Inhomogeneity structure and the applicability of effective medium approximations in calculating light scattering by inhomogeneous particles

摘要

Atmospheric aerosols commonly occur as inhomogeneous particles with significant internal variations of refractive indices. For the purpose of light-scattering calculations, effective medium approximations (EMAs) have been developed that seek to model an inhomogeneous particle with an internal variation of refractive index using a homogeneous particle with a single "effective" refractive index, one whose value is expected to yield approximately the same scattering properties. Here the applicability of four EMAs is investigated in the relatively simple case that the particle is a mixture of materials having just two different indices of refraction, and consideration is given to the effects of the spatial arrangement of these on the reliability of results obtained using the EMAs. The EMAs considered are based on the Bruggeman theory, the Maxwell-Garnett theory, and two different Wiener bounds. The mixing states considered are relatively regular forms of "layering," as well as states that involve varying degrees of more irregular "mixing." Just two overall particle shapes are considered: spheres and spheroids. For each inhomogeneous particle two sets of scattering calculations are performed: calculations treating the inhomogeneous particle itself exactly, and calculations treating an effective homogeneous particle with the same shape and size but a single refractive index determined using one of the EMAs. The first kind is done using the core-mantle Mie theory in the case of stratified composition and the pseudo-spectral time-domain method in the more irregular mixing cases. The second kind of calculation is done using the Lorenz-Mie or T-matrix method.