Atmos. Chem. Phys. Discuss., 10, 7601-7639, 2010
© Author(s) 2010. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Brown carbon and water-soluble organic aerosols over the southeastern United States
A. Hecobian1, X. Zhang1, M. Zheng1, N. Frank2, E. S. Edgerton3, and R. J. Weber1
1Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, Georgia, USA
2Office of Air Quality Planning & Standards, US Environmental Protection Agency, 109 TW Alexander Drive, Research Triangle Park, NC, USA
3Atmospheric Research and Analysis, Inc., Durham, North Carolina, USA

Abstract. Fine particle (PM2.5) light absorption characteristics of aqueous extracts over wavelengths of 250 to 700 nm were investigated based on two data sets; 24-h Federal Reference Method (FRM) filter extracts from 15 southeastern US monitoring sites over the year of 2007 (900 filters), and online measurements from a Particle-Into-Liquid Sampler deployed from July to mid-August 2009 in Atlanta, Georgia. Three main sources of soluble chromophores were identified, biomass burning, mobile source emissions, and compounds linked to Secondary Organic Aerosol (SOA) formation. Absorption spectra of aerosol solutions from different sources were similar. Angstrom exponents were ~7±1 for biomass burning and non-biomass burning-impacted samples (delineated by a levoglucosan concentration of 50 ng m−3) at both rural and urban sites. The absorption coefficient from measurements averaged between wavelength 360 and 370 nm (Abs365, in units m−1) was used as a measure of overall brown carbon absorptivity. Biomass-burning-impacted samples were highest during colder months and Abs365 was correlated with levoglucosan at all sites. During periods of little biomass burning in summer, light absorbing compounds were still ubiquitous and correlated with fine particle Water-Soluble Organic Carbon (WSOC), but comprised a much smaller fraction of the WSOC, where Abs365/WSOC (i.e., mass absorption efficiency) was typically ~3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs365 to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for ~10% of the regional yearly average Abs365. Summertime diurnal profiles of Abs365 and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs365/WSOC. After noon this ratio substantially increased, indicating that either some fraction of the non-light absorbing fresh SOA was rapidly (within hours) converted to chromophores heterogeneously, or that SOA from gas-particle partitioning later in the day was more light-absorbing. Factor analysis associated ~20 to 30% of Abs365 over 2007 with a secondary source that was highest in summer and also the main source for oxalic acid, suggesting that aqueous phase reactions may account for the light-absorbing fraction of WSOC observed throughout the southeastern US in summer.

Citation: Hecobian, A., Zhang, X., Zheng, M., Frank, N., Edgerton, E. S., and Weber, R. J.: Brown carbon and water-soluble organic aerosols over the southeastern United States, Atmos. Chem. Phys. Discuss., 10, 7601-7639, doi:10.5194/acpd-10-7601-2010, 2010.
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