Long-Term Estimation of Rainfall Acidity in Major African Ecosystems

The paper examines the acidity of precipitation in three major ecosystems in Western and Central Africa, using data from 413 rainfall samples in the dry savanna of Banizoumbou, Niger; 1,056 samples in the wet savanna of Lamto, Ivory Coast; and 578 samples in the equatorial forest of Zoetele, Cameroon. Analyzed at the Toulouse Aerology Laboratory, the findings indicate that rain in Banizoumbou is alkaline (mean pH 6.19), while at Lamto and Zoetele it is acidic (mean pH 5.24 and 5.45, respectively). Despite significant potential acidity, actual acidity (measured by H+ ion concentration) remains low across all sites. The study highlights the importance of a multiphasic process in rainwater neutralization, involving the capture of acidic gases by mineral dust and gas-particle interactions primarily influenced by ammonia (NH3). Notably, the relationship between pH and the ratio of cations to anions varies: in Banizoumbou, they move inversely, whereas in Lamto and Zoetele, they change in the same direction. This suggests that terrestrial dust influences dominate in the dry savanna, while gas-particle conversion processes are more prominent in the wet savanna and equatorial forest.