Dependence of SOA oxidation on organic aerosol mass concentration and OH exposure: experimental PAM chamber studies
1Department of Meteorology, Pennsylvania State University, University Park, PA 16802, USA
2Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO 80309-0311, USA
3Department of Earth and Environmental Science, Korea University, Seoul, 136-701, Korea
Abstract. The oxidation of secondary organic aerosol (SOA) is studied with mass spectra analysis of SOA formed in a Potential Aerosol Mass (PAM) chamber, a small flow-through photo-oxidation chamber with extremely high OH and ozone levels. Oxidation for a few minutes in the PAM chamber is equivalent to days to weeks in the atmosphere. The mass spectra were measured with a Quadrupole Aerosol Mass Spectrometer (Q-AMS) for SOA formed from oxidation of α-pinene, m-xylene, p-xylene, and a mixture of the three. The organic mass fraction of m/z 44 (CO2+) and m/z 43 (mainly C2H3O+), named f44 and f43, respectively, are used as indicators of the degree of organic aerosol (OA) oxidation that occurs as the OA mass concentration or the OH exposure are varied. The degree of oxidation is sensitive to both. For a fixed OH exposure, the degree of oxidation initially decreases rapidly and then more slowly as the OA mass concentration increases. For fixed initial precursor VOC amounts, the degree of oxidation increases linearly with OH exposure, with linear f44 increase and f43 decrease. The degree of oxidation seen in this study is similar to that seen in large environmental chambers for the least oxidized OA and similar to the atmosphere for the most oxidized OA. These results, while sensitive to the determination of f44 and f43, provide evidence that characteristics of atmospheric OA oxidation can be generated in a PAM chamber. For all measurements in this study, the sum of f44 and f43 is 0.25± 0.03, so that the slope of a linear regression is approximately −1 on an f44 vs. f43 plot. This constancy of the sum suggests that these ions are complete proxies for organic mass in the OA studied.