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

Abstract

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.

A critical review of filter transmittance measurements for aerosol light absorption, and de novo calibration for a decade of monitoring on PTFE membranes

“…Absorption is sensitive to iron's oxidation state and other distinctions seldom analyzed in PM 2.5 samples (Lafon et al 2006), and to the physical dispersal of the absorbing minerals in the rest of the dust matrix (Sokolik and Toon 1999). Absorption by iron oxides varies strongly with wavelength (Bedidi and Cervelle 1993), and the available refractive index data that might be used to reconcile measurements at different wavelengths are spectrally inconsistent (Zhang et al 2015). Finally, absorption and mass are incommensurable quantities measured by distinct systems, so any MAC is sensitive to details of sample selection for both measurements.…”

Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D

“…3. For Saharan dust, hematite and goethite are considered to be the principal compositions of absorbing free iron (Zhang et al, 2015). Laboratory experiments of goethite show no detectable signal from the SP2 due to its very weak absorption at 1064 nm.…”

“…Secondly, since the composition of the dust is highly source dependent, the identification of the source of the dust is crucial to determine its properties at the receptor location. In particular, the free iron content such as the goethite / hematite ratio contained in Saharan dust crucially determines its absorbing properties (Zhang et al, 2015). Previous studies have used offline analysis of filter measurements to determine the iron content using X-ray diffraction and have found a diversity of iron content across different dust source regions (Linke et al, 2006), and the offline composition results have been associated with variations in the optical properties of dust (McConnell et al, 2010).…”

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…According to previous studies, iron oxides are strong absorbers at visible wavelengths and might play a critical role in climate perturbation caused by dust aerosols (Sokolik and Toon, 1999;X. L. Zhang et al, 2015).…”