Open AccessCharacterization of iron oxides in mineral dust aerosols: Implications for light absorption
Author(s) -
Lafon Sandra,
Sokolik Iri.,
Rajot Jean Louis,
Caquineau Sandrine,
Gaudichet Annie
Publication year - 2006
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2005jd007016
Subject(s) - hematite , goethite , mineral dust , mineralogy , iron oxide , single scattering albedo , extinction (optical mineralogy) , mineral , albedo (alchemy) , asian dust , materials science , aerosol , wavelength , environmental science , chemistry , metallurgy , art , organic chemistry , optoelectronics , adsorption , performance art , art history
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, ω 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 ω 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, ω 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 ω 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.