The aerosol quality monitoring system in most mega-cities is based on a network of ground-based stations essentially measuring aerosol mass concentrations in one or two particle size ranges (usually PM10 and PM2.5). Satellite retrieval of aerosol optical depth (AOD) over urban areas has become an extremely important additional source of aerosol data as urbanization increases exponentially worldwide, and is critical in megacities without ground-based networks. Progress in this satellite domain will provide an alternative tool to complement the monitoring of urban air-pollution control efforts. Recent studies have compared quantitative AOD data from the MODIS sensor on the Terra and Aqua satellite (10 km x 10 km pixels) with data from ground-based particulate matter monitoring networks and have shown that the remote sensing product can be used quantitatively to estimate PM2.5 under certain conditions. Nevertheless, major improvements in the accuracy and spatial resolution of the satellite products are required for air quality applications in urban areas. We discuss the possibilities and limitations of the use of satellite AOD products with high spatial resolution over urban areas as air quality monitoring tools. The AOD product from satellites was compared specifically to the measured ground based PM2.5 mass concentrations of aerosols in different urban areas including Mexico City and Sao Paulo. We will present the results of a study evaluating the use of the remote sensing of aerosols as a complementary tool to the ground based monitoring of aerosols in the atmosphere in urban areas.