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

摘要

Light absorption of fine particle (PM) aqueous extracts betweenwavelengths of 200 and 800 nm were investigated from two data sets: 24-hFederal Reference Method (FRM) filter extracts from 15 Southeastern USmonitoring sites over the year of 2007 (900 filters), and onlinemeasurements from a Particle-Into-Liquid Sampler deployed from July tomid-August 2009 in Atlanta, Georgia. Three main sources of solublechromophores were identified: biomass burning, mobile source emissions, andcompounds linked to secondary organic aerosol (SOA) formation. Absorptionspectra of aerosol solutions from filter extracts were similar for differentsources. Angstrom exponents were ~7±1 for biomass burning andnon-biomass burning-impacted 24-h filter samples (delineated by alevoglucosan concentration of 50 ng m) at both rural and urban sites.The absorption coefficient from measurements averaged between wavelength 360and 370 nm (Abs, in units m) was used as a measure of overallbrown carbon light absorption. Biomass-burning-impacted samples were highestduring winter months and Abs was correlated with levoglucosan at allsites. During periods of little biomass burning in summer, light absorbingcompounds were still ubiquitous and correlated with fine particleWater-Soluble Organic Carbon (WSOC), but comprised a much smaller fractionof the WSOC, where Abs/WSOC (i.e., mass absorption efficiency) wastypically ~3 times higher in biomass burning-impacted samples. Factoranalysis attributed 50% of the yearly average Abs to biomass burningsources. Brown carbon from primary urban emissions (mobile sources) was alsoobserved and accounted for ~10% of the regional yearly averageAbs. Summertime diurnal profiles of Abs and WSOC showed thatmorning to midday increases in WSOC from photochemical production wereassociated with a decrease in Abs/WSOC. After noon, this ratiosubstantially increased, indicating that either some fraction of thenon-light absorbing fresh SOA was rapidly (within hours) converted tochromophores heterogeneously, or that SOA from gas-particle partitioninglater in the day was more light-absorbing. Factor analysis on the 24-hintegrated filter data associated ~20 to 30% of Abs over 2007with a secondary source that was highest in summer and also the main sourcefor oxalate, suggesting that aqueous phase reactions may account for thelight-absorbing fraction of WSOC observed throughout the Southeastern USin summer.