Assessing the impact of anthropogenic pollution on isoprene-derived secondary organic aerosol formation in PM2.5 collected from the Birmingham, Alabama ground site during the 2013 Southern Oxidant and Aerosol Study

Abstract

In the southeastern U.S., substantial emissions of isoprene from deciduous trees undergo atmospheric oxidation to form secondary organic aerosol (SOA) that contributes to fine particulate matter (PM2.5). Laboratory studies have revealed that anthropogenic pollutants, such as sulfur dioxide (SO2), oxides of nitrogen (NOx), and aerosol acidity, can enhance SOA formation from the hydroxyl radical (OH)-initiated oxidation of isoprene; however, the mechanisms by which specific pollutants enhance isoprene SOA in ambient PM2.5 remain unclear. As one aspect of an investigation to examine how anthropogenic pollutants influence isoprene-derived SOA formation, high-volume PM2.5 filter samples were collected at the Birmingham, Alabama (BHM) ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). Sample extracts were analyzed by gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and ultra performance liquid chromatography coupled to an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR QTOFMS) to identify known isoprene SOA tracers. Tracers quantified using both surrogate and authentic standards were compared with collocated gas- and particle-phase data as well as meteorological data provided by the Southeastern Aerosol Research and Characterization (SEARCH) network to assess the impact of anthropogenic pollution on isoprene-derived SOA formation. Results of this study reveal that isoprene-derived SOA tracers contribute a substantial mass fraction of organic matter (OM) (~7 to ~20%). Isoprene-derived SOA tracers correlated with sulfate (SO42-) (r2 = 0.34, n = 117), but not with NOx. Moderate correlation between methacrylic acid epoxide and hydroxymethyl-methyl-α-lactone (MAE/HMML)-derived SOA tracers and nitrate radical production (P[NO3]) (r2 = 0.57, n = 40) were observed during nighttime, suggesting a potential role of NO3 radical in forming this SOA type. However, the nighttime correlation of these tracers with nitrogen dioxide (NO2) (r2 = 0.26, n = 40) was weaker. Ozone (O3) correlated strongly with MAE/HMML-derived tracers (r2 = 0.72, n = 30) and moderately with 2-methyltetrols (r2 = 0.34, n = 15) during daytime only, suggesting that a fraction of SOA formation could occur from isoprene ozonolysis in urban areas. No correlation was observed between aerosol pH and isoprene-derived SOA. Lack of correlation between aerosol acidity and isoprene-derived SOA indicates that acidity is not a limiting factor for isoprene SOA formation at the BHM site as aerosols were acidic enough to promote multiphase chemistry of isoprene-derived epoxides throughout the duration of the study. All in all, these results confirm the reports that anthropogenic pollutants enhance isoprene-derived SOA formation.

Full text

1
Assessing the impact of anthropogenic pollution on isoprene-derived secondary organic 1
aerosol formation in PM2.5 collected from the Birmingham, Alabama ground site during the 2
2013 Southern Oxidant and Aerosol Study 3
4
W. Rattanavaraha1, K. Chu1, S. H. Budisulistiorini1,a, M. Riva1, Y.-H. Lin1,b, E. S. Edgerton2, K. 5
Baumann2, S. L. Shaw3, H. Guo4, L. King4, R. J. Weber4, E. A. Stone5, M. E. Neff5, J. H. 6
Offenberg6, Z. Zhang1, A. Gold1, and J. D. Surratt1,* 7
8
1 Department of Environmental Sciences and Engineering, Gillings School of Global Public 9
Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA 10
2 Atmospheric Research & Analysis, Inc., Cary, NC, USA 11
3 Electric Power Research Institute, Palo Alto, CA, USA 12
4 Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, GA, USA 13
5 Department of Chemistry, University of Iowa, Iowa City, IA, USA 14
6 Human Exposure and Atmospheric Sciences Division, United States Environmental Protection 15
Agency, Research Triangle Park, NC, USA 16
a now at: Earth Observatory of Singapore, Nanyang Technological University, Singapore 17
b now at: Michigan Society of Fellows, Department of Chemistry, University of Michigan, Ann 18
Arbor, MI, USA 19
20
* To whom correspondence should be addressed. Email: surratt@unc.edu 21
For Submission to: Atmospheric Chemistry & Physics Discussions 22
23
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Abstract 24
In the southeastern U.S., substantial emissions of isoprene from deciduous trees undergo 25
atmospheric oxidation to form secondary organic aerosol (SOA) that contributes to fine particulate 26
matter (PM2.5). Laboratory studies have revealed that anthropogenic pollutants, such as sulfur 27
dioxide (SO2), oxides of nitrogen (NOx), and aerosol acidity, can enhance SOA formation from 28
the hydroxyl radical (OH)-initiated oxidation of isoprene; however, the mechanisms by which 29
specific pollutants enhance isoprene SOA in ambient PM2.5 remain unclear. As one aspect of an 30
investigation to examine how anthropogenic pollutants influence isoprene-derived SOA 31
formation, high-volume PM2.5 filter samples were collected at the Birmingham, Alabama (BHM) 32
ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). Sample extracts were 33
analyzed by gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) with prior 34
trimethylsilylation and ultra performance liquid chromatography coupled to an electrospray 35
ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-36
QTOFMS) to identify known isoprene SOA tracers. Tracers quantified using both surrogate and 37
authentic standards were compared with collocated gas- and particle-phase data as well as 38
meteorological data provided by the Southeastern Aerosol Research and Characterization 39
(SEARCH) network to assess the impact of anthropogenic pollution on isoprene-derived SOA 40
formation. Results of this study reveal that isoprene-derived SOA tracers contribute a substantial 41
mass fraction of organic matter (OM) (~7 to ~20%). Isoprene-derived SOA tracers correlated with 42
sulfate (SO42-) (r2 = 0.34, n = 117), but not with NOx. Moderate correlation between methacrylic 43
acid epoxide and hydroxymethyl-methyl-α-lactone (MAE/HMML)-derived SOA tracers and 44
nitrate radical production (P[NO3]) (r2 = 0.57, n = 40) were observed during nighttime, suggesting 45
a potential role of NO3 radical in forming this SOA type. However, the nighttime correlation of 46
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these tracers with nitrogen dioxide (NO2) (r2 = 0.26, n = 40) was weaker. Ozone (O3) correlated 47
strongly with MAE/HMML-derived tracers (r2 = 0.72, n = 30) and moderately with 2-methyltetrols 48
(r2 = 0.34, n = 15) during daytime only, suggesting that a fraction of SOA formation could occur 49
from isoprene ozonolysis in urban areas. No correlation was observed between aerosol pH and 50
isoprene-derived SOA. Lack of correlation between aerosol acidity and isoprene-derived SOA 51
indicates that acidity is not a limiting factor for isoprene SOA formation at the BHM site as 52
aerosols were acidic enough to promote multiphase chemistry of isoprene-derived epoxides 53
throughout the duration of the study. All in all, these results confirm the reports that anthropogenic 54
pollutants enhance isoprene-derived SOA formation. 55
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1. Introduction 56
Fine particulate matter, suspensions of liquid or solid aerosol in a gaseous medium that are 57
less than or equal to 2.5 μm in diameter (PM2.5), play a key role in physical and chemical 58
atmospheric processes. They influence climate patterns both directly, through the absorption and 59
scattering of solar and terrestrial radiation, and indirectly, through cloud formation (Kanakidou et 60
al., 2005). In addition to climatic effects, PM2.5 has been demonstrated to pose a potential human 61
health risk through inhalation exposure (Pope and Dockery, 2006; Hallquist et al., 2009). Despite 62
the strong association of PM2.5 with climate change and environmental health, there remains a need 63
to more fully resolve its composition, sources, and chemical formation processes in order to 64
develop effective control strategies to address potential hazards in a cost-effective manner 65
(Hallquist et al., 2009; Boucher et al., 2013; Nozière et al., 2015). 66
Atmospheric PM2.5 are comprised in a large part (up to 90% by mass in some locations), 67
of organic matter (OM) (Carlton et al., 2009; Hallquist et al., 2009). OM can be derived from many 68
