LIGHT-SCATTERING BY POLYDISPERSE ROTATIONALLY SYMMETRICAL NONSPHERICAL PARTICLES - LINEAR-POLARIZATION

Since most solid particles in the Earth and planetary atmospheres have irregular shapes, quantifying the effects of particle nonsphericity on the results of remote sensing of the atmosphere is an important problem. In this paper, we perform a general theoretical survey of linear polarization of light scattered by polydisperse, randomly oriented, rotationally symmetric particles of size comparable to the wavelength of radiation. Our paper deals with polydispersions of nonspherical particles because (1) averaging light-scattering characteristics over sizes provides more realistic modeling of natural particle ensembles and (2) comparisons of scattering properties of particles of a single size are usually meaningless because of the complicated interference structure and high-frequency ripple of monodisperse scattering patterns. In our computations, we use the T-matrix approach, as extended recently to randomly oriented particles by Mishchenko V. Opt. Soc. Amer. A 8, 871 (1991)]. Following Hansen and Travis [Space Sic. Rev. 16, 527 (1974)], we assume that the scattering properties of polydisperse particles depend primarily on only the effective size parameter and effective variance of the size distribution, the particular shape of the distribution being of minor importance. Therefore, to describe the dispersion of particle sizes in the ensemble, we employ a convenient power law distribution of particle equivalent-sphere size parameters. Size-averaged light-scattering characteristics are calculated by numerically integrating monodisperse quantities using a Gaussian quadrature formula. The results of extensive numerical calculations for particles of different shape and refractive index are presented in the form of color contour diagrams of linear polarization as a function of scattering angle and effective equivalent-sphere size parameter. The influence of particle size distribution, shape, and refractive index on the polarization patterns is examined in detail and implications for polarimetric remote sensing of nonspherical aerosols are discussed. The diagrams displayed include calculations for over 150,000 different monodisperse particles in random orientation with equivalent-sphere size parameters up to 30 and may be used to interpret results of laboratory measurements and remote observations of light scattering by small particles.