Rainwater Chemistry and Wet Deposition over the Wet Savanna Ecosystem of Lamto (Cote d'Ivoire)

摘要

From the IGAC-DEBITS Africa network (IDAF), data sets on precipitation chemistry collected from the 'wet savanna ecosystem' site of Lamto (Cote d'lvoire), are analyzed (1995-2002). Inorganic (Ca~(2+), Mg~(2+), Na~+, K~+, NH_4~+ Cl~-, SO_4~(2-), NO_3~-) and organic (HCOO~-, CH_3COO~-) ions content were determined using Ion Chromatography. The analyzed 631 rainfall events represent 8420.9 mm of rainfall from a 9631.1 mm total. The precipitation chemistry at Lamto is influenced by four main sources: natural biogenic emissions from savanna soils (NO_x and NH_3), biomass burning (savanna and domestic fires), terrigeneous particles emissions from dry savanna soils, and marine compounds embedded in the summer monsoon. The inter-annual variability of the weighted volume mean concentration of chemical species linked with wet deposition fluctuates by ～20% over the period. Ammonium concentration is found to be the highest (17.6 μeq.l~(-1)) from all IDAF sites belonging to the West Africa ecosystems. Ammonia sources are from domestic animals, fertilizers and biomass burning. In spite of the high potential acidity of 30.5 μeq.l~(-1) from NO_3~-, SO_4~(2-), HCOO~-and CH_3COO~-, a relatively weak acidity is measured: 6.9 μeq.l~(-1). The 40% acid neutralization is explained by the acid gas - alkaline soil particles interaction. The remaining neutralization is from inclusion of gaseous ammonia. When results from Lamto, are compared with those from Banizoumbou (dry savanna) and Zoetele (equatorial forest), a regional view for wet tropospheric chemistry processes is obtained. The high concentration of the particulate phase in precipitation emphasizes the importance of multiphases processes between gases and particles in the atmospheric chemistry of the West Africa ecosystems. For example, the nss Ca~(2+) precipitation content, main indicator of terrigeneous particles, goes from 30.8 μeq.l~(-1) in dry savanna to 9.2 μeq.l~(-1) at Lamto and 8.9 μeq.l~(-1) in the Cameroon forest. A similar gradient is obtained for rainfall mineral particles precipitation content with contribution of 80% in dry savanna, 40% in wet savanna, and 20% in the equatorial forest.