Effect of Heterogeneity and Shape on Optical Properties of Urban Dust Based on 3-Dimensional Modeling of Individual Particles

Published: October 05, 2017

Author(s)

Joseph M. Conny, Diana Ortiz-Montalvo

Abstract

Ground- and satellite-based radiometric monitoring of dust aerosol optical properties is essential for reducing aerosol radiative forcing uncertainties. However, the algorithms for retrieving aerosol optical behavior rely on models of particles as homogeneous spheres or spheroids, and thus may not fully account for the variation in composition heterogeneity and shape of particles. Here we show the effect of heterogeneity and shape on the optical properties of urban dust particles based on the 3-dimensional spatial and optical modeling of individual particles. Using scanning electron microscopy/x-ray spectroscopy (SEM/EDX) and focused ion-beam (FIB) tomography, 3-D spatial models accounted for surface features, inclusions, and voids, as well as overall composition and shape. Using the discrete dipole approximation method and voxel data from spatial models, we report extinction efficiency, asymmetry parameter, backscattering fraction, and single scattering albedo (SSA). Results of models show particle shape to be a more important factor for determining extinction efficiency than spatially distinguishing the separate phases in models, regardless of whether absorption or scattering dominated. In addition to homogeneous models that accurately represented particle morphology, the single tetrahedron geometric model provided better extinction accuracy than spherical or cubic models. For iron-containing heterogeneous particles, all models showed the asymmetry parameter as well as SSA varying with the composition of the iron-containing phase, even if <10 % of the particle’s volume. For particles containing loosely held phases with widely varying refractive indexes (i.e., exhibiting “severe” compositional and optical heterogeneity), only models that account for phase heterogeneity may sufficiently determine SSA.
Citation: Journal of Geophysical Research-Atmospheres
Pub Type: Journals

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Keywords

atmospheric aerosol, urban dust, light scattering, discrete dipole approximation, focused ion-beam scanning electron microscopy, FIB-SEM, FIB tomography, energy-dispersive x-ray spectroscopy, EDX
Created October 05, 2017, Updated August 26, 2019