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Reflectance Spectroscopy of Global Parent Soils: Application to Aerosol Mineral Dust
  • Mohammad Reza Sadrian,
  • Wendy Calvin,
  • Johann Engelbrecht
Mohammad Reza Sadrian
Department of Geological Sciences and Engineering, University of Nevada, Reno, Reno, 89557, USA

Corresponding Author:[email protected]

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Wendy Calvin
Department of Geological Sciences and Engineering, University of Nevada, Reno, Reno, 89557, USA
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Johann Engelbrecht
Desert Research Institute (DRI), Reno, Nevada 89512-1095, USA
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Mineral dust particles originate from a variety of arid regions around the world. Mineral dust directly modifies the Earth’s radiative balance through absorption and scattering. This radiative forcing varies strongly with mineral composition, yet there is still limited knowledge on the mineralogy of global dust source regions. Previously, 65 surface soil samples were collected worldwide, sieved to < 38 μm fraction and analyzed using XRD, SEM and re-suspended to determine scattering and absorption coefficients at three visible wavelengths (Engelbrecht et al. Atmos. Chem. Phys. 16, 2016). This dust collection represents global surface soils with comparable mineral compositions to windblown dust. For this research, we measured spectra of 26 of these samples selected from major dust source regions with compositional diversity. We measured these samples using laboratory reflectance spectroscopy in the visible and near-infrared (0.4 to 2.5 μm, VNIR) and long-wave infrared (2.5 to 25 μm, LWIR). These data are relevant to satellite imaging spectrometers, but will particularly inform measurements planned for EMIT and SBG. We compared the measured spectra to standard spectral libraries to identify dominant materials and to compare and contrast these with the major minerals identified via XRD. VNIR spectral analysis detected diagnostic absorptions for minerals such kaolinite, calcite, hematite, and goethite. Common silicates, quartz and feldspar, are abundant in the majority of these samples and are expected to have diagnostic features in the LWIR. LWIR reflectance is strongly dependent on particle size (e.g. Salisbury et al., 1991), though the 0-74 μm grain size fraction of pure silicate minerals still show characteristic signatures between 8 and 10 µm. Surprisingly, diagnostic silicate features were not observed in many of the samples. We identified quartz absorptions between 4.8 and 5.4 μm and at 14.3 μm. We are still trying to understand why the fundamental vibrational features of silicates are obscured, but it may be related to multiple overlapping features in mixtures, grain coatings, or anomalous dispersion. LWIR spectra also revealed numerous diagnostic carbonate features, particularly those near 4, 5.6, and 11.4 μm. We also identified carbonate in several samples where it had not been identified with XRD.