Inferring iron-oxide species content in atmospheric mineral dust from DSCOVR EPIC observations
Authors: Go Sujung , Lyapustin Alexei , Schuster Gregory L. , Choi Myungje , Ginoux Paul , Chin Mian , Kalashnikova Olga , Dubovik Oleg , Kim Jhoon , da Silva Arlindo , Holben Brent , Reid Jeffrey S. 
Published: January 2022

Abstract. The iron-oxide content of dust in the atmosphere and most
notably its apportionment between hematite (α-Fe2O3) and
goethite (α-FeOOH) are key determinants in quantifying dust's light
absorption, its top of atmosphere ultraviolet (UV) radiances used for dust monitoring, and
ultimately shortwave dust direct radiative effects (DREs). Hematite and
goethite column mass concentrations and iron-oxide mass fractions of total
dust mass concentration were retrieved from the Deep Space Climate
Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) measurements
in the ultraviolet–visible (UV–Vis) channels. The retrievals were
performed for dust-identified aerosol plumes over land using aerosol optical
depth (AOD) and the spectral imaginary refractive index provided by the
Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm over
six continental regions (North America, North Africa, West Asia, Central
Asia, East Asia, and Australia). The dust particles are represented as an
internal mixture of non-absorbing host and absorbing hematite and goethite.
We use the Maxwell Garnett effective medium approximation with carefully
selected complex refractive indices of hematite and goethite that produce
mass fractions of iron-oxide species consistent with in situ values found in the
literature to derive the hematite and goethite volumetric/mass
concentrations from MAIAC EPIC products. We compared the retrieved hematite
and goethite concentrations with in situ dust aerosol mineralogical content
measurements, as well as with published data. Our data display variations
within the published range of hematite, goethite, and iron-oxide mass
fractions for pure-mineral-dust cases. A specific analysis is presented for
15 sites over the main dust-source regions. Sites in the central Sahara,
Sahel, and Middle East exhibit a greater temporal variability of iron oxides
relative to other sites. The Niger site (13.52∘ N, 2.63∘ E)
is dominated by goethite over the Harmattan season with a median of
∼ 2 weight percentage (wt %) of iron oxide. The Saudi Arabia
site (27.49∘ N, 41.98∘ E) over the Middle East also
exhibited a surge of goethite content with the beginning of the shamal season. The
Sahel dust is richer in iron oxide than Saharan and northern China dust
except in summer. The Bodélé Depression area shows a distinctively
lower iron-oxide concentration (∼ 1 wt %) throughout the
year. Finally, we show that EPIC data allow the constraining of the hematite
refractive index. Specifically, we select 5 out of 13 different hematite refractive indices that are widely variable in published laboratory studies
by constraining the iron-oxide mass ratio to the known measured values.
The provided climatology of hematite and goethite mass fractions across the main
dust regions of Earth will be useful for dust shortwave DRE studies and
climate modeling.