Reflectance Spectroscopy of Global Parent Soils: Application to Aerosol
Mineral Dust
Abstract
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.