Monitoring of Sahelian Aerosol and Atmospheric Water Vapor Content Characteristics from Sun Photometer Measurements

Abstract

Atmospheric measurements in two Sahelian sites in West Africa are presented and analyzed. The measurements were performed using a sun photometer with five bands in the visible and near-infrared range of the solar spectrum. This instrument measures spectral values of the solar irradiances that are used to derive the aerosol optical thickness in three bands; the two other bands are used to derive the integrated atmospheric water vapor content using a differential absorption method. The Ångström exponent, which is an estimate of the aerosol particle size, is derived from the spectral dependence of the optical thickness. Although the sites were located far from Sahara Desert aerosol sources, the observed aerosol optical thicknesses were high, with a mean annual value of 0.5 at 550 nm. The spectral dependence of aerosol optical thickness is generally low, with a mean annual value of Ångström's exponent of 0.4. The aerosol optical thickness and the atmospheric water vapor content are both characterized by high temporal variability and exhibit seasonal cycles. From these measurements, climatological values and associated probability distribution laws are proposed.