取向数对非球形沙尘气溶胶光学特性的影响

Effect of Orientation Numbers on Optical Properties of Nonspherical Dust Aerosols

利用离散偶极子近似法研究了旋转椭球体沙尘气溶胶粒子在尺度参数变化范围为0.1~23时(波长0.55!m对应有效半径为0.01~2!m)的光学特性,并通过分析比较不同取向数下光学特性的差别,研究取向数对非球形沙尘粒子光学特性的影响。对单分散系,较少的取向数使旋转椭球体沙尘气溶胶粒子的各光学参量对于取向的选取敏感,但随着取向数的增多,沙尘粒子的各光学量逐步趋于一致。对多分散系,受粒子谱分布积分效果的影响,旋转椭球体沙尘气溶胶粒子对取向的敏感度,相比单分散系要稍低一些。粒子的非球形程度对获得随机取向光学特性取向数的选取也有影响,粒子非球形特征越明显,获得随机取向光学特性所需要的取向数越多。以多分散的旋转椭球体沙尘粒子为例,要获得随机取向沙尘粒子的消光效率因子、单次散射反照率和不对称因子(相对偏差在1%以内),轴半径比为16的非球形程度所需的取向数超过500,而轴半径比为2的非球形程度所需的取向数则仅超过100。

Orientation-averaged optical properties of spheroid dust aerosol particles are carried out based on discrete dipole approximation for size parameters from 0.1 to 23（corresponding to effective radius from 0.01 to 2 tam at wavelength of 550 nm）. The effects of orientation numbers on optical properties of nonspherical dust aerosols are carried out by using different orientation numbers. For monodisperse particles, optical properties of dust spheroid are sensi- tive to the orientation when using few orientation numbers. However,for polydisperse parti- cles, they are less sensitive to the orientation with the effect of size distribution. For the effect of asphericity degree,the larger it is,the more orientation numbers should be utilized for ori- entation-averaged optical properties. Take the polydisperse spheroid dust particle as an exam- pie,to obtain the orientation-averaged extinction efficiency factor, single scattering albedo and asymmetry parameter（differences less than 1 ~）, the orientation number should be larger than 500 for the aspect ratio of 16,but it is larger than 100 for the aspect ratio of 2.