Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 5 2009 10.5194/acp-9-1579-2009 http://www.atmos-chem-phys.net/9/1579/2009/ http://www.atmos-chem-phys.net/9/1579/2009/acp-9-1579-2009.html http://www.atmos-chem-phys.net/9/1579/2009/acp-9-1579-2009.pdf 1579 1595 2009-03-03 Long term precipitation chemistry and wet deposition in a remote dry savanna site in Africa (Niger) C. Galy-Lacaux lacc@aero.obs-mip.fr D. Laouali L. Descroix N. Gobron C. Liousse Laboratoire d'Aérologie, 14, Av. Edouard Belin, 31400 Toulouse, France Université Abdou Moumouni, Faculté des Sciences, Département de Physique, BP 10662, Niamey, Niger Institut de Recherche pour le Développement (IRD) Niger, Laboratoire d'Étude des Transferts en Hydrologie et Environnement (UMR 5564), France European Commission – Joint research Centre, Institute for Environment and Sustainability Global Environment Monitoring Unit, TP 440, via E. Fermi, 21020 Ispra (VA), Italy Long-term precipitation chemistry have been recorded in the rural area of Banizoumbou (Niger), representative of a semi-arid savanna ecosystem. A total of 305 rainfall samples ~90% of the total annual rainfall) were collected from June 1994 to September 2005. From ionic chromatography, pH major inorganic and organic ions were detected. Rainwater chemistry is controlled by soil/dust emissions associated with terrigeneous elements represented by SO₄^2−, Ca²⁺, Carbonates, K⁺ and Mg²⁺. It is found that calcium and carbonates represent ~40% of the total ionic charge. The second highest contribution is nitrogenous, with annual Volume Weighed Mean (VWM) for NO₃^− and NH₄⁺ concentrations of 11.6 and 18.1 μeq.l^−1, respectively. This is the signature of ammonia sources from animals and NO_x emissions from savannas soil-particles rain-induced. The mean annual NH₃ and NO₂ air concentration are of 6 ppbv and 2.6 ppbv, respectively. The annual VWM precipitation concentration of sodium and chloride are both of 8.7 μeq.l^−1 which reflects the marine signature of monsoonal and humid air masses. The median pH value is of 6.05. Acidity is neutralized by mineral dust, mainly carbonates, and/or dissolved gases such NH₃. High level of organic acidity with 8μeq.l^−1 and 5.2 μeq.l^−1 of formate and acetate were also found. The analysis of monthly Black Carbon emissions and Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) values show that both biogenic emission from vegetation and biomass burning could explain the rainfall organic acidity content. The interannual variability of the VWM concentrations around the mean (1994–2005) is between ±5% and ±30% and mainly due to variations of sources strength and rainfall spatio-temporal distribution. From 1994 to 2005, the total mean wet deposition flux in the Sahelian region is of 60.1 mmol.m^−2.yr^−1 ±25%. 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