随机分布烟尘团簇粒子辐射特性研究

基于分形理论,采用蒙特卡罗方法对随机分布的烟尘团簇粒子结构进行了仿真模拟,利用离散偶极子近似(discrete dipole approximation, DDA)方法研究了随机分布的烟尘团簇粒子的辐射特性,分析讨论了分形维数、原始微粒粒径和数量以及复折射率对随机分布烟尘团簇粒子辐射特性的影响.研究表明,在给定分形维数的情况下,烟尘团簇粒子的辐射特性取决于原始微粒粒径、数量及复折射率;原始微粒较小的团簇粒子,当分形维数较小时,吸收截面变化不明显,但当分形维数大于2时,吸收截面骤然增大,然而,对于具有比较大的原始微粒粒径、数量及复折射率的烟尘团簇粒子,吸收截面随着分形维数的增大而单调递减;随着分形维数的增大,团簇粒子的散射截面、消光截面及单次散射反照率均单调递增;从整体上来讲,团簇粒子的辐射特性与等效球形粒子的辐射特性存在着比较大的差别,并且这种差别随着分形维数的增大而减小.该工作对研究气溶胶粒子的辐射及气候效应具有重要的科学价值. 关键词： 烟尘团簇粒子 辐射特性 离散偶极子近似方法     

Laboratory measurements of single light scattering by ensembles of randomly oriented small irregular particles in air. A review

In this paper we present an overview of light scattering experiments devoted to measure one or more elements of the scattering matrix as functions of the scattering angle of ensembles of randomly oriented small irregular particles in air. A summary of the most important findings in light scattering experiments on ensembles of randomly oriented particles in air is given. The particles of interest are relevant for studies of atmospheres of planets and satellites and also for other astronomical bodies and environments. Some applications of light scattering experiments are also presented.     

激光在随机分布烟尘团簇粒子中的衰减特性

基于分形理论,采用团簇—团簇凝聚模型对随机分布烟尘团簇粒子的分形结构进行了模拟.利用离散偶极子近似方法研究了不同类型的随机分布烟尘团簇粒子的单次散射特性.利用蒙特卡罗方法研究了激光信号在随机分布烟尘团簇粒子中的传输衰减特性.讨论了入射角、激光波长、烟尘粒子数密度以及组成单个烟尘团簇粒子的原始微粒粒径和数目等参量对激光衰减特性的影响.研究结果对激光在复杂随机介质中的传输衰减特性分析提供了理论依据.     

Determination of the soot aggregate size distribution from elastic light scattering through Bayesian inference

Recovering the size distribution of aerosolized soot aggregates from multiangle elastic light scattering data requires the inversion of an integral equation, which is a mathematically ill-posed problem. This paper demonstrates how maximum a posteriori (MAP) inference can be used to stabilize the inversion by introducing prior information about the size distribution of the soot aggregates. Results show that the size distribution can be recovered using only simple smoothness and non-negativity priors if the aggregate number density is known, but otherwise it is necessary to specify additional information about the presumed distribution shape.     

分形结构对随机取向烟尘团簇粒子光散射特性的影响

利用蒙特卡罗方法对不同分形维数和分形前向因子的随机取向烟尘团簇粒子的分形结构进行了仿真,采用离散偶极子近似（DDA）方法对随机取向烟尘团簇粒子的缪勒矩阵元进行了数值计算,并与球形粒子模型进行了比较,深入探讨了烟尘团簇粒子的分形维数和分形前因子对其散射特性的影响。研究表明,等效球形粒子的光散射特性与随机取向烟尘团簇粒子的光散射特性存在很大差别,并且此差别随着团簇粒子的分形维数以及分形前向因子的增大而减小;分形维数对表征团簇粒子散射特性的缪勒矩阵元的影响在一定散射角范围内均比较明显,分形前向因子对团簇粒子的缪勒矩阵元角分布的影响与分形维数的影响类似,不过其影响相对分形维数较弱。     

用蒙特卡罗方法计算光通过薄层随机分布粒子的散射

本文利用蒙特卡罗方法计算准直光束通过薄层随机分布粒子散射的透射和反射光强,并和输运理论的扩散近似结果做了比较。当散射接近各向同性时两者符合良好。当散射明显地成为各向异性时,蒙特卡罗方法的结果是合理的,而输运理论的扩散近似失效。     

团簇自然气溶胶散射相函数的数值分析

针对不同比例的灰尘和海盐团簇自然气溶胶,利用离散偶极子近似(DDA)法,通过考察成分的影响,使用波长为0.55 m,尺度参数变化范围为0.1~25时,研究了散射相函数和激光雷达比的影响,结果表明二者的变化趋势大体相同。尺度参数为13~24时,团簇自然气溶胶散射相函数有明显的后向散射加强,其中尺度参数为18附近,后向散射加强效应最明显。灰尘比海盐对散射相函数影响更大,二者的影响主要集中在尺度参数为15~25范围内,但除了散射角为180附近的后向外,影响的散射角方向略有不同。不同比例下的团簇自然气溶胶激光雷达比在尺度参数为3左右有最大值,其值约为180。在尺度参数为0.2~25范围内,成分对激光雷达比的影响不大,相对偏差小于10%,特别是尺度参数为0.5~2时的影响可以忽略,相对偏差小于1%。     