sources. Primary organic aerosol (POA) is emitted from both natural (e.g., fungal spores, 69
vegetation, vegetative detritus) and anthropogenic sources (fossil fuel and biomass burning) prior 70
to atmospheric processing. As a result of large anthropogenic sources, POA is abundant largely in 71
urban areas. Processes such as biomass burning and combustion also yield volatile organic 72
compounds (VOCs), which have high vapor pressures and can undergo atmospheric oxidation to 73
form secondary organic aerosol (SOA) through gas-to-particle phase partitioning (condensation or 74
nucleation) with subsequent particle-phase (multiphase) chemical reactions (Grieshop et al., 75
2009). 76
At around 600 Tg emitted per year into the atmosphere, isoprene (2-methyl-1,3-butadiene, 77
C5H8) is the most abundant volatile non-methane hydrocarbon (Guenther et al., 2012). The 78
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abundance of isoprene is particularly high in the southeastern U.S. due to emissions from broadleaf 79
deciduous tree species (Guenther et al., 2006). Research over the last decade has revealed that 80
isoprene, via hydroxyl radical (OH)-initiated oxidation, is a major source of SOA (Claeys et al., 81
2004; Edney et al., 2005; Kroll et al., 2005 ; Kroll et al., 2006; Surratt et al., 2006; Lin et al., 2012; 82
Lin et al., 2013a). In addition, it is known that SOA formation is enhanced by anthropogenic 83
emissions, namely oxides of nitrogen (NOx) and sulfur dioxide (SO2), that are a source of acidic 84
aerosol onto which photochemical oxidation products of isoprene are reactively taken up to yield 85
a variety of SOA products (Edney et al., 2005; Kroll et al., 2006; Surratt et al., 2006; Surratt et al., 86
2007b; Surratt et al., 2010; Lin et al., 2013b;) . 87
Recent work has begun to elucidate some of the critical intermediates of isoprene oxidation 88
that lead to SOA formation through acid-catalyzed heterogeneous chemistry (Kroll et al., 2005; 89
Surratt et al., 2006). Under low-NOx conditions, such as in a pristine environment, isomeric 90
isoprene epoxydiols (IEPOX) have been demonstrated to be critical to the formation of isoprene 91
SOA. On advection of IEPOX to an urban environment and mixing with anthropogenic emissions 92
of acidic sulfate aerosol, SOA formation is enhanced (Surratt et al., 2006; Lin et al., 2012; Lin et 93
al., 2013b). This pathway has been shown to yield 2-methyltetrols as major SOA constituents of 94
ambient PM2.5 (Claeys et al, 2004; Surratt et al., 2010; Lin et al., 2012). Further work has revealed 95
a number of additional IEPOX-derived SOA tracers, including C5-alkene triols (Wang et al., 2005; 96
Lin et al., 2012), cis- and trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) (Lin et 97
al., 2012; Zhang et al., 2012), IEPOX-derived organosulfates (OSs) (Lin et al., 2012), and IEPOX-98
derived oligomers (Lin et al., 2014). Some of the IEPOX-derived oligomers have been shown to 99
contribute to aerosol components known as brown carbon that absorb light in the near ultraviolet 100
(UV) and visible ranges (Lin et al., 2014). Under high-NOx conditions, such as encountered in an 101
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urban environment, isoprene is oxidized to methacrolein and SOA formation occurs via the further 102
oxidation of methacrolein (MACR) (Kroll et al., 2006; Surratt et al., 2006) to methacryloyl 103
peroxynitrate (MPAN) (Chan et al., 2010; Surratt et al., 2010; Nguyen et al., 2015). It has recently 104
been shown that when MPAN is oxidized by OH it yields at least two SOA precursors, methacrylic 105
acid epoxide (MAE) and hydroxymethyl-methyl-α-lactone (HMML) (Surratt et al., 2006; Surratt 106
et al., 2010; Lin et al., 2013a; Nguyen et al., 2015). Whether SOA precursors are formed under 107
high- or low-NOx conditions, aerosol acidity is a critical parameter that enhances the reaction 108
kinetics through acid-catalyzed reactive uptake and multiphase chemistry of either IEPOX or 109
MAE/HMML ( Surratt et al., 2007b; Surratt et al., 2010; Lin et al., 2013b). 110
Due to the considerable emissions of isoprene, an SOA yield of even 1% would contribute 111
significantly to ambient SOA (Carlton et al., 2009; Henze et al., 2009). This conclusion is 112
supported by measurements showing that up to a third of total fine OA mass can be attributed to 113
IEPOX-derived SOA tracers in Atlanta, GA (JST) during summer months (Budisulistiorini et al., 114
2013; Budisulistiorini et al., 2015). A recent study in Yorkville, GA (YRK), similarly found that 115
IEPOX-derived SOA tracers comprised 12-19% of the fine OA mass (Lin et al., 2013b). Another 116
SOAS site at Centreville, Alabama (CTR) revealed IEPOX-SOA contributed 18% of total OA 117
mass (Xu et al., 2015). The individual ground sites corroborate recent aircraft-based measurements 118
made in the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling 119
by Regional Surveys (SEAC4RS) aircraft campaign, which estimates an IEPOX-SOA contribution 120
of 32% to OA mass in the southeastern U.S. (Hu et al., 2015). 121
It is clear from the field studies discussed above that particle-phase chemistry of isoprene-122
derived oxidation products plays a large role in atmospheric SOA formation. However, much 123
remains unknown regarding the exact nature of its formation, limiting the ability of models to 124
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accurately account for isoprene SOA (Carlton et al., 2010b; Foley et al., 2010). Currently, 125
traditional air quality models in the southeastern U.S. do not incorporate detailed particle-phase 126
chemistry of isoprene oxidation products (IEPOX or MAE/HMML) and generally under-predict 127
isoprene SOA formation (Carlton et al., 2010a). Recent work demonstrates that incorporating the 128
specific chemistry of isoprene epoxide precursors into models increases the accuracy of isoprene 129
SOA prediction (Pye et al., 2013; Karambelas et al., 2014), suggesting that understanding the 130
formation mechanisms of biogenic SOA, especially with regard to the effects of anthropogenic 131
emissions, such as NOx and SO2, will be key to more accurate models. More accurate models are 132
needed in order to devise cost-effective control strategies for reducing PM2.5 levels. Since isoprene 133
is primarily biogenic in origin, and therefore not controllable, the key to understanding the public 134
health and environmental implications of isoprene SOA lies in resolving the effects of 135
anthropogenic pollutants. 136
This study presents results from the 2013 Southeastern Oxidant and Aerosol Study 137
(SOAS), where several well-instrumented ground sites dispersed throughout the southeastern U.S. 138
made intensive gas- and particle-phase measurements from June 1 – July 16, 2013. The primary 139
purpose of this campaign was to examine, in greater detail, the formation mechanisms, 140
composition, and properties of biogenic SOA, including the effects of anthropogenic emissions. 141
This study pertains specifically to the results from the BHM ground site, where the city’s ample 142
urban emissions mix with biogenic emissions from the surrounding rural areas, creating an ideal 143
location to investigate such interactions. The results presented here focus on analysis of PM2.5 144
collected on filters during the campaign by gas chromatography interfaced to electron ionization-145
mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography interfaced with 146
electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-147
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HR-QTOFMS). The analysis of PM2.5 was conducted in order to measure quantities of known 148
isoprene SOA tracers and using collocated air quality and meteorological measurements to 149
investigate how anthropogenic pollutants including NOx, SO2, aerosol acidity (pH), PM2.5 sulfate 150
(SO42-), and O3 affect isoprene SOA formation. These results, along with the results presented 151
from similar studies during the 2013 SOAS campaign, seek to elucidate the chemical relationships 152
between anthropogenic emissions and isoprene SOA formation in order to provide better 153
parameterizations needed to improve the accuracy of air quality models in this region of the U.S. 154
2. Methods 155
2.1. Site description and collocated data 156
Filter samples were collected in the summer of 2013 as part of the SOAS field campaign 157
at the BHM ground site (33.553N, 86.815W). In addition to the SOAS campaign, the site is also 158
