Remote sensing of aerosol optical characteristics in sub-Sahel, West Africa

We have determined the characteristics of sub-Sahelian aerosols from a 2-ye ar record of continuous ground-based measurements, made at the University o f Ilorin, Ilorin (08 degrees 19'N, 04 degrees 20'E), Nigeria, in cooperatio n with the Aerosol Robotic Network. Observations of spectral aerosol optica l depths during the dusty harmattan season indicate more than a twofold inc rease, when compared to other seasons. Retrieved columnar volume size distr ibutions show the existence of bimodality with a dominant coarse mode. The retrieved size distributions were grouped according to different ranges of aerosol optical depths to characterize the aerosols for this particular reg ion. Monthly means of retrieved single-scattering albedos show a sharp decr ease from similar to0.95 to similar to0.85 at 500 nm from the preharmattan to the harmattan season when biomass burning is also practiced, increasing the presence of absorbing aerosols. On the basis of these comprehensive obs ervations, we propose to augment existing desert aerosol models, as present ed in the literature, to better characterize the dust outbreak season in We st Africa, which is quite prolonged and overlaps with the biomass burning s eason.