Modeling optical properties of particles with small-scale surface roughness: combination of group theory with a perturbation approach

Michael Kahnert,Tom Rother

doi:10.1364/oe.19.011138

Abstract

A T-matrix method for scattering by particles with small-scale surface roughness is presented. The method combines group theory with a perturbation expansion approach. Group theory is found to reduce CPU-time by 4-6 orders of magnitude. The perturbation expansion extends the range of size parameters by a factor of 5 compared to non-perturbative methods. An application to optically hard particles shows that small-scale surface roughness changes scattering in side- and backscattering directions, and it impacts the single-scattering albedo. This can have important implications for interpreting remote sensing observations, and for the climate impact of mineral aerosols.

Highlights

Small-scale surface roughness is a morphological property that is encountered in many types of aerosols in planetary atmospheres, as well as in mineral particles in the interplanetary and interstellar medium
Mineral aerosols in planetary atmospheres typically have large size parameters in the visible part of the spectrum. (The size parameter is defined as x = 2πr/λ, where r is the particle radius, and λ is the wavelength of light.) geometric optics, which is an approximate method valid for large size parameters, cannot be applied to such particles owing to the small size-scale of the surface perturbations
We test our approach by implementing the perturbation expansion method into the Tsym program, which is a T-matrix code for scattering by 3D targets that has been made for accounting for point-group symmetries [6, 7]

Summary

Introduction

Small-scale surface roughness is a morphological property that is encountered in many types of aerosols in planetary atmospheres, as well as in mineral particles in the interplanetary and interstellar medium. Mineral aerosols in planetary atmospheres typically have large size parameters in the visible part of the spectrum. Previous computational studies of particles with small-scale surface roughness have been limited to rather moderate size parameters. A recent modeling study of hematite aerosols at a wavelength of λ = 633 nm considered Chebyshev particles up to r = 1.4 μm [1], which corresponds to a size parameter of 14. Current computational methods for particles with small-scale surface roughness are severely limited in the range of size parameters for which numerical computations are sufficiently stable and expedient. To the best of our knowledge, all previous studies based on T-matrix methods have been limited to model particles with axisymmetric geometries (e.g. [1,2,3])

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