Mie理论在静态光散射粒度测量的应用下限研究

测量下限是光散射颗粒测试技术的关键问题。本文通过理论分析、比较归一化散射光强的分布图和构造方差函数F(d)对颗粒散射光的光强分布进行了定性和定量的讨论,对Mie散射向Rayleigh散射趋近的情况进行了分析,讨论了散射光光强大小的分布,分析了测量不同粒径的颗粒的可行性,最终得到在入射光源是波长为0.6328μm的He-Ne激光器的情况下,当粒径d取200nm以上时,不同粒径颗粒的M ie散射光强分布有较大差别,适合用静态光散射的方法来判断颗粒粒径。     

超细颗粒物近场散射光场分布特性研究

为研究超细颗粒物近场散射光强分布的特性,应用了时域有限差分方法系统对不同尺寸微小颗粒的近场散射光强进行分析。对于单颗粒的散射光场,颗粒越小,散射光主要表现为各向同性,随着颗粒增大,前向散射光将占主要部分,这与传统的Mie散射理论相一致。对于紧靠的两个颗粒,由于多重散射的作用,散射光场表现出更多的复杂性,两个颗粒的排列方式不同也会对结果有较大影响。这些结果为进一步分析复杂颗粒系统的光散射特性提供了基础。     

Rise-2008型激光粒度仪／粒度分析仪

Rism2008型激光粒度分析仪采用全量程米氏散射理论,充分考虑到被测颗粒和分散介质的折射率等光学性质,根据大小不同的颗粒在各角度上散射光强的变化反演出颗粒群的粒度分布数据。     

Rise-2008型激光粒度仪／粒度分析仪

工作原理： Rise-2008型激光粒度分析仪采用全量程米氏散射理论，充分考虑到被测颗粒和分散介质的折射率等光学性质，根据大小不同的颗粒在各角度上散射光强的变化反演出颗粒群的粒度分布数据。     

Rise-2008型激光粒度仪／粒度分析仪

工作原理：Rise-2008型激光粒度分析仪采用全量程米氏散射理论，充分考虑到被测颗粒和分散介质的折射率等光学性质，根据大小不同的颗粒在各角度上散射光强的变化反演出颗粒群的粒度分布数据。     

基于Mie散射理论的微球体颗粒数值模拟计算和实验研究

应用Mie散射理论,在散射角0～π范围内,模拟微球体颗粒的散射光强度与粒径大小的变化关系,分析了相对折射率的大小对散射光强度的影响;在实验装置条件下,分别模拟了散射光强度与粒径大小的变化关系及2um、5um、10um三种粒子的散射光强度与波长的变化关系。实验验证了数值模拟与实验结果基本一致,也证明实验中粒子散射遵从Mie散射理论模型。     

前向大角度法测量亚微米级颗粒

采用Mie散射理论,分析了亚微米级颗粒散射光角度分布特征,研究了粒径、折射率等参数对散射光分布特点的影响.研究发现,将散射光强度的角度分布进行归一化后,前向30°部分范围内的分布特性近似为线性,且呈现斜率与颗粒粒径相关,而与折射率无关的特点.在此基础上,本文中提出一种新的测量方法--前向大角度法,即通过测量散射光在前向30°范围内的光强线性变化的斜率来直接得到亚微米级颗粒的平均粒径.文中讨论了该方法的适用范围、近似表达式、误差分析等.     

球形粒子对平面偏振光散射的数值计算与分析

论文在Mie理论基础上,给出了球形粒子对平面偏振光的散射强度和散射系数公式,利用连分式递推算法进行了编程计算,重点对1.06μm激光的模拟结果进行了分析.从得到的散射图像可以看出,散射强度角分布与散射粒子尺度有密切关系,随着粒子尺度的逐渐增大,散射光强主要集中到前、后向散射方向,集中的角度越来越窄,模拟结果明显出现了散射强度最弱的极值角,且该极值角随粒径的增大而增大,最后逼近90°方向.散射强度角分布与波长有关,当它们在同一数量级时达到最大值,与散射粒子折射率无关.该递推算法因为每一步计算都是独立的,与前后项的准确性没有关系,不存在不稳定、发散等情况,能够计算粒径参数范围从10-4开始,对上限不受任何限制.     

