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 in Ilorin, Nigeria, 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, which is confirmed by aerosol optical depth (AOD) (675 nm) and Ångström exponent (AE) (440–870 nm) values with 1 ^st and 99 ^th percentile values of 0.08 and 2.16, and 0.11 and 1.47, respectively. The 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 periods of 2005–2009 and 2011–2015. The DRF and RFE of the dominant aerosol classes—desert dust (DD), biomass burning (BB), urban (UB) and gas flaring (GF)—have been estimated. The median (± standard deviation) values of the DRF at the top of the 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 those of the RFE are –26.2 ± 4.1 Wm ^–2 δ ^–1 , –35.2 ± 4.6 Wm ^–2 δ ^–1 , –31.0 ± 8.4 Wm ^–2 δ ^–1 and –37.0 ± 10.3 Wm ^–2 δ ^–1 , respectively. Arguably due to its high SSA and assymetric values, the DD aerosol class shows the largest DRF but the smallest RFE. Its smallest AOD notwithstanding, the GF class can cause greater perturbation of 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 similar in magnitude to that of biomass burning and urban aerosol in West Africa.