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Spatial and seasonal variations of fine particle water-soluble organic carbon (WSOC) over the southeastern United States: Implications for secondary organic aerosol formation

by X. Zhang, Z. Liu, A. Hecobian, M. Zheng, N. H. Frank, E. S. Edgerton, R. J. Weber
Atmospheric Chemistry and Physics ()
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Secondary organic aerosol (SOA) in the southeastern US is investigated\nby analyzing the spatial-temporal distribution of water-soluble organic\ncarbon (WSOC) and other PM2.5 components from 900 archived 24-h Teflon\nfilters collected at 15 urban or rural EPA Federal Reference Method\n(FRM) network sites throughout 2007. Online measurements of WSOC at an\nurban/rural-paired site in Georgia in the summer of 2008 are contrasted\nto the filter data. Based on FRM filters, excluding biomass-burning\nevents (levoglucosan < 50 ng m(-3)), WSOC and sulfate were highly\ncorrelated with PM2.5 mass (r(2)similar to 0.7). Both components\ncomprised a large mass fraction of PM2.5 (13% and 31%, respectively,\nor similar to 25% and 50% for WSOM and ammonium sulfate). Sulfate and\nWSOC both tracked ambient temperature throughout the year, suggesting\nthe temperature effects were mainly linked to faster photochemistry\nand/or synoptic meteorology and less due to enhanced biogenic\nhydrocarbon emissions. FRM WSOC, and to a lesser extent sulfate, were\nspatially homogeneous throughout the region, yet WSOC was moderately\nenhanced (27%) in locations of greater predicted isoprene emissions in\nsummer. A Positive Matrix Factorization (PMF) analysis identified two\nmajor source types for the summer WSOC; 22% of the WSOC were associated\nwith ammonium sulfate, and 56% of the WSOC were associated with brown\ncarbon and oxalate. A small urban excess of FRM WSOC (10%) was observed\nin the summer of 2007, however, comparisons of online WSOC measurements\nat one urban/rural pair (Atlanta/Yorkville) in August 2008 showed\nsubstantially greater difference in WSOC (31%) relative to the FRM\ndata, suggesting a low bias for urban filters. The measured Atlanta\nurban excess, combined with the estimated boundary layer heights, gave\nan estimated Atlanta daily WSOC production rate in August of 0.55 mgC\nm(-2) h(-1) between mid-morning and mid-afternoon. This study\ncharacterizes the regional nature of fine particles in the southeastern\nUS, confirming the importance of SOA and the roles of both biogenic and\nanthropogenic emissions.

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