Secondary organic aerosol in the Southeastern United States: A summary of findings from ambient studies

Fine particles over the southeastern U.S. are known to have adverse health impacts and an influence on regional climate. Secondary organic aerosol (SOA) is a significant fraction of this fine PM, (only sulfate is more important), yet SOA remains one of the least understood components of ambient particles. Recent work has shown that SOA formed within anthropogenic emissions produce vastly (x10 or so) more organic aerosol than what can be explained by air quality models. Radiocarbon dating of SOA (Weber et al., 2007) and a positive correlation of SOA with temperature (Hennigan et al., 2009) in the southeast are consistent with SOA composed of biogenic VOCs. However, in contrast to this, airborne measurements have shown high spatial correlations between anthropogenic CO and SOA (Weber et al, 2007) and substantially higher SOA is often observed in Atlanta compared to a rural site during periods of low wind speed (Zhang et al., 2009). Currently, a widely accepted physicochemical mechanism that may explain how most of the SOA is formed is lacking. Our results from experiments over the last five years in Atlanta and the surrounding region, however, are consistent with a mechanism involving the role of low molecular weight SOA-precursor vapors (Sullivan et al, 2006), that may be formed from the photo-oxidation (Hennigan et al., 2009) of either anthropogenic or biogenic VOCs, that partition to aerosol water through a process of reactive uptake (Hennigan et al., 2008; 2009). This paper will summarize the findings from these studies. These issues have significant implications for regulating PM in the southeastern US.

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Hennigan, C. J., M. H. Bergin, A. G. Russell, A. Nenes, R. J. Weber, Gas/particle partitioning of water-soluble organic aerosol in Atlanta, Atm. Chem. Phys., 9, 3613-3628, 2009.

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Zhang, X., R. J. Weber, On the spatial distribution of SOA in the Southeastern United States, manuscript in preparation, 2009.