Satellite data of moderate spatial resolution (MODIS and MERIS) were used to retrieve the aerosol optical depth (AOD) over the urban area of Athens. MODIS products were obtained at a horizontal resolution of 10 by 10？ centered over Athens, while the differential textural analysis (DTA) code was applied to MERIS images to retrieve relative-to-reference AOD with a resolution of 260？m by 290？m. The possibility of exploiting the full resolution of MERIS data in retrieving AOD over a grid of a few hundreds metres was thereby investigated for the first time. MERIS-based AOD, centred at 560？nm, showed strong positive correlation to ground-based data ( = 0.85), while MODIS AOD products were in agreement with both MERIS and . Back trajectories were used to study the impact of atmospheric conditions prevailing during the examined days. Days associated with Saharan air masses corresponded to enhanced AOD and predominance of coarse-mode particles. The results suggest that, at least for the case of Athens, AOD retrieved by MERIS images using the DTA code over cloud-free areas can be related to . The accuracy of retrieval mainly depends on the successful selection of the reference satellite data, namely, an image being least contaminated by tropospheric aerosols. 1. Introduction Various types of aerosol can be found in the Mediterranean, namely, desert dust, originated from the Saharan desert, pollution particles, emittedmainly by urban and industrial activities, marine aerosol, formed continuously over the Mediterranean, and biomass-burning smoke, produced by seasonal forest fires during summer. and have been accounted for health impairment of citizens in urban areas [1, 2], and high levels of such particulate pollutants, along with gaseous pollutants (nitrogen oxides, carbon monoxide, and ozone) have been reported in Athens (Greece) over the last two decades [3]. Therefore, air quality improvement and monitoring in the capital of Greece is of great concern to the government since reduction of air pollutant exposures will have both short- and long-term public health benefits. Furthermore, Greece has to comply with the related directives of the European Union. Aerosol parameters can be measured in-situ or remotely sensed from ground, aircraft, or satellite. In particular, satellite remote sensing has been increasingly used to map aerosols in the atmosphere [4]. It constitutes a recent but powerful tool for assessing aerosol spatial distribution and properties due to its major benefit of providing complete and synoptic views of large areas in single snapshots. The
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