The Influence of Fine-Mode Aerosols on MODIS–AERONET Aerosol Optical Depth Disparities in the Sahel West Africa

In the present paper, the disparities between aerosol optical depths (AOD) retrieved using the Moderate Resolution Infrared Spectro-radiometer (MODIS) and AErosol RObotic NETwork (AERONET) are presented for five Sahel sites based on the R 2 correlation metric of their multiyear data. The sites are Banizoumbou (13.54N; 2.66E), Dakar (14.39N; 16.95 W), Djougou (9.76N; 1.59E), Ilorin (8.32N; 4.34E), and Ouagadougou (12.2N; 1.4 W). The overall and seasonal MODIS–AERONET AOD disparities for the locations were evaluated and compared with aerosol parameters associated with fine-mode prevalence at the sites, namely, total AOD ( AOD 500nm ), precipitable water ( PW ), Angstrom derivative ( α 1 ), and fine mode fraction ( FMF ) for the seasons MAM, JJA, SON, and DJF. The largest overall disparity (least R 2 ) was found for Ilorin ( R 2 = 0.021 ± 0.295), while the least overall disparity (largest R 2 ) was found for Banizoumbou ( R 2 = 0.660 ± 0.132). The FMF was found to have the strongest correlations with the overall and seasonal disparities ( R 2 * FMF = – 0.626 (overall), R 2 * FMF = – 0.350 (MAM), R 2 * FMF = – 0.9 (JJA), R 2 * FMF = – 0.602 (SON), R 2 * FMF = 0.084 (DJF). The largest seasonal disparity occurred at Ilorin in JJA ( R 2 = 0.014) when very low AOD 500nm (0.352) was associated with significant fine-mode occurrence ( α 1 = 0.433; FMF = 0.581), while the least seasonal disparity occurred in Djougou in MAM ( R 2 = 0.866) when very high AOD 500nm (0.814) was associated with the least fine-mode occurrence ( α 1 ~ 0.00; FMF = 0.278). In Banizoumbou, Dakar, Djougou, and Ouagadougou, fine-mode aerosols occurred more significantly in the dry period (low PW ), suggesting a biomass burning source, and at Ilorin, the fine-mode occurred more significantly in the monsoon season (high PW ), suggesting a non-biomass burning source. Since the AOD at Ilorin correlated strongest with monsoon-season meridional wind ( AOD 500nm *MW wet = 0.611), compared to other seasonal wind fields, it is inferred that the large monsoon-season MODIS–AERONET AOD disparities at the site were due to southwesterly wind, which transports fine-mode aerosols from coastal industrial sites, as indicated in previous studies.