Diurnal and nocturnal measurements of aerosol optical depth at a desert site in Namibia

The Namibian desert is a candidate site for astrophysical observations with ground-based instrumentation, such as the High Energy Stereoscopic System experiment. For this kind of application, the characterization of the atmospheric transmittance is mandatory. In this context, a first campaign of both solar and lunar direct irradiance measurements has been performed from 7 July 1998 to 10 July 1998 on a desert plateau in Namibia. The aerosol optical depth in the visible range (330-700 nm) has been measured; we found daily values (at 483 nm) ranging from 0.10 to 0.26, while the nightly ones range from 2.5 x 10(-3) up to 0.86. By means of a least square fitting procedure, the a Angstrom turbidity parameters have been estimated. (alpha daily values 0.92 divided by 3.64, alpha nightly values 0.008 divided by 4.2; beta daily values 0.06 divided by 0.56, beta nightly values 0.001 divided by 0.68). Furthermore, a relationship between a Angstrom parameters and meteorological variables such as relative humidity and wind speed has been investigated. The results do not highlight any correlation, except for two diurnal data sets that show a negative correlation between the optical thickness and the wind speed. Although preliminary, our measurements allow us to have a first insight into characterizing the aerosol optical properties of the Namibian background aerosol. On the other hand, an extended campaign of measurements is needed for a full characterization of the site.