Diurnal variation of the aerosol optical depth for two distinct cases in the Athens area, Greece

This work uses the Ångström turbidity coefficient to investigate the variation of the aerosol optical depth (AOD) in the Athens area during "aerosol-clear" (low particulate-matter concentration) and "aerosol-burden" (high particulate-matter concentration) atmospheric conditions. The AOD is estimated at wavelengths in the band 310-1100 nm from direct-beam spectral irradiance measurements using ground-based instrumentation during an experimental campaign performed in the period 20 September to 1 October 2002. All data collected were recorded under clear-sky conditions near the city center of Athens; the AODs were estimated with calculated optical air masses in the range 1.3-2.0. The study shows that the AOD is influenced by meteorological parameters such as humidity and wind speed and direction. Also, traffic seems to play a significant role on the concentration of the aerosols in the atmosphere of urban environments. Therefore, the AODs are higher with low relative humidity, low south wind, high traffic and human activity. On the contrary, the AODs are found to be low with high relative humidity, higher south winds and low human activity and traffic. Also, the AOD levels show a gradual decrease with wavelength and time of the day.