Complex refractive index and single scattering albedo of Icelandic dust in the shortwave part of the 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, Zongbo Shi

*Corresponding author for this work

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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 370nm to 0.94-0.96 at 950nm in Icelandic dust from the different hotspots, which falls within the range of mineral dust from northern Africa and eastern Asia. 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 be independent of wavelength. The imaginary part (k(λ)) was almost constant with wavelength and was found to be around 0.004 at 370nm and 0.002-0.003 at 950nm. 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 part of the 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 towards the near-infrared. In Icelandic dust, k(λ) between 660-950nm 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 that has not been accounted for in climate predictions.

Original languageEnglish
Pages (from-to)7975-8000
Number of pages26
JournalAtmospheric Chemistry and Physics
Volume23
Issue number14
DOIs
Publication statusPublished - 18 Jul 2023

Bibliographical note

Publisher Copyright:
© 2023 Clarissa Baldo et al.

ASJC Scopus subject areas

  • Atmospheric Science

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