The objective of this study is to evaluate the impact of meteorological input data on calculated gas and aerosol concentrations. We use two different meteorological models (MM5 and WRF) together with the chemistry transport model CHIMERE. We focus on the Po valley area (Italy) for January and June 2005. <br><br> Firstly we evaluate the meteorological parameters with observations. The analysis shows that the performance of both models is similar, however some small differences are still noticeable. <br><br> Secondly, we analyze the impact of using MM5 and WRF on calculated PM<sub>10</sub> and O<sub>3</sub> concentrations. In general CHIMERE/MM5 and CHIMERE/WRF underestimate the PM<sub>10</sub> concentrations for January. The difference in PM<sub>10</sub> concentrations for January between CHIMERE/MM5 and CHIMERE/WRF is around a factor 1.6 (PM<sub>10</sub> higher for CHIMERE/MM5). This difference and the larger underestimation in PM<sub>10</sub> concentrations by CHIMERE/WRF are related to the differences in heat fluxes and the resulting PBL heights calculated by WRF. In general the PBL height by WRF meteorology is a factor 2.8 higher at noon in January than calculated by MM5. This study showed that the difference in microphysics scheme has an impact on the profile of cloud liquid water (CLW) calculated by the meteorological driver and therefore on the production of SO<sub>4</sub> aerosol. <br><br> A sensitivity analysis shows that changing the Noah Land Surface Model (LSM) for the 5-layer soil temperature model, the calculated monthly mean PM<sub>10</sub> concentrations increase by 30%, due to the change in the heat fluxes and the resulting PBL heights. <br><br> For June, PM<sub>10</sub> calculated concentrations by CHIMERE/MM5 and CHIMERE/WRF are similar and agree with the observations. Calculated O<sub>3</sub> values for June are in general overestimated by a factor 1.3 by CHIMERE/MM5 and CHIMRE/WRF. The reason for this is that daytime NO<sub>2</sub> concentrations are a higher than the observations and nighttime NO concentrations (titration effect) are underestimated.