Quantification of iron oxides in desert aerosol

For assessing the impact of desert aerosols on climate, it is necessary to quantify their content in iron oxides minerals. These minerals are well known for their light absorbing properties. Iron contained in them—the so-called “free-iron” by soil scientists—represents only a part of the total-iron content. Indeed, the latter also includes the iron trapped in the crystal lattice of several desert aerosol mineralogical species. In this paper we propose an adapted technique to quantify the free-iron in desert aerosol based on a selective extraction method classically used in soil science. Adaptations are made necessary because of the low aerosol load collected on filter samples under field conditions (masses typically lower than 0.5 mg). Validation is obtained by comparing results of the adapted and classical methods when applied to the same geostandards. We analysed natural samples from three main desert source areas : Sahel (Niger during local erosion events), Sahara (Niger during Harmattan events) and Gobi desert (China). For these samples, the free-to-total iron ratio varied between 0.4 and 0.7. This natural variability indicates that the free-iron in aeolian dust is not directly proportional to the total-iron. As a consequence, total iron cannot be used directly to calculate dust light absorbing properties. The percentages of free-iron relative to the total estimated aerosol mass is 2.8%, 3.7%, and 5.0% in Saharan, Chinese, and Sahelian samples respectively.