Secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), compounds that are produced from isoprene oxidation under low-NO conditions. IEPOX-SOA can account for a substantial fraction of organic aerosol (OA) in biogenic-influenced areas. In this study, IEPOX-SOA was identified from measurements at the forested southeast U.S. supersite (Centreville, AL) during the Southern Oxidant and Aerosol Study (SOAS) using Positive Matrix Factorization (PMF) of aerosol mass spectrometer (AMS) measurements. The SOAS results clearly show that the fraction of OA measured at C5H6O+ (fC5H6O) in AMS spectra is a good tracer for IEPOX-SOA and correlates with the well-known methyltetrol tracers of this chemistry. Other field and chamber studies are included in the analysis to investigate the robustness of fC5H6O as an IEPOX-SOA tracer in AMS data. We observed clearly higher fC5H6O (3×10-3 - 25×10-3) in OA from regions with strong isoprene emissions vs those from urban and biomass-burning plumes (0 - 3.5×10-3 with an average of 1.75×10-3). In isoprene-influenced areas, fC5H6O in ambient OA positively correlates with the relative contribution of IEPOX-SOA to OA and decreases with OA aging. The kOH for destruction of IEPOX-SOA via heterogeneous oxidation in aerosol phase is approximately 5.3×10-13 cm3 molecular-1 s-1, with life time is around 14.5 days (assumming avrage OH concentration=1.5×106 molecular cm-3). Volatility analysis using a thermal denuder shows that IEPOX-SOA is not more volatile than bulk SOA, contrasting with the high volatility of the tracers identified. Finally, we develop a simplified method to estimate ambient IEPOX-SOA mass concentrations as a function of fC5H6O, which is shown to perform well compared to the full PMF method. When only unit mass resolution data is available as in ACSM data, the method performs less well because of increased interferences from other ions at m/z 82. Estimated IEPOX-SOA concentrations in the southeastern U.S. from an aircraft campaign (SEAC4RS) correlate well with the IEPOX-related species detected by other techniques over a wide range of conditions, which confirms the usefulness of our method. IEPOX-SOA accounts for 14 - 17% of the OA in the SE U.S. during the summer according to both the SEAC4RS and SOAS results.

Characterization of a real-time tfor isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) from aerosol mass spectrometer measurements

249th National Meeting (2015)

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