Light scattering by needle-type and disk-type particles

A powerful tool to analyze light scattering by 3D arbitrary-shaped homogeneous or inhomogeneous obstacles located in free space is based on volume integral equation. In this paper we apply a weak form of volume integral equation to simulate light scattering by needle- and disk-type particles such as straight and curved cylinders, cylindrical plate and hexagonal prism with high aspect ratio and low and high values of refractive indexes. For problems where discrete sources method could be applied, we calculated differential scattering cross-section using both methods and got excellent agreement in results.     

The T-matrix technique for calculations of scattering properties of ensembles of randomly oriented particles with different size

We develop a modification of the T-matrix method that allows efficient studies of scattering properties of ensembles of independent irregular particles of different size. The advantage of the modification is quick calculations using the so-called shape-matrices (Sh-matrices), which allow more rapid calculations of scattering by particles of different size and can be used for averaging scattering properties over particle size. To illustrate the advantage we calculate the scattering-angle dependence of the intensity and degree of linear polarization of ensembles of cubes and Chebyshev particles of different size using both the new and traditional methods. Our time savings in calculating scattering properties for the particles with the new methodology is approximately a factor of ten when calculating scattering properties of one hundred of the same type of particles with different size parameter. As can be anticipated, increasing the size interval results in a smoothing of the structure of the photometric curves and a decrease in the linear polarization.     

Characteristics of light scattering by smoke particles based on spheroid models

Light scattering models of smoke particles play an important role on the development of photoelectric smoke detection. Aiming at the influence of morphology of smoke particles, spheroid models are introduced to analyze the Stokes scattering matrix of smoke particles, which are lognormal size distributions. Under the condition of random orientations, the effects of refractive indexes and mean size of smoke particles are considered. The results show that after averaging of the orientation and size, the nonsphericity of smoke particles has a considerable effect on their light scattering. Additionally, the nonsphericity of gray smoke particles generated from smoldering fires is more important than soot from flaming fires for analyzing the light scattering.     

Porous irregular aggregates of sub-micron sized grains to reproduce cometary dust light scattering observations

The present study intends to interpret some of the characteristic features of the light scattered by cometary dust, such as phase angle and wavelength dependence of its polarization, through simulations using Ballistic Cluster-Cluster Aggregation (BCCA) or Ballistic Particle-Cluster Aggregation (BPCA) aggregates of up to 128 sub-micron sized grains (spherical and spheroidal with a possible size distribution) of various composition (silicates, organics, silicates core with organics mantle). The dependence of the linear polarization with the size parameter is shown to depend highly on the size and composition of the constitutive grains. Internal interactions induced by shape or orientation averaging of the grains may lessen this dependence, leading to results comparable to those observed on cometary dust for fluffy aggregates of grains with a size parameter in the 1.3–1.8 range. A size distribution of realistically shaped particles (aggregates of spheroids and larger spheroids) following a power law size distribution with a power index of -3, the smallest grains radius by 0.03– and the largest spheroids effective radius by , gives a very good fit to the Hale-Bopp observed phase curves. The best silicates–organics ratio ranges from about 50–75% organics and 25–50% silicates in volume considering the eventual presence of core-mantle grains.     

Light scattering by aggregates of varying porosity and the opposition phenomena observed in the low-albedo particulate media

The rule that the opposition phenomena in brightness and linear polarization observed in many regolith surfaces usually accompany each other is violated in the cases of very dark asteroids and particulate samples: practically no nonlinear surge of brightness to opposition is observed while the branch of negative polarization at small phase angles exists. To explain this fact, we model the light scattering by particulate media with ensembles of spherical particles (with size comparable to the wavelength) of varying packing density and refractive index. The superposition T-matrix method is used. The increase in the absorption and/or packing density diminishes the amplitude of the brightness opposition peak, and its profile becomes wider. The influence on the branch of negative polarization is more complex and depends on the relation between the size parameters of the constituents, the refractive index, and the porosity. However, the feature common to all considered cases is that the negative branch changes its shape and the polarization minimum moves to the inversion point. This behavior radically differs from that observed in nonabsorbing ensembles of particles and reflects the fact that the efficiency of the coherent backscattering, which mainly determines these characteristics in nonabsorbing ensembles (to the packing density of about 30%), decreases. Moreover, since the angular profiles are not simply damped, but the polarization minimum changes its angular position, we may conclude that the near-field interaction of the constituents becomes important: the shielding of particles by each other eliminates many constituents from the scattering and the near-field effects promote the negative polarization and smooth the backscattering brightness surge. Due to this, when the packing density exceeds 10-20%, the opposition phenomena in absorbing ensembles are caused not only by the coherent backscattering, and situations, when the opposition brightness surge is practically suppressed, but the negative branch of polarization still survives, are possible. This may explain the fact that the dark regolith surfaces show no brightness opposition effect, but produce the branch of negative polarization with the minimum shifted from opposition.     

