Number, Mass and Volume Distributions of Mineral Aerosol and Soils of the Sahara

Abstract

A direct method will be described to determine the complete mineral size distribution in aerosol (xylene-insoluble component) and soils (water-insoluble component) covering a size range from 0.01 up to 100 ?m and 1000 ?m radius, respectively, by using a combination of a scanning electron microscope, optical microscope and sieving. Aerosol and soil samples from the Sahara have been investigated. All mineral aerosol size distributions indicate a maximum between 0.06 and 0.08 ?m radius and mineral particles have been found in the Aitken size range down to 0.02 ?m radius. The concentration decrease toward larger particles is not uniform and shows considerable variations below 0.5 ?m and above 5 ?m radius. Volume distributions show a fairly stable mode around 3 ?m and a highly variable mode around 30 ?m radius. Particles below 5 ?m radius can be considered as a well mixed mineral background aerosol, traveling long distances, whereas larger particles seem to be of local origin, activated under strong wind conditions. Soil size distributions show an absolute maximum at 0.1 ?m radius and confirm earlier results of a secondary maximum between 20 and 30 ?m radius, which hypothesizes a particle loss for the size range of r < 20 ?m due to erosion. Soil volume distributions also show a bimodal structure with two maxima, one for particles less than 5 ?m radius and the other for larger particles. This might indicate different mineral composition due to weathering and subsequent removal by water and wind erosion.