Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids
Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids
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DOI：
10.1029/96jd02110
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发表时间：
1997-07-27
影响因子：
4.4
通讯作者：
West, RA
中科院分区：
文献类型：
--
作者：
Mishchenko, MI;Travis, LD;West, RA
实验室和现场测量表明，自然非球形颗粒的散射特性可以显着不同的体积或表面等效球体，从而表明米氏理论可能不适合解释卫星反射率测量尘埃对流层气溶胶。在本文中，我们使用严格的T-矩阵方法来广泛计算光散射的形状分布的多分散，随机取向的球状体的折射率和尺寸分布代表自然发生的尘埃气溶胶。我们的计算表明，即使在大小和取向平均，一个单一的球形形状总是产生一个独特的，形状特定的相位函数明显不同于其他球形形状所产生的。然而，相函数平均在一个宽的纵横比分布的长球形和扁球形是光滑的，无特色的，几乎平坦的侧散射角，非常类似于天然土壤和尘埃颗粒的测量。因此，虽然自然尘埃颗粒，当然，不是完美的球体，他们总是高度可变的形状的混合物，和他们的相函数可以充分使用一个宽的纵横比分布的长球形和扁球形颗粒建模。我们的非球形与投影面积等效的球形粒子的比较表明，球形非球形的散射相函数的差异可以是大的，因此可以导致显着的错误，检索气溶胶光学厚度，如果米氏理论是用来分析非球形气溶胶的反射率测量。另一方面，在总的光学截面，单次散射的散射系数，相函数的不对称参数，和后向散射分数的差异是小得多，在大多数情况下不超过10%。这可能表明，对于一个给定的气溶胶光学厚度的粒子形状对气溶胶辐射强迫的影响可以忽略不计。球形-非球形的反射-后向散射比的差异非常大，在反演类尘气溶胶的激光雷达测量中应明确考虑。
Laboratory and in situ measurements show that scattering properties of natural nonspherical particles can be significantly different from those of volume- or surface-equivalent spheres, thus suggesting that Mie theory may not be suitable for interpreting satellite reflectance measurements for dustlike tropospheric aerosols. In this paper we use the rigorous T-matrix method to extensively compute light scattering by shape distributions of polydisperse, randomly oriented spheroids with refractive indices and size distributions representative of naturally occurring dust aerosols. Our calculations show that even after size and orientation averaging, a single spheroidal shape always produces a unique, shape-specific phase function distinctly different from those produced by other spheroidal shapes. However, phase functions averaged over a wide aspect-ratio distribution of prolate and oblate spheroids are smooth, featureless, and nearly flat at side-scattering angles and closely resemble those measured for natural soil and dust particles. Thus, although natural dust particles are, of course, not perfect spheroids, they are always mixtures of highly variable shapes, and their phase function can be adequately modeled using a wide aspect-ratio distribution of prolate and oblate spheroidal grains. Our comparisons of nonspherical versus projected-area-equivalent spherical particles show that spherical-nonspherical differences in the scattering phase function can be large and therefore can cause significant errors in the retrieved aerosol optical thickness if Mie theory is used to analyze reflectance measurements of nonspherical aerosols. On the other hand, the differences in the total optical cross sections, single-scattering albedo, asymmetry parameter of the phase function, and backscattered fraction are much smaller and in most cases do not exceed 10%. This may suggest that for a given aerosol optical thickness the influence of particle shape on the aerosol radiative forcing is negligibly small. Spherical-nonspherical differences in the extinction-to-backscatter ratio are very large and should be explicitly taken into account in inverting lidar measurements of dustlike aerosols.