Seasonal characteristics of spectral aerosol optical properties at a sub-Saharan site

The seasonal dependence of the Aerosol Optical Depth (AOD) and the associated Angstrom exponent (α) at Ilorin Nigeria (8°32′N; 4°34′E) are studied using the NASA–AERONET measurements for 6 years (1998–2003). Both the AOD and its sensitivity to wavelength are highest at the peak of dry season months (January, February, March) and lowest during the wet season months (June through September). Using fitted Gaussian models to the AOD and α distributions; it is shown that a wide spectrum of aerosol particle sizes is enclosed in a largely coarse mode aerosol regime. The particle distribution is found to be more homogenous in the wet season (0.53 < α < 0.70), with fine dust as the major constituent whereas in the dry season there is a larger spectrum of particles (0.36 < α < 1.00) which is differentiated between a lower α mode (0.36 < α < 0.41) mainly of Sahara dust and a higher α mode (0.85 < α < 1.00) possibly of biomass smoke from bush burning. The results are well correlated with size distribution data for the site and corroborate previous conclusions on aerosol apportionment for the region. A range of weather factors is used to explain the observed seasonal variability. The results amongst other benefits reinforce the existing understanding about aerosol flux patterns in the area.