On the variability of desert aerosol radiative characteristics

Size distributions of desert aerosol observed in arid regions of Senegal, Mali, and Niger under different weather conditions are classified in terms of visibility ranges into three categories and converted to lognormal distributions. The resulting curves are considered to represent average distributions encountered during fair weather, wind carrying dust, and sandstorm episodes. By means of the Mie theory, main radiative characteristics, i.e., attenuation coefficients, single scattering albedo, asymmetry factor, and phase functions, are computed for the spectral range between 0.3 and 40 μm. Regardless of the prevailing atmospheric conditions, extinction and scattering coefficients indicate a nearly wavelength independent pattern in the solar spectral range due to the large and effective particle scattering diameter. All size distributions yield strong absorption peaks at 10 μm, clogging the atmospheric window. Moreover, the temporal variation of single scattering albedo and asymmetry factor is most sensitive in the solar spectrum. For climate modeling purposes, average values are computed for seven spectral bands of the European Centre for Medium Range Weather Forecast radiation scheme. In addition, phase functions for eight characteristic wavelengths are presented and show predominantly forward scattering properties. Optical depth measurements performed in the framework of the African turbidity monitoring network between November 1980 and February 1984 indicate a high space and time fluctuation of aerosol concentration over the Sahara and surrounding areas. Daily average optical depth values (base e ) between 0.03 in the Hoggar mountains and 3.5 during sandstorms in Boutilimit (Mauritania) have been observed for the mandatory wavelength 0.5 μm. Turbidity values in Zaria (Nigeria) indicate that the Sahel region is an insignificant area for mineral dust production.