Spectroscopy of Global Dust/Soils Particles and their Optical Constants for use in Radiative Transfer Models and Remote Sensing
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Authors
Sadrian , Mohammadreza
Issue Date
2023
Type
Dissertation
Language
Keywords
Characterization of mineral dust , Global mineral dust aerosols , Mineral dust complex spectral properties , Mineral dust infrared complex refractive index
Alternative Title
Abstract
Mineral dust particles dominate aerosol mass in the atmosphere and directly modify Earth’s radiative balance through absorption and scattering. This radiative forcing varies strongly with mineral composition and optical properties (e.g., optical constants), yet there is still limited knowledge about these aspects of atmospheric mineral dust. In this study we examined both airfall dust and surface soils from dust entrainment regions in order to characterize their constituent mineralogy, spectral and optical properties. We first measured 37 airfall dust samples using visible, shortwave infrared (VSWIR) reflectance spectroscopy, combined with a Hapke radiative transfer model to identify their spectral diversity, mineralogy, and mineral abundances. We next investigated the diversity in the mineralogical composition and spectra of 26 global surface soil samples using VSWIR and longwave infrared (LWIR) reflectance, as well as LWIR transmission and conducted linear spectral mixture (LSM) modeling on the transmission data. We compared the mineralogy and mineral abundances derived using spectroscopy with results obtained with x-ray diffraction (XRD) and optical mineralogy (OM). The results for soil and airfall dust samples show that VSWIR is very sensitive to iron-bearing minerals and non-mineral components and better identifies their type compared to XRD and OM. Although clay minerals and carbonates have a multiple SWIR absorption signatures, they frequently overlap, making it difficult to ascribe specific spectral features to them. For the airfall dust, the derived abundance values for total clays and carbonate obtained from XRD and SWIR spectroscopy correlate, however the former technique somewhat underpredicts clays and the latter technique significantly underpredicts carbonates. LWIR reflectance for soils easily distinguishes various carbonates from the band center shifts, even in trace amounts that were not identified with other techniques. LWIR reflectance for silicates are more complicated to interpret and to model using LSM. On the other hand, the strongest absorptions in LWIR transmission spectra of soils are caused by the interaction and overlapping of absorptions of multiple silicates. Despite this, there is often good agreement between the determined mineralogy from LSM modeling of transmission and the results from XRD. In the final step, we employed transmission spectral measurements in order to estimate optical constants for same the 26 global soil samples. Because mineral mixtures were successfully retrieved by LSM modeling of LWIR transmission spectra we anticipate that the estimated dust aerosol optical constants (imaginary (k) and real (n) indices of refraction) features are representative of these mineral mixtures. The k spectra were derived from transmission spectra of KBr pellets containing dispersed dust and the n spectra were estimated with the subtractive Kramers–Kronig (SKK) method. Our values were compared to past studies where we observe additional spectral diversity in optical constants for these samples. The magnitude and spectral shape of k and n obtained from our soil samples are in good agreement with previously published values that used dust aerosols re-suspended in a chamber. This suggests these soils are a good analog to suspended mineral dust. We note the SKK method can offer a reliable mechanism for measuring IR spectra of both optical constants for soils. The spectral measurements and optical constants presented in this research can be used to improve global soil atlases and to support interpretation and validation of data from current and future remote sensing instruments.