Journal article

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

Desboeufs K, Journet E, Rajot J, Chevaillier S, Triquet S, Formenti P, Zakou A ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 19 (2010) pp. 9283-9293

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This paper documents the chemical composition of 7 rain events
associated with mesoscale convective systems sampled at the supersite of
Banizoumbou, Niger, during the first special observation periods
(June-July 2006) of the African Monsoon Multidisciplinary Analyses
(AMMA) experiment. Time-resolved rain sampling was performed in order to
discriminate the local dust scavenged at the beginning of rain event
from the aerosol particles incorporated in the cloud at the end of the
rain. The total elemental composition is dominated by Al, Si, Fe and Ca,
indicating a high influence of dust and limited marine or anthropogenic
contribution. After the aerosol wash-out, the elemental concentrations
normalized to Al and the microscopic observations of diatoms, a tracer
of the Bodele depression, both indicate that the total elemental
composition of rainwater is controlled by dust originating from
North-eastern Saharan sources and probably incorporated in the
convective cloud from the Harmattan layer. The low variability of the
rain composition over the measurement period indicates a regional and
temporal homogeneity of dust composition in the Harmattan layer. In the
dissolved phase, the dominant anions are nitrate (NO(3)(-)), sulphate
(SO(4)(2-)) and chloride (Cl(-)). However, between June and July we
observe an increasing contribution of the organic anions (formate,
acetate, oxalate) associated with biogenic emissions to the total ion
composition. These results confirm the large influence of biogenic
emissions on the rain composition over Sahel during the wet season. The
paper concludes on the capacity of mesoscale convective systems to carry
simultaneously dust and biogenic compounds originating from different
locations and depose them jointly. It also discusses the potential
biogeochemical impact of such a phenomenon.

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  • K. Desboeufs

  • E. Journet

  • J. L. Rajot

  • S. Chevaillier

  • S. Triquet

  • P. Formenti

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