FACTORS REGULATING OZONE OVER THE UNITED-STATES AND ITS EXPORT TO THEGLOBAL ATMOSPHERE

The factors regulating summertime O3 over the United States and its ex port to the global atmosphere are examined with a 3-month simulation u sing a continental scale, three-dimensional photochemical model. It is found that reducing NO(x) emissions by 50% from 1985 levels would dec rease rural O3 concentrations over the eastern United States by about 15% under almost all meteorological conditions, while reducing anthrop ogenic hydrocarbon emissions by 50% would have less than a 4% effect e xcept in the largest urban plumes. The strongly NO(x)-limited conditio ns in the model reflect the dominance of rural areas as sources of O3 on the regional scale. The correlation between O3 concentrations and t emperature observed at eastern U.S. sites is attributed in part to the association of high temperatures with regional stagnation, and in par t to an actual dependence of O3 production on temperature driven prima rily by conversion of NO(x) to peroxyacetylnitrate (PAN). The net numb er of O3 molecules produced per molecule of NO(x) consumed (net O3 Pro duction efficiency, accounting for both chemical production and chemic al loss of O3) 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 becaus e of lower NO(x) concentrations in the west. Approximately 70% of the net chemical production of O3 in the U.S. boundary layer is exported ( the rest is deposited). Only 6% of the NO(x) emitted in the United Sta tes is exported out of the U.S. boundary layer as NO(x) or PAN, but th is export contributes disproportionately to total U.S. influence on gl obal tropospheric O3 because of the high O3 production efficiency per unit NO(x) in the remote troposphere. It is estimated that export of U .S. pollution supplies 8 Gmol O3 d-1 to the global troposphere in summ er, including 4 Gmol d-1 from direct export of O3 out of the U.S. boun dary layer and 4 Gmol d-1 from production of O3 downwind of the United States due to exported NO(x). This U.S. pollution source can be compa red to estimates of 18-28 Gmol d-1 for the cross-tropopause transport of O3 over the entire northern hemisphere in summer.