Modeling the optical properties of forsterite particles using different irregularly shaped particles

Abstract

Light scattering by four different types of irregularly shaped particles with refractive index m = 1.6 + 0.0005i is investigated over a wide range of size parameter x = 1–48 calculated by means of the discrete dipole approximation. The packing density of the constituent materials varies by a factor of ∼4.6 between the densest morphology of the pocked spheres and the fluffiest morphology of the loose clusters. The modeling reveals a great deal of similarity in the light-scattering response in all four types of particles, despite their dramatically different morphology. The resemblance is further enhanced when the light-scattering response is averaged over all available sizes of particles. However, the influence of particles whose size parameter exceeds 32 appears to be hardly noticeable. All four types of particles are capable of reasonably good fits to the experimental optical measurements of the forsterite particles with a simple power-law size distribution. It is significant that the power index providing these best fits appears to be squeezed to a very narrow range of values n = 3.075 ± 075. This finding potentially has a great impact on remote sensing as it raises confidence in the retrievals obtained with any model of irregularly shaped particles whose shape attains a high level of disorder. Interestingly, when the size distribution of laboratory-measured forsterite particles is considered, only the agglomerated debris particles immediately yield a reasonably good fit to the optical measurements. This suggests the morphology may more closely resemble that of the forsterite particles.