Shift of the linear polarization angle of light scattered by anisotropic aerosol and hydrosol particles

Abstract

We simulate single scattering by two types of compositions of scatterers, soot-aerosol-like and organic/mineral-hydrosol-like, with three different overall shapes, ideal amorphous solid spherical aggregates, fractal aggregates, and helices. We expand our recently developed flexible scattering order formulation of the discrete dipole approximation approach to allow birefringent as well as isotropic polarizabilities. Using this formulation for simulating birefringence and the multiple sphere T-matrix (MSTM) model for simulating intrinsic chirality, we contrast the scattering by particles with anisotropic refractive indices, both optically active and birefringent. Then for each polarizability, we calculate the shift of the linear polarization angle (SoLPA) of the scattered light relative to the scattering plane. We investigate the influence of chirality strength, absorption, birefringence strength, and scatterer size on the SoLPA. We find that for single scattering, the SoLPA resulting from intrinsic optical activity is relatively low ($\lesssim 1^{\circ }$), while the SoLPA resulting from birefringence can be relatively large (>1°) or even very large in some cases (>10°). We find that the SoLPA increases approximately linearly with chirality strength, and for low to moderate birefringence strengths, the SoLPA is also increases approximately linearly with birefringence strength. However, for higher birefringence strengths, the linearity of the SoLPA with birefringence strength can break down. We also find that the spatial patterns of the variations in SoLPA as a function of intrinsic chirality strength and birefringence strength tend to be similar for the fractal aggregate and helical scatterers but differ significantly for the ideal amorphous solid aggregate as compared to the other two shapes. This result has implications for any scattering particle whose constituent parts are not arranged on a regular, crystalline-like grid. The SoLPA examined here can become even more significant in multiple scattering environments and thus could have important implications for remote sensing and understanding of polarization sensitive marine species.