Extended calculation of polarization and intensity of fractal aggregates based on rigorous method for light scattering simulations with numerical orientation averaging

Abstract

The intensity and polarization of fractal aggregates have been investigated using both rigorous and approximate methods for light scattering simulations. However, previous studies using the analytical orientation averaging version of the rigorous method were generally limited to a few hundred monomers when the monomer size parameter was around 1.7. In this study, we propose using numerical orientation averaging instead of analytical orientation averaging. The numerical averaging is performed together with a fixed orientation version of the rigorous T-matrix method for clusters of spheres. This approach enables increasing the number of monomers by a factor of 2–7 or the size of monomers by a factor of 8–10 compared to the analytical orientation averaging version. We investigated the influence of monomer size and the number of monomers on the light scattering of silicate aggregates (refractive index m = 1.68 + 0.03 i ) for incident light with a wavelength of 0.6 μ m . We considered ballistic particle–cluster aggregates (BPCA) and ballistic cluster–cluster aggregates (BCCA) composed of 128, 256, 512, and 1024 monomers with radii between 0.11 and 0.17 μ m . Our results show that the size of monomers plays an important role in reproducing the negative polarization branch for all the BPCA and BCCA. Silicate aggregates with the monomer radius of less than 0.17 μ m contribute to reproducing the negative polarization branch, while aggregates with monomers larger than 0.17 μ m do not have the negative polarization branch. Polarization oscillation with scattering angle occurs for larger monomers (i.e., monomer radius ⩾ 0.3 μ m ). The maximum polarization decreases for increasing monomer radius between 0.11 and 0.17 μ m . However, the negative polarization branch is generally enhanced for monomer radii up to around 0.15 μ m , and reduced for further increase of monomer size. The number of monomers also has a large influence on the negative polarization branch in the case of BPCA. The increase in the number of monomers from 128 to 1024 shifts the scattering angle of minimum polarization to larger angles for BPCA. In addition, the increase in the number of monomers reduces the values of negative polarization for BPCA while the variation with the number of monomers for BCCA is small and is not monotonic.