Abstract
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.
Highlights
Eolian dust, suspended in the troposphere at a rate of 1– 4 Pg yr−1, persists for 1–7 d or longer depending on particle size (Boucher et al, 2013)
Airborne dust contributes to the direct radiative effect (DRE) by absorbing or scattering solar and terrestrial radiation in the shortwave (SW, 0.185–4.0 μm) and longwave (LW, 3.33–1000 μm) spectral regions, respectively (Di Biagio et al, 2020)
Kok et al (2017) suggested that the net (SW + LW) dust DRE is cooling at −0.20 W m−2 with an uncertainty range of −0.48 to +0.20 W m−2, based on the complex refractive index from the Optical Properties of Aerosols and Clouds (OPAC) database (Hess et al, 1998; Volz, 1973)
Summary
Eolian dust, suspended in the troposphere at a rate of 1– 4 Pg yr−1, persists for 1–7 d or longer depending on particle size (Boucher et al, 2013). A few Earth-system models (ESMs, i.e., coupled climate models) have adopted a regionally and temporally variable spectral refractive index of dust by parameterization with common soil mineralogy components (Scanza et al, 2015; Perlwitz et al, 2015a, b). The rationale for this is that dust aerosols are soil particles suspended in the atmosphere (Scanza et al, 2015). The results of this work are summarized in the concluding Sect. 5
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