Airborne observations of mineral dust over Western Africa in the summer monsoon season: spatial and vertical variability of physico-chemical and optical properties
1LISA, UMR CNRS 7583 Université Paris Est Créteil et Université Paris Diderot Institut Pierre Simon Laplace, Créteil, France
2BIOEMCO, UMR IRD 211, Paris, France
3Laboratoire d'Aérologie, Université de Toulouse/CNRS 5560, Toulouse, France
Abstract. 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 single scattering albedo of Sahelian dust is lower than that of Saharan dust, owing to different mineralogy and size distribution. Nonetheless this difference is within 0.02 (single scattering albedo unit) and does not significantly alter the radiative perturbation at the surface or at the top of the atmosphere. Our investigation provides with further experimental evidence of the limited dynamical evolution of the number size distribution during two days of transport. Whereas the chemical composition is clearly depending on the origin of mineral dust, no indications are of a possible dependence on the number size distribution.