Assessment of biomass burning impacts on ambient PM2.5 over the Southeastern U.S. in 2007 from analysis of archived FRM filters

In order to assess the spatiotemporal distribution of biomass burning and its impacts on regional air quality in the southeastern U.S., 24-h integrated Federal Reference Method (FRM) PM2.5 filter samples collected at 15 sites (eight urban sites and seven rural sites in Georgia, Alabama and South Carolina) throughout the year of 2007 were analyzed for biomass-burning tracers. A total of ~900 samples were analyzed for a suite of water-soluble components, including water-soluble organic carbon (WSOC), levoglucosan and water-soluble K+. Levoglucosan and K+ were studied in conjunction with MODIS aqua fire counts data to compare their performances as biomass burning tracers. Temporally, levoglucosan concentrations agree better with the seasonal variations of fire counts than K+. Levoglucosan was observed to have a distinct seasonal variation, characterized by the maximum concentration in winter and spring, when fire counts suggest frequent biomass burning events, and a minimum during summer, when biomass burning is more rarely detected. In contrast, no apparent seasonal trends were found for K+. Furthermore, levoglucosan and K+ are well correlated (R2 = 0.588) in winter, suggesting their common origin from biomass burning, whereas poorly correlated in other seasons, indicating the existence of other K+ sources. Spatially, levoglucosan concentrations and the ratios of [levoglucosan]/[PM2.5 mass] at urban sites are both higher than those at rural sites during enhanced biomass burning seasons, implying larger biomass burning impacts in urban regions. Additionally, strong correlations between adjacent sampling sites indicate biomass burning has larger effects on air quality locally, rather than regionally. Finally, the contributions of biomass burning to ambient WSOC and PM2.5 mass concentrations were quantitatively estimated using a tracer-based approach. The results indicate biomass burning as a major WSOC and PM2.5 contributor in winter and spring, whereas SOA (secondary organic aerosol) formation dominated in summer.