Single scattering by realistic, inhomogeneous mineral dust particles with stereogrammetric shapes

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Abstract

Light scattering by single, inhomogeneous mineral dust particles was simulated based on shapes and compositions derived directly from measurements of real dust particles instead of using a mathematical shape model. We demonstrate the use of the stereogrammetric shape retrieval method in the context of single-scattering modelling of mineral dust for four different dust types-all of them inhomogeneous-ranging from compact, equidimensional shapes to very elongated and aggregate shapes. The three-dimensional particle shapes were derived from stereo pairs of scanning-electron microscope images, and inhomogeneous composition was determined by mineralogical interpretation of localized elemental information based on energy-dispersive spectroscopy. Scattering computations were performed for particles of equal-volume diameters, from 0.08 μm up to 2.8 μm at 550 nm wavelength, using the discrete-dipole approximation. Particle-to-particle variation in scattering by mineral dust was found to be quite considerable and was not well reproduced by simplified shapes of homogeneous spheres, spheroids, or Gaussian random spheres. Effective-medium approximation results revealed that particle inhomogeneity should be accounted for even for small amounts of absorbing media (here up to 2% of the volume), especially when considering scattering by inhomogeneous particles at size parameters 3 x <8. When integrated over a log-normal size distribution, the linear depolarization ratio and single-scattering albedo were also found to be sensitive to inhomogeneity. The methodology applied is work-intensive and the light-scattering method used quite limited in terms of size parameter coverage. It would therefore be desirable to find a sufficiently accurate but simpler approach with fewer limitations for single-scattering modelling of dust. For validation of such a method, the approach presented here could be used for producing reference data when applied to a suitable set of target particles. © 2014 Author(s).

Figures

  • Fig. 1. Mineral dust particles used in this study (from left to right): scanning-electron microscope images, the retrieved TIN model of the surface, and the final volume representation from two different viewing angles.
  • Fig. 2. Energy-dispersive spectroscopic images of the particle Agg I, based on which the elemental distribution and the composition of the particle were estimated (volume representation of the retrieved particle on the right). O and Na images show a pronounced shadowing effect towards the detector due to absorption of low-energy X-ray radiation inside the particle volume.
  • Table 1. Mineralogical composition of the selected dust particles, and respective refractive indices at λ= 550 nm. MinDat stands for the web-based mineralogical database, http://www.mindat.org, where the information was retrieved in 2012.
  • Fig. 3. A schematic presentation of the stereogrammetric shape retrieval method. A detailed description of each of the phases is given in Sect. 4.1.
  • Fig. 4. Keypoints (blue crosses) and interest points (red dots) on the first SEM image of the Agg I particle.
  • Fig. 5. Horizontal disparities reflecting the particle’s feature elevation over the substrate plane at the matched points of the Agg I particle.
  • Fig. 6. Shapes of a prolate spheroid and five Gaussian random spheres derived as simplified models for each dust particle. The three black cross sections of each dust particle were used in deriving the corresponding Gaussian random sphere statistics.
  • Table 2. Effective refractive indices meff and shape model parameters. The covariance functions for the Gaussian random sphere are not listed.

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CITATION STYLE

APA

Lindqvist, H., Jokinen, O., Kandler, K., Scheuvens, D., & Nousiainen, T. (2014). Single scattering by realistic, inhomogeneous mineral dust particles with stereogrammetric shapes. Atmospheric Chemistry and Physics, 14(1), 143–157. https://doi.org/10.5194/acp-14-143-2014

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