[1] We report on measurements that were specifically designed to determine iron oxides in mineral dust aerosols needed for improved optical modeling. Atmospheric dust samples as well as samples generated in a wind tunnel from soils were analyzed by a number of analytical techniques for their total and free iron content (bulk and size resolved), hematite and goethite, mineralogy, and size distribution. These samples are representative of several important dust sources in East Asia and northern Africa. A novel data set generated from these measurements enables us to perform an in-depth modeling study of dust optical properties in the solar spectrum. We modeled the iron oxide-clay aggregates, which are the key light-absorbing species, as well as their mixtures with nonabsorbing minerals. A volume fraction of iron oxide in aggregates was determined from measurements. Significant differences in the single-scattering albedo, ! 0 , were found between hematite-and goethite-clay aggregates, although these calculations involved several important assumptions about the partition of hematite and goethite in size-resolved aggregates. Furthermore, we found that variability of the free iron content is large enough to cause important differences in ! 0 of mineral dust originating from different sources. In contrast, this variability has little effect on the extinction coefficient and optical depth. We demonstrate that for the same size distribution, ! 0 calculated from data obtained for Chinese and Tunisian samples show higher values and more distinct wavelength dependence than those of Niger dust. All the above ! 0 differ from ones calculated using the refractive indices of Patterson et al. (1977) or the OPAC model (Hess et al., 1998), which are often used in radiative transfer studies. We conclude that information on a size-resolved content of free iron and a fraction of hematite and goethite in aggregates will need to be known on a regional basis to improve the prediction of the single-scattering albedo at solar wavelengths and hence the radiative impact of atmospheric mineral dust.

[1] To understand the impact of dust deposition on ocean primary productivity, the evaluation of dust input is not sufficient, and the soluble iron supply is also needed. Measurements of iron in aerosols suggest a much higher solubility than in soil, implying substantial atmospheric processing of chemical or physical nature. Here, we have investigated the effect of the mineralogy on iron solubility. We have measured iron solubility from some minerals commonly found in dust (clay, feldspar and iron (hydr-)oxides). Firstly, we observe a greater solubility of iron resulting from clays (~4%) in comparison to iron (hydr-)oxides (<1%), independently of specific surface of minerals. Secondly, by considering amount of dissolved iron and the mineralogical composition of dust, our results indicate that dissolved iron fraction mainly comes from clay dissolution in contrast to that assumed in some biogeochemical models. In consequence, it appears that iron solubility is closely linked to the mineralogical composition of aerosol. Citation: Journet, E., K. V.

This paper presents data on elemental and mineralogical composition of mineral dust from various source regions of Africa collected during the African Monsoon Multidisciplinary Analyses (AMMA) SOP0/DABEX and Dust Ouflow and Deposition to the Ocean (DODO) DODO1 experiments (January–February 2006), and the DODO2 campaign (August 2006). Bulk filter samples were collected at the AMMA supersite of Banizoumbou, Niger, as well as on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe‐146 aircraft. Both mineral dust and biomass burning in external mixing occurred in surface and elevated layers during the winter field phase of the campaign. However, mineral dust was overwhelming, accounting for 72% of the estimated aerosol mass in aged elevated biomass burning layers and up to 93% in plumes of mineral dust, which generally occurred in the boundary layer. A number of well‐defined episodes of advection of mineral dust could be identified both at the ground and on the aircraft. The elemental and mineralogical composition varied depending on source region. This variability could be well traced by the calcium content, which is enhanced in dust from North Africa but depleted in dust from the Sahel. Iron oxides in the form of hematite and goethite are enriched in dust emitted within Sahel and in Mauritania, whereas dust from the Bodélé depression is iron‐oxide depleted. Iron oxides represented between 2.4% and 4.5% of the total estimated dust oxide mass. This regional variability will have to be taken into account in estimating the optical properties of absorption of mineral dust from western Africa.

[1] On the basis of daily Saharan dust samples collected at Sal Island (Cape Verde Archipelagos) and Barbados (Caribbean Sea) over 3 years, this study focuses on the mineralogical signature of the African sources providing dust over the tropical North Atlantic Ocean. First, the sources of the collected dust were localized by using relative clays abundance (illite-to-kaolinite ratio) combined with Meteosat infrared imagery, horizontal visibility, and backward trajectories of dusty air masses. Then, each identified source was linked to a single value of the illite-to-kaolinite ratio. Those results highlight that the clay content of the emitted dust depends directly on both the latitude and the longitude of the source. Dust originating from northwestern sources exhibits illite-tokaolinite ratios higher than those from Sahelian regions. Likewise, illite-to-kolinite ratio decreases from west to east.

