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.

Weruka Rattanavaraha,Rodney J Weber,Jason D Surratt,John H Offenberg,Matthieu Riva,Ying-Hsuan Lin,Eric S Edgerton,Laura King,Avram Gold,Karsten Baumann,Hongyu Guo,Miranda E Neff,Elizabeth A Stone,Stephanie L Shaw,Zhenfa Zhang,Sri Hapsari Budisulistiorini,Kevin Chu

doi:10.5194/acp-16-4897-2016

Abstract

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 (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 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 () (r2 = 0.34, n = 117) but not with NOx. Moderate correlations between methacrylic acid epoxide and hydroxymethyl-methyl-α-lactone (together abbreviated MAE/HMML)-derived SOA tracers with nitrate radical production (P[NO3]) (r2 = 0.57, n = 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) (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 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.

Highlights

Fine particulate matter, suspensions of liquid or solid aerosol in a gaseous medium that are less than or equal to 2.5 μm in diameter (PM2.5), play a key role in physical and chemical atmospheric processes
The analysis of PM2.5 was conducted in order to determine quantities of known isoprene secondary organic aerosol (SOA) tracers using collocated air quality and meteorological measurements to investigate how anthropogenic pollutants including NOx, SO2, aerosol acidity, PM2.5 sulfate (SO24−), and O3 affect isoprene SOA formation. These results, along with the results presented from similar studies during the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign, seek to elucidate the chemical relationships between anthropogenic emissions and isoprene SOA formation in order to provide better parameterizations needed to improve the accuracy of air quality models in this region of the US
This study examined isoprene SOA tracers in PM2.5 samples collected at the BHM ground site during the 2013 SOAS campaign and revealed the complexity and potential multitude of chemical pathways leading to isoprene SOA formation

Summary

Introduction

Suspensions of liquid or solid aerosol in a gaseous medium that are less than or equal to 2.5 μm in diameter (PM2.5), play a key role in physical and chemical atmospheric processes They influence climate patterns both directly, through the absorption and scattering of solar and terrestrial radiation, and indirectly, through cloud formation (Kanakidou et al, 2005). As a result of large anthropogenic sources, POA is abundant largely in urban areas Processes such as natural plant growth, biomass burning, and combustion yield volatile organic compounds (VOCs), which have high vapor pressures and can undergo atmospheric oxidation to form secondary organic aerosol (SOA) through gas-to-particle phase partitioning (condensation or nucleation) with subsequent particle-phase (multiphase) chemical reactions (Grieshop et al, 2009)

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Results

Conclusion