part of the Southeastern Aerosol Research and Characterization Study (SEARCH) (Figure S1 of 159
the Supplement), an observation and monitoring program initiated in 1998. SEARCH and this site 160
are described elsewhere in detail (Hansen et al., 2003; Edgerton et al., 2006). The BHM site is 161
surrounded by significant transportation and industrial sources of PM. West of BHM are US-31 162
and I-65 highways. To the north, northeast and southwest of BHM several coking ovens and an 163
iron pipe foundry are located (Hansen et al., 2003). 164
2.2. High-Volume filter sampling and analysis methods 165
2.2.1. High-Volume filter sampling 166
From June 1 – July 16, 2013, PM2.5 samples were collected onto TissuquartzTM Filters 167
(8 x 10 in, Pall Life Sciences) using high-volume PM2.5 samplers (Tisch Environmental) operated 168
at 1 m3 min-1 at ambient temperature described in detail elsewhere (Budisulistiorini et al. 2015; 169
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Riva et al., 2015). All quartz filters were pre-baked prior to collection. The procedure consisted of 170
baking filters at 550 °C for 18 hours followed by cooling to 25 °C over 12 hours. 171
The sampling schedule is given in Table 1. Either two or four samples were collected per 172
day. The regular schedule consisted of two samples per day, one during the day, the second at 173
night, each collected for 11 hours. On intensive sampling days, four samples were collected, with 174
the single daytime sample being subdivided into three separate periods. The intensive sampling 175
schedule was conducted on days when high levels of isoprene, SO42- and NOx where forecast by 176
the National Center for Atmospheric Research (NCAR) using the Flexible Particle dispersion 177
model (FLEXPART) (Stohl et al., 2005) and Model for Ozone and Related Chemical Tracers 178
(MOZART) (Emmons et al., 2010) simulations. Details of these simulations have been 179
summarized in Budisulistiorini et al. (2015); however, these model data were only used 180
qualitatively to determine the sampling schedule. The intensive collection frequency allowed 181
enhanced time resolution for offline analysis to examine the effect of anthropogenic emissions on 182
the evolution of isoprene SOA tracers throughout the day. 183
In total, 120 samples were collected throughout the field campaign with a field blank filter 184
collected every 10 days to identify errors or contamination in sample collection and analysis. All 185
filters were stored at -20 °C in the dark until extraction and analysis. In addition to filter sampling 186
of PM2.5, SEARCH provided a suite of additional instruments at the site collecting measurements 187
of a variety of variables, including meteorology, gas, and continuous PM monitoring. The variables 188
with respective instrumentation are summarized in Table S1 of the Supplement. 189
2.2.2. Isoprene-derived SOA analysis by GC/EI-MS 190
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SOA collected in the field on quartz filters was extracted and isoprene tracers quantified 191
by GC/EI-MS with prior trimethylsilylation. A 37-mm diameter circular punch from each filter 192
was extracted in a pre-cleaned scintillation vial with 20 mL of high-purity methanol (LCMS 193
CHROMASOLV-grade, Sigma-Aldrich) by sonication for 45 minutes. The extracts were filtered 194
through PTFE syringe filters (Pall Life Science, Acrodisc®, 0.2-µm pore size) to remove insoluble 195
particles and residual quartz fibers. The filtrate was then blown dry under a gentle stream of N2 at 196
room temperature. The dried residues were immediately trimethylsilylated by reaction with 100 197
μL of BSTFA + TMCS (99:1 v/v, Supelco) and 50 μL of pyridine (anhydrous, 99.8 %, Sigma-198
Aldrich) at 70 °C for 1 hour. Derivatized samples were analyzed within 24 hours after 199
trimethylsilylation using a Hewlett-Packard (HP) 5890 Series II Gas Chromatograph coupled to a 200
HP 5971A Mass Selective Detector. The gas chromatograph was equipped with an Econo-Cap®-201
EC®-5 Capillary Column (30 m x 0.25 mm i.d.; 0.25-μm film thickness) to separate trimethylsilyl 202
derivatives before MS detection. 1 μL aliquots were injected onto the column. Operating 203
conditions and procedures have been described elsewhere (Surratt et al., 2010). 204
Extraction efficiency was assessed and taken into account for the quantification of all SOA 205
tracers. Efficiency was determined by analyzing 4 pre-baked filters spiked with 50 ppmv of 2-206
methyltetrols, 2-methylglyceric acid, levoglucosan, and cis- and trans-3-MeTHF-3,4-diols. 207
Extraction efficiency was above 90% and used to correct the quantification of samples. Extracted 208
ion chromatograms (EICs) of m/z 262, 219, 231, 335 were used to quantify the cis-/trans-3-209
MeTHF-3,4-diols, 2-methyltetrols and 2-methylglyceric acid, C5-alkene triols, and IEPOX-210
dimers, respectively (Surratt et al., 2006). 211
2-Methyltetrols were quantified using an authentic reference standard that consisted of a 212
mixture of racemic diasteroisomers. Similarly, 3-MeTHF-3,4-diol isomers were also quantified 213
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using authentic standards; however, 3-MeTHF-3,4-diol isomers were detected in few field 214
samples. 2-Methylglyceric acid was also quantified using an authentic standard. Procedures for 215
synthesis of the 2-methyltetrols, 3-MeTHF-3,4-diol isomers, and 2-methylglyceric acid have been 216
described elsewhere (Zhang et al., 2012; Budisulistiorini et al., 2015). C5-alkene triols and IEPOX-217
dimers were quantified using the average response factor of the 2-methyltetrols. 218
2.2.3. Isoprene-derived SOA analysis by UPLC/ ESI-HR-QTOFMS 219
A 37-mm diameter circular punch from each quartz filter was extracted following the same 220
procedure described in section 2.2.1 for GC/EI-MS analysis. The dried residues were reconstituted 221
with 150 µl of a 50:50 (v/v) solvent mixture of methanol (LC-MS CHROMASOVL-grade, Sigma-222
Aldrich) and high-purity water (Milli-Q, 18.2 MΩ). The extracts were immediately analyzed by 223
the UPLC/ESI-HR-QTOFMS (6520 Series, Agilent) operated in the negative ion mode. Detailed 224
operating conditions have been described elsewhere (Riva et al., 2015). Mass spectra were 225
acquired at a mass resolution 7000-8000 over the range m/z 200 – 400. 226
Extraction efficiency was determined by analyzing 3 pre-baked filters spiked with propyl 227
sulfate and octyl sulfate (electronic grade, City Chemical LLC). Extraction efficiencies were in the 228
range 86 – 95%. EICs of m/z 215, 333 and 199 were used to quantify the IEPOX-derived OS, 229
IEPOX-derived dimer OS and the MAE/HMML-derived OS, respectively (Surratt et al., 2007a). 230
EICs were generated with a ± 5 ppm tolerance. All accurate masses for all measured 231
organosulfates were within ± 5 ppm. For simplicity, only the nominal masses are reported in the 232
text when describing these products. IEPOX-derived OS and IEPOX-derived dimer OS were 233
quantified by authentic standards (Zhang et al., 2012). The MAE/HMML-derived OS was 234
quantified using authentic MAE/HMML-derived OS synthesized in-house by a procedure to be 235
described in a forthcoming publication (1H NMR trace, Figure S2). 236
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EICs of of m/z 155, 169 and 139 were used to quantify the glyoxal-derived OS, 237
methylglyoxal-derived OS, and the hydroxyacetone-derived OS, respectively (Surratt et al., 238
2007a). In addition, EICs of m/z 211, 260 and 305 were used to quantify other known isoprene-239
derived OSs (Surratt et al., 2007a). Glycolic acid sulfate synthesized in-house was used as a 240
standard to quantify the glyoxal-derived OS (Galloway et al., 2009) and propyl sulfate, was used 241
as a surrogate standard to quantify the remaining isoprene-derived OSs. 242
2.2.4. OC and WSOC analysis 243
A 1.5 cm2 square punch from each quartz filter was analyzed for total organic carbon (OC) 244
and elemental carbon (EC) by the thermal-optical method (Birch and Cary, 1996) on a Sunset 245
Laboratory OC/EC instrument (Tigard, OR) at the National Exposure Research Laboratory 246
(NERL) at the U.S. Environmental Protection Agency, Research Triangle Park, NC. The details 247
of the instrument and analytical method have been described elsewhere (Birch and Cary, 1996). In 248
addition to the internal calibration using methane gas, four different mass concentrations of sucrose 249
