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

Abstract

<p><strong>Abstract.</strong> The optical properties of airborne mineral dust depend on its mineralogy, size distribution, shape, and 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 (SSA) 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). These were selected to represent the global scale variability of the dust mineralogy. Dust was re‒suspended into a 4.2 m<sup>3</sup> smog chamber where its spectral shortwave scattering (β<sub>sca</sub>) and absorption (β<sub>abs</sub>) 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 β<sub>sca</sub> and β<sub>abs</sub> measurements. Our results show that the imaginary part of the refractive index (k) and the SSA largely vary from sample to sample, with values for k in the range 0.001 to 0.009 at 370 nm and 0.0003 to 0.002 at 950 nm, and values for SSA in the range 0.70 to 0.96 at 370 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’s 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. As an intrinsic property of matter, k is independent of size. When the iron oxide content exceeds > 3 %, the SSA linearly decreases with increasing fraction of coarse particles at short wavelengths (< 600 nm).</p> <p>We recommend that source‒dependent values of the SW spectral refractive index and SSA are used in models and remote sensing retrievals instead of generic values. In particular, the close relationships found between k/SSA and the iron content in dust enable establishing predictive rules for spectrally‒resolved SW absorption based on particle composition.</p>

Dust Constraints from joint Observational-Modelling-experiMental analysis (DustCOMM): comparison with measurements and model simulations

“…Therefore, in regions with significant dust loading, accurate estimates of dust size distribution can be key to accurate simulations of precipitation initiation and aerosolcloud interactions, including dust aerosol indirect and semidirect effects (e.g. Sassen, 2003;Doherty and Evan, 2014). Since DustCOMM represents the dust size distribution more accurately than model simulations, it could be used to improve the simulated dust impacts on clouds, precipitation, and aerosol-cloud interactions.…”

“…Specifically, dust affects global climate directly by influencing the amount of radiation that can reach or leave the atmosphere and the surface (Haywood et al, 2003;Kok et al, 2017) or indirectly by changing the amount, reflectivity, and lifetime of clouds (e.g. Lohmann and Diehl, 2006;Doherty and Evan, 2014;Amiri-Farahani et al, 2017). In addition, dust also impacts global biogeochemistry through deposition of iron and phosphorous-rich micronutrients (Mahowald et al, 2008(Mahowald et al, , 2009Ito et al, 2019), both of which are linked to the ability of ocean and land ecosystems to absorb atmospheric carbon dioxide (e.g.…”

The influence of simulated surface dust lofting and atmospheric loading on radiative forcing

“…Aerosol extinction, and thus AOD, is a function of the real part of the index of refraction. A representative real refractive index for dust of 1.53 at 550 nm was assigned based on surface observations (such as AERONET; Dubovik et al, 2006;Giles et al, 2012), radiative closure studies Christopher and Wang, 2003), and laboratory studies (Di Biagio et al, 2019). This value matches that used in the RAMS parameterization of dust radiative effects.…”

Capability of Superspheroids for Modeling PARASOL Observations Under Dusty‐Sky Conditions

“…Although better agreement could be achieved for the spheroidal model if we tune the real part of dust refractive index, a reasonable dust refractive index should be seriously considered. A real part of 1.43 or 1.45 at 865‐nm wavelength is uncommon in the laboratory measurements (Biagio et al., 2019). Thus, the superspheroidal model shows more capability than the conventional model in modeling the PARASOL measured polarized radiance with a more reasonable dust refractive index.…”