Water-Soluble Organic Aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States

• Main Authors: A. Hecobian, X. Zhang, M. Zheng, N. Frank, E. S. Edgerton, R. J. Weber
• Format: Article
• Language: English
• Published: Copernicus Publications 2010-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/10/5965/2010/acp-10-5965-2010.pdf

Light absorption of fine particle (PM<sub>2.5</sub>) aqueous extracts between wavelengths of 200 and 800 nm were investigated from 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 filter extracts were similar for different sources. Angstrom exponents were ~7±1 for biomass burning and non-biomass burning-impacted 24-h filter samples (delineated by a levoglucosan concentration of 50 ng m<sup>−3</sup>) at both rural and urban sites. The absorption coefficient from measurements averaged between wavelength 360 and 370 nm (Abs<sub>365</sub>, in units m<sup>−1</sup>) was used as a measure of overall brown carbon light absorption. Biomass-burning-impacted samples were highest during winter months and Abs<sub>365</sub> 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 Abs<sub>365</sub>/WSOC (i.e., mass absorption efficiency) was typically ~3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs<sub>365</sub> to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for ~10% of the regional yearly average Abs<sub>365</sub>. Summertime diurnal profiles of Abs<sub>365</sub> and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs<sub>365</sub>/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 on the 24-h integrated filter data associated ~20 to 30% of Abs<sub>365</sub> over 2007 with a secondary source that was highest in summer and also the main source for oxalate, suggesting that aqueous phase reactions may account for the light-absorbing fraction of WSOC observed throughout the Southeastern US in summer.