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Chemistry of rain events in West Africa: Evidence of dust and biogenic influence in convective systems

by K. Desboeufs, E. Journet, J. L. Rajot, S. Chevaillier, S. Triquet, P. Formenti, A. Zakou
Atmospheric Chemistry and Physics ()
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This paper documents the chemical composition of 7 rain events\nassociated with mesoscale convective systems sampled at the supersite of\nBanizoumbou, Niger, during the first special observation periods\n(June-July 2006) of the African Monsoon Multidisciplinary Analyses\n(AMMA) experiment. Time-resolved rain sampling was performed in order to\ndiscriminate the local dust scavenged at the beginning of rain event\nfrom the aerosol particles incorporated in the cloud at the end of the\nrain. The total elemental composition is dominated by Al, Si, Fe and Ca,\nindicating a high influence of dust and limited marine or anthropogenic\ncontribution. After the aerosol wash-out, the elemental concentrations\nnormalized to Al and the microscopic observations of diatoms, a tracer\nof the Bodele depression, both indicate that the total elemental\ncomposition of rainwater is controlled by dust originating from\nNorth-eastern Saharan sources and probably incorporated in the\nconvective cloud from the Harmattan layer. The low variability of the\nrain composition over the measurement period indicates a regional and\ntemporal homogeneity of dust composition in the Harmattan layer. In the\ndissolved phase, the dominant anions are nitrate (NO(3)(-)), sulphate\n(SO(4)(2-)) and chloride (Cl(-)). However, between June and July we\nobserve an increasing contribution of the organic anions (formate,\nacetate, oxalate) associated with biogenic emissions to the total ion\ncomposition. These results confirm the large influence of biogenic\nemissions on the rain composition over Sahel during the wet season. The\npaper concludes on the capacity of mesoscale convective systems to carry\nsimultaneously dust and biogenic compounds originating from different\nlocations and depose them jointly. It also discusses the potential\nbiogeochemical impact of such a phenomenon.

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