Mineral dust and carbonaceous aerosols in West Africa: Source assessment and characterization

As part of the AMMA (African Monsoon Multidisciplinary Analysis) international research project, an intensive field campaign called “Special Observing Period” (SOP 0) was carried out in West Africa during the dry season (February 2006), near M’Bour, Senegal. The aim of the ground-based sampling experiment was to determine the chemical composition of dust and carbonaceous aerosols in the surface layer, then to investigate the main source areas influencing the chemical composition of the particles. Major elements (Al, Ca and Fe), total (TC) and black carbon (BC), and water soluble ion (Na+, K+, Mg2+, NH4+, Cl−, NO3−, SO42− and PO43−) concentrations were measured. Total mass, number concentration and aethalometer measurements of PM-10 were also obtained. Mineral dust in the surface layer is principally present in the coarse fraction representing 75–90% of the collected mass (wt.%). Dust, suggested by backward trajectories of the air masses and supported by the variations of Ca/Al ratios, originates mainly from Northern Sahara. Particulate organic matter (POM) concentrations are more variable, but POM is mainly present in the fine fraction (up to 77 wt.%). Its presence is due to local sources such as domestic fire emissions rather than to remote sources as open-field vegetation fires in the Sahelian zone. Comparisons of Black Carbon (BC) concentrations measured with an aethalometer in the Ultra-violet and the Near-infrared wavelengths, show that POM originating from the adjacent Western African coast contains less aromatics than POM transported from the main biomass burning areas of the Sahelian zone. Thus, smouldering, the main combustion process for locally emitted carbon aerosols appears to generate less aromatic compounds than burning of vegetation.

► In winter, dust source regions in Senegal are located in the Northern Sahara.
► Carbonaceous aerosols have their main origin in smouldering processes.
► Smouldering produce less aromatics compounds in aerosols than flaming of vegetation.