Desert Aerosol: Characteristics and Effects on Climate

Abstract

Major sources for crustal - derived aerosol are the earth’s arid and semiarid regions. Soil size distributions of different locations in the Sahara have, therefore, been analysed and allow us to hypothesize a particle loss mostly for the size range of radius smaller than 20 µm due to erosion. Cumulative mass distributions emphasize an apparent difference in the productivity of the soil types considered. Furthermore, the physical properties and radiative characteristics of desert dust such as size distribution, source strength, deposition rate, extinction, scattering, and absorption coefficients, single scattering albedo, asymmetry factor, and optical depth, that are relevant quantities required to estimate the aerosol impact on present day climate and likely to reconstruct the earth’s past climate, have been observed or computed, and discussed. About 600 – 700 Tg of crustal material are mobilized from the Sahara and 1800 – 2000 Tg worldwide and injected into the atmosphere each year. A considerable part of that amount contributes to the sediments of the Atlantic, the Mediterranean, and the Pacific. Both computed and observed data indicate desert dust as one of most prominant aerosol types with the highest variability in its microphysical components as well as in its radiative characteristics, and the best absorber in the atmospheric transparency window. It has been pointed out that the presence of desert dust leads to a warming due to the absorption of solar radiation in the dust layer and above the dust layer, a corresponding cooling due to the backscattered solar radiation, and a challenging warming due to the absorption of the thermal infrared radiation below the dust cloud.