Airborne observations of mineral dust over western Africa in the summer Monsoon season: spatial and vertical variability of physico-chemical and optical properties

We performed airborne measurements of aerosol particle concentration, composition, size distribution and optical properties over Western Africa in the corridor 2-17° N and 3-5° E. Data were collected on board the French ATR-42 research aircraft in June-July 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) intensive field phases in June-July 2006 using the AVIRAD airborne aerosol sampling system. The aerosol vertical distribution was documented on an almost daily basis. In particular, the vertical distribution of mineral dust emitted locally by Mesoscale Convective Systems (MSC) was distinguished from that of mineral dust that was transported from the Saharan by the African Easterly Jet (AEJ). Mineral dust emitted in the Sahel by convection-driven erosion was mostly confined in the boundary layer. One episode of injection of Sahelian mineral dust in the AEJ was observed. The elemental composition was found to be an element of difference between episodes, whereas the volume size distribution was more uniform. For non-mixed dust, the single scattering albedo ranged between 0.88-0.93 at 370 nm and between 0.91-0.99 at 950 nm, lower values being observed for Sahelian dust. In correspondence, the imaginary part of the complex refractive index varied between 0.002-0.004 at 370 nm and between 0.001-0.002 at 950 nm.