SIMULATION OF SUMMERTIME OZONE OVER NORTH-AMERICA

The concentrations of O3 and its precursors over North America are sim ulated for three summer months with a three-dimensional, continental-s cale photochemical model using meteorological input from the Goddard I nstitute for Space Studies (GISS) general circulation model (GCM). The model has 4-degreesx5-degrees grid resolution and represents non line ar chemistry in urban and industrial plumes with a subgrid nested sche me. Simulated median afternoon O3 concentrations at rural U.S. sites a re within 5 ppb of observations in most cases, except in the south cen tral 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 nonme thane hydrocarbons are generally in good agreement with observations, concentrations of NO(x) are underpredicted by 10-30%, and concentratio ns of peroxyacylnitrates (PANs) are overpredicted by a factor of 2 to 3. The overprediction of PANs is attributed to flaws in the photochemi cal mechanism, including excessive production from oxidation of isopre ne, 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.