The factors regulating summertime O₃ 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 O₃ 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 O₃ on the regional scale. The correlation between O₃ 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 O₃ production on temperature driven primarily by conversion of NO_x to peroxyacetylnitrate (PAN). The net number of O₃ molecules produced per molecule of NO_x consumed (net O₃ production efficiency, accounting for both chemical production and chemical 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 because 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 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 O₃because 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 summer, including 4 Gmol d⁻¹ from direct export of O₃ out of the U.S. boundary 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 compared to estimates of 18–28 Gmol d⁻¹ for the cross-tropopause transport of O₃ over the entire northern hemisphere in summer.

The concentrations of O₃ and its precursors over North America are simulated for three summer months with a three-dimensional, continental-scale photochemical model using meteorological input from the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The model has 4°×5° grid resolution and represents non linear chemistry in urban and industrial plumes with a subgrid nested scheme. Simulated median afternoon O₃ concentrations at rural U.S. sites are within 5 ppb of observations in most cases, except in the south central United States where concentrations are overpredicted by 15–20 ppb. The model captures successfully the development of regional high-O₃ episodes over the northeastern United States on the back side of weak, warm, stagnant anticyclones. Simulated concentrations of CO and nonmethane hydrocarbons are generally in good agreement with observations, concentrations of NO_x are underpredicted by 10–30%, and concentrations of peroxyacylnitrates (PANs) are overpredicted by a factor of 2 to 3. The overprediction of PANs is attributed to flaws in the photochemical mechanism, including excessive production from oxidation of isoprene, and may also reflect an underestimate of PANs deposition. Subgrid nonlinear chemistry as captured by the nested plumes scheme decreases the net O₃ production computed in the United States boundary layer by 8% on average.