real atmosphere Stefano Alessandrini*, Enrico Ferrero** Urban and regional air qualità: advances in chemistry transport modelling, experiences in USA and Italy Milano, 6 Novembre 2008 PRESENTAZIONE POWER POINT * CESI RICERCA ** UNIVERSITA’ DEL PIEMONTE ORIENTALE In this work a lagrangian particle model able to account for simple chemical reactions between NO ad O 3 (Alessandrini et al. 2007) has been improved in order to consider the photolysis of NO 2. A system of chemical equations is numerically solved on an eulerian grid, while the particles trajectories are moved in a lagrangian frame. In order to test the sensitivity of the model respect to some parameters (especially to the rate of photolysis), simulations of a wind tunnel experiment have been carried out. Then the NO x emissions of a power plant in real atmosphere, situated in a complex topography environment, have been considered. The meteorological model RAMS has been applied to build the wind field together with the interface code MIRS to compute turbulence parameter requested for the dispersion simulations. The plume transitions over an air quality station, allowing for a comparison between the measured and computed concentrations of all the reaction’s compounds (NO, NO 2 and O 3 ), have been simulated by the lagrangian particle model. Some of the simulated episodes refer to the diurnal time, when the ultraviolet radiation activates the NO 2 photolysis making necessary the model complete set of chemical equations. In order to reduce computational cost and improve the accuracy of the background O 3 concentration representation, the concept of concentration holes carried by the particles is proposed and tested. This new method does not need to release a big amount of particles filling the whole domain, but only the inside plume particles should be accounted for. Comparison between NO, NO 2 and O 3 concentrations are presented in term of scatter plot and statistical indexes analysis. The satisfactory results suggest that the model can be used in practical applications in real atmosphere also for regulatory purposes when the NO 2 concentration limit are imposed by the legislation.