Contrasting mineral dust abundances from X-ray diffraction and reflectance spectroscopy

[摘要] Mineral dust particles dominate aerosol mass in the atmosphere and directlymodify Earth's radiative balance through absorption and scattering. Thisradiative forcing varies strongly with mineral composition, yet there isstill limited knowledge on the mineralogy of atmospheric dust. In thisstudy, we performed X-ray diffraction (XRD) and reflectance spectroscopymeasurements on 37 different dust deposition samples collected as airfall inan urban setting to determine mineralogy and the relative proportions ofminerals in the dust mixture. Most commonly, XRD has been used tocharacterize dust mineralogy; however, without prior special samplepreparation, this technique is less effective for identifying poorlycrystalline or amorphous phases. In addition to XRD measurements, weperformed visible and short-wave infrared (VSWIR) reflectance spectroscopyfor these natural dust samples as a complementary technique to determinemineralogy and mineral abundances. Reflectance spectra of dust particles area function of a nonlinear combination of mineral abundances in the mixture.Therefore, we used a Hapke radiative transfer model along with a linearspectral mixing approach to derive relative mineral abundances fromreflectance spectroscopy. We compared spectrally derived abundances withthose determined semi-quantitatively from XRD. Our results demonstrate thattotal clay mineral abundances from XRD are correlated with those fromreflectance spectroscopy and follow similar trends; however, XRDunderpredicts the total amount of clay for many of the samples. On the otherhand, calcite abundances are significantly underpredicted by SWIR comparedto XRD. This is caused by the weakening of absorption features associatedwith the fine particle size of the samples, as well as the presence of darknon-mineral materials (e.g., asphalt) in these samples. Another possibleexplanation for abundance discrepancies between XRD and SWIR is related tothe differing sensitivity of the two techniques (crystal structure vs.chemical bonds). Our results indicate that it is beneficial to use both XRDand reflectance spectroscopy to characterize airfall dust because theformer technique is good at identifying and quantifying the SWIR-transparentminerals (e.g., quartz, albite, and microcline), while the latter techniqueis superior for determining abundances for clays and non-mineral components.

[学科分类] 内科医学