Water-Soluble Organic Aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States
Light absorption of fine particle (PM(2.5)) 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 similar to 7 +/- 1 for biomass burning and non-biomass burning-impacted 24-h filter 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 (Abs(365), in units m(-1)) was used as a measure of overall brown carbon light absorption. Biomass-burning-impacted samples were highest during winter 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 Abs(365)/WSOC (i.e., mass absorption efficiency) was typically similar to 3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs(365) to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for similar to 10% of the regional yearly average Abs(365). Summertime diurnal profiles of Abs(365) and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs(365)/WSOC. After noon, this ratio substantially increased, indicating that either some fraction of the nonlight 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 similar to 20 to 30% of Abs(365) 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.