颗粒层状结构对散射特性及激光粒度测量的影响

根据Aden-Kerker理论,对双层颗粒前向散射特性进行了研究。在此基础上,分别用衍射理论、Mie理论和Aden-Kerker理论计算了散射系数矩阵并采用非独立模式算法进行反演。数值计算结果表明,双层颗粒的散射光强由颗粒折射率和内外径决定,在实际的激光前向散射粒度测量中,如果沿用Fraunhofer衍射理论或Mie理论计算系数矩阵,在测量双层颗粒时,反演结果会有一定的误差。相比于Mie理论,衍射理论的反演误差更大。     

火灾烟雾颗粒的光学散射特性研究

从理论和实验两个方面研究了几种常见的燃烧烟雾在不同波长激光下的散射特性。从Mie散射理论出发，比较几种Mie散射算法的优缺点，采用一种改进的连分式算法对火灾烟雾颗粒的散射光强分布进行计算，得出不同粒径大小和波长下光强分布图。结合理论计算，设计一套实验装置，测量并计算在不同角度下3种烟雾颗粒和面粉气溶胶散射光的相对光强比，实验测量值与理论计算值吻合较好。研究结果表明不同种类烟雾散射光相对光强比互不相同，火灾烟雾与非烟雾气溶胶差距较大，从而表明散射光相对光强比是区分不同烟雾特定的物性参数，为火灾烟雾探测技术发     

Light scattering and absorption by soot in the presence of sulfate aerosols

The radiative properties of aerosol-soot mixtures, both internal and external, are determined in the visible and near-infrared bands by use of exact indirect mode-matching solutions to electromagnetic-wave scattering from a sphere with an eccentric spherical inclusion and from a cluster of spheres. Spherical sulfate droplets are assumed to represent aerosol particles. Soot particles are represented by volume-equivalent carbon spheres, the size distribution of which is obtained from the number distribution of the primary carbon particles that aggregate into soot grains. The mean gram-specific absorption cross section and the mean albedo of aerosol-soot mixtures are obtained by integration of the corresponding characteristics of composite sulfate-carbon particles over the size range of carbon spheres. Enhanced absorption of light by soot in aerosol-soot mixtures, a result of lensing by sulfate droplets, is highlighted by maps of the electromagnetic field in a sulfate-carbon particle.     

Effective medium theories for irregular fluffy structures: aggregation of small particles

The extinction efficiencies as well as the scattering properties of particles of different porosity are studied. Calculations are performed for porous pseudospheres with small size (Rayleigh) inclusions using the discrete dipole approximation. Five refractive indices of materials covering the range from 1.20+0.00i to 1.75+0.58i were selected. They correspond to biological particles, dirty ice, silicate, and amorphous carbon and soot in the visual part of the spectrum. We attempt to describe the optical properties of such particles using Lorenz-Mie theory and a refractive index found from some effective medium theory (EMT) assuming the particle is homogeneous. We refer to this as the effective model. It is found that the deviations are minimal when utilizing the EMT based on the Bruggeman mixing rule. Usually the deviations in the extinction factor do not exceed approximately 5% for particle porosity P = 0 - 0.9 and size parameters x(porous) = 2 pi r(s,porous)/lambda < or approximately = 25. The deviations are larger for scattering and absorption efficiencies and smaller for particle albedo and the asymmetry parameter. Our calculations made for spheroids confirm these conclusions. Preliminary consideration shows that the effective model represents the intensity and polarization of radiation scattered by fluffy aggregates quite well. Thus the effective models of spherical and nonspherical particles can be used to significantly simplify the computations of the optical properties of aggregates containing only Rayleigh inclusions.     

Finite-difference time-domain solution of light scattering by dielectric particles with large complex refractive indices

The finite-difference time-domain (FDTD) technique is examined for its suitability for studying light scattering by highly refractive dielectric particles. It is found that, for particles with large complex refractive indices, the FDTD solution of light scattering is sensitive to the numerical treatments associated with the particle boundaries. Herein, appropriate treatments of the particle boundaries and related electric fields in the frequency domain are introduced and examined to improve the accuracy of the FDTD solutions. As a result, it is shown that, for a large complex refractive index of 7.1499 + 2.914i for particles with size parameters smaller than 6, the errors in extinction and absorption efficiencies from the FDTD method are generally less than ~4%. The errors in the scattering phase function are less than ~5%. We conclude that the present FDTD scheme with appropriate boundary treatments can provide a reliable solution for light scattering by nonspherical particles with large complex refractive indices.     

