Long term precipitation chemistry and wet deposition in a remote dry savanna site in Africa (Niger)

摘要

Long-term precipitation chemistry have been recorded in the rural area ofBanizoumbou (Niger), representative of a semi-arid savanna ecosystem. Atotal 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 iscontrolled by soil/dust emissions associated with terrigeneous elementsrepresented by SO₄2−, Ca2+, Carbonates, K+ andMg2+. 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 andNO_x emissions from savannas soil-particles rain-induced. The meanannual NH₃ and NO₂ air concentration are of 6 ppbv and 2.6 ppbv,respectively. The annual VWM precipitation concentration of sodium andchloride are both of 8.7 μeq.l−1 which reflects the marinesignature of monsoonal and humid air masses. The median pH value is of 6.05.Acidity is neutralized by mineral dust, mainly carbonates, and/or dissolvedgases such NH₃. High level of organic acidity with 8μeq.l−1and 5.2 μeq.l−1 of formate and acetate were also found. Theanalysis of monthly Black Carbon emissions and Fraction of AbsorbedPhotosynthetically Active Radiation (FAPAR) values show that both biogenicemission from vegetation and biomass burning could explain therainfall organic acidity content. The interannual variability of the VWMconcentrations around the mean (1994–2005) is between ±5% and ±30%and mainly due to variations of sources strength and rainfallspatio-temporal distribution. From 1994 to 2005, the total mean wetdeposition flux in the Sahelian region is of 60.1 mmol.m−2.yr−1±25%. Finally, Banizoumbou measurements are compared to otherlong-term measurements of precipitation chemistry in the wet savanna ofLamto (C?te d'Ivoire) and in the forested zone of Zoétélé(Cameroon). The total chemical loading presents a maximum in the dry savannaand a minimum in the forest (from 143.7, 100.2 to 86.6 μeq.l−1),associated with the gradient of terrigeneous sources. The wet depositionfluxes present an opposite trend, with 60.0 mmol.m−2.yr−1 inBanizoumbou, 108.6 mmol.m−2.yr−1 in Lamto and 162.9mmol.m−2.yr−1 in Zoétélé, controlled by rainfallgradient along the ecosystems transect.