MOBILIZATION, TRANSPORT AND DYNAMICS OF WEST AFRICAN AEROSOLS AS RETRIEVED FROM GROUND AND SATELLITE OBSERVATIONS

Abstract

West Africa due to its climatological and geographical conditions is an essential domain for the characterization atmospheric aerosol optical properties across the globe. This research provides a comparative and analytical study of aerosol optical properties which includes the Aerosol Optical Depth (AOD440nm), Angstrom exponent (α440-870), Wind trajectory and particle size distribution, were investigated using satellite and ground-based sun-photometer measurements between 2004-2014 at six West African sites; Agoufou (15°21'N, 1°29′W), Banizoumbou (13°45′N, 02°39′E), Ouagadougou (12°22'N, 1°31′W), Dakar (14°42'N, 17°29′W), Djougou (9°42′N, 1°40′E) and Ilorin (8°32′N, 4°34′E). The day-to-day variation of AOD revealed days of severe AOD occurrences in each of the sites which were examined using geopotential height at 850 hpa, relative humidity at 850 hpa, surface mean wind and backward trajectories of air masses at 500m, 1500m and 3000m, arriving at the study the sites during the period of maximum AOD value. The intercomparison of satellite derived data of CALIPSO, MISR, MODISSTD, MODISDB, and OMI with ground-based AERONET measurements revealed good degree of correlation at the six study sites. The MODISSTD – AERONET comparisons showed an overall high correlation (R2 > 60), and the MODISDB – AERONET comparisons indicated relatively overall low correlation of R2 < 60 at the six investigated sites. Results from multiyear observations indicates that monthly average AOD generally exhibits primary and secondary maximum in March and June respectively. Furthermore, the seasonal cycle of Angstrom exponent shows maximum AODs during the pre-monsoon (March-April-May) and monsoon (June-July- August) periods and are often associated with dust storm in all sites. It suggests that fine mode aerosols influence the daily aerosol optical even during the local harmattan season when biomass/urban pollution aerosol mixes with coarse mode dust aerosols originating at the sites. Trajectory analysis revealed that winds originating from the Sahara are predominantly distributions of dust particles transported especially from the source regions over West Africa. Regarding periods of severe AOD occurrences, strong synoptic-scale ridge developed which caused a shift in the position of the anticyclone over the desert region. This study demonstrates that localized sources of anthropogenic aerosols, desert dust, and smoke from forest fires can influence regions in West Africa that are far removed from the emission sources