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The formation of photooxidants and aerosols in the Milan area (Po Valley, Italy) has been simulated for 13 May 1998 with an Eulerian model. The high‐pressure conditions and the weak synoptic circulation favor the formation of strongly enhanced photooxidants and aerosol levels downwind of the metropolitan area of Milan. The polluted air masses are transported northward where the highest ozone (O₃) concentrations of up to 200 ppb are reached 30 to 40 km away from the city center of Milan. The core of the Milan plume has an extension of only 10 km in length and 15 km in width at 1600 LT. The focus of the simulation was the sensitivity of the O₃ formation to the reduction of nitrogen oxides (NO_x) and volatile organic compounds (VOC) emissions. The model results are compared to the measurements in order to evaluate the quality of the simulation. The spatial and temporal variations of the concentrations of O₃ …