In recent years, long term simulations with Chemical Transport Models (CTM) have been increasingly used for air quality assessment and to evaluate emission reduction policies. For this reason the CityDelta open model intercomparison exercise has been organised by the Joint Research Centre of Ispra as a contribution to the modelling activities in the CAFE (Clean Air For Europe) Programme. Within this frame, five Italian groups cooperated in order to perform one-year simulations of PM10 concentrations over the Milan area (Northern part of Italy), by use of two CTM (CAMx and TCAM). The computational domain ([300 x 300] km2) includes a large flat area surrounded by mountains along three sides: it is therefore a complex test bench, in which low wind speeds and summery intense solar radiation cause high ozone and PM concentration levels. The models have shared the same input fields (gridded emissions, boundary and initial concentrations, meteorological fields), provided mainly by Joint Research Centre to all the participant. Besides, a common set of pre-processors for input data was developed and integrated in the two modelling system. The PM simulated concentrations have been analysed from a statistical point of view and compared with a dataset of experimental PM10 mean daily concentrations at four air quality monitoring stations located in the inner part of the domain, near Milan. The performances of the two models are quite satisfactory, and good enough to assure coherent and adequate long term scenario analysis. The simulated PM10 values are generally different, but the spatial distribution are quite similar. On the other hand, more efforts are needed to understand the different behaviours of the two models in terms of inorganic ions and PM2.5 percentages. As for the secondary inorganic components, for CAMx the most important role is played by sulphates, while nitrates and ammonium are the main components for TCAM. However, the percentage of the sum of sulphates, nitrates and ammonium on total PM10 mass is quite similar for both models and is more than 50% of the total PM10. PUBBLICATO A5021346 (PAD – 643973)PUBBLICATO A5021346 (PAD – 643973)