Spectral aerosol optical properties from AERONET Sun-photometric measurements over West Africa

Aerosol optical properties including Aerosol Optical Depth (AOD, τaλ), Angstrom exponent (α), size distribution, single scattering albedo (wo) were examined using long-term ground-based radiometric measurements at five West African sites; Dakar (14°42'N 17°29′W), Banizoumbou (13°45′N, 02°39′E), Agoufou (15°21'N,1°29′W), Ilorin (08°32′N, 04°34′E), and Ouagadougou (12°22′N, 1°31′W). Also included were observations made at the Cape Verde Islands (16° 45′N, 22° 57′W) to investigate the aerosol transport over the Atlantic Ocean. Results from multiyear observations show that monthly average AOD generally exhibits primary and secondary maximum in March and June respectively except Ilorin that shows peak value in January. The seasonal cycle of Angström exponent (α440–870 nm) shows that minimum values are often associated with dust storm in all sites however, the monthly mean α440–870 nm yields high values at Ilorin, even for peak dust seasons. It suggests that fine mode aerosols dominate the daily aerosol optical influence even during the ‘harmattan’ season when biomass/urban pollution aerosol mixes with coarse mode dust originating aerosols at the sites. The retrieved aerosol size distributions for predominately fine mode particles (cases with α440–870 nm> 0.5) at Ilorin and Cape Verde show an obvious steady increase in particle size as τa440 nm increased in both the modal maximum value and expansion of the fine mode boundary towards larger radius. The volume median radius of the size distribution of the coarse mode desert dust at Ilorin ranged from ∼ 1.9 to 3.2 μm with geometric standard deviation of 1.61 to 1.83. The coarse particle mode at Cape Verde shows very little dynamic changes as a function of aerosol optical depth. Results from estimated single scattering albedo (wo) show wide differences in the observed magnitudes of the fine/coarse mode particle absorption at the sites under study. The differences in dust absorption may be adduced to the possible interactions between dust and fine mode pollution aerosols and variability of the dust optical properties from different source regions in West Africa. Air mass backward trajectories based on multiyear data were developed to explain the transport and identification of different aerosol types in this region of study.