随机取向非球形沙尘气溶胶粒子的光学特性

利用离散偶极子近似法分析了一种随机取向旋转椭球体沙尘气溶胶粒子模型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,研究了沙尘粒子非球形性程度对其光学特性的影响,并考察了非球形粒子的随机取向能否用等体积球体来代替。就随机取向单分散和多分散旋转椭球体沙尘气溶胶而言,粒子非球形特征越明显,消光效率因子、不对称因子和单次散射反照率基本上偏离其等体积球体越大;对于相同的非球形,不对称因子偏离其等体积球体的相对偏差要比消光效率因子和单次散射反照率要大。非球形粒子的随机取向并不能使其光学特性严格等效为其等体积球体的光学特性。如果粒子形状偏离球体较小,则非球形粒子的随机取向的平均效果能使其消光效率因子、不对称因子和单次散射反照率近似用等体积球体的对应光学参量来等效;而如果粒子形状偏离球形较大,仅有单次散射反照率可以近似用等体积球体的单次散射反照率来等效,例如,轴半径比为16的旋转椭球体沙尘粒子的单次散射反照率偏离其等体积球体仅在3%以内。

Orientation-Averaged Optical Properties of Nonspherical Dust Aerosols

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 μm at wavelength of 550 nm).The effects of the asphericity degree on aerosol optical properties and whether properties of orientation-averaged nonspherical particles can be replaced by those of effective isovolumetric spherical particles are also studied.For monodisperse and polydisperse spheroid dust,distinctive asphericity can generally induce distinctive deviations from their effective isovolumetric spherical particles for extinction efficiency factor,asymmetry parameter and single scattering albedo.For the same asphericity,the deviations of asymmetry parameter of nonspherical particles from their isovolumetric spheres are larger than those of extinction efficiency factor and single scattering albedo.Optical properties of orientation-averaged nonspherical particles cannot be rigorously replaced by those of their effective isovolumetric spheres.Orientation-averaged extinction efficiency factors,asymmetry parameters and single scattering albedos of nonspherical aerosols can be approximately replaced by those of their effective isovolumetric spheres,if the asphericity is small.However,for large asphericity,the approximate replacement happens only for the single scattering albedo with its deviation within 3％ for spheroid dust at aspect ratio of 16 as an example.