Distribution and Mixing State of Saharan Dust in the Vertical Column - Airborne observations over the Atlantic Ocean in the Cape Verde region

During the Saharan Mineral Dust Experiment (SAMUM) 2008 field campaign, particles with geometric diameters between approximately 0.1 to 25 µm were collected on board of the Falcon sampling aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR, German Aerospace Centre). To investigate the spatial and vertical distribution of aerosol in the Cape Verde region, sampling was performed along the West African coastline and in the Cape Verde region. The chemical composition of these samples was determined by means of electron microscope single particle analysis. The major task of the conducted flights was to observe mixing of Saharan desert dust and biomass burning aerosol from the south Sahel belt. On all days investigated a distinct layer structure of biomass burning aerosol and desert dust layers was observed. The aerosol composition of the small particles (geometric diameter < 0.5 µm) was highly variable and in case of biomass burning strongly dominated by soot with up to 90 % relative number abundance. Internal mixtures of soot particles with mineral dust were not detected. Soot was only observed to mix with secondary sulphate. Biomass burning aerosol particles were enriched in potassium and chlorine originating from the vegetation matrix, and sulphur. After long travel times (aging), particles appeared to be depleted in chlorine and enriched in sulphur. The coarse particles (d > 0.5 µm) were dominated by silicates, which appeared to be mixtures of different minerals. In the Cape Verde region mineral dust is well mixed. For the aerosol arriving from central West Africa a determination of source regions by elemental or mineralogical composition was generally not possible. However, when the air mass followed another transport path, with longer travel time as observed for some high altitude trajectories, the dust origin appeared to be more in the south east of central West Africa, probably in the Bodélé. Refractive indices were calculated for each flight level, revealing a large variance in light absorption, resulting from the high number abundance of soot particles in the biomass burning layers. For a detailed modeling of radiation forcing and resulting effects on climate, the observed layer structure displaying different chemical and physical properties at the respective levels should be taken into account and might lead to some modification of the heretofore known results.

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