solution were used to verify the accuracy of instrument during the analysis. 250
Water-soluble organic carbon (WSOC) was measured in aqueous extracts of quartz fiber filter 251
samples using a total organic carbon (TOC) analyzer (Sievers 5310C, GE Water & Power) 252
equipped with an inorganic carbon remover (Sievers 900). To maintain low background carbon 253
levels, all glassware used was washed with water, soaked in 10% nitric acid, and baked at 500˚C 254
for 5 h and 30 min prior to use. Samples were extracted in batches that consisted of 12-21 PM2.5
255
samples and field blanks, one laboratory blank, and one spiked solution. A 17.3 cm2 filter portion 256
was extracted with 15 mL of purified water (> 18 MΩ, Barnstead Easypure II, Thermo Scientific) 257
by ultra-sonication (Branson 5510). Extracts were then passed through a 0.45 µm PTFE filter to 258
remove insoluble particles. The TOC analyzer was calibrated using potassium hydrogen phthalate 259
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(KHP, Sigma Aldrich) and was verified daily with sucrose (Sigma Aldrich). Samples and standards 260
were analyzed in triplicate; the reported values correspond to the average of the second and third 261
trials. Spiked solutions yielded recoveries that averaged (± one standard deviation) 96 ± 5 % (n = 262
9). All ambient concentrations were field blank subtracted. 263
2.2.5. Estimation of aerosol pH by ISORROPIA 264
Aerosol pH was estimated using a thermodynamic model, ISORROPIA-II (Nenes et al., 265
1998). SO42-, nitrate (NO3-), and ammonium (NH4+) ion concentrations measured in PM2.5
266
collected from BHM, as well as relative humidity (RH), temperature and gas-phase ammonia 267
(NH3) were used as inputs into the model. These variables were obtained from the SEARCH 268
network at BHM, which collected the data during the period covered by the SOAS campaign. The 269
ISORROPIA-II model estimates particle hydronium ion concentration per unit volume of air (H+, 270
μg m−3), aerosol liquid water content (LWC, μg m−3), and aqueous aerosol mass concentration (μg 271
m−3). The model-estimated parameters were used in the following formula to calculate the aerosol 272
pH: 273
Aerosol pH = −log= −log( 

 ×  × 1000 ) 274
where  is H+ activity in the aqueous phase (mol L-1), LMASS is total liquid-phase aerosol mass 275
(μg m−3) and  is aerosol density. Details of the ISORROPIA-II model and its ability to predict 276
pH, LWC, and gas-to-particle partitioning are not the focus of this study and are discussed 277
elsewhere and (Fountoukis et al., 2009). 278
2.2.6. Estimation of nighttime NO3 279
Nitrate radical (NO3) production (P[NO3]) was calculated using the following equation: 280
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[] = [][] 281
where [NO2] and [O3] correspond to the measured ambient NO2 and O3 concentrations (mol 282
cm-3), respectively, and k is the temperature-dependent rate constant (Herron and Huie, 1974; 283
Graham and Johnston, 1978). Since no direct measure of NO3 radical was made at this site during 284
SOAS, P[NO3] was used as a proxy for NO3 radicals present in the atmosphere to examine if there 285
is any association of it with isoprene-derived SOA tracers. 286
3. Results and Discussion 287
3.1. Overview of the study 288
The campaign extended from June 1 through July 16, 2013. Temperature during this period 289
ranged from a high of 32.6 °C to a low of 20.5 °C, with an average of ~26.4 °C. RH varied from 290
37-96% throughout the campaign, with an average of 71.5%. Rainfall occurred intermittently over 291
2-3 day periods and averaged 0.1 inches per day. Wind analysis reveals that air masses approached 292
largely from the south-southeast at an average wind speed of 2 m s-1. Summaries of meteorological 293
conditions as well as wind speed and direction during the course of the campaign are given in 294
Table 2 and illustrated in Figures 1 and 2. 295
The average concentration of carbon monoxide (CO), a combustion byproduct, was 208.7 296
ppbv. The mean concentration of O3 was significantly higher (t-test, p-value < 0.05) on intensive 297
sampling days (37.0 ppbv) than regular sampling days (25.2 ppbv). Concentrations of NOx, NH3, 298
and SO2 were lower averaging 7.8, 1.9, and 0.9 ppbv, respectively. On average, OC and WSOC 299
levels were 7.2 (n = 120) and 4 µg m-3 (n = 100), respectively. The largest inorganic component of 300
PM2.5 was SO42-, which averaged 2 μg m-3 with excursions between 0.4 and 4.9 μg m-3 during the 301
campaign. NH4+ and NO3- were present at low levels, averaging 0.66 and 0.14 μg m-3, respectively. 302
Time series of gas and PM2.5 components are shown in Figure 2. WSOC accounted for 35% of OC 303
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mass (Figure S3a), and was smaller than that recently reported in rural areas during SOAS 304
(Budisulistiorini et al., 2015; Hu et al., 2015), but consistent with previous observations at the 305
BHM site (Ding et al., 2008). WSOC/OC ratios are commonly lower in urban than rural areas, as 306
a consequence of higher primary OC emissions; thus, PM at BHM probably contains increased 307
OC. 308
Diurnal variation of meteorological parameters, trace gases, and PM2.5 components are 309
shown in Figure S4 of the Supplement. Temperature dropped during nighttime, and reached a 310
maximum in the afternoon (Figure S4a). Conversely, RH was low during day and high at night. 311
High-NOx levels were found in the early morning and decreased during the course of the day 312
(Figure S4c), most likely in conjunction with rising O3 levels. O3 reached a maximum 313
concentration between 12 - 3 pm due to photochemistry (Figure S4b). SO2 was slightly higher in 314
the morning (Figure S4c), but decreased during the day most likely as a result of planetary 315
boundary layer (PBL) dynamics. NH3 remained fairly constant throughout the day (Figure S4c). 316
No significant diurnal variation was found in the concentration of inorganic PM2.5 components, 317
including SO42-, NO3-, and NH4+ (Figure S4d). Unfortunately, a measurement of isoprene could 318
not be made at BHM during the campaign. However, the diurnal trend of isoprene levels might be 319
similar to the data at the CTR site (Xu et al., 2015), which is only 61 miles away from BHM. Xu 320
et al. (2015) observed the highest levels of isoprene (~ 6 ppb) at CTR in the mid-afternoon (3 pm 321
local time) and its diurnal trend was similar to isoprene-OA measured by the Aerodyne Aerosol 322
Mass Spectrometer (AMS) during the SOAS campaign. 323
3.2 Characterization of Isoprene SOA 324
Table 3 summarizes the mean and maximum concentrations of known isoprene-derived 325
SOA tracers detected by GC/EI-MS and UPLC/ESI-HR-QTOFMS. Levoglucosan was also 326
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analyzed as a tracer for biomass burning. Among the isoprene-derived SOA tracers, the highest 327
mean concentration was for 2-methyltetrols (376 ng m-3), followed by the sum of C5-alkene triols 328
(181 ng m-3) and the IEPOX-derived OS (165 ng m-3). The concentrations account for 3.8%, 1.8% 329
and 1.6%, respectively, of total OM mass. Noteworthy is that maximum concentrations of 2-330
methylerythritol (a 2-methyltetrol isomer; 1049 ng m-3), IEPOX-derived OS (865 ng m-3) and (E)-331
2-methylbut-3-ene-1,2,4-triol (879 ng m-3) were attained during the intensive sampling period 4-7 332
pm local time on June 15, 2013, following five consecutive days of dry weather (Figure 2a and 333
2d) when high levels of isoprene, SO42-, and NOx were forecast. 334
Together, the IEPOX-derived SOA tracers, which represent SOA formation from isoprene 335
oxidation predominantly under the low-NOx pathway, comprised 92.5% of the total detected 336
isoprene-derived SOA tracer mass at the BHM site. This contribution is slightly lower than 337
observations reported at rural sites located in Yorkville, GA (97.5%) and Look Rock, Tennessee 338
(LRK) (97%) (Lin et al., 2013b; Budisulistiorini et al., 2015). 339
The sum of MAE/HMML-OS and 2-MG, which represent SOA formation from isoprene 340
oxidation predominantly under the high-NOx pathway, contributed 3.25% of the total isoprene-341
derived SOA tracer mass, while the OS derivative of glycolic acid (GA sulfate) contributed 3.3%. 342
The contribution of GA sulfate was consistent with the level of GA sulfate measured by the 343
airborne NOAA Particle Analysis Laser Mass Spectrometer (PALMS) over the continental U.S. 344
