real atmosphere Stefano Alessandrini*, Enrico Ferrero** HARMO 12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes Cavtat, Croatia 6-9 Ottobre 2008 *CESI RICERCA ** UNIVERSITÀ DEL PIEMONTE ORIENTALE In this work a lagrangian particle model able to account for simple chemical reactions between NO and O3 (Alessandrini et al., 2007) has been improved in order to consider the photolysis of NO2. A system of chemical equations is numerically solved on an eulerian grid, while the particles trajectories are moved in a lagrangian frame. The NOx emissions of a power plant in real atmosphere, situated in a complex topography environment, have been considered as a test case. The meteorological model RAMS has been applied to build the wind field together with the interface code MIRS to compute turbulence parameters, 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, NO2 and O3), have been simulated by the lagrangian particle model. The simulated episodes refer to the diurnal time, when the ultraviolet radiation activates the NO2 photolysis making necessary the model complete set of chemical equations. In order to reduce computational cost and improve the accuracy of the background O3 concentration representation, the concept of concentration deficit 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. Comparisons between NO/NO2’s concentrations ratio are presented in term of scatter plots 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 NO2 concentration limits are imposed by the legislation.