What is the real role of iron oxides in the optical properties of dust aerosols?

Abstract

Iron oxide compounds constitute an important component of mineral dust aerosols. Several previous studies have shown that these minerals are strong absorbers at visible wavelengths and thus that they play a critical role in the overall climate perturbation caused by dust aerosols. When compiling a database of complex refractive indices of possible mineral species of iron oxides to study their optical properties, we found that uniformly continuous optical constants for a single type of iron oxide in the wavelength range between 0.2 and 50 μm are very scarce, and that the use of hematite to represent all molecular or mineral iron-oxides types is a popular hypothesis. However, the crucial problem is that three continuous data sets for complex refractive indices of hematite are employed in climate models, but there are significant differences between them. Thus, the real role of iron oxides in the optical properties of dust aerosols becomes a key scientific question, and we address this problem by considering different refractive indices, size distributions and more logical weight fractions and mixing states of hematite. Based on the microscopic observations, a semi-external mixture that employs an external mixture between Fe aggregates and other minerals and partly internal mixing between iron oxides and aluminosilicate particles is advised as the optimal approximation. The simulations demonstrate that hematite with a spectral refractive index from Longtin et al. (1988) shows approximately equal absorbing capacity to the mineral illite over the whole wavelength region from 0.55 to 2.5 μm, and only enhances the optical absorption of aerosol mixture at λ < 0.55 μm. Using the data set from Querry (1985) may overestimate the optical absorption of hematite at both visible and near-infrared wavelengths. More laboratory measurements of the refractive index of iron oxides, especially for hematite and goethite in the visible spectrum, should therefore be taken into account when assessing the effect of mineral dust on climate forcing.