power plant pollution dispersion in complex terrain Stefano Alessandrini*, Enrico Ferrero** ITM – NATO/SPS International Technical Meeting on Air Pollution Modelling and its Application, San Francisco, 18-22 Maggio 2009 * CESI RICERCA ** UNIVERSITÀ DEL PIEMONTE ORIENTALE A modeling system based on the regional atmospheric model RAMS (Pielke et al,1992) and the Lagrangian particle model SPRAY (Tinarelli et al, 2000) is adopted to forecast the concentration of pollutants around the CERCS power plant, situated in a complex terrain area of Spain. To calculate also the NO2 concentration, we introduced in the Lagrangian dispersion model an hybrid Lagrangian-Eulerian scheme for chemical reactions between nitrogen oxides an ozone including the NO2 photolysis. This new model was tested in controlled conditions in a previous work (Alessandrini and Ferrero, 2009). It is worth noticing that the small distance of the air quality stations from the power plant stacks gives the possibility to study also the cases where the plume has not still reached the photochemical equilibrium condition. In these situations the photochemical algorithm used shows considerably important advantages compared to more simple approaches generally adopted. The results of the simulations carried out using the modeling system are presented analyzing some particularly interesting episodes. The comparison between the predicted and measured concentrations of all the emitted substances shows theability of the chemical scheme to account for the short term turbulent dispersion.