Biomass burning impact on PM2.5 over the southeastern US during 2007: Integrating chemically speciated FRM filter measurements, MODIS fire counts and PMF analysis
Archived Federal Reference Method (FRM) Teflon filters used by state regulatory agencies for measuring PM<sub>2.5</sub> 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<sup>+</sup> 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<sup>+</sup> concentrations had no apparent seasonal trend and poor correlation with fire counts. Levoglucosan and K<sup>+</sup> only correlated well in winter (<i>r</i><sup>2</sup>=0.59) when biomass burning emissions were highest, whereas in other seasons they were not correlated due to the presence of other K<sup>+</sup> sources. Levoglucosan also exhibited larger spatial variability than K<sup>+</sup>. Both species were higher in urban than rural sites (mean 44% higher for levoglucosan and 86% for K<sup>+</sup>). Positive Matrix Factorization (PMF) was applied to analyze PM<sub>2.5</sub> 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<sub>2.5</sub> 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<sup>+</sup>.