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Guillaume A. d'Almeida


In this paper the source strength and the deposition rate of the dust emerging from the Sahara are assessed. For this purpose a multichannel sunphotometer has been developed and a turbidity network covering 11 stations has been set up in the Sahara, in the Sahel region and the surrounding southern area for a duration of about two years. A correlation analysis connecting observed aerosol turbidity parameters and mineral dust mass concentration has been performed during a four-week field campaign in Agadez (Niger). An appropriate box model including the aerosol turbidity parameters, actual wind field data of the source regions, the general circulation pattern over Africa and dry and wet deposition reveals a total mass production of about 630 × 106 and 710 × 106 t yr−1 for all suspended particulate matter, 80 × 106 and 90 × 106 t yr−1 for aerosol particles smaller than 5 μm radius for the years 1981 and 1982 respectively. About 60% of the mass moves southward to the Gulf of Guinea, 28% westward to the equatorial North Atlantic Ocean and 12% northward to Europe. A considerable part is deposited in the Atlantic Ocean and the Mediterranean forming deep-sea sediments.

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Guillaume A. d'Almeida and Lothar Schütz


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 μm and 1000 μm 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 μm radius and mineral particles have been found in the Aitken size range down to 0.02 μm radius. The concentration decrease toward larger particles is not uniform and shows considerable variations below 0.5 μm and above 5 μm radius. Volume distributions show a fairly stable mode around 3 μm and a highly variable mode around 30 μm radius. Particles below 5 μm 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 μm radius and confirm earlier results of a secondary maximum between 20 and 30 μm radius, which hypothesizes a particle loss for the size range of r < 20 μm due to erosion. Soil volume distributions also show a bimodal structure with two maxima, one for particles less than 5 μm radius and the other for larger particles. This might indicate different mineral composition due to weathering and subsequent removal by water and wind erosion.

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