Laboratory measurements of mineral dust scattering phase function and linear polarization

With the goal of improving our understanding of how small mineral dust particles scatter light at visible and near-infrared wavelengths, we measured the scattering phase function and linear polarization of small mineral dust particles over the scattering angle range 15 degrees-170 degrees at three wavelengths (0.47, 0.652, and 0.937 mu m). Particle samples were obtained from Duke Scientific Corp., and include aluminum oxide, silicon carbide, aluminum silicate, antimony oxide, calcium carbonate, and cerium oxide. Particle equivalent-sphere radii range from a few tenths of a micron to about 10 mu m. The particles were injected into a laboratory chamber, where they scattered light as they fell through the air. They were collected on a scanning electron micrograph (SEM) substrate. Particle shapes and sizes were then measured from the SEM images. We compare measured phase functions with those calculated for spheroids with a distribution of axial ratios and sizes, random orientation, and refractive index 1.53 + 0.008i [Mishchenko et al., this issue]. Two of the samples (one of which has a refractive index close to that used in theoretical computations) produced scattering phase functions that were quite similar to those for spheroids. Two samples produced phase functions whose variation between 15 degrees and 170 degrees was much less than that for the spheroids or for the other samples. We suspect this difference may be due to the very high refractive index of those particles, although differences in particle microstructure may also be important. Two samples produced positive linear polarization which had a single broad maximum near 100 degrees scattering angle, and a magnitude greater than 40% at some wavelengths. Two samples had generally positive linear polarization but a more complicated structure, and two samples produced mostly negative polarization whose amplitude was small. We do not have numerical results for the appropriate refractive index and size parameter with which to compare the polarization measurements. We hope the questions raised by this work will stimulate additional effort to develop and test numerical codes for scattering by nonspherical particles.