Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition

P Formenti,A Klaver,J L Rajot,S Caquineau,E Journet,K Desboeufs,S Chevaillier

doi:10.5194/acp-14-10663-2014

Abstract

Abstract. In the last few years, several ground-based and airborne field campaigns have allowed the exploration of the properties and impacts of mineral dust in western Africa, one of the major emission and transport areas worldwide. In this paper, we explore the synthesis of these observations to provide a large-scale quantitative view of the mineralogical composition and its variability according to source region and time after transport. This work reveals that mineral dust in western Africa is a mixture of clays, quartz, iron and titanium oxides, representing at least 92% of the dust mass. Calcite ranged between 0.3 and 8.4% of the dust mass, depending on the origin. Our data do not show a systematic dependence of the dust mineralogical composition on origin; this is to be the case as, in most of the instances, the data represent the composition of the atmospheric burden after 1–2 days after emission, when air masses mix and give rise to a more uniform dust load. This has implications for the representation of the mineral dust composition in regional and global circulation models and in satellite retrievals. Iron oxides account for 58 ± 7% of the mass of elemental Fe and for between 2 and 5% of the dust mass. Most of them are composed of goethite, representing between 52 and 78% of the iron oxide mass. We estimate that titanium oxides account for 1–2% of the dust mass, depending on whether the dust is of Saharan or Sahelian origin. The mineralogical composition is a critical parameter for estimating the radiative and biogeochemical impact of mineral dust. The results regarding dust composition have been used to estimate the optical properties as well as the iron fractional solubility of Saharan and Sahelian dust. Data presented in this paper are provided in numerical form upon email request while they are being turned into a public database, the Dust-Mapped Archived Properties (DUST-MAP), which is an open repository for compositional data from other source regions in Africa and worldwide.

Highlights

Mineral dust from wind-driven soil erosion is an important player in the climate system
To provide the quantitative estimate of the mineralogical composition of mineral dust on a regional scale, we report in this paper the synthesis of data from various aircraft and ground-based field campaigns which took place in western Africa in 2006 and 2007
Our results indicate the dominance of clays in the dust composition when only the aerosol fraction is measured

Summary

Introduction

Mineral dust from wind-driven soil erosion is an important player in the climate system. Mineral dust has several climatic and environmental impacts, related to its ability to scatter and absorb radiation (both in the solar and the terrestrial spectrum) to act as giant cloud and ice nuclei, to alter the concentrations of some gaseous pollutants (such as ozone) and to provide nutrients to the ecosystems via dry and wet deposition, whereby changing the surface albedo (Shao et al, 2011; Mahowald et al, 2011) The relevance of these phenomena can be expressed in terms of radiative forcing, whose magnitude, in terms of central values, is estimated to be as high as 0.5–1 W m−2 on the global scale (Mahowald et al, 2011). These sources are relevant on the global scale as their emissions are transported across the Atlantic Ocean towards South and Central America (Reid et al, 2003; Koren et al, 2006; Ben-Ami et al, 2009, 2010)

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Conclusion