Assessing the impact of anthropogenic pollution on isoprene-derived secondary organic aerosol formation in PM_2.5 collected from the Birmingham, Alabama, ground site during the 2013 Southern Oxidant and Aerosol Study

• Main Authors: W. Rattanavaraha, K. Chu, S. H. Budisulistiorini, M. Riva, Y.-H. Lin, E. S. Edgerton, K. Baumann, S. L. Shaw, H. Guo, L. King, R. J. Weber, M. E. Neff, E. A. Stone, J. H. Offenberg, Z. Zhang, A. Gold, J. D. Surratt
• Format: Article
• Language: English
• Published: Copernicus Publications 2016-04-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/16/4897/2016/acp-16-4897-2016.pdf

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_2.5). Laboratory studies have revealed that anthropogenic pollutants, such as sulfur dioxide (SO₂), oxides of nitrogen (NO_<i>x</i>), 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_2.5 remain unclear. As one aspect of an investigation to examine how anthropogenic pollutants influence isoprene-derived SOA formation, high-volume PM_2.5 filter samples were collected at the Birmingham, Alabama (BHM), ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). Sample extracts were analyzed by gas chromatography–electron ionization-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) to identify known isoprene SOA tracers. Tracers quantified using both surrogate and authentic standards were compared with collocated gas- and particle-phase data as well as meteorological data provided by the Southeastern Aerosol Research and Characterization (SEARCH) network to assess the impact of anthropogenic pollution on isoprene-derived SOA formation. Results of this study reveal that isoprene-derived SOA tracers contribute a substantial mass fraction of organic matter (OM) ( ∼  7 to  ∼  20 %). Isoprene-derived SOA tracers correlated with sulfate (SO₄²⁻) (<i>r</i>² = 0.34, <i>n</i> = 117) but not with NO_<i>x</i>. 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[NO₃]) (<i>r</i>² = 0.57, <i>n</i> = 40) were observed during nighttime, suggesting a potential role of the NO₃ radical in forming this SOA type. However, the nighttime correlation of these tracers with nitrogen dioxide (NO₂) (<i>r</i>² = 0.26, <i>n</i> = 40) was weaker. Ozone (O₃) correlated strongly with MAE/HMML-derived tracers (<i>r</i>² = 0.72, <i>n</i> = 30) and moderately with 2-methyltetrols (<i>r</i>² = 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.