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Spheroids have been frequently used to model light scattering by nonspherical particles. In particular, Kahnert FM 2004 found that all diagonal elements of the scattering matrix of nonspherical particles can be approximated much better with the spheroidal particle shape model than with the spherical model. Nousiainen T et al 2006, who used spheres, spheroids, and polyhedral prisms, also found that spheroids best approximate the scattering matrix of small feldspar aerosol [a log-normal PSD with an average radius, r = 0.167 and SD(log r) = 2.32, and refractive index of 1.5 - 0.001i at a wavelength of 632 nm]. Feldspar is a common silicate-based mineral in the Earth crust.
However, as shown by Mishchenko MI et al 1997, even after size and orientation averaging, a spheroidal particle shape always generates a phase function distinctly different from those characteristic of other spheroidal shapes. Encouraged by the results of Hill SC et al 1984a in modeling of the phase function of soil-based aerosol, they proposed to use a shape-distribution of spheroids in modeling the phase function of nonspherical particles.
In the studies of Nousiainen T et al 2006 and Kahnert FM 2004 which both considered size- as well as shape-distributed spheroids, elongated spheroids and flattened spheroids were found to be more important than the nearly-spherical spheroids. This finding questions the validity of a frequent assumption of an equiprobable shape distribution for such irregular particles. Nousiainen et al propose instead the following particle shape distribution for fitting the scattering matrix data:
| f(ξ) ~ |ξ q| | (1) |
where ξ is a shape parameter that equals (1 - 1/ε) for elongated (prolate) spheroids, with ε < 1 being the spheroid aspect ratio, and where the parameter q = 2 or 3. Nousiainen T et al 2006 point out that, given the similarities between scattering matrices of irregular mineral particles (Volten H et al 2001), the preference for the spheroidal particle shape model of irregular mineral particles in light scattering may be applicable on a more general basis.
| CITATION: Jonasz M. 2006. Effective optical particle shape: A spheroid (www.tpdsci.com/Tpc/EfOptPtShpSphd.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 05-Apr-2006 Modified: 07-Apr-2006 Peer-reviewed: 06-Apr-2006 |
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