Chemical composition of mineral dust aerosol during the Saharan Dust Experiment (SHADE) airborne campaign in the Cape Verde region, September 2000



[1] During the Saharan Dust Experiment (SHADE) in September 2000, we collected filter samples on board the Met Office C-130 aircraft. The main study area was over the Atlantic Ocean between Sal Island and Senegal. Dust was encountered in the altitude range from 0.5–1 to 4–4.5 km. Sublayers with different particle size distributions and different source areas can be distinguished within the main dust plume. Diminution of O3 in anticorrelation with the dust layer suggests removal of O3 by dust particles. Dust loadings containing up to 54 μg m−3 Al (submicron plus supermicron fraction) were measured during particularly intense dust events. Mixing of dust with anthropogenic aerosols, mainly NH4HSO4, was observed in the fine fraction. It was associated with air masses that had originated over Europe and then traveled over North Africa. The ratio of nitrate to non-sea-salt sulfate was around 0.3. This argues against any significant influence of biomass burning emissions, which have much higher nitrate. However, we also encountered aged fossil fuel pollution plumes, likely from North America. The geochemical signature of mineral dust was consistent with previous results in the area. Si, Fe, and Ti were not enriched with respect to the soil composition, while other elements, such as Ca and S, were. Ca is prevalently present as calcite in African soils, but it is also found as calcium sulfate in the atmosphere.