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Discussion papers
https://doi.org/10.5194/acp-2019-145
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-145
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Mar 2019

Research article | 12 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Claudia Di Biagio1, Paola Formenti1, Yves Balkanski2, Lorenzo Caponi1,3, Mathieu Cazaunau1, Edouard Pangui1, Emilie Journet1, Sophie Nowak4, Meinrat O. Andreae5,6, Konrad Kandler7, Thuraya Saeed8, Stuart Piketh9, David Seibert10, Earle Williams11, and Jean-Francois Doussin1 Claudia Di Biagio et al.
  • 1Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR7583, CNRS, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre et Simon Laplace, Créteil, France
  • 2Laboratoire des Sciences du Climat et de l’Environnement, CEA CNRS UVSQ, 91191, Gif sur Yvette, France
  • 3PM_TEN srl, Piazza della Vittoria 7/14, 16121, Genoa, Italy
  • 4Plateforme RX UFR de chimie, Université Paris Diderot, Paris, France
  • 5Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. box 3060, 55020, Mainz, Germany
  • 6Geology and Geophysics Department, King Saud University, Riyadh, Saudi Arabia
  • 7Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Schnittspahnstr. 9, 64287 Darmstadt, Germany
  • 8Science department, College of Basic Education, Public Authority for Applied Education and Training, Al-Ardeya, Kuwait
  • 9Climatology Research Group, Unit for Environmental Science and Management, North-West University, Potchefstroom, South Africa
  • 10Walden University, Minneapolis, Minnesota, USA
  • 11Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Abstract. 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 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. 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).

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.

Claudia Di Biagio et al.
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Short summary
This paper presents a new dataset of laboratory measurements of the shortwave spectral complex refractive index and single scattering albedo (SSA) for global mineral dust aerosols of varying origin and composition. Our results show that the dust refractive index and SSA vary strongly from source to source mostly due to particle iron content changes. We recommend that source‒dependent values of the SW spectral refractive index and SSA are used in models and remote sensing applications.
This paper presents a new dataset of laboratory measurements of the shortwave spectral complex...
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