Characterization of iron oxides in mineral dust aerosols: Implications for light absorption

Abstract

[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.

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

“…Our results extend earlier research on the occurrences of iron-bearing minerals in Bodélé surface sediments and elsewhere in North Africa that aimed to establish linkages between dust mineralogy and potential environmental effects of dust (Lafon et al, , 2006Alfaro et al, 2004;Moreno et al, 2006;Mounkaila, 2006;Journet et al, 2008Journet et al, , 2104Chudnovsky et al, 2009;Lázaro et al, 2011;Maher, 2011;Shi et al, 2011;Moosmüller et al, 2012;Adetunji, 2014;Formenti et al 2014 a, b). These studies employed many methods for dust and soil mineralogy, including magnetic analyses, X-ray absorption analysis, X-ray diffraction, microscopy, elemental and isotopic chemistry combined with leaching experiments, as well spectroscopic techniques in the laboratory and applied to satellite retrievals.…”

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…Koepke et al, 1997;Hess et al, 1998). However, the recent findings indicated a possibility of strong deviations of these literature values from the real ones, which are controlled by dust chemical composition (especially percentage of hematite; Lafon et al, 2006) and mixing with other primary (black carbon) and/or secondary aerosols (organics, sulphate, ...). For example, the remote sensing study of Dubovik et al (2002) reported much lower desert dust absorption in visible spectral range than commonly used in aerosol properties databases although more recent work proposed by Sinyuk et al (2007) seems indicating larger dust absorption close to typical model values.…”

Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

“…In the same way, the partitioning of iron oxides into their major forms (haematite, Fe 2 O 3 and goethite, FeOOH) is neglected and, in accordance with Lafon et al (2006) and McConnell et al (2010), the iron oxide content is either considered to be entirely in the form of haematite or in the form of goethite. (b) For lightly loaded samples (mass ≤800 µg), as is the case most of the time for aircraft sampling, the mineralogical composition can be estimated indirectly by using the elemental concentrations measured by XRF or PIXE (Lafon et al, 2006;McConnell et al, 2010). In this approach, the dust mineralogical composition is approximated by four components, namely clays, quartz, calcite and iron oxides.…”

“…The elemental concentrations of Al and Si are used to apportion clays and quartz, Ca to account for calcite and Fe(ox) and Fe ratio to account for the iron oxides content. See Lafon et al (2006) for the explicit formulae. In this case too, clays and iron oxides were approximated by their pure forms (kaolinite, illite, haematite, goethite).…”

“…First, we considered that minerals are externally mixed with the exception of iron oxides, which we considered to be internally mixed with clays (Sokolik and Toon, 1999;Lafon et al, 2006).…”