Modeling the interaction of dust with long-wave (LW) radiation is still a challenge because of the scarcity of information on the complex refractive index of dust from different source regions. In particular, little is known about the variability of the refractive index as a function of the dust mineralogical composition, which depends on the specific emission source, and its size distribution, which is modified during transport. As a consequence, to date, climate models and remote sensing retrievals generally use a spatially invariant and time-constant value for the dust LW refractive index. In this paper, the variability of the mineral dust LW refractive index as a function of its mineralogical composition and size distribution is explored by in situ measurements in a large smog chamber. Mineral dust aerosols were generated from 19 natural soils from 8 regions: northern Africa, the Sahel, eastern Africa and the Middle East, eastern Asia, North and South America, southern Africa, and Australia. Soil samples were selected from a total of 137 available samples in order to represent the diversity of sources from arid and semi-arid areas worldwide and to account for the heterogeneity of the soil composition at the global scale. Aerosol samples generated from soils were re-suspended in the chamber, where their LW extinction spectra (3–15 µm), size distribution, and mineralogical composition were measured. The generated aerosol exhibits a realistic size distribution and mineralogy, including both the sub- and super-micron fractions, and represents in typical atmospheric proportions the main LW-active minerals, such as clays, quartz, and calcite. The complex refractive index of the aerosol is obtained by an optical inversion based upon the measured extinction spectrum and size distribution. Results from the present study show that the imaginary LW refractive index (k) of dust varies greatly both in magnitude and spectral shape from sample to sample, reflecting the differences in particle composition. In the 3–15 µm spectral range, k is between ∼ 0.001 and 0.92. The strength of the dust absorption at ∼ 7 and 11.4 µm depends on the amount of calcite within the samples, while the absorption between 8 and 14 µm is determined by the relative abundance of quartz and clays. The imaginary part (k) is observed to vary both from region to region and for varying sources within the same region. Conversely, for the real part (n), which is in the range 0.84–1.94, values are observed to agree for all dust samples across most of the spectrum within the error bars. This implies that while a constant n can be probably assumed for dust from different sources, a varying k should be used both at the global and the regional scale. A linear relationship between the magnitude of the imaginary refractive index at 7.0, 9.2, and 11.4 µm and the mass concentration of calcite and quartz absorbing at these wavelengths was found. We suggest that this may lead to predictive rules to estimate the LW refractive index of dust in specific bands based on ...

Abstract. The aim of this study is to find a tracer allowing retrieval of the regional origin of mineral dust for Saharan aerosols transported over the North Atlantic Ocean. Because of physical and chemical ffactionation processes occurring at the soil-atmosphere interface mid during the atmospheric transport of dust, clay mineral species seein to be the best candidate. This study shows that the ratio between relative abundance of illite and kaolinire (I/K ratio) is the parameter that is the most sensitive to the regional origin of Saharan dust collected on Sal Island (Cape Verde). By comparing the I/K ratio measured in dust enfitted from the same Saharan source and collected along its transport both on Sal Island and Barbados (Caribbean Sea), we show that this ratio seems to remain unchanged after long-range transport.

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.

This paper reports on the X-ray absorption analysis of samples of mineral dust emitted from or transported to Western Africa. We found that iron oxides account, by mass, for 38% to 72% of the total elemental iron. They are composed of minerals in the Fe(III) oxidation state: goethite (FeO·OH) and hematite (Fe 2 O 3 ). The apparent fraction of iron oxide attributed to goethite is higher than hematite regardless of the source region from which the dust originated. The goethite percent content of iron oxides is in the range 52-78% (by mass), the highest values being measured for dust originating in the Sahel. The limited number of samples analyzed and the sample-to-sample variability prevent us from concluding firmly on the regional variability of the goethite-to-hematite ratio. Based on the experimental data on mineralogical composition and on concurrent measurements of the number size distribution, the optical properties of mineral dust have been calculated in a Mie approximation for homogeneous spherical particles. At 550 nm, the single-scattering albedo ω 0 ranges between 0.89 and 0.93, the asymmetry factor g ranges between 0.76 and 0.8 and the mass extinction efficiency k ext varies between 0.5 and 1.1 m 2 g À1 ; these values are all in the range of those from independent direct measurements. Neglecting the partitioning between hematite and goethite and the assimilation of iron oxides by hematite, as it is often done with models, lowers the single-scattering albedo and increases the asymmetry factor in the UV-visible spectral region below 550 nm. The mass extinction efficiency is insensitive to the nature of the iron oxides but rather responds to variations in the number size distribution. The mineralogy of iron oxides should therefore be taken into account when assessing the effect of mineral dust on climate and atmospheric chemistry, in particular via interactions involving photolysis.