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Optical Modeling of Single Asian Dust and Marine Air Particles: A Comparison with Geometric Particle Shapes for Remote Sensing

Published

Author(s)

Joseph M. Conny, Robert D. Willis, Diana Ortiz-Montalvo

Abstract

With aerosol models used in remote sensing to determine aerosol optical thickness, the choice of particle shape has historically been based on the average optical behavior of particle ensembles. Here, we compare the optical properties of various simplified geometric shapes with single atmospheric particles, specifically Asian dust and marine background air particles collected at Mauna Loa Observatory. Exact 3-dimensional representations of the particles were acquired with focused ion-beam (FIB) tomography, a technique involving FIB milling of individual particles followed by imaging and elemental mapping with scanning electron microscopy. Particles were heterogeneous with mainly dolomite or calcite and a minor amount of iron; marine air particles largely contained gypsum but no iron. Optical properties were calculated with the discrete dipole approximation method. Geometric shapes were grouped as spheroids (sphere, spheroid, ellipsoid), cuboids (cube, square prism, rectangular prism), and pyramids (tetrahedron, triangular pyramid). Each group represented a progression of 1, 2, or 3 axes to replicate particle dimensions. Most geometric shapes underestimated extinction and overestimated the backscatter fraction. Not surprisingly, extinction and backscatter fraction of the single-axis spherical and cubic models were furthest from those of the particles. While the ellipsoid and rectangular prism were closer dimensionally to the particles, the extinction and backscatter fraction for the spheroid and square prism, respectively, were often closer to the particles. Overall, the tetrahedron and triangular pyramid performed remarkably well. The tetrahedron has the advantage that parameterization of the aerosol model does not require an aspect ratio. Surface roughness in the particles invariably decreased the backscatter fraction. While surface roughness typically contributes a minor part to overall scattering, in some cases the additional surface area of the tetrahedron and t
Citation
Journal of Quantitative Spectroscopy and Radiative Transfer

Keywords

atmospheric aerosol, Asian dust, scattering, extinction, backscattering, particle shape, surface roughness, scanning electron microscopy, focused ion-beam tomography, FIB-SEM, discrete dipole approximation

Citation

Conny, J. , Willis, R. and Ortiz-Montalvo, D. (2020), Optical Modeling of Single Asian Dust and Marine Air Particles: A Comparison with Geometric Particle Shapes for Remote Sensing, Journal of Quantitative Spectroscopy and Radiative Transfer, [online], https://doi.org/10.184343/M32263 (Accessed May 19, 2022)
Created June 30, 2020, Updated March 1, 2021