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

Abstract

The factors regulating summertime O 3 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 x emissions by 50% from 1985 levels would decrease rural O 3 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 x‐limited conditions in the model reflect the dominance of rural areas as sources of O 3 on the regional scale. The correlation between O 3 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 3 production on temperature driven primarily by conversion of NO x to peroxyacetylnitrate (PAN). The net number of O 3 molecules produced per molecule of NO x consumed (net O 3 production efficiency, accounting for both chemical production and chemical loss of O 3) 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 x concentrations in the west. Approximately 70% of the net chemical production of O 3 in the U.S. boundary layer is exported (the rest is deposited). Only 6% of the NO x emitted in the United States is exported out of the U.S. boundary layer as NO x or PAN, but this export contributes disproportionately to total U.S. influence on global tropospheric O 3 because of the high O 3 production efficiency per unit NO x in the remote troposphere. It is estimated that export of U.S. pollution supplies 8 Gmol O 3 d−1 to the global troposphere in summer, including 4 Gmol d−1 from direct export of O 3 out of the U.S. boundary layer and 4 Gmol d−1 from production of O 3 downwind of the United States due to exported NO x. This U.S. pollution source can be compared to estimates of 18–28 Gmol d−1 for the cross‐tropopause transport of O 3 over the entire northern hemisphere in summer.