Complex refractive indices and single-scattering albedo of global dust aerosols in the shortwave spectrum and relationship to size and iron content

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.

Differences and Similarities of Central Asian, African, and Arctic Dust Composition from a Single Particle Perspective

“…The Arctic sample ( Figure 3f) has a much higher iron contribution to the single particles, potentially related to the different weathering regime and geological basement. While the total iron content optical properties cannot be derived directly with high accuracy due to the varying contribution of structural iron [51,65], it was shown that estimates of the imaginary part of the refractive index obtained from composition measurements agree reasonably well with optical measurements [6,51]. For the size range of 4-20 µm (transported cross-section maximum), the total deposited aerosol imaginary part of the refractive index for 470 nm wavelength k 470 is 0.4 × 10 −3 at Tenerife and (0.6-0.7) × 10 −3 at all other locations except for Svalbard, where it is 0.9 × 10 −3 .…”

“…For |Fe| > 0.2, only the Fe index is used as a classification criterion, whereas for |Fe| < 0.2, an additional condition is used for ruling out the classification of particles dominated by non-silicate components ( Mg + |Al| + |Si| + |P| + |Fe| > 0.3), for which this approach would lead to high uncertainty. In addition, from the iron content of the single particles, the imaginary part of the index of refraction at 470 nm wavelength (k 470 ) is estimated following an empirical approach from Di Biagio et al [51], who established a relationship between the iron content and the optical properties. To estimate the single particle iron content, the dust compounds Na, Mg, Al, Si, P, K, Ti, and Fe are assumed to exist in their oxidized form.…”

Impact of Dust Shortwave Absorbability on the East Asian Summer Monsoon

“…As illustrated by the most recent laboratory measurements of the dust refractive index by Di Biagio et al (2019), dust shortwave absorbability is closely related to the mass concentration of iron oxides (FeOx), and the imaginary part of the dust refractive index can be empirically inferred given the mass concentration of FeOx by a formula. In this study, mass concentrations of 1.2%, 2.4%, and 4.8% FeOx values are used, where 2.4% is the global mean value in the measurements of Di Biagio et al (2019).…”

“…In addition to the uncertainties stemming from the theoretical and experimental methods for obtaining the refractive indices, the mineral composition variability from different dust source regions significantly impacts the imaginary parts of the complex refractive indices (Balkanski et al, 2007; Sokolik & Toon, 1999), which are mainly determined by the iron oxide content (shortwave) and clay, quartz, and calcium‐rich components (longwave). For example, Di Biagio et al (2019) reveal the close relationship between iron oxides (FeOx) in mineral composition and the imaginary parts of the dust refractive indices in shortwave bands; most importantly, their results show that the imaginary parts of dust refractive indices were overestimated in previous literature (e.g., Hess et al, 1998; Shettle & Fenn, 1979). This lower dust absorption finding is consistent with some other studies (e.g., Balkanski et al, 2007; Kim et al, 2011).…”

“…Dust aerosols can alter the radiation budget via interactions with radiation in both shortwave (scattering and absorbing solar radiation) and longwave (absorbing and emitting infrared radiation) spectral regions, though this direct radiative effect (DRE) still remains uncertain (Boucher et al, 2013; Kok et al, 2017; Sokolik et al, 2001; Tuccella et al, 2020). One of the major reasons is associated with the complex refractive index uncertainties of dust aerosols (Di Biagio et al, 2014, 2017, 2019; Dubovik et al, 2002; Shettle & Fenn, 1979; Wagner et al, 2012). In addition to the uncertainties stemming from the theoretical and experimental methods for obtaining the refractive indices, the mineral composition variability from different dust source regions significantly impacts the imaginary parts of the complex refractive indices (Balkanski et al, 2007; Sokolik & Toon, 1999), which are mainly determined by the iron oxide content (shortwave) and clay, quartz, and calcium‐rich components (longwave).…”