Abstract
Abstract. During the 2006 and 2007 special observing periods of the African Monsoon Multidisciplinary Analysis campaign an original experimental system has been implemented in Banizoumbou (Niger) for measuring the size-resolved dust emission flux in natural conditions and documenting the possible influence of wind speed on its size distribution. The instrumental set-up, associated methodology, and the quality tests applied to the data set are described before the results acquired during 2 events of the Monsoon type and 1 of the convective type are analyzed in detail. In good agreement with the theory of sandblasting, it is found in all cases that saltation must take place for a vertical emission flux to be detected. During a particular erosion event, the magnitude of the vertical flux is controlled by the surface roughness, which conditions the saltation threshold, and by the wind friction velocity. The dust flux released by the high energy convective event is also found to be much richer in very fine (<2 µm) particles than those of the relatively moderate Monsoon event, which shows that aerodynamic conditions definitely influence the initial size distribution of the erosion flux as previously suggested by wind tunnel experiments. However, the size distribution of the dust released by a given event is fairly constant and insensitive to even relatively important variations of u*. This is interpreted as a possible result of the rather long duration (15 min) over which wind fluctuations must be averaged for computing u*, which could make it an inadequate parameter for representing the very short response-time physical processes that are at the origin of fine dust emission at the measurement sites.
Highlights
Huge amounts of mineral particles are released by wind erosion in arid and semi arid areas
The model derived from these observations (Alfaro and Gomes, 2001) has been validated indirectly by comparing the total dust flux measured on the field with the one obtained by summing the masses predicted in each size class (Gomes et al, 2003; Rajot et al, 2003; Alfaro et al, 2004) but comparing the laboratory findings with size-resolved measurements performed in natural source areas is essential to increase the accuracy of models describing the dust cycle and its impacts
After summarizing briefly the current understanding of the two wind erosion processes that lead to emission of fine dust particles by arid and semi-arid soils the first section of this paper describes the original experimental set-up implemented at the Banizoumbou African Monsoon Multidisciplinary Analysis (AMMA) supersite during the dust emission season
Summary
Huge amounts of mineral particles are released by wind erosion in arid and semi arid areas. Jaenicke et al, 1971), it is usually considered that only the finest dust particles – those with sizes smaller than 20 μm and hereinafter referred to as FD – can remain suspended in the troposphere for several days and be transported very far from their source During this suspension phase they interact with solar and terrestrial radiation having a direct impact on climate. At the other end of the dust cycle, input of mineral particles in deposition areas can enhance the development of terrestrial or marine ecosystems (Swap, 1992; Jickells et al, 2005) Because it conditions at the same time the potential of air suspended particles for long range transport and the way these particles interact with solar and terrestrial radiation, the initial size distribution of dust emitted in source areas is a crucial parameter for the quantification of its impacts. This simplification allowed the use of fixed “standard” size-distribution – the most frequently used being the one Published by Copernicus Publications on behalf of the European Geosciences Union
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