<html>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</html>
Creators: name: Zhang, Zhenfa
name: King, Laura
name: Lin, Ying-Hsuan
name: Neff, Miranda E.
name: Budisulistiorini, Sri Hapsari
name: Guo, Hongyu
name: Weber, Rodney J.
name: Offenberg, John H.
name: Gold, Avram
name: Baumann, Karsten
name: Shaw, Stephanie L.
name: Rattanavaraha, Weruka
name: Surratt, Jason D.
name: Chu, Kevin
name: Edgerton, Eric S.
name: Riva, Matthieu
name: Stone, Elizabeth A.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2016

description: <html>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 (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>x</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 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 (SO<sub>4</sub><sup>2-</sup>) (r<sup>2</sup> = 0.34, n = 117), but not with NO<sub>x</sub>. Moderate correlation between methacrylic acid epoxide and hydroxymethyl-methyl-α-lactone (MAE/HMML)-derived SOA tracers and nitrate radical production (P[NO<sub>3</sub>]) (r<sup>2</sup> = 0.57, n = 40) were observed during nighttime, suggesting a potential role of NO<sub>3</sub> radical in forming this SOA type. However, the nighttime correlation of these tracers with nitrogen dioxide (NO<sub>2</sub>) (r<sup>2</sup> = 0.26, n = 40) was weaker. Ozone (O<sub>3</sub>) correlated strongly with MAE/HMML-derived tracers (r<sup>2</sup> = 0.72, n = 30) and moderately with 2-methyltetrols (r<sup>2</sup> = 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.</html>