Sources of carbonaceous aerosols over the United States and implications for natural visibility

emissions, captures most of the variance in the observations (R 2 = 0.84 for EC, 0.67 for OC) with a low bias of 15% for EC and 26% for OC. Multiple linear regression to fit the IMPROVE data yields best estimates of 1998 U.S. sources of 0.60 Tg year 1 EC and 0.52 Tg year 1 OC from fossil fuel; 0.07 Tg year 1 EC and 0.89 Tg year 1 OC from biofuel; 0.08 Tg year 1 EC and 0.60 Tg year 1 OC from wildfires; and 1.10 Tg year 1 OC from vegetation. We find that fires in Mexico and Canada contributed 40–70% of annual mean natural EC in the United States for 1998 and 20–30% of annual mean natural OC. Transpacific transport from Asian pollution sources amounted to less than 10% of the natural EC and less than 2% of the natural OC; in contrast to ozone, we find that intercontinental transport of anthropogenic carbonaceous aerosols does not enhance significantly the natural background. IMPROVE observations and model simulations for the summer of 1995 show that Canadian fire emissions can produce large events of elevated EC and OC in the southeastern United States. Our best estimates of mean natural concentrations of EC and OC in the United States, using a model simulation with climatological monthly mean fire emissions, are 2–3 times higher than the default values recommended by the U.S. Environmental Protection Agency for visibility calculations, except for OC in the eastern United States (16% lower). INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: carbonaceous aerosols, natural visibility, natural aerosol concentrations, trans-Pacific transport of aerosol, transboundary transport of aerosol, biomass burning aerosol

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