during the Deep Convective Clouds and Chemistry Experiment and SEAC4RS (Liao et al., 2015). 345
However, the contribution of GA sulfate to the total OM at BHM (0.3%) is lower than aircraft-346
based measurements made by Liao et al. (2015) near the ground in the eastern U.S. (0.9%). GA 347
sulfate can form from biogenic and anthropogenic emissions other than isoprene, including 348
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glyoxal, which is thought to be a primary source of GA sulfate (Galloway et al., 2009). For this 349
reason, GA sulfate will not be further discussed in this study. 350
Isoprene SOA contribution to total OM was estimated by assuming the OM/OC ratio 1.6 351
based on the recent studies (El-Zanan et al., 2009; Simon et al., 2011; Ruthenburg et al., 2014; 352
Blanchard et al., 2015). On average, isoprene-derived SOA tracers (sum of both IEPOX- and 353
MAE/HMML-derived SOA tracers) contributed ~7% (ranging to ~ 20% at times) of the total 354
particulate OM mass. The average contribution is lower than measured at other sites in the S.E. 355
USA, including both rural LRK, (Budisulistiorini et al., 2015; Hu et al., 2015) and urban Atlanta, 356
GA (Budisulistiorini et al., 2013). The contribution of SOA tracers to OM in the current study was 357
estimated on the basis of offline analysis of filters, while tracer estimates in the two earlier studies 358
was based on online ACSM/AMS measurements. The low isoprene SOA/OM ratio is consistent 359
with the low WSOC/OC reported in section 3.1, suggesting an increased contribution of primary 360
OA or secondary OM to the total OM at BHM. However, it should be noted that total IEPOX-361
derived SOA mass at BHM may actually be closer to ~14% since recent measurements by the 362
Aerodyne ACSM at LRK indicated that tracers could only account for ~50% of the total IEPOX-363
derived SOA mass resolved by the ACSM (Budisulistiorini et al., 2015). Unfortunately, an 364
Aerodyne ACSM or AMS was not available at the BHM site, precluding confirmation of that 365
IEPOX-derived SOA mass at BHM might account for 14% (on average) of the total OM mass. 366
Levoglucosan, a biomass-burning tracer, averaged 1% of total OM with spikes up to 8%, the same 367
level measured for 2-methylthreitol and (E)-2-methylbut-3-ene-1,2,4-triol (Table 3). The ratio of 368
average levoglucosan at BHM relative to CTR was 5.4 suggesting significantly more biomass 369
burning impacting the BHM site. 370
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IEPOX- and MAE/HMML-derived SOA tracers accounted for 18% and 0.4% of the 371
WSOC mass, respectively (Figure S3b), lower than the respective contributions of 24% and 0.7% 372
measured at LRK (Budisulistiorini et al., 2015). 373
Figure S5 shows no diurnal variation for the average day and night concentrations of 374
isoprene-derived SOA tracers. Thus cooler nighttime temperatures also do not appear to enhance 375
gas-to-particle partitioning at the BHM site. Figures S6 and S7 show the variation of isoprene-376
derived SOA tracers during intensive sampling periods. The highest concentrations were usually 377
observed in samples collected from 4 pm – 7 pm, local time; however, no statistical significance 378
were observed between intensive periods. This observation illustrates the importance of the higher 379
time-resolution of the tracer data during intensive sampling periods over course of the campaign 380
(Table S2-S6). An additional consequence of the intensive sampling periods was resolution of a 381
significant correlation between isoprene SOA tracers and O3 to be discussed in more detail in 382
section 3.3.2. 383
3.3 Influence of anthropogenic emissions on isoprene-derived SOA 384
3.3.1 Effects of reactive nitrogen-containing species 385
During the campaign, no isoprene-derived SOA tracers, including MAE/HMML-derived 386
OS and 2-MG, correlated with NOx or NOy (r2 = 0, n = 120). This is inconsistent with the current 387
understanding of SOA formation from isoprene oxidation pathways under high-NOx conditions, 388
which proceeds through uptake of MAE (Lin et al., 2013a), and, as recently suggested, HMML 389
(Nguyen et al., 2015), to yield 2-MG and its OS derivative. Plume age, as a ratio of NOx:NOy,, in 390
this study was highly correlated with O3 (r2 = 0.79, n = 120). This correlation might be explained 391
by the photolysis of NO2, which is abundant due to traffic at the urban ground site, resulting in 392
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formation of tropospheric O3. A negative correlation coefficient (r = - 0.47, n = 120) between 393
plume age and 2-MG abundance was found, suggesting that formation of some 2-MG may be 394
associated with ageing of air masses. 395
A previous study supported a major role for NO3 in the nighttime chemistry of isoprene 396
(Ng et al., 2008). Correlation of IEPOX- and MAE/HMML-derived SOA with nighttime NO2, O3, 397
and P[NO3] were examined in this study (Figures 3 and 4). As shown in Figure 3f, a moderate 398
correlation between MAE/HMML-derived SOA and nighttime P[NO3] (r2 = 0.57, n = 40) was 399
observed. The regression analysis revealed a significant correlation at the 95% confidence interval 400
(p-value < 0.05) (Table S7). This finding suggests that some MAE/HMML-derived SOA may form 401
locally from the reaction of isoprene with NO3 radical at night. A field study reported a peak 402
isoprene mixing ratio in early evening (Starn et al., 1998) as the PBL height decreases at night. As 403
a result, lowering PBL heights could concentrate the remaining isoprene, NO2, and O3 that can 404
continue to react during the course of the evening. 2-MG formation has been reported to be NO2-405
dependent via the formation and further oxidation of MPAN (Surratt et al., 2006; Chan et al., 406
2010). Hence, decreasing PBL may be related to nighttime MAE/HMML-derived SOA formation 407
through isoprene oxidation by both P[NO3] and NO2. 408
Although P[NO3] depends on both NO2 and O3 levels, O3 correlates moderately with 409
MAE/HMML-derived SOA tracers during day (r2 = 0.48, n = 75), but not at night (r2 = 0.08, n = 410
45). The effect of O3 on isoprene-derived SOA formation during daytime will be discussed further 411
in section 3.3.2. NO2 levels correlate only weakly with MAE/HMML-derived SOA tracers, (r2 = 412
0.26, n = 45) indicating that NO2 levels alone do not explain the moderate correlation of P[NO3] 413
with these tracers. To our knowledge, correlation of P[NO3] with high-NOx SOA tracers has not 414
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been observed in previous field studies., indicating that further work is needed to examine the 415
potential role of nighttime NO3 radicals in forming these SOA tracers. 416
As shown in Figure 4f, IEPOX-derived SOA was weakly correlated (r2 = 0.26, n = 40) with 417
nighttime P[NO3]. The correlation appears to be driven by the data at the low end of the scale and 418
could therefore be misleading. However, Schwantes et al. (2015) demonstrated that NO3-initaited 419
oxidation of isoprene yields isoprene nitrooxy hydroperoxides (INEs) through nighttime reaction: 420
RO2 + HO2, which on further oxidation yielded isoprene nitrooxy hydroxyepoxides (INHEs). The 421
INHEs undergo reactive uptake onto acidic sulfate aerosol to yield SOA constituents similar to 422
those of IEPOX-derived SOA. The present study raises the possibility that a fraction of IEPOX-423
derived SOA comes from NO3-initiated oxidation of isoprene at night. The work of Ng et al. 424
(2008) does not explain the weak association we observe here between IEPOX-derived SOA 425
tracers and P[NO3] as a consequence of the reactions RO2 + RO2 and RO2 + NO3 reactions 426
dominating in those experiments. It is now thought that RO2 + HO2 should dominate in field studies 427
(Schwantes et al., 2015; Paulot et al., 2009). 428
3.3.2 Effect of O3
429
During the daytime, O3 was moderately correlated (r2 = 0.48, n = 75) with total 430
MAE/HMML-derived SOA (Figure 3b). This correlation was stronger (r2 = 0.72, n = 30, p-value 431
< 0.05, Table S7) when filters taken during regular daytime sampling periods are considered, 432
suggesting that formation of MACR (a precursor to MAE and HMML) (Lin et al., 2013b; Nguyen 433
et al., 2015) was enhanced by oxidation of isoprene by O3 (Kamens et al., 1982). O3 was not 434
correlated (r2 = 0.08, n = 45) with MAE/HMML-derived SOA at night (Figure 3e). The latter 435
finding is consistent with the absence of photolysis to drive the production of O3. However, 436
