MODELING PHASE FUNCTIONS FOR DUSTLIKE TROPOSPHERIC AEROSOLS USING A SHAPE MIXTURE OF RANDOMLY ORIENTED POLYDISPERSE SPHEROIDS

Laboratory and in situ measurements show that scattering properties of natural nonspherical particles can be significantly different from th ose of volume- or surface-equivalent spheres, thus suggesting that Mie theory may not be suitable for interpreting satellite reflectance mea surements for dustlike tropospheric aerosols. In this paper we use the rigorous T-matrix method to extensively compute light scattering by s hape distributions of polydisperse, randomly oriented spheroids with r efractive indices and size distributions representative of naturally o ccurring dust aerosols. Our calculations show that even after size and orientation averaging, a single spheroidal shape always produces a un ique, shape-specific phase function distinctly different from those pr oduced by other spheroidal shapes. However, phase functions averaged o ver a wide aspect-ratio distribution of prolate and oblate spheroids a re smooth, featureless, and nearly flat at side-scattering angles and closely resemble those measured for natural soil and dust particles. T hus, although natural dust particles are, of course, not perfect spher oids, they are always mixtures of highly variable shapes, and their ph ase function can be adequately modeled using a wide aspect-ratio distr ibution of prolate and oblate spheroidal grains. Our comparisons of no nspherical versus projected-area-equivalent spherical particles show t hat spherical-nonspherical differences in the scattering phase functio n can be large and therefore can cause significant errors in the retri eved aerosol optical thickness if Mie theory is used to analyze reflec tance measurements of nonspherical aerosols. On the other hand, the di fferences in the total optical cross sections, single-scattering albed o, asymmetry parameter of the phase function, and backscattered fracti on are much smaller and in most cases do not exceed 10%. This may sugg est that for a given aerosol optical thickness the influence of partic le shape on the aerosol radiative forcing is negligibly small. Spheric al-nonspherical differences in the extinction-to-backscatter ratio are very large and should be explicitly taken into account in inverting l idar measurements of dustlike aerosols.