Factors regulating ozone over the United States and its export to the global atmosphere

Abstract

The factors regulating summertime O over the United States and its export to the global atmosphere are examined with a 3‐month simulation using a continental scale, three‐dimensional photochemical model. It is found that reducing NO emissions by 50% from 1985 levels would decrease rural O concentrations over the eastern United States by about 15% under almost all meteorological conditions, while reducing anthropogenic hydrocarbon emissions by 50% would have less than a 4% effect except in the largest urban plumes. The strongly NO‐limited conditions in the model reflect the dominance of rural areas as sources of O on the regional scale. The correlation between O concentrations and temperature observed at eastern U.S. sites is attributed in part to the association of high temperatures with regional stagnation, and in part to an actual dependence of O production on temperature driven primarily by conversion of NO to peroxyacetylnitrate (PAN). The net number of O molecules produced per molecule of NO consumed (net O production efficiency, accounting for both chemical production and chemical loss of O) has a mean value of 6.3 in the U.S. boundary layer; it is 3 times higher in the western United States than in the east because of lower NO concentrations in the west. Approximately 70% of the net chemical production of O in the U.S. boundary layer is exported (the rest is deposited). Only 6% of the NO emitted in the United States is exported out of the U.S. boundary layer as NO or PAN, but this export contributes disproportionately to total U.S. influence on global tropospheric O because of the high O production efficiency per unit NO in the remote troposphere. It is estimated that export of U.S. pollution supplies 8 Gmol O d to the global troposphere in summer, including 4 Gmol d from direct export of O out of the U.S. boundary layer and 4 Gmol d from production of O downwind of the United States due to exported NO. This U.S. pollution source can be compared to estimates of 18–28 Gmol d for the cross‐tropopause transport of O over the entire northern hemisphere in summer.