Complex refractive indices and single-scattering albedo of global dust aerosols in the shortwave spectrum and relationship to size and iron content
Authors: Di Biagio Claudia , Formenti Paola , Balkanski Yves , Caponi Lorenzo , Cazaunau Mathieu , Pangui Edouard , Journet Emilie , Nowak Sophie , Andreae Meinrat O. , Kandler Konrad , Saeed Thuraya , Piketh Stuart , Seibert David , Williams Earle , Doussin Jean-François 
Published: December 2019

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.