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

We use a global 3-D model simulation of elemental carbon (EC) and organic carbon (OC) aerosols to interpret year-round observations for 1998 at a network of 45 U.S. sites (IMPROVE) and from there to better quantify the natural and anthropogenic U.S. sources of these aerosols. Our resulting best estimate of 1998 sources in the contiguous United States are 0.60 Tg yr EC and 0.52 Tg yr OC from fossil fuel; 0.07 Tg yr EC and 0.89 Tg yr OC from biofuel; 0.08 Tg yr EC and 0.60 Tg yr OC from wildfires; and 1.10 Tg yr 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 2030% of annual mean natural OC. Transpacific transport from Asian sources amounted to less than 10% of the natural EC and less than 2% of the natural OC over the United States. 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, with major implications for seasonal mean concentrations. Our best estimates of mean natural concentrations of EC and OC in the United States are 2-3 times higher than the default values recommended by the U.S. Environmental Protection Agency (EPA) for visibility calculations, except for OC in the eastern United States (16% lower). Higher natural OC concentrations in the western United States imply a natural visibility that is 20% lower than that estimated from EPA default natural concentrations.