Abstract

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 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.

Highlights

  • The role of mineral dust on climate change is prominent but still largely uncertain, as pointed out by the latest Intergovernmental Panel on Climate Change’s report (Forster et al, 2007)

  • Data are presented for three atmospheric levels: below 1 km, corresponding to the mean boundary layer depth during the campaign; between 1 and 3 km, corresponding to the mean extent of the African Easterly Jet (AEJ) and to the expected maximum dust concentration in the Saharan Air Layer (SAL); and above 3 km, for decreasing concentrations

  • The elemental concentrations show the same large variability than during previous airborne campaigns in western Africa. They are lower than those measured during wintertime in the Niamey area during the SOP0 phase (Formenti et al, 2008) and only comparable to those observed for short-range transport conditions such as those encountered offshore Senegal during the Saharan Dust Experiment (SHADE) and Dust Outflow and Deposition to the Ocean (DODO1) airborne campaigns (Formenti et al, 2003; McConnell et al, 2008)

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Summary

Introduction

The role of mineral dust on climate change is prominent but still largely uncertain, as pointed out by the latest Intergovernmental Panel on Climate Change’s report (Forster et al, 2007). Mineral dust steaming out of Africa towards the Atlantic ocean and the Mediterranean sea is emitted from various localized sources in the arid and semi-arid areas of the Sahara and Sahel, differing in intensity, seasonality and frequency (Goudie and Middleton, 2001; Prospero et al, 2002; Laurent et al, 2008). 1997; Goudie and Middleton, 2001) shows that the atmospheric content of mineral dust in the Sahelian part of western Africa is high and shows a very pronounced seasonal cycle despite high variability, both at the daily and inter-annual scales This is largely explained by the alternance of two meteorological regimes. Exploration of the area south of 13◦ N is presented to illustrate the time evolution of the meridian extent of the dust transport region with the progression of the Monsoon season

Instrumentation
Aerosol particle chemical composition
Aerosol particle size distribution
Aerosol particle scattering coefficients
Aerosol particle absorption coefficients
Area and period of study
Aerosol spatial variability
Aerosol vertical profiles
Discussion of case studies
June 6 June 7 June 8 June 11 June 12 June
Advection of southern hemispheric biomass burning
Local erosion events
Saharan dust transport
Sahelian dust transport
Particle size distribution
Optical properties
Discussion and conclusions
Full Text
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