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

Abstract

Abstract. In the last few years, several ground-based and airborne field campaigns have allowed exploring 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 with a large-scale quantitative view of the mineralogical composition and its variability with 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 with origin, likely as in most of the cases they represent the composition of the atmospheric burden after 1–2 days after emission, when air masses mix and give raise to a more uniform dust load. This has implications for the representation of the mineral dust composition in regional and global circulation models, and satellite retrievals. Iron oxides account for 58 ± 7% of the mass of elemental Fe, and 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 to estimate the radiative and biogeochemical impact of mineral dust. The results on dust composition have been applied to estimate the optical properties as so 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 implemented as a public database, the Dust-Mapped Archived Properties (DUST-MAP), an open repository for compositional data from other source regions in Africa and worldwide.