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

摘要

Iron oxide compounds constitute an important component of mineral dustaerosols. Several previous studies have shown that these minerals are strongabsorbers at visible wavelengths and thus that they play a critical role inthe overall climate perturbation caused by dust aerosols. When compiling adatabase of complex refractive indices of possible mineral species ofiron oxides to study their optical properties, we found that uniformlycontinuous optical constants for a single type of iron oxide in thewavelength range between 0.2 and 50 μm are very scarce, and thatthe use of hematite to represent all molecular or mineral iron-oxides typesis a popular hypothesis. However, the crucial problem is that threecontinuous data sets for complex refractive indices of hematite are employedin climate models, but there are significant differences between them. Thus,the real role of iron oxides in the optical properties of dust aerosolsbecomes a key scientific question, and we address this problem byconsidering different refractive indices, size distributions and morelogical weight fractions and mixing states of hematite. Based on themicroscopic observations, a semi-external mixture that employs an externalmixture between Fe aggregates and other minerals and partly internal mixingbetween iron oxides and aluminosilicate particles is advised as the optimalapproximation. The simulations demonstrate that hematite with a spectralrefractive index from Longtin et al. (1988) shows approximately equalabsorbing capacity to the mineral illite over the whole wavelength regionfrom 0.55 to 2.5 μm, and only enhances the optical absorptionof aerosol mixture at λ < 0.55 μm. Using the data set fromQuerry (1985) may overestimate the optical absorption of hematite at bothvisible and near-infrared wavelengths. More laboratory measurements of therefractive index of iron oxides, especially for hematite and goethite in thevisible spectrum, should therefore be taken into account when assessing theeffect of mineral dust on climate forcing.