In the frame of Citydelta Project four different atmospheric chemistry-transport models (CALGRID[1], STEM[2], CAMx[3] and TCAM) have been applied to 1999 summer season over a 300×300 km2 domain situated in the Northern Italy. The validation phase shows that all the models ensure good agreement with data. To better understand the behaviour of the different models, a sensitivity analysis has been carried out with regard to the performances of chemical mechanisms and numerical integrators. The sensitivity analysis has been performed with regard to ozone and considering the mean and frequency distribution concentration. The analysis has been carried out comparing the results obtained with 4 chemical mechanisms (SAPRC 90/99, CBIV 90/99) and 3 different numerical integrators (QSSA, IEH, CMC[3]). With regard to solvers comparison, the results shows that the IEH (STEM, TCAM) ensure better performance than QSSA and CMC, but it require higher computational time. The fully-explicit, fast integrator QSSA (Calgrid) is not able to correctly reproduce high concentration during the simulation period. The CMC integrator (CAMx), shows performances comparable to IEH during the day, but heavily underestimate ozone concentration during the night. Concerning the chemical mechanism, it could be noted that models using the same scheme presents similar spatial mean distribution over the domain. The simulations performed using CBIV mechanisms, show higher ozone concentration in the rural area of the domain, due to the different VOC chemistry. In conclusion, the analysis suggests that (1) all the models used in the comparison are able to reproduce the ozone production in the simulation domain, (2) the integrator algorithms have a deep impact over the peak concentration, and (3) the chemical scheme mainly affects the spatial distribution of the concentration over the domain. [1] Yamartino, R. J., Scire J.S., Charcichael G. R. and Chang Y. S. 1991. The Calgrid mesoscale photochemical grid model – I. Model formulation. Atmospheric Environment Vol. 26, 3,1493-1512. [2] Silibello, C., Calori, G., Pirovano, G. and Carmichael, G.R., 2001,Development of STEM-FCM modelling system: Chemical mechanisms sensitivity evaluated on a photochemical episode, Proc. of APMS’01- Int. Conference, Paris, April 2001 [3] ENVIRON (1997) User’s guide to the comprehensive air quality model with extensions (CAMx). Version 1.10. Report prepared by ENVIRON International Corporation, Novato, CA PUBBLICATO A5017547 (PAD – 652192)PUBBLICATO A5017547 (PAD – 652192)