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residual O3 may play an important role at night to form MAE/HMML-derived SOA via the P[NO3] 437
pathway discussed in section 3.3.1. 438
O3 was not correlated (r2= 0.10, n = 75) with IEPOX-derived SOA during daytime (Figure 439
4b), but weakly correlated with 2-methylerythritol (r2 = 0.25, n = 30) as shown in Table S2, 440
especially during intensive 3 sampling periods (r2 = 0.34, n = 15, Table S5). An important 441
observation with regard to this result is that no correlation has been found between O3 and 2-442
methyltetrols (r2 < 0.01) in previous field studies (Lin et al., 2013b; Budisulistiorini et al., 2015). 443
Isoprene ozonolysis yielded 2-methyltetrols in chamber studies in the presence of acidified sulfate 444
aerosol (Riva et al., 2015) but C5-alkene-triols were not formed by this pathway. The greatest 445
abundance of isoprene-derived SOA tracers in daytime samples was generally observed in 446
intensive 3 samples; however, there was no statistical significance observed between intensive 447
samples. The moderate correlation (r2 = 0.34, n = 15, p-value < 0.05) between O3 and the 2-448
methyltetrols observed in intensive 3 samples occurred when O3 reached maximum levels, 449
suggesting that ozonolysis of isoprene plays a role in 2-methyltetrol formation. Lack of correlation 450
between O3 and C5-alkene triols during intensive 3 sampling (r2 = 0.10, n = 15) supports this 451
contention. A putative pathway is formation of hydroperoxides that partition to wet acidic sulfate 452
aerosols and react further to yield 2-methyltetrols. Additional work using authentic standards is 453
needed to validate this tentative hypothesis. 454
3.3.3 Effect of particle SO42-
455
SO42- was moderately correlated with IEPOX-derived SOA (r2 = 0.36, n = 117) and 456
MAE/HMML-derived SOA (r2 = 0.33, n = 117) at the 95% confidence interval as shown in Table 457
S7. The strength of the correlations was consistent with studies at other sites across the 458
Southeastern U.S. (Budisulistiorini et al., 2013; Lin et al., 2013b; Budisulistiorini et al., 2015; Xu 459
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et al., 2015). Aerosol surface area provided by acidic SO42- has been demonstrated to control the 460
uptake of isoprene-derived epoxides (Lin et al., 2012; Gaston et al., 2014; Nguyen et al., 2014; 461
Riedel et al., 2015). 462
Furthermore, SO42- is proposed to enhance IEPOX-derived SOA formation by providing 463
particle water (H2Optcl) required for IEPOX uptake (Xu et al., 2015). Aerosol SO42- also promotes 464
acid-catalyzed ring-opening reactions of IEPOX by H+, proton donors such as NH4+, and 465
nucleophiles (e.g., H2O, SO42-, or NO3-) (Surratt et al., 2010; Nguyen et al., 2014). Since SO42- 466
tends to drive both particle water and acidity (Fountoukis and Nenes, 2007), the extent to which 467
each influences isoprene SOA formation during field studies remains unclear. Multivariate linear 468
regression analysis on SOAS data from the CTR site and the SCAPE dataset revealed a statistically 469
significant positive linear relationship between SO42- and the isoprene (IEPOX)-OA factor 470
resolved by positive matrix factorization (PMF). On the basis of this analysis the abundance of 471
SO42- was concluded to control directly the isoprene SOA formation over broad areas of the 472
Southeastern U.S. (Xu et al., 2015), consistent with previous reports (Lin et al., 2013; 473
Budisulistiorini et al., 2013; Budisulistiorini et al., 2015). Another potential pathway for SO42-
474
levels to enhance isoprene SOA formation is through salting-in effects; however, systematic 475
investigations of this effect are lacking and further studies are warranted (Xu et al., 2015). 476
3.3.4 Effect of aerosol acidity 477
The aerosol at BHM was acidic throughout the SOAS campaign (pH range 1.60  1.94, 478
average 1.76) in accord with a study by Guo et. al. (2014) that found aerosol pH ranging from 479
0  2 throughout the southeastern U.S. However, no correlation of pH with isoprene SOA 480
formation was observed at BHM, also consistent with previous findings using the thermodynamic 481
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models to estimate aerosol acidity in many field sites across the southeastern U.S. region, including 482
Yorkville, GA (YRK) (Lin et al., 2013b), Jefferson Street, GA (JST) (Budisulistiorini et al., 2013), 483
and LRK (Budisulistiorini et al., 2015). However, it is important to point out that the lack of 484
correlation between SOA tracers and acidity may stem from the small variations in aerosol acidity 485
throughout the campaign. Gaston et al. (2014) and Riedel et al. (2015) recently demonstrated that 486
an aerosol pH < 2 at atmospherically-relevant aerosol surface areas would allow reactive uptake 487
of IEPOX onto acidic (wet) sulfate aerosol surfaces to be competitive with other loss processes 488
(e.g., deposition and reaction of IEPOX with OH). In fact, it was estimated that under such 489
conditions IEPOX would have a lifetime of ~ 5 hr. The constant presence of acidic aerosol has 490
also been observed at other field sites in the southeastern U.S. (Budisulistiorini et al., 2013; 491
Budisulistiorini et al., 2015; Xu et al., 2015), supporting a conclusion that acidity is not the limiting 492
variable in forming isoprene SOA. 493
3.4 Comparison among different sampling sites during 2013 SOAS campaign 494
Table 5 summarizes the mean concentration and contribution of each isoprene SOA tracer 495
at BHM, CTR, and LRK. BHM is an industrial-residential area, LRK and CTR are rural areas, 496
although LRK is influenced by a diurnal upslope/downslope cycle of air from an urban locality 497
(Knoxville) (Tanner et al., 2005). IEPOX-derived SOA was predominant at all three sites during 498
the SOAS campaign, while MAE/HMML-derived SOA constituted a minor contribution. The 499
average ratio of 2-methyltetrols to C5-alkene triols at BHM was 2.2, nearly double that of CTR 500
(1.3) and LRK (1.1). Although 2-methyltetrols and C5-alkene triols are considered to form readily 501
from the acid-catalyzed reactive uptake and multiphase chemistry of IEPOX (Edney et al., 2005; 502
Surratt et al., 2006), Riva et al. (2015) recently demonstrated that only 2-methyltetrols can be 503
formed via isoprene ozonolysis in the presence of acidic sulfate aerosol. The higher levels of the 504
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2-methyltetrols observed at the urban BHM site indicates a likely competition between the IEPOX 505
uptake and ozonolysis pathways. Together, these findings suggest that urban O3 may play an 506
important role in forming the 2-methyltetrols observed at BHM. There were notable trends found 507
among the three sites: (1) average C5-alkene triol concentrations were higher at CTR (214.1 ng m-
508
3) than at BHM (169.7 ng m-3) and LRK (144.4 ng m-3); (2) average isomeric 3-MeTHF-diol 509
concentrations were lower at CTR (0.2 ng m-3) than the BHM (15.4 ng m-3) or LRK (4.4 ng m-3) 510
sites. Except for the 2-methyltetrols, reasons for the differences observed for the other tracers 511
between sites remains unclear and warrant future investigations. 512
513
4. Conclusions 514
This study examined isoprene SOA tracers in PM2.5 samples collected at the BHM ground 515
site during the 2013 SOAS campaign and revealed the complexity and potential multitude of 516
chemical pathways leading to isoprene SOA formation. Isoprene SOA contributed up to ~20% 517
(~7% on average) of total OM mass. IEPOX-derived SOA tracers were responsible for 92% of the 518
total quantified isoprene SOA tracer mass, with 2-methyltetrols being the major component (47%). 519
Differences in the relative contributions of IEPOX- and MAE/HMML-derived SOA tracers at 520
BHM and the rural CTR and LRK sites (Budisulistiorini et al., 2015) during the 2013 SOAS 521
campaign, support suggestions that anthropogenic emissions effect isoprene SOA formation. The 522
correlation between 2-methyltetrols and O3 at BHM is in accord with work by Riva et al. (2015), 523
demonstrating a potential role of O3 in generating isoprene-derived SOA in addition to the 524
currently accepted IEPOX multiphase pathway. 525
At BHM, the statistical correlation of particulate SO42- with IEPOX- (r2 = 0.36, n = 117, p 526
< 0.05) and MAE-derived SOA tracers (r2 = 0.33, n = 117, p < 0.05) suggests that SO42- plays a 527
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role in isoprene SOA formation. Although none of isoprene-derived SOA tracers correlated with 528
