Sources of carbonaceous aerosols over the United
States and implications for natural visibility
Rokjin J. Park, Daniel J. Jacob, Mian Chin, and Randall V. Martin
Submitted for publication in J. Geophys. Res.
Abstract
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-1
EC and 0.52 Tg yr-1 OC from fossil fuel; 0.07 Tg yr-1 EC
and 0.89 Tg yr-1 OC from biofuel; 0.08 Tg yr-1 EC and
0.60 Tg yr-1 OC from wildfires; and 1.10 Tg yr-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 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.
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