Multi-year analysis of aerosol optical properties at various timescales using AERONET data in tropical West Africa

The optical properties of aerosols are investigated using multi-year analysis from the Ilorin AERONET site (8.320 N, 4.340 E) in Nigeria, in tropical West Africa. Characterization at five wavelengths (440, 550, 675, 870 and 1020 nm) were carried out to investigate the absorption and scattering tendencies of aerosol properties at multi-timescale. Results show large aerosol optical thickness (AOT) at 550 nm in a mixed aerosol load with 74% coarse mode dominance and 24% fine mode particles densely distributed in the dry-cold/Harmattan season (DJF) and sparsely distributed in the wet season (June-Oct). The volume size distribution (VSD) pattern shows higher variations and dominance for the coarse mode in the dry season than for the accumulation mode. The AngstroEurom exponent (AE) alpha and tau 550 show regular variations, with seasonal averaged maximum alpha > 1 and tau 550 peak at similar to 1.0. Values of alpha > 1 were observed in wet seasons; similar to 1.1 (Aug), 1.04 (Sep). Averaged single scattering albedo (SSA) is confined to values between 0.83 and 0.97, with the highest values in October and lowest values in DJF due to biomass burning (BB) emissions mixed with transported desert dust that are composed of iron oxides (hematite) absorbing mineral. Seasonal omega 0 increases with increasing wavelength and absorption was found to be the highest at 440 nm, with a total absorption of 0.16. The asymmetry parameter g shows minor interannual variations. The highest value of >0.70 was found at the shortest wavelength (440 nm), and was found to decrease as wavelengths increased. The real part of the refractive index (RR) ranges between 1.17 and 1.44 depending on seasons, and the imaginary part of the refractive index (RI) shows higher absorption for dry season (0.009).