Aerosol optical depth at 550？nm ( ) and fine-mode (FM) fraction data from Terra-MODIS were obtained over the Greater Athens Area covering the period February 2000–December 2005. Based on both and FM values three main aerosol types have been discriminated corresponding to urban/industrial aerosols, clean maritime conditions, and coarse-mode, probably desert dust, particles. Five main sectors were identified for the classification of the air-mass trajectories, which were further used in the analysis of the ( and FM data for the three aerosol types). The HYSPLIT model was used to compute back trajectories at three altitudes to investigate the relation between -FM and wind sector depending on the altitude. The accumulation of local pollution is favored in spring and corresponds to air masses at lower altitudes originating from Eastern Europe and the Balkan. Clean maritime conditions are rare over Athens, limited in the winter season and associated with air masses from the Western or Northwestern sector. The coarse-mode particles origin seems to be more complicated proportionally to the season. Thus, in summer the Northern sector dominates, while in the other seasons, and especially in spring, the air masses belong to the Southern sector enriched with Saharan dust aerosols. 1. Introduction Atmospheric aerosols play a crucial role in the Earth’s climate through scattering and absorption both solar and thermal radiation (direct effect), thus influencing the radiative and energy balance and/or acting as cloud condensation nuclei (indirect effect), affecting cloud albedo and lifetime, precipitation rate, and hydrology cycle [1]. The great effort spent over the last decades by the scientific community focusing on aerosol climatology and optical properties reflects the importance attributed to aerosols in controlling the Earth’s climate [2]. Major advancements in this field have been achieved by the new generation of space borne instruments (e.g., Along Track Scanning Radiometer (ATSR-2) on board the European Space Agency (ESA-ERS2), Moderate Resolution Imaging Spectroradiometer (MODIS), and Multiangle Imaging Spectroradiometer (MISR) on board the NASA Terra, POLarization and Directionality of the Earth's Reflectances (POLDER) on board the ADEOS), specifically designed to provide detection and characterization of atmospheric aerosols [3]. The Mediterranean Sea is one of the most affected areas regarding aerosol load in the world [4]. Due to the variety of the regions surrounding the Mediterranean basin, different aerosol types can be found within the basin, having
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