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The extinction efficiency factor associated with the scattering of a plane electromagnetic wave impinging on a basal face of a dielectric disk or a cylindrical particle is investigated by employing the physical-geometric optics hybrid (PGOH) method and the discrete-dipole approximation (DDA) method. It is found that the derived extinction efficiency factor from the PGOH is a function of the thickness of the disk, or the length of the cylinder, and the refractive index, but is independent of the diameter and shape of the cross section of the basal face of the particle. Furthermore, the oscillations of the extinction efficiency factor versus the thickness or length of the particle do not diminish if the particle is not absorptive. The values of the extinction efficiency factor simulated from the DDA method are quite different from those computed from the PGOH, although the size parameter of the particle is in the commonly recognized geometric optics regime. To explain the difference, the concept of the edge effect associated with the tunneling rays in the semiclassical scattering theory is generalized from the case of spherical particles to that of nonspherical particles based on the localization principle. Accordingly, the edge-effect contribution can be distinguished and removed from the extinction cross section calculation by the DDA method. The remaining part of the extinction cross section, associated with the interference between the transmitted rays and incident rays, agrees well with the results computed from the PGOH, and the agreement illustrates the presence of the edge effect in the case of nonspherical particles with surfaces that have no curvature along the incident direction. It is found that the asymptotic extinction efficiency factor may not necessarily converge to 2, but it depends on the specific physical processes of the interference between diffracted and transmitted light and of the edge effect. [c] 2010 Optical Society of America OCIS codes: 290.1310, 010.0010.

We explore the use of nonsymmetric geometries to simulate the single-scattering properties of airborne dust particles with complicated morphologies. Specifically, the shapes of irregular dust particles are assumed to be nonsymmetric hexahedra defined by using the Monte Carlo method. A combination of the discrete dipole approximation method and an improved geometric optics method is employed to compute the single-scattering properties of dust particles for size parameters ranging from 0.5 to 3000. The primary optical effect of eliminating the geometric symmetry of regular hexahedra is to smooth the scattering features in the phase function and to decrease the backscatter. The optical properties of the nonsymmetric hexahedra are used to mimic the laboratory measurements. It is demonstrated that a relatively close agreement can be achieved by using only one shape of nonsymmetric hexahedra. The agreement between the theoretical results and their measurement counterparts can be further improved by using a mixture of nonsymmetric hexahedra. It is also shown that the hexahedron model is much more appropriate than the 'equivalent sphere' model for simulating the optical properties of dust particles, particularly, in the case of the elements of the phase matrix that associated with the polarization state of scattered light. [c] 2010 Optical Society of America OCIS codes: 290.0290, 080.0080, 260.0260, 010.0010.

The single-scattering properties of randomly oriented triaxial ellipsoids with size parameters from the Rayleigh to geometric-optics regimes are investigated. A combination of the discrete dipole approximation (DDA) technique and an improved geometric optics method (IGOM) is applied to the computation of ellipsoidal particle scattering properties for a complete range of size parameters. Edge effect contributions to the extinction and absorption efficiencies are included in the present IGOM simulation. It is found that the extinction efficiency, single-scattering albedo, and asymmetry factor computed from the DDA method for small size parameters smoothly transition to those computed from the IGOM for moderate-to-large size parameters. The phase matrix elements computed from these two methods are also quite similar when size parameters are larger than 30. Thus, the optical properties of ellipsoidal particles can be computed by combing the DDA and the IGOM for small-to-large size parameters. Furthermore, we also examine the applicability of the ellipsoid model to the simulation of the scattering properties of realistic aerosol particles by comparing the theoretical and experimental results for feldspar aerosols. It is shown that the ellipsoid model is better than the commonly used spheroid model for simulating dust particle optical properties, particularly, their polarization characteristics, realistically. OCIS codes: 290.0290, 080.0080, 260.0260, 010.0010.