gas-phase NOx and NOy, MAE/HMML-derived SOA tracers correlated with nighttime P[NO3] (r2 529
= 0.57, n = 400), indicating that NO3 may affect local MAE/HMML-derived SOA formation. 530
Nighttime P[NO3] was weakly correlated (r2 = 0.26, n = 40) with IEPOX-derived SOA tracers, 531
lending some support to recent work by Schwantes et al. (2015) showing that isoprene + NO3 532
yields INHEs that can by undergo reactive uptake to yield IEPOX tracers and contribute to IEPOX-533
derived SOA tracer loadings. In addition, nighttime 2-methyltetrol levels in the urban atmosphere 534
deviate from the conventional understanding of isoprene SOA formation in terms of segregated 535
NOx dependent regimes. The correlation of daytime O3 with MAE/HMML-derived SOA and with 536
2-methyltetrols offers a new insight into influences on isoprene SOA formation. Notably, O3 has 537
not been reported to correlate with isoprene-derived SOA tracers in previous field studies (Lin et 538
al., 2013b; Budisulistiorini et al., 2015). In this study, the strong correlation (r2 = 0.72, n = 30) at 539
the 95% confidence interval of O3 with MAE/HMML-derived SOA tracers during the regular 540
daytime sampling schedule indicates that O3 likely oxidizes some isoprene to MACR as precursor 541
of 2-MG at BHM. The weak correlation (r2 = 0.16, n = 75) between O3 and 2-methyltetrols early 542
in the day as well as the better correlation (r2 = 0.34, n = 15) later in the day (intensive 3, 4-7 PM 543
local time) are consistent with recent laboratory studies demonstrating that 2-methyltetrols can be 544
formed via isoprene ozonolysis in the presence of acidified sulfate aerosol (Riva et al., 2015). 545
Although urban O3 and nighttime P[NO3] may have a role in local formation of 546
MAE/HMML- and IEPOX-derived SOA tracers at BHM, this does not appear to explain the 547
majority of the SOA tracers, since no significant day-night variation of the entire group of tracers 548
was observed during the campaign. The majority of IEPOX-derived SOA was likely formed when 549
isoprene SOA precursors (IEPOX) were generated upwind and transported to the BHM site. Wind 550
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26
directions during the campaign are consistent with long-range transport of isoprene SOA 551
precursors from southwest of the site, which is covered by forested areas. The absence of a 552
correlation of aerosol acidity with MAE/HMML- and IEPOX-derived SOA tracers indicates that 553
acidity is not the limiting variable that controls formation of these compounds. However, the lack 554
of correlation between SOA tracers and acidity may stem from nearly invariant aerosol acidity 555
throughout the campaign. Hence, despite laboratory studies demonstrating that aerosol acidity can 556
enhance isoprene SOA formation (Surratt et al., 2007; Surratt et al., 2010; Lin et al., 2012), the 557
effect may not be significant in the southeastern U.S. during the summer months due to the constant 558
acidity of aerosols. Future work should examine how well current models can predict the isoprene 559
SOA levels observed during this study, especially since urban emissions are directly present. 560
Furthermore, explicit models are now available to predict the isoprene SOA tracers measured here 561
(McNeill et al., 2012; Pye et al., 2013), which will allow the modeling community to test the 562
current parameterizations that are used to capture the enhancing effect of anthropogenic pollutants 563
on isoprene-derived SOA formation. In addition, the significant correlations of isoprene-derived 564
SOA tracers with P[NO3] observed during this study indicate a need to better understand nighttime 565
chemistry of isoprene. Lastly, although O3 appears to have an enhancing effect on isoprene-566
derived SOA tracers, the intermediates are unknown. Hydroperoxides suggested by Riva et al. 567
(2015) may be key, but chamber experiments with authentic precursors are needed to test this 568
hypothesis. 569
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27
Acknowledgements 570
This work was funded by the U.S. Environmental Protection Agency (EPA) through grant number 571
835404. The contents of this publication are solely the responsibility of the authors and do not 572
necessarily represent the official views of the U.S. EPA. Further, the U.S. EPA does not endorse 573
the purchase of any commercial products or services mentioned in the publication. The authors 574
would also like to thank the Electric Power Research Institute (EPRI) for their support. This study 575
was supported in part by the National Oceanic and Atmospheric Administration (NOAA) Climate 576
Program Office’s AC4 program, award number NA13OAR4310064. The authors thank the 577
Camille and Henry Dreyfus Postdoctoral Fellowship Program in Environmental Chemistry for 578
their financial support. The authors thank Louisa Emmons and Christoph Knote for their assistance 579
with chemical forecasts made available during the SOAS campaign. We would like to thank 580
Annmarie Carlton, Joost deGouw, Jose Jimenez, and Allen Goldstein for helping to organize the 581
SOAS campaign and coordinating communication between ground sites. UPLC/ESI-HR-Q-582
TOFMS analyses were conducted in the UNC-CH Biomarker Mass Facility located within the 583
Department of Environmental Sciences and Engineering, which is a part of the UNC-CH Center 584
for Environmental Health and Susceptibility supported by National Institute for Environmental 585
Health Sciences (NIEHS), grant number 5P20-ES10126. WSOC measurements at the University 586
of Iowa were supported through EPA STAR grant 8354101. The authors thank Theran Riedel for 587
useful discussions. We also thank SCG Chemicals Co., Ltd., Siam Cement Group, Thailand, for 588
the full support for W. Rattanavaraha attending UNC, Chapel Hill. 589
590
591
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28
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Table 1. Sampling schedule during SOAS at the BHM ground site. 810
No. of samples/ day Sampling schedule Dates
2 (regular) Day: 8 am – 7 pm June 1 – June 9
Night: 8 pm – 7 am next day June 13,
June 17 – June 28,
July 2- July 9,
July 15
4 (intensive) Intensive 1: 8 am – 12 pm, June 10 – June 12,
Intensive 2: 1 pm – 3 pm, June 14 – June 16,
Intensive 3: 4 pm – 7 pm, June 29 – June 30,
Intensive 4: 8 pm – 7 am next day July 1,
July 9 – July 14
811
Table 2. Summary of collocated measurements of meteorological variables, gaseous species, and 812
PM2.5 constituents. 813
814 Category Condition Average
SD
Minimum
Maximum
Meteorology Rainfall (in) 0.1
0.2
0.0
1.4
Temp (°C) 26.4
3.0
20.5
32.7
RH (%) 71.5
15.0
36.9
96.1
BP (mbar) 994.2
3.9
984.2
1002.4
SR (W m-2) 303.7
274.5
7.0
885.0
Trace gas (ppbv) O3 31.1
14.8
8.3
62.2
CO 208.7
72.0
99.6
422.9
SO2 0.9
0.8
0.1
3.7
NO 1.3
1.2
0.1
7.0
NO2 6.6
5.1
1.0
22.7
NOx 7.8
6.0
1.3
29.7
NOy 9.1
5.8
2.2
30.4
HNO3 0.3
0.2
0.1
1.0
NH3 1.9
0.8
0.7
4.0
PM2.5 (μg m
-
3
)
OC 7.2
3.2
1.4
14.9
EC 0.6
0.5
0.1
2.7
WSOC 4.0
1.8
0.5
7.5
SO42- 2.0
0.9
0.4
4.9
NO3- 0.1
0.1
0.0
0.8
NH4+ 0.7
0.3
0.2
1.2
Aerosol pH 1.8
0.1
1.6
1.9
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37
Table 3. Summary of isoprene-derived SOA tracers measured by GC/EI-MS and UPLC/ESI-HR-QTOFMS 815
816
817
818
819
820
821
SOA tracers m/z
Frequency
of detection
(%)a
Max
concentration
(ng/m3)
Mean
concentration
(ng/m3)
Isoprene SOA
Mass fraction
(%)b
% of
total OMc
Measured by GC/EI
-
MS
2
-
methylerythritol
219
9
9.2
1048.
9
26
9.0
33.
8
2.
7
2-methylthreitol 219 100.0
388.9
107.3
13.5 1.1
(E)-2-methylbut-3-ene-1,2,4-triol 231 96.7
878.9
112.7
14.2 1.1
(Z)
-
2
-
methylbut
-
3
-
ene
-
1,2,4
-
triol
231
9
5.8
287.
8
38.9
4.
9
0.
4
2-methylbut-3-ene-1,2,3-triol 231 94.2
503.3
28.9
3.6 0.3
2
-
methylglyceric acid
219
9
3.3
35.
0
10.
8
1.
4
0.
1
cis-3-MeTHF-3,4-diol 262 22.5
98.9
6.9
0.9 0.1
trans-3-MeTHF-3,4-diol 262 10.0
137.6
8.6
1.1 0.1
IEPOX
-
derived dimer
333
1
0
.0
2.
2
0.
0
0.
0
0.
0
Levoglucosan 204 100.0
922.6
98.7
- 1.0
Measured by UPLC/ESI-HR-
QTOFMS
IEPOX-derived OSs
C
5
H
11
O
7
S
-
215 100.0
864.9
164.5
20.7 1.6
C
10
H
21
O
10
S
-
333
1
.7
0.
3
0.0
0.0
0.
0
MAE-derived OS
C
4
H
7
O
7
S
-
199 100.0
35.7
7.2
1.9 0.1
GA sulfate
C
2
H
3
O
6
S
-
155 100.0
75.2
26.2
3.3 0.3
Methylglyoxal-derived OS
C
3
H
5
O
6
S
-
169 97.5
10.5
2.7
0.3 0.0
Isoprene-derived OSs
C
5
H
7
O
7
S
-
211 97.5
5.2
1.4
0.2 0.0
C
5
H
10
NO
9
S
-
260
9
0
.0
3.
9
0.
3
0.0
0.
