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 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.

Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

“…5), also reported in Table 3, ranged from 2.8 % (Namibia) to 7.3 % (Australia). These are in the range of values reported in the literature, taking into account that differences might be also due to the method (direct measurement/calculation) and/or the size fraction over which the total dust mass concentration is estimated (Chiapello et al, 1997;Reid et al, 1994;Derimian et al, 2008;Formenti et al, 2001Formenti et al, , 2011Formenti et al, , 2014aScheuvens et al, 2013). The agreement of MC Fe% values obtained by the XRF analysis of polycarbonate filters (Eq.…”

“…The values obtained during our experiments are reported in Table 3. There is a very good correspondence between the values obtained for the Mali, Libya, Algeria and (to a lesser extent) Morocco experiments to values found in environmental aerosol samples by Chiapello et al (1997) and Formenti et al (2011Formenti et al ( , 2014a. These authors indicate that dust from local erosion of Sahelian soils, such as from Mali, has Si / Al ratios in the range of 2-2.5 and Fe / Ca ratios in the range of 3-20, depending on the time proximity to the erosion event.…”

“…Here we use the Fe / Ca and Si / Al elemental ratios obtained from XRF analysis to discriminate the origin of dust samples. These ratios have been extensively used in the past to discriminate the origin of African dust samples collected in the field (Chiapello et al, 1997;Formenti et al, 2011Formenti et al, , 2014a. The values obtained during our experiments are reported in Table 3.…”

“…Dust from sources in the Sahara, such as Libya and Algeria, shows Si / Al ratios in the range of 2-3 and Fe / Ca ratios in the range of 0.7-3, whereas dust from Morocco has Si / Al ratios around 3 and Fe / Ca ratios around 0.4. The only major difference is observed for the Bodélé experiment, for which the Fe / Ca ratio is enriched by a factor of 6 with respect to the values of 1 found during the field observations (Formenti et al, , 2014a. This could reflect the fact that the Bodélé aerosol in the chamber is generated from a sediment sample and not from a soil.…”

“…For samples from areas other than northern Africa, the largest variability is observed for the Fe / Ca values, ranging from 0.1 to 8, whereas the Si / Al ratio varied only between 2.5 and 4.8. In this case, values are available in the literature for comparison (e.g., Cornille et al, 1990;Reid et al, 1994;Eltayeb et al, 2001;Lafon et al, 2006;Shen et al, 2007;Radhi et al, 2010Radhi et al, , 2011Formenti et al, 2011Formenti et al, , 2014aScheuvens et al, 2013, and references within). Values in the PM 2.5 fraction are very consistent with those obtained in the PM 10.6 : their linear correlation has a slope of 1.03 (±0.05) and a R 2 equal to 0.97, suggesting that the elemental composition is relatively size independent.…”