8 th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes ______________________________________________________________________________________ - 24 - A FLEXIBLE MODEL SYSTEM FOR AIR POLLUTION SIMULATION AND CONTROL IN NORTHERN ITALY Edoardo Decanini 1 and Marialuisa Volta 2 1 Scuola Normale Superiore, Pisa, ITALY 2 Facoltà di Ingegneria, Università di Brescia, Brescia, ITALY INTRODUCTION Northern Italy, characterised by a complex terrain, high and urban and industrial emissions and a close road net, is often affected by severe photochemical and particulate pollution episodes, mainly during summer season. Photochemical pollution is a mixture of pollutants, as ozone, NO 2 , PAN and formaldehyde, produced by a complex chemical reaction system. The main compounds inducing these reactions are NO x and VOC coming from road traffic and industrial combustion plants. An essential role in the chemical process activation is played by high solar radiation and stagnating meteorological conditions. These prerogatives make photochemical smog to be typically a summer phenomenon, mainly occurring in domains nearby large urban areas. The cause-effect relation between precursors and photochemical pollutant is quite complex and non-linear, due to accumulation processes and to the large number of variables affecting the photo-oxidant production, including their integration and the consequent feedback multiple reactions. As a consequence of such complexity, the development of effective strategies of emission reduction requires suitable tools that enable to take into account all the variables involved in gas and aerosol phenomenology. In this work an integrated modelling system that can include several chemical mechanisms representing the gas-phase photochemistry has been set up to support the analysis of emission control strategies. Preliminary simulations and comparison between the different mechanisms results are presented. The system will be integrated with an aerosol processor and a particle emission module for particulate matter analysis. APPROACH The modelling system includes three main processors and post-processing tools: 1. The photochemical model CALGRID, Yamartino, R.J., et al. (1992) which is an Eulerian three-dimensional model that implements an accurate advection-diffusion scheme in terrain- following co-ordinates with vertical variable spacing; a resistance-based dry deposition algorithm takes into account pollutant properties, local meteorology and terrain features; the CALGRID chemical module uses SAPRC-90 mechanism, Carter W.P.L. (1990), including 54 chemical species with 129 reactions and the QSSA (Quasi Steady State Approximations) solver for the integration of kinetic equations. The CALGRID model has been modified to allow for changes in chemical schemes, Decanini E. and Volta M. (2002), and this goal has been obtained implementing the Flexible Chemical Mechanism interface, Kumar N. et al. (1995). The chemical mechanisms that have been implemented are two: SAPRC-97, Carter W.P.L. et al. (1997), which considers 82 chemical species and 184 reactions; Carbon Bond IV, Gery M. et al. (1988), with 37 chemical species and 78 reactions. The original integration solver of the CALGRID, the QSSA, has been substituted with the IEH solver of Sun P. et al. (1994) as the FCM makes possible to distinguish among fast and slow reacting species: the IEH integration method uses the implicit scheme LSODE of Hindmarsh A.C. (1980) to solve for the fast reacting species and an explicit second order scheme to solve the slow reacting species. 2. The meteorological pre-processor CALMET; Scire J.S. et al. (1990), which provides 3D meteorological fields by means of a three-step procedure: the available local measurements (SYNOP reports and upper air sounding data; wind and temperature profiles) and the ECMWF fields are collected and analysed; the background wind field is reconstructed,