Analysis of radiative properties and direct radiative forcing estimates of dominant aerosol clusters over an urban-desert region in West Africa

The strategic location of the AERONET site (Ilorin) makes it possible to obtain information on several aerosol types and their radiative effects. The strong reversal of wind direction occasioned by the movement of the ITCZ during the West Africa Monsoon (WAM) plays a major role in the variability of aerosol nature at this site. Aerosol optical depth (AOD) (675 nm) and Angstrom exponent (AE) (440-870 nm) with 1st and 99th percentile values of 0.08 and 2.16, and 0.11 and 1.47, respectively, confirms the highly varying nature of aerosol at this site. Direct radiative forcing (DRF) and radiative forcing efficiency (RFE) of aerosol as retrieved from the AERONET sun-photometer measurements are estimated using radiative transfer calculations for the period 2005-2009 and 2011-2015. The DRF and RFE of dominant aerosol classes - desert dust (DD), biomass burning (BB), urban (UB) and gas flaring (GF) - have been estimated. Median (± standard deviation) values of DRF at top-of-atmosphere (TOA) for the DD, BB, UB and GF aerosol classes are -27.5 ± 13.2 Wm⁻², -27.1 ± 8.3 Wm⁻², -11.5 ± 13.2 Wm⁻² and -9.6 ± 8.0 Wm⁻², respectively. While that of RFE for DD, BB, UB and GF aerosol classes are -26.2 ± 4.1 Wm⁻²δ⁻¹, -35.2 ±4 .6 Wm⁻²δ⁻¹, -31.0 ± 8.4 Wm⁻²δ⁻¹ and -37.0 ± 10.3 Wm⁻²δ⁻¹, respectively. The DD aerosol class showed the largest DRF but the smallest RFE, arguably, due to the high SSA and asymmetry factor values for this aerosol type. Its smallest AOD notwithstanding, the GF class could cause more perturbation to the Earth-Atmosphere system in the sub-region both directly and indirectly possibly due to the presence of black carbon and other co-emitted aerosol and the ageing of the GF aerosols. This study presents the first estimate of DRF for aerosols of gas flaring origin and shows that its radiative potential can be of similar magnitude to biomass burning, and urban aerosol in West Africa.