Three years (2006–2008) of ground-based observations of the Aerosol OpticalDepth (AOD) in the urban environment of Athens, in the EasternMediterranean, are analysed in this work. Measurements were acquired with aMulti-Filter Rotating Shadowband Radiometer at five wavelengths. The dailyaverage AOD at 500 nm is 0.23, and the mean Ångström coefficientcalculated between 415 and 867 nm is 1.41. The annual variability of AOD hasa spring maximum dominated by coarse dust particles from the Sahara (AOD0.34–0.42), while the diurnal pattern is typical for urban sites, with AODsteadily increasing throughout the day. The greatest contribution to theannually averaged AOD, accounting for almost 40%, comes from regional and local sources (namely theIstanbul metropolitan area, the extended areas of biomass burning around thenorth coast of the Black Sea, power plants spread throughout the Balkans andthe industrial area in the Po valley, with average daily AOD in the range of0.25–0.35). An additional importantcontribution (23%) is dust from Africa, whereas the rest of Europecontributes another 22%. The geographical distribution of the abovesources in conjunction with the prevailing synoptic situation andcontribution of local sources, lead to mixed types of aerosols over Athens,with highly variable contribution of fine and coarse particles to AOD in therange 10%–90%. This is the first long-term, ground based data setavailable for Athens, and it has also been used for the validation of satellitederived AOD by MODIS, showing good agreement on an annual basis, but with anoverestimation of satellite AODs in the warm period.
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