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

Abstract

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 × 10-3 up to 0.86. By means of a least square fitting procedure, the Ångström turbidity parameters have been estimated. (α daily values 0.92 ÷ 3.64, α nightly values 0.008 ÷ 4.2; β daily values 0.06 ÷ 0.56, β nightly values 0.001 ÷ 0.68). Furthermore, a relationship between Ångström 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.