Simulation of the ozone formation in the northern part of the Po Valley

Abstract

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 (O 3) 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 03 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 O 3 and other trace compounds are reasonably well reproduced. Several reasons for systematic deviations have been analyzed in order to understand their possible influence on the sensitivity analysis. Moreover, the simulation showed that indicator species such as the H 2 O 2 /HNO 3 or O 3 /NO y ratio could be applied to characterize the VOC/NO x regime of the O 3 production in the Milan area, but that it is necessary to modify the indicator ratios to include the aerosol nitrate. Highest O 3 values appear at the transition from VOC to NO x sensitivity approximately 30 km downwind of the highest emissions. Copyright 2002 by the American Geophysical Union.