Abstract
Abstract. The optical properties of airborne mineral dust depend on its mineralogy, size distribution, and shape, and they might vary between different source regions. To date, large differences in refractive index values found in the literature have not been fully explained. In this paper we present a new dataset of complex refractive indices (m=n-ik) and single-scattering albedos (SSAs) for 19 mineral dust aerosols over the 370–950 nm range in dry conditions. Dust aerosols were generated from natural parent soils from eight source regions (northern Africa, Sahel, Middle East, eastern Asia, North and South America, southern Africa, and Australia). They were selected to represent the global-scale variability of the dust mineralogy. Dust was resuspended into a 4.2 m3 smog chamber where its spectral shortwave scattering (βsca) and absorption (βabs) coefficients, number size distribution, and bulk composition were measured. The complex refractive index was estimated by Mie calculations combining optical and size data, while the spectral SSA was directly retrieved from βsca and βabs measurements. Dust is assumed to be spherical in the whole data treatment, which introduces a potential source of uncertainty. Our results show that the imaginary part of the refractive index (k) and the SSA vary widely from sample to sample, with values for k in the range 0.0011 to 0.0088 at 370 nm, 0.0006 to 0.0048 at 520 nm, and 0.0003 to 0.0021 at 950 nm, as well as values for SSA in the range 0.70 to 0.96 at 370 nm, 0.85 to 0.98 at 520 nm, and 0.95 to 0.99 at 950 nm. In contrast, the real part of the refractive index (n) is mostly source (and wavelength) independent, with an average value between 1.48 and 1.55. The sample-to-sample variability in our dataset of k and SSA is mostly related to differences in the dust iron content. In particular, a wavelength-dependent linear relationship is found between the magnitude of k and SSA and the mass concentrations of both iron oxide and total elemental iron, with iron oxide better correlated than total elemental iron with both k and SSA. The value of k was found to be independent of size. When the iron oxide content exceeds 3 %, the SSA linearly decreases with an increasing fraction of coarse particles at short wavelengths (< 600 nm). Compared to the literature, our values for the real part of the refractive index and SSA are in line with past results, while we found lower values of k compared to most of the literature values currently used in climate models. We recommend that source-dependent values of the SW spectral refractive index and SSA be used in models and remote sensing retrievals instead of generic values. In particular, the close relationships found between k or SSA and the iron content in dust enable the establishment of predictive rules for spectrally resolved SW absorption based on particle composition.
Highlights
We investigate the relationship of k and single-scattering albedos (SSAs) to the iron content to provide a parameterization of the dust absorption as a function of its mineralogy, which can be applied to climate models
The relationships found between k, SSA, and the iron oxides or elemental iron content in dust create an opportunity to establish predictive rules to estimate the spectrally resolved SW absorption of dust based on composition
We recommend the use of iron oxide content rather than iron content as it is better correlated with k and SSA
Summary
With teragram amounts of annual emissions, a residence time of about 1–2 weeks in the atmosphere, and a planetary-scale transport, mineral dust aerosols are a global phenomenon (Uno et al, 2009; Ginoux et al, 2012) and contribute significantly to the global and regional aerosol loading (Ridley et al, 2016) and direct radiative effect (Miller et al, 2014).large uncertainties still persist on the magnitude and overall sign of the dust direct radiative effect (Boucher et al, 2013; Highwood and Ryder, 2014; Kok et al, 2017). One of the major sources of this uncertainty is our insufficient knowledge of dust absorption properties in the shortwave (SW) and longwave (LW) spectral ranges (e.g., Balkanski et al, 2007; Samset et al, 2018), given that mineral dust contains large particles and a variety of minerals absorbing over both spectral regions (e.g., iron oxides, clays, quartz, and calcium-rich species; Sokolik and Toon, 1999; Lafon et al, 2006; Di Biagio et al, 2014a, b). Dust aerosols of different origins should be more or less absorbing in the SW and have a different imaginary spectral refractive index and SSA. The real part (n) of the dust refractive index, mostly related to particle scattering, is less variable, with values between 1.47 and 1.56 at 550 nm (e.g., Volz, 1972; Patterson et al, 1977; Balkanski et al, 2007; Petzold et al, 2009)
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