Analysis of spatial-temporal heterogeneity in remotely sensed aerosol properties observed during 2005-2015 over three countries along the Gulf of Guinea Coast in Southern West Africa

In the present study, the spatial-temporal distribution and estimation of trends of different aerosol optical properties, and related impact factors were investigated over three countries: Ghana, Togo, and Benin along the Gulf of Guinea Coast in Southern West Africa (SWA). For this purpose, long-term satellite derived aerosol optical properties (aerosol optical depth at 550 nm; AOD₅₅₀, Ångström exponent at 470-660 nm; AE_470-660, and absorption aerosol index; AAI) retrieved from the Moderate-resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI) during January 2005-December 2015 were utilized. The annual mean spatial distribution of AOD₅₅₀ was found to be high (>0.55) over the southern coastal area, moderate-to-high (0.35-0.55) over the central, and low (<0.35) over northern parts of the study domain. The seasonal mean variations showed high (low) values of AOD₅₅₀ and AAI during the Harmattan or dry (wet) season. Whereas, low (high) AE_470-660 values were characterized during the Harmattan (wet) season. Linear trend analysis revealed a decreasing trend in AOD₅₅₀ and AAI, and increasing trend in AE_470-660. Further, an investigation on the potential drivers to AOD distribution over the SWA revealed that precipitation, NDVI, and terrain were negatively correlated with AOD. Finally, the HYSPLIT derived back trajectory analyses revealed diverse transport pathways originated from the North Atlantic Ocean, Sahara Desert, and Nigeria along with locally generated aerosols.