Trends in rainwater ions (Na+, K+, Cl-) under the influence of the West African Monsoon in the wet savanna of Lamto, Côte d’Ivoire

Abstract

Atmospheric chemical composition is strongly influenced by natural and anthropogenic emission sources, which may vary with season. In this study, we investigated the trends in the impact of the West African Monsoon (WAM) on the variability of sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) ions in rainfall over the humid savanna of Lamto in Côte d’Ivoire. Over the 22 years (1994-2015) of the study period, a combination of in-situ rainfall measurements and satellite-based Climate Hazards Group InfraRed Precipitation with Station (CHIRPS) precipitation data was used to understand the rainfall patterns associated with the WAM on the concentrations of these ions. Non-parametric statistical analyses were also performed. Results revealed a significant increasing trend in all three ion concentrations at the event scale, suggesting that in addition to the natural sources such as the Atlantic Ocean, the Sahel and Saharan desert, other anthropogenic factors such as road construction and urbanisation may influence the presence of these ions in the rainwater. Moreover, the strong positive correlations between Na⁺ and Cl⁻ suggest a common marine source transported by the WAM flow still predominates in Lamto. However, the increasing trend in K⁺, which is a good tracer of biomass combustion, highlights that Lamto is also influenced by local surrounding anthropogenic sources such as biomass burning. An improved assessment of the impact of these local sources will be needed for efficient mitigation strategies in the area.