LIGHT-SCATTERING BY POLYDISPERSIONS OF RANDOMLY ORIENTED SPHEROIDS WITH SIZES COMPARABLE TO WAVELENGTHS OF OBSERVATION

We report the results of an extensive study of the scattering of light by size and size-shape distributions of randomly oriented prolate and oblate spheroids with the index of refraction 1.5 + 0.02i typical of some mineral terrestrial aerosols. The scattering calculations have be en carried out with Waterman's T-matrix approach, as developed recentl y by Mishchenko [J. Opt. Soc. Am. A 8, 871 (1991); Appl. Opt. 32, 4562 (1993)]. Our main interest is in light scattering by polydisperse mod els of nonspherical particles because averaging over sizes provides mo re realistic modeling of natural ensembles of scattering particles and washes out the interference structure and ripple typical of monodispe rse scattering patterns, thus enabling us to derive meaningful conclus ions about the effects of particle nonsphericity on light scattering. Following Hansen and Travis [Space Sci. Rev. 16, 527 (1974)], we show that scattering properties of most physically plausible size distribut ions of randomly oriented nonspherical particles depend primarily on t he effective equivalent-sphere radius and effective variance of the di stribution, the actual shape of the distribution having a minor influe nce. To minimize the computational burden, we have adopted a computati onally convenient power law distribution of particle equivalent-sphere radii n(r) is-proportional-to r-3, r1 less-than-or-equal-to r less-th an-or-equal-to r2. The effective variance of the size distribution is fixed at 0.1, and the effective size parameter continuously varies fro m 0 to 15. We present results of computer calculations for 24 prolate and oblate spheroidal shapes with aspect ratios from 1.1 to 2.2. The e lements of the scattering matrix for the whole range of size parameter s and scattering angles are displayed in the form of contour plots. Co mputational results are compared with analogous calculations for surfa ce-equivalent spheres, and the effects of particle shape on light scat tering are discussed in detail.