Biomass burning impact on PM _2.5 over the southeastern US during 2007: integrating chemically speciated FRM filter measurements, MODIS fire counts and PMF analysis

Abstract. Archived Federal Reference Method (FRM) Teflon filters used by state regulatory agencies for measuring PM_2.5 mass were acquired from 15 sites throughout the southeastern US and analyzed for water-soluble organic carbon (WSOC), water-soluble ions and carbohydrates to investigate biomass burning contributions to fine aerosol mass. Based on over 900 filters that spanned all of 2007, levoglucosan and K⁺ were studied in conjunction with MODIS Aqua fire count data to compare their performances as biomass burning tracers. Levoglucosan concentrations exhibited a distinct seasonal variation with large enhancement in winter and spring and a minimum in summer, and were well correlated with fire counts, except in winter when residential wood burning contributions were significant. In contrast, K⁺ concentrations had no apparent seasonal trend and poor correlation with fire counts. Levoglucosan and K⁺ only correlated well in winter (r²=0.59) when biomass burning emissions were highest, whereas in other seasons they were not correlated due to the presence of other K⁺ sources. Levoglucosan also exhibited larger spatial variability than K⁺. Both species were higher in urban than rural sites (mean 44% higher for levoglucosan and 86% for K⁺). Positive Matrix Factorization (PMF) was applied to analyze PM_2.5 sources and four factors were resolved: biomass burning, refractory material, secondary light absorbing WSOC and secondary sulfate/WSOC. The biomass burning source contributed 13% to PM_2.5 mass annually, 27% in winter, and less than 2% in summer, consistent with other souce apportionment studies based on levoglucosan, but lower in summer compared to studies based on K⁺.