Effect of iron oxides on radiative properties of mineral dust

Soil-derived mineral dust play an important role in the radiative balance of the Earth's system, often exerting the largest radiative impact compared to other atmospheric aerosols. Because of large uncertainties in its physiochemical properties, the magnitude of diverse radiative effects caused by dust remains highly uncertain. Previous studies suggested that the amount of iron oxides is a key factor controlling the ability of dust to absorb sunlight and, hence, its radiative forcing (Sokolik and Toon, 1999). Here we use new data on iron oxides to further examine this issue. The size-segregated iron oxide content was measured with a modified CBD-extraction method (Lafon et al., 2004) in a number of dust samples representative of different dust sources in China and North Africa. Furthermore, the relative proportion between hematite and goethite was determined in each dust sample using diffuse reflectance spectrometry. The data were used to perform an extensive modeling of spectral optical properties of dust at the solar wavelengths. The role of the iron oxide amount (total iron vs. free iron), iron oxide speciation (hematite vs. goethite), and differences in the regional mineralogical composition in affecting dust optical properties was examined. Radiative transfer calculations were carried out in to order to explore the extent to which these factors can affect the radiative forcing of mineral dust. The paper will present the results of this study and discuss implications for the prediction of dust radiative impacts at the regional scale.