Assessing the impact of anthropogenic pollution on isoprene-derived secondary organic aerosol formation in PM<sub>2.5</sub> collected from the Birmingham, Alabama, ground site during the 2013 Southern Oxidant and Aerosol Study
In the southeastern US, substantial emissions of isoprene from deciduous trees undergo atmospheric oxidation to form secondary organic aerosol (SOA) that contributes to fine particulate matter (PM<sub>2.5</sub>). Laboratory studies have revealed that anthropogenic pollutants, such as...
Main Authors: | , , , , , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-04-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/16/4897/2016/acp-16-4897-2016.pdf |
Summary: | In the southeastern US, substantial emissions of isoprene from deciduous
trees undergo atmospheric oxidation to form secondary organic aerosol (SOA)
that contributes to fine particulate matter (PM<sub>2.5</sub>). Laboratory studies
have revealed that anthropogenic pollutants, such as sulfur dioxide
(SO<sub>2</sub>), oxides of nitrogen (NO<sub><i>x</i></sub>), 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 PM<sub>2.5</sub> remain unclear. As one aspect of an investigation to
examine how anthropogenic pollutants influence isoprene-derived SOA
formation, high-volume PM<sub>2.5</sub> filter samples were collected at the
Birmingham, Alabama (BHM), ground site during the 2013 Southern Oxidant and
Aerosol Study (SOAS). Sample extracts were analyzed by gas chromatography–electron
ionization-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and
ultra performance liquid chromatography coupled to electrospray ionization
high-resolution quadrupole time-of-flight mass spectrometry
(UPLC/ESI-HR-QTOFMS) to identify known isoprene SOA tracers. Tracers
quantified using both surrogate and authentic standards were compared with
collocated gas- and particle-phase data as well as meteorological data
provided by the Southeastern Aerosol Research and Characterization (SEARCH)
network to assess the impact of anthropogenic pollution on isoprene-derived
SOA formation. Results of this study reveal that isoprene-derived SOA tracers
contribute a substantial mass fraction of organic matter (OM) ( ∼ 7 to
∼ 20 %). Isoprene-derived SOA tracers correlated with sulfate
(SO<sub>4</sub><sup>2−</sup>) (<i>r</i><sup>2</sup> = 0.34, <i>n</i> = 117) but not with NO<sub><i>x</i></sub>. Moderate
correlations between methacrylic acid epoxide and
hydroxymethyl-methyl-<i>α</i>-lactone (together
abbreviated MAE/HMML)-derived SOA tracers with nitrate radical
production (P[NO<sub>3</sub>]) (<i>r</i><sup>2</sup> = 0.57, <i>n</i> = 40) were observed during
nighttime, suggesting a potential role of the NO<sub>3</sub> radical in forming
this SOA type. However, the nighttime correlation of these tracers with
nitrogen dioxide (NO<sub>2</sub>) (<i>r</i><sup>2</sup> = 0.26, <i>n</i> = 40) was weaker. Ozone
(O<sub>3</sub>) correlated strongly with MAE/HMML-derived tracers (<i>r</i><sup>2</sup> = 0.72,
<i>n</i> = 30) and moderately with 2-methyltetrols (<i>r</i><sup>2</sup> = 0.34, <i>n</i> = 15)
during daytime only, suggesting that a fraction of SOA formation could occur
from isoprene ozonolysis in urban areas. No correlation was observed between
aerosol pH and isoprene-derived SOA. Lack of correlation between aerosol
acidity and isoprene-derived SOA is consistent with the observation that
acidity is not a limiting factor for isoprene SOA formation at the BHM site
as aerosols were acidic enough to promote multiphase chemistry of
isoprene-derived epoxides throughout the duration of the study. All in all,
these results confirm previous studies suggesting that anthropogenic
pollutants enhance isoprene-derived SOA formation. |
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ISSN: | 1680-7316 1680-7324 |