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In the present study we investigate the spatial and temporal variation of aerosol optical thickness (AOT) in the eastern Mediterranean basin, and more specifically in the area extending from 28.5 degrees N to 42.5 degrees N and from 18.5 degrees E to 35.5 degrees E, which includes large urban areas and megacities such as Cairo, Istanbul, Athens, Izmir, Ankara, and Thessaloniki. For this purpose we use long-term AOT data from the Total Ozone Mapping Spectrometer (TOMS) version2 converted to 500 nm (AOT(500)) for the period 1980-2001 and Collection 005 AOT data at lambda = 550 nm (AOT(550)) from Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites for the periods 2000-2005 and 2002-2005, respectively. The spatial and temporal variation of AOT shows a good agreement between TOMS and MODIS, in terms of geographical patterns, which maximizes the usefulness of TOMS AOT product given its long temporal coverage (climatological). According to MODIS-Terra, the annual mean AOT550 over the studied region equals 0.22 +/- 0.05 (monthly values ranging from 0.14 to 0.32) and shows strong spatial inhomogeneities. The smallest values (down to 0.1) occur over the western, and especially in the northwestern, part of Greece, over the northern part of Anatolian peninsula and also over the sea of east Mediterranean. The largest AOT values (up to about 1.0) occur over northern Africa, Middle East, and the adjacent coasts, and over the Anatolian plateau. These high aerosol loadings are of natural origin, mainly desert dust. However, very large values (up to 0.8) are also found over large urban areas surrounding megacities, associated with anthropogenic, apart from natural desert, aerosols. By using TOMS AOT data as proxy for the dust source, and the difference of MODIS-TOMS AOT as an indicator of the anthropogenic aerosol component, the relative contribution of natural versus anthropogenic sources of aerosols has been derived for the eastern Mediterranean area. Limitations of this approach are discussed and the associated uncertainties are also evaluated and discussed.