Single-scattering modeling of thin, birefringent mineral-dust flakes using the discrete-dipole approximation

Abstract

The morphology and composition of mineral-dust particles in two surface samples collected in the Sahara desert are investigated by electron microscope imaging, X-ray diffractography, and energy-dispersive spectroscopy. Most submicron particles in the samples are found to be flake-like and thin, and appear to be composed of either calcite or dolomite. The single-scattering properties of these flake-like particles are modeled using the discrete-dipole approximation for size parameters from 1 to 10. For simplicity, the particles are assumed to be composed of calcite, as the properties of dolomite are very similar. The birefringence of calcite is fully accounted for in the simulations, but to assess the importance of birefringence on scattering, two equivalent, isotropic cases analogous to internal and external mixing also are considered. The results show that both isotropic analogs scatter similarly, whereas birefringence affects the light scattering considerably. In particular, the asymmetry parameter is systematically smaller for birefringent than for isotropic flakes. The birefringence also weakens the negative-polarization branch (NPB) near the backscattering direction. Flakes are found to scatter light very differently from volume-equivalent spheroids, spheres, or roundish irregularly shaped particles: their asymmetry parameters tend to be much higher, and their backscattering enhancement and their NPBs are much weaker. The results demonstrate that the single-scattering properties of different mineral-dust types can be remarkably different. Whether the findings have a significant impact on the assessment of the radiative impact of atmospheric dust depends on the abundance of flake-like particles in the atmosphere, which is not known. Copyright 2009 by the American Geophysical Union.