MARCH 2004: Transboundary pollution influences on aerosol concentrations in the United States

We use a global 3-D coupled oxidant-aerosol model (GEOS-CHEM) to quantify natural and transboundary pollution influences on sulfate-nitrate-ammonium aerosol concentrations in the United States. This work is motivated in part by the Regional Haze Rule of the U.S. Environmental Protection Agency (EPA), which requires immediate action to improve visibility in U.S. wilderness areas along a linear trajectory towards an endpoint of "natural visibility conditions" by 2064.

Left figure shows an illustrate example of such an action of visibility improvement (top) and a corresponding U.S. anthropogenic emission reduction required (bottom) toward a 2064 endpoint of natural visibility either from our results or from the EPA default. We also estimate "background" aerosol concentrations (visibility degradation) in the United States, representing contributions from both natural and transboundary pollution sources not amenable by the U.S. EPA, using the model simulation with shutting off U.S. anthropogenic emissions. Shown in the right is the global distribution of Asian pollution influence on sulfate-nitrate-ammonium aerosol concentrations in surface air, as determined by difference between the standard simulation and the sensitivity simulation with anthropogenic Asian emissions shut off for 2001. We find that transpacific transport of Asian pollution accounts for 30% of background sulfate in both the western and eastern United States and is comparable in magnitude with that of transboundary transport from Canada and Mexico. Our estimates of background concentrations are several-fold higher than those of natural concentrations because of these transboundary pollution influences. The large transboundary pollution influence means that a natural visibility objective cannot be achieved by the domestic emission control alone. Moreover, consideration of a background rather than natural visibility 2064 endoint would have an important implication of the Regional Haze Rule. It would require, at least in the west, a significantly slower schedule of U.S. anthropogenic emission reductions shown in left figure (solid line). More information about this work can be found in Park et al. [2004] (pdf).