10 Mar 2023
 | 10 Mar 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Complex refractive index and single scattering albedo of Icelandic dust in the shortwave spectrum

Clarissa Baldo, Paola Formenti, Claudia Di Biagio, Gongda Lu, Congbo Song, Mathieu Cazaunau, Edouard Pangui, Jean-Francois Doussin, Pavla Dagsson-Waldhauserova, Olafur Arnalds, David Beddows, A. Robert MacKenzie, and Zongbo Shi

Abstract. Icelandic dust can impact the radiative budget in high-latitude regions directly by affecting light absorption and scattering and indirectly by changing the surface albedo after dust deposition. This tends to produce a positive radiative forcing. However, the limited knowledge of the spectral optical properties of Icelandic dust prevents an accurate assessment of these radiative effects. Here, the spectral single scattering albedo (SSA) and the complex refractive index (m = n - ik) of Icelandic dust from five major emission hotspots were retrieved between 370–950 nm using online measurements of size distribution and spectral absorption (βabs) and scattering (βsca) coefficients of particles suspended in a large-scale atmospheric simulation chamber. The SSA(λ) estimated from the measured βabs and βsca increased from 0.90–0.94 at 370 nm to 0.94–0.96 at 950 nm in Icelandic dust from the different hotspots, which falls within the range of mineral dust from northern Africa and eastern Asian. The spectral complex refractive index was retrieved by minimizing the differences between the measured βabs and βsca and those computed using the Mie theory for spherical and internally homogeneous particles, using the size distribution data as input. The real part of the complex refractive index (n(λ)) was found to be 1.60-1.61 in the different samples and independent on wavelength. The imaginary part (k(λ)) was almost constant with wavelength and was found to be around 0.004 at 370 nm and 0.002–0.003 at 950 nm. The estimated complex refractive index was close to the initial estimates based on the mineralogical composition, also suggesting that the high magnetite content observed in Icelandic dust may contribute to its high absorption capacity in the shortwave spectrum. The k(λ) values retrieved for Icelandic dust are at the upper end of the reported range for low-latitude dust (e.g., from the Sahel). Furthermore, Icelandic dust tends to be more absorbing toward the near-infrared. In Icelandic dust, k(λ) between 660–950 nm was 2–8 times higher than most of the dust samples sourced in northern Africa and eastern Asia. This suggests that Icelandic dust may have a stronger positive direct radiative forcing on climate which has not been accounted for in climate predictions.

Clarissa Baldo et al.

Status: open (until 21 Apr 2023)

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Clarissa Baldo et al.


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Short summary
This paper presents new shortwave spectral complex refractive index and single scattering albedo data for Icelandic dust. Our results show that the imaginary part of the complex refractive index of Icelandic dust is at the upper end of the range of low-latitude dust. Furthermore, we observed that Icelandic dust is more absorbing towards the near-infrared, which we attribute to its high magnetite content. These findings are important for modelling dust aerosol radiative effects in the Arctic.