随机分布烟尘簇团粒子缪勒矩阵的数值计算

利用蒙特卡罗方法根据团簇-团簇凝聚（CCA）模型对由球形原始粒子凝聚而成的烟尘簇团粒子进行了模拟,用离散偶极子近似（DDA）方法对随机分布的烟尘簇团粒子的缪勒矩阵元素进行了数值计算,给出了不同入射波长情况下随机分布烟尘簇团粒子的缪勒矩阵元素与组成簇团粒子的基本粒子的粒径和数目的数值关系,为进一步研究随机分布簇团粒子的形成机理、形态特性、散射特性提供了一种理论方法. 关键词： 烟尘簇团粒子 缪勒矩阵 离散偶极子近似方法     

A study of radiative properties of fractal soot aggregates using the superposition T-matrix method

We employ the numerically exact superposition T-matrix method to perform extensive computations of scattering and absorption properties of soot aggregates with varying state of compactness and size. The fractal dimension, D_f, is used to quantify the geometrical mass dispersion of the clusters. The optical properties of soot aggregates for a given fractal dimension are complex functions of the refractive index of the material m, the number of monomers N_S, and the monomer radius a. It is shown that for smaller values of a, the absorption cross section tends to be relatively constant when D_f<2 but increases rapidly when D_f>2. However, a systematic reduction in light absorption with D_f is observed for clusters with sufficiently large N_S, m, and a. The scattering cross section and single-scattering albedo increase monotonically as fractals evolve from chain-like to more densely packed morphologies, which is a strong manifestation of the increasing importance of scattering interaction among spherules. Overall, the results for soot fractals differ profoundly from those calculated for the respective volume-equivalent soot spheres as well as for the respective external mixtures of soot monomers under the assumption that there are no electromagnetic interactions between the monomers. The climate-research implications of our results are discussed.     

Monte Carlo simulation of light scattering from size distributed sub-micron spherical CdS particles in a volume element

Simulation of polarized light scattering by spherical particles having modal radius of 180 nm is presented in this paper. A Monte Carlo method which is based on the Stokes-Mueller formalism developed in ANSI Standard C-language is used for simulation. Single scattering is considered in our program with monodispersed sub-micron sized spherical CdS particles. We have considered only θ dependent scattering as described by Mie theory for spherical CdS particles. The experiments for studying light scattering properties of these particles were conducted in a designed and developed laser based light scattering studies setup. The simulation results were compared with experimental results and theoretical results obtained purely from Mie theory. The closeness of agreement or disagreement between these results is discussed in this paper.     

团簇自然气溶胶散射相函数的数值分析

针对不同比例的灰尘和海盐团簇自然气溶胶,利用离散偶极子近似(DDA)法,通过考察成分的影响,使用波长为0.55 m,尺度参数变化范围为0.1~25时,研究了散射相函数和激光雷达比的影响,结果表明二者的变化趋势大体相同。尺度参数为13~24时,团簇自然气溶胶散射相函数有明显的后向散射加强,其中尺度参数为18附近,后向散射加强效应最明显。灰尘比海盐对散射相函数影响更大,二者的影响主要集中在尺度参数为15~25范围内,但除了散射角为180附近的后向外,影响的散射角方向略有不同。不同比例下的团簇自然气溶胶激光雷达比在尺度参数为3左右有最大值,其值约为180。在尺度参数为0.2~25范围内,成分对激光雷达比的影响不大,相对偏差小于10%,特别是尺度参数为0.5~2时的影响可以忽略,相对偏差小于1%。     

Limiting possibilities of the analysis of the microparticle parameters in the emulsions by the light scattering methods

Two most important characteristics of a light scattering instrument for the measurement of the microparticle parameters in emulsions and gases are discussed. These characteristics are the resolution and the accuracy of particle size measurement. In order to obtain the most reliable data, likely interpretation methods of the measurement results are compared.     

Stability in Debye series calculation for light scattering by absorbing particles and bubbles

The Debye-series decomposition is of importance for understanding of light scattering features and for the validity of the geometrical optics approximation to light scattering. The numerical stability and accuracy for calculating light scattering with Debye series is studied and an improved algorithm is proposed in this work. The ratios of the Riccati-Bessel functions and the logarithmic derivatives of the Riccati-Bessel functions are employed and calculated with proper recurrences. Exemplifying results are provided to show the improvement of the algorithm.     

Paramagnetic magnetochiral effect in the H h and V h components of Rayleigh light scattering by molecular systems

A static magnetic field, acting on an isotropic medium composed of paramagnetic chiral molecules perpendicular to the propagation direction of the optical beam, is shown to induce an axial differential effect—magnetochiral Rayleigh light scattering. Diamagnetic and paramagnetic contributions to the effect are given in terms of the linear and nonlinear electric dipole, magnetic dipole and electric quadrupole polarizabilities responsible also for natural optical activity, the Faraday effect as well as magnetochiral birefringence. Specific cases are discussed.