Growing attention has been paid over recent years to the possibility of monitoring surface particulate matter (PM) concentrations through the use of satellite observations. Satellite remote sensing of both aerosol and trace gas constituents can be usefully employed in air quality monitoring (AQ). The MODIS (Terra-Aqua/NASA platforms) aerosol optical properties were used here in a semi-empirical approach to estimate PM2.5 content at ground level. PM2.5 samplings were employed to convert aerosol optical depth AOD into PM estimates, considering meteorological fields simulated by MM5. Thus, daily maps of satellite-based PM2.5 concentrations over Northern Italy were derived. Comparison with daily PM2.5, sampled on the ground during the QUITSAT project over six validation sites of the Po valley, showed good agreement (R2 Â¿ 0.68 and R2 Â¿ 0.59 for MODIS/Terra and MODIS/Aqua, respectively), with the satellite-based concentrations tending to underestimate the values by at most ~20%. Monthly averaged values were also compared providing good agreement, with R2 Â¿ 0.7 for each platform.