Impacts of Aerosols on Precipitation over West Africa

Abstract

The aim of this study is to investigate the interaction between West African aerosols and convective, stratiform and shallow clouds, which affect precipitation quality and amount. Gridded satellite observation precipitation datasets Tropical Rainfall Measuring Mission (TRMM), in-situ precipitation data, reanalysis aerosol data ECMWF Atmospheric Composition Reanalysis 4 (EAC4) and rainwater samples were analysed to study the high aerosol concentration impacts on precipitation amount, frequency and quality over the Sahel and Guinea region. Mann-kendal Test was used to verify the trend in time series data. Sperman correlation was used to identify statistical links between aerosols and precipitation. To investigate the quantitative effects of aerosols on rain frequency over West Africa, the precipitation data was categorized into three groups: light (r < 2.5 mm/h), moderate (2.5 mm/h ≤ r < 10 mm/h), and heavy rain (10 mm/h ≤ r < 50 mm/h). Rain events from 12 stations located across West Africa were classified into two groups, based on aerosol concentration percentile. The first group contained precipitation data with the 10th percentile aerosol concentration considered as clean condition. While the second group contained data with the 90th percentile aerosol concentration considered as polluted condition. Each group was then further divided based on the percentile of aerosol events to obtain the rainfall frequency percentage. Precipitation reduction is observed during the seasons of December - January - February (DJF) and June - July - August (JJA) when black carbon and organic matter concentrations increased over West Africa. Additionally, results suggest that precipitation enhancement is observed during March - April - May (MAM) and September - October - November (SON) seasons when sulphate concentration increased. This may suggest that the West African atmospheric black carbon, organic matter and sulphate variation affect the amount of seasonal rainfall. However, the increase in sea salt particles during SON, when there is higher rainfall, could be related to the role of sea salt particles as cloud condensation nuclei, which could influence the amount and timing of rainfall in the region. Results show that dust is the dominant aerosol over all stations used in this study. Organic matter is the second most significant aerosol in this region, which contributes to atmospheric pollution. A comparison between the Clean-case (C-case) and Polluted-case (P-case) shows that aerosol concentration variations affect precipitation class frequency over West Africa. Results suggest that aerosol pollution mostly decreases light rain and increases moderate and heavy rain over West Africa. Chemical laboratory rainwater quality analysis illustrates that Ouagadougou experienced acid rain during the rainy season of 2022. It indicates that there may be sources of sulphur emissions in these areas that are contributing to the elevated sulphate levels in the rainwater. The rainwater sample from Dakar showed sodium levels above the threshold, which could be an indication of potential sources of sodium contamination in the area. It indicates also that the levels of nitrate in the rainwater samples were within the normal range in most cities, but rainwater in some cities shows that the levels of nitrate in the rainwater samples were below the nitrate thresholds of 1.5 and 5 mg/L.