Dust deposition and particle size in Mali, West Africa

Abstract

Dust deposition samples were collected during April and May (1990) at four sites on the inland delta region of Mali in West Africa. High dust deposition rates were measured and characterised by considerable variations between sites. A major dust plume passed over the area, following a dust storm near Gao, 500 km to the east, however the contribution of this plume to overall deposition was surprisingly small. Dust particle-size characteristics indicate mixing of dusts from three sources. Long-distance dust deposition producing fine deposits (mainly < 5 μm); dust from regional sources producing deposits mainly in the size range 20–40 μm; and local dust arising from human activities, particularly vehicle and livestock movements, producing relatively coarse material in the size range 50–70 μm, plus some fines. Local dusts appear to have made the greatest contribution to deposition, with a moderate (though less than expected) contribution from the plume and a minor contribution from long-distance dusts. These results have implications for dust deposition studies aimed at quantifying dust contributions to soils. Although these Mali dust deposition rates are high (2.5–28.6 t −1 km −2 day −1), the significant proportion of locally derived dust means that new dust inputs to the soils of the Inland Delta region of Mali are small.