area on the basis of source apportionment results Guido Pirovano*, Marco Bedogni**, Simone Casadei** 7th International Conference on Air Quality – Science and Application (formerly Urban Air Quality Conference) Istanbul, 24-27 Marzo 2009 PRESENTAZIONE POWER POINT *CESI RICERCA ** AMA AGENZIA MOBILITÀ E AMBIENTE DI MILANO The Po valley, a flat area with peculiar orographical and meteorological features placed in Northern Italy, often suffers high PM concentrations and it has been identified by the European Commission as one of the main areas where pollutant levels will still remain problematic despite of the implementation of the CAFE 2020 emission reduction scenarios. In the city of Milan, placed in the middle of the Po Valley, field campaigns highlighted that fine particulate matter is mainly composed by secondary aerosol, confirming that the definition of effective emission reduction policies is a challenging task. To focalise local physical and chemical phenomena, the CAMx model and PSAT tool have been applied to reconstruct the contribution of the main emission sources to PM2.5 concentration in Milan within the framework of the CityDeltaIII project. PSAT (PM Source Apportionment Technology) algorithm implements an efficient and mass-conserving approach that allows to discriminate the contribution of different emission sectors and areas to the modelled PM concentrations. Obtained results highlight that road transport is the most relevant sector contributing to the PM2.5 yearly mean in Milan, followed by agriculture and domestic heating, and that 65% of the PM2.5is due to emissions outside of Milan. Besides, a quantitative analysis of the processes governing the Secondary Inorganic Aerosols (SIA) production has been carried out by means of some synthetic indicators, proving that SIA production arises in a very complex framework being influenced, among others, by plume age and meteorological conditions. On the basis of source apportionment findings a set of emission reduction scenarios has been defined and evaluated. The scenarios have been defined in order to compare the effectiveness of local versus regional policies as well as to rank the influence of the different kind of precursor species. Finally, obtained results have been discussed with respect to the indicator values.