0
C
5
H
9
N
2
O
11
S
-
305 5.0
3.3
2.9
0.4 0.0
Hydroxyacetone-derived OS
C
2
H
3
O
5
S
-
139 30.8
2.6
0.2
0.0 0.0
a
Total filters = 120
b Mass fraction is the contribution of each species among total known isoprene-derived SOA mass detected by GC/EI
MS and UPLC/ESI-HR-QTOFMS
c OM/OC = 1.6
d
d
d
d
d
d
d
d
d
OA tracers quantified by authentic standards
e SOA tracers quantified by 2-methyltetrols as a surrogate standard
f SOA tracer quantified by IEPOX-derived OS (m/z 215) as a surrogate standard
g SOA tracers quantified by propyl sulfate as a surrogate standard
d
e
e
e
e
f
g
g
g
m/z
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38
Table 4. Overall correlation (r2) of isoprene-derived SOA tracers and collocated measurements at 822
BHM during 2013 SOAS campaign. 823
SOA tracers CO O 3 NOx NOy SO2 NH3 SO4 NO3 NH4 OC WSOC pH
MAE/HMML-derived SOA
tracers 0.07 0.26 0.00 0.01 0.06 0.11 0.33 0.01 0 .18 0.47 0.20 0.00
2-methylglyceric acid 0.01 0.26 0.01 0.00 0.01 0.07 0.10 0.00 0.06 0.19 0.02 0.00
MAE-derived OS 0.10 0.14 0.00 0.02 0.07 0.09 0.38 0.01 0.18 0.32 0.23 0.01
IEPOX-derived SOA
tracers
0.04 0.05 0.00 0.01 0.05 0.01 0.36 0.00 0.21 0.24 0.12 0.00
2-methylerythritol 0.00 0.16 0.03 0.02 0.01 0.00 0.30 0.02 0.1 8 0.18 0.19 0.00
2-methylthreitol 0.00 0.13 0.02 0.03 0.02 0.00 0.20 0.01 0.16 0.17 0.15 0.00
(E)-2-methylbut-3-ene-1,2,4-triol 0.07 0.00 0.02 0 .01 0.07 0.00 0.15 0.00 0.19 0.11 0.04 0.00
(Z)-2-methylbut-3-ene-1,2,4-triol 0.04 0.00 0.00 0.00 0.06 0.00 0.28 0.00 0.20 0.04 0.00 0.00
2-methylbut-3-ene-1,2,3-triol 0.02 0.00 0.03 0.00 0.00 0.02 0.32 0.01 0.03 0.17 0.04 0.00
IEPOX-derived OS 0.02 0 .14 0.03 0.00 0.00 0.00 0.27 0.00 0.16 0.29 0.29 0.00
IEPOX dimer 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Other isoprene SOA tracers
GA sulfate
C2H3O6S
-
0.30 0.23 0.01 0.00 0.08 0.09 0.27 0.00 0.19 0.38 0.18 0.00
Methylglyoxal-derived OS
C3H5O6S
-
0.14 0.04 0.02 0.03 0.03 0.07 0.31 0.02 0 .25 0.21 0.24 0.00
Isoprene-derived OSs
C5H7O7S
-
0.01 0.23 0.03 0.01 0.00 0.02 0.21 0.00 0.16 0.31 013 0.00
C5H10NO9S
-
0.17 0.00 0.12 0.14 0.10 0.14 0.31 0.16 0.23 0.20 0.07 0.00
C5H9N2O11S
-
*
0.32 0.71 0.66 0.58 0.42 0.02 0.68 0.50 0.42 0.00 0.50 0.00
Hydroxyacetone-derived OS
C2H3O5S
-
0.02 0.10 0.08 0.07 0.05 0.00 0.00 0.03 0.00 0.01 0.01 0.00
Other tracer
Levoglucosan 0.00 0.09 0.02 0 .01 0.02 0 .00 0.00 0.02 0 .00 0.08 0.04 0.01
* Found only in 6 of 120 filters 824
The correlations in this table are positive. 825
826
827
828
829
830
831
832
833
Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2015-983, 2016
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39
Table 5. Summary of isoprene-derived SOA tracers from the three SOAS ground sites: BHM, 834
CTR, and LRK. 835
836
SOA tracers
Urban Rural
BHM CTR LRK
Mean
(ng m-3)
Average
amount
detected
tracers
(%)
Mean
(ng m-3)
Average
amount
detected
tracers
(%)
Mean
(ng m-3)
Average
amount
detected
tracers
(%)
MAE/HMML derived SOA
MAE/HMML
-
derived
OS
7.2
1.1
10.2
1.3
8.2
.1 8
2
-
methylglyceric acid
10.4
1.7
5.1
0.7
7.5
.1 6
IEPOX derived SOA
IEPOX
-
derived
OS
164.5
24.3
207.1
26.8
139.2
.30 3
IEPOX
-
derived dimer
OS
0.04
0.00
0.7
0.1
1.1
.0 2
2
-
methylerythritol
266.7
37.9
204.8
26.5
120.7
.26 3
2
-
methylthreitol
107.3
15.8
73.7
9.5
42.4
.9 2
(E)
-
2
-
methylbut
-
3
-
ene
-
1,2,4
-
triol
109.0
12.3
137.3
17.8
98.8
.21 5
(Z)
-
2
-
methylbut
-
3
-
ene
-
1,2,4
-
triol
37.3
4.1
50.7
6.6
29.1
.6 1
2
-
methylbut
-
3
-
ene
-
1,2,3
-
triol
23.4
2.5
26.1
3.4
16.5
.3 6
trans
-
3
-
MeTHF
-
3,4
-
diol
8.6
1.0
0.0
0.0
2.7
.0 6
cis
-
3
-
MeTHF
-
3,4
-
diol
6.8
1.0
0.2
0.0
1.7
.0 4
837
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Manuscript under review for journal Atmos. Chem. Phys.
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40
838
839
840
841
842
843
844
845
846
847
848
849
Figure 1. Wind rose illustrating wind direction during the campaign at the BHM site. Bars indicate 850
direction of incoming wind, with 0 degrees set to geographic north. Length of bar size indicates 851
frequency with color segments indicating the wind speed in m s-1. 852
Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2015-983, 2016
Manuscript under review for journal Atmos. Chem. Phys.
Published: 19 January 2016
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41
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
Figure 2. Time series of (a) meteorological data, (b) trace gases, (c) PM2.5 constituents, (d) 874
MAE/HMML-derived SOA tracers and (e) IEPOX-derived SOA tracers during the 2013 SOAS 875
campaign at the BHM site. 876
2000
1500
1000
500
0
Mass conc. (ng m-3)
11/06/2013 21/06/2013 01/07/2013 11/07/2013
Date and Time (Local)
100
80
60
40
20
0
Mass conc. (ng m-3)
90
80
70
60
50
40
RH (%)
32
28
24
Temp (C)
4
3
2
1
0
Rainfall (inch)
50
40
30
20
10
0
SO2, NOx, NH3 (ppb)
400
200
0
-200
-400
CO (ppb)
120
80
40
0
O3 (ppb)
Total MAE/HMML-SOA trace rs
(a)
Meteor
ology
(b) Trace gas
(c) PM
2.5
composition
(d) MAE/HMML-derived SOA
(e) IEPOX-derived SOA
RH, Temperature, Rainfall
CO
O3, NOx, SO2, NH3
OC, WSOC
SO4, NH4, NO3
2-methylglyceric aid (2-MG)
MAE-derived organosulfate (MAE-OS)
2-methylthrietol
2-methylerythritol
(z)-2-methylbut-3-ene-1,2,4,triol
2-methylbut-3-ene-1,2,3-triol
(E)-2-methylbut-3-ene-1,2,4-triol
IEPOX-derived organosulfate
Total IEPOX-SOA tracer s
10
8
6
4
2
0
SO4, NO3, NH4 (ug m-3
)
-20
-10
0
10
20
OC, WSOC (ug m-3)
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42
877
878
879
880
881
882
(a) (b) (c) 883
884
885
886
887
888
889
890
(d) (e) (f) 891
892
Figure 3. Correlation of MAE-derived SOA tracers with (a) daytime NO2, (b) daytime O3, (c) 893
daytime P[NO3], (d) nighttime NO2, (e) nighttime O3, and (f) nighttime P[NO3]. Nighttime P[NO3] 894
correlation suggests that NO3 radical chemistry could explain some fraction of the MAE/HMML-895
derived SOA tracer concentrations. 896
897
898
5x106
4321
Daytime P[NO3]
r2 = 0.07
6050403020
Daytime O3 (ppb)
r2 = 0.48
100
80
60
40
20
0
Mass conc. (ng m-3)
8642
Daytime NO2 (ppb)
r2 = 0.04
100
80
60
40
20
0
Mass conc. (ng m-3)
2015105
Nighttime NO2 (ppb)
r2 = 0.26
6050403020100
Nighttime O3 (ppb)
r2 = 0.08
6x106
5432
Nighttime P[NO3]
r2 = 0.57
Daytime NO
2
(ppb)
D
aytime
O
3
(ppb)
D
aytime P[NO
3
]
Nighttime
NO
2
(ppb)
Nighttime
O
3
(ppb)
Nighttime P[NO
3
]
Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2015-983, 2016
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43
899
900
901
902
903
904
(a) (b) (c) 905
906
907
908
909
910
911
(d) (e) (f) 912
Figure 4. Correlation of IEPOX-derived SOA tracers with (a) daytime NO2, (b) daytime O3 , (c) 913
daytime P[NO3], (d) nighttime NO2, (e) nighttime O3, and (f) nighttime P[NO3]. Nighttime P[NO3] 914
correlation suggests that NO3 radical chemistry could explain some fraction of the IEPOX-derived 915
SOA tracer concentrations. 916
2000
1500
1000
500
0
Mass conc. (ng m-3)
8642
r2 = 0.00
6050403020
r2 = 0.10
40353025201510
r2 = 0.09
2000
1500
1000
500
0
Mass conc. (ng m-3)
2015105
r2 = 0.04
6x10
6
5432
r2 = 0.26
5x10
6
4321
r2 = 0.06
Daytime NO
2
(ppb)
Daytime O
3
(ppb)
Daytime P[NO
3
]
Nighttime NO
2
(ppb)
Nighttime O
3
(ppb)
Nighttime P[NO
3
]
Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2015-983, 2016
Manuscript under review for journal Atmos. Chem. Phys.
Published: 19 January 2016
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