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Modeling optical properties of non-cubical sea salt particles
  • Franz Kanngießer,
  • Michael Kahnert
Franz Kanngießer
Chalmers University of Technology

Corresponding Author:franz.kanngiesser@chalmers.se

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Michael Kahnert
Swedish Meteorological and Hydrological Institute
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Dried sea salt aerosol is commonly represented in aerosol-optics models by ideal cubical particles, while samples reveal that marine aerosol particles frequently display distorted cubical shapes, and they can have more or less rounded edges. In this study three types of non-ideal cuboidal model geometries are investigated, namely, convex polyhedra, Gaussian random cubes, and superellipsoids. Optical calculations were performed at a wavelength of 532 nm using the discrete dipole approximation and the T-matrix method. The main focus is on optical properties relevant to lidar remote sensing, namely, the linear depolarization ratio in the backscattering direction, and the extinction-to-backscatter or lidar ratio. Gaussian random distortions tend to increase the depolarization ratio in relation to that of perfect cubes, while superellipsoids mimicking cubes with rounded edges generally decrease the depolarization ratio. Convex polyhedra can describe randomly distorted cubes. Their computed depolarisation ratios display random fluctuations about those computed for ideal cubes. The results suggest that Gaussian random cubes and superellipsoids are most consistent with the observations if the geometries deviate only mildly from that of an ideal cube. Gaussian random cubes that strongly diverge from cubical shape pose a risk of overestimating both depolarization and extinction-to-backscatter ratio. Superellipsoids that approach octahedral shape yield unrealistically high depolarization ratios. Investigation of size-averaged optical properties of superellipsoids demonstrate that the presence of absorbing material in marine aerosols can have a dramatic effect on the lidar ratio, and its effect on the depolarization ratio can be of comparable magnitude as that caused by rounding of edges.
27 Feb 2021Published in Journal of Geophysical Research: Atmospheres volume 126 issue 4. 10.1029/2020JD033674