飞尘气溶胶粒子散射特性分析：对比实验和米散射方法

A study of scattering properties of fly ash aerosols:comparison of laboratory and Lorenz-Mie results

采用Mie散射理论计算了可见光波段等效球飞尘气溶胶粒子的Stokes散射矩阵,并与实验得到的空间随机取向的非球形飞尘气溶胶粒子结果进行了对比分析;由理论与实验方法得到的散射相函数,采用离散坐标法计算了两者的双向反射函数(BRDF),并对此结果进行了分析研究。结果表明:实验测量的非球形飞尘气溶胶粒子群的散射矩阵和基于球形粒子假设的Mie散射理论计算结果在大多数散射角上都不相同,但是不对称因子却大致相同;球形-非球形粒子群的BRDF随反射角的变化趋势基本一致,但是球形粒子群的BRDF曲线分布具有更大的波动趋势;随着光学厚度的增加,球形-非球形粒子群的BRDF曲线分布均趋于平坦,计算结果趋于一致。因此在飞尘气溶胶粒子散射特性研究中,当光学厚度较小时,用球形假设的方法会造成一定的误差,BRDF相对误差最大可以达到60%,需考虑粒子非球形特性造成的影响;而当光学厚度较大时,BRDF相对误差一般不会超过10%,采用球形假设的方法具有一定的适用性。

This paper compared the results of laboratory measurements of the Stokes scattering matrix of non spherical fly ash aerosols at a visible wavelength with the results of Lorenz Mie computations for projected area equivalent spheres. Bidirection al reflectance functions （BRDFs） were calculated and analyzed by DISORT with scattering phase function calculated using Lorenz Mie theory and measured in an experiment. The results show poor agreement between experimental scattering matrix of non spherical fly ash aerosol particles and that of Lorenz Mie calculation for most scattering angles. However, their asymmetry factors are similar. The variation trend of BRDF of spherical particles is basically consistent with that of non spherical particles, but the BRDF curve of spherical particles is more variable. With the increasing of aerosol optical depth, the curves of BRDF of spherical particles and non spherical particles go to a steady and similar value. Assumption of spherical particles will result in a certain error under thin optical depth, the relative error of BRDF will grow up to 60%, therefore the influence of non spherical characteristics must be taken into consideration. However, when the optical depth is larger, the relative error of BRDF is usually lower than 10%, spherical particle assumption has certain applicability.