Fossil sources of ambient aerosol carbon based on 14C measurements

Organic and elemental carbonaceous material constitute significant portions of PM10 and PM2.5 mass on a global basis and are typically 35% and 10%, respectively, in urban areas of the southeastern United States. The organic portion of this is a complex mixture of primary compounds and secondary materials formed in situ from primary emissions. These materials derive from a variety of sources that are dynamically changing in time and space, making difficult the quantitative characterization of their sources. Results of measurements of the C-14 content of organic aerosol particles (PM10 and TSP) sampled at Look Rock, TN are reported in this work with the aim of better estimating bounds to fossil and biogenic sources that contribute to PM mass concentrations. The fraction of fossil-derived carbon in these particles has been estimated as 1 minus the fraction of "modern" carbon, the latter by definition biologically synthesized in equilibrium with atmospheric, cosmic ray-derived (CO2)-C-14. Fossil carbon fractions vary in a wide range from less than 10 to about 60% during spring, summer, and fall seasons at this ridge-top site near the Great Smoky Mountains National Park. The implications of these findings on control strategies for the organic carbon fraction of PM2.5 mass are discussed.