两种方法测量大气气溶胶折射率的对比分析

针对气溶胶折射率在分析大气气溶胶光学特性中的重要性,介绍两种综合利用黑碳仪、浊度计、光学粒子计数器和微脉冲激光雷达测量大气气溶胶折射率的新方法。两种方法都是根据球形粒子的Mie散射理论计算大气气溶胶的折射率,使用以上两种方法对厦门地区气溶胶折射率进行了计算和对比分析,证明了它们的合理性,分析了它们的测量精度和误差来源。     

Modeling the optical properties of mineral particles suspended in seawater and their influence on ocean reflectance and chlorophyll estimation from remote sensing algorithms

The optical properties of mineral particles suspended in seawater were calculated from the Mie scattering theory for different size distributions and complex refractive indices of the particles. The ratio of the spectral backscattering coefficient to the sum of the spectral absorption and backscattering coefficients of seawater, b(b)(lambda)/[a(lambda) + b(b)(lambda)], was analyzed as a proxy for ocean reflectance for varying properties and concentrations of mineral particles. Given the plausible range of variability in the particle size distribution and the refractive index, the general parameterizations of the absorption and scattering properties of mineral particles and their effects on ocean reflectance in terms of particle mass concentration alone are inadequate. The variations in the particle size distribution and the refractive index must be taken into account. The errors in chlorophyll estimation obtained from the remote sensing algorithms that are due to the presence of mineral particles can be very large. For example, when the mineral concentration is 1 g m(-3) and the chlorophyll a concentration is low (0.05 mg m(-3)), current global algorithms based on a blue-to-green reflectance ratio can produce a chlorophyll overestimation ranging from approximately 50% to as much as 20-fold.     

Optical absorption by randomly oriented carbon spheroids

The optical properties of carbon spheroids are compared with those of carbon spheres for all size regimes. In general, the absorption cross section/unit volume is increased by axial elongation, particularly away from the resonance region. The results are specific to carbon since the effect of shape change in a given size regime can depend crucially on the value of the refractive index.     

Optical properties of dispersed aerosols in the near ultraviolet (355 nm): measurement approach and initial data

An aerosol albedometer combining cavity ring-down spectroscopy (CRDS) with integrating sphere nephelometry was developed for use at λ = 355 nm. The instrument measures extinction and scattering coefficients of dispersed particulate matter in the near ultraviolet (UV) spectral region. Several samples have been analyzed, including: ammonium sulfate, secondary organic aerosols (SOA) resulting from the ozonolysis of α-pinene and photooxidation of toluene, redispersed soil dust samples, biomass burning aerosols, and ambient aerosols. When particle size and number density were experimentally controlled, extinction coefficients and scattering coefficients were found to have a linear relationship with particle number concentration, in good agreement with light scattering theory. For ammonium sulfate and pinene samples, extinction cross sections for size-selected (D(p) = 300 nm) samples were within the range of 1.65-2.60 × 10(-9) cm(2) with the largest value corresponding to ammonium sulfate and the lowest value for pinene SOA. The scattering cross sections of pinene and ammonium sulfate aerosols were indistinguishable from the extinction cross sections, indicating that these particle types had minimal light absorption at 355 nm. However, soil dusts and biomass burning aerosols showed significant absorption with single scatter albedo (SSA) between 0.74 and 0.84. Ambient aerosols also had transient absorption at 355 nm that correlated well with a particle-soot absorption photometer (PSAP) measuring visible light absorption.     

纳米及微米磁性颗粒在长波长下的光散射特性分析

根据Mie散射理论,分析计算了纳米级、微米级磁性颗粒的光散射特性。针对目前在导航、遥控及雷达等行业广泛应用的纳米、微米磁性颗粒,在Mie散射系数中引入磁导率变量,对比分析了磁性与非磁性颗粒、吸收性与非吸收性颗粒的散射特性规律。数值计算结果表明,磁导率的变化对具有吸收性磁性颗粒的散射特性造成影响,随着磁导率的增大,颗粒的散射光强及吸收性能将逐渐增大,同时磁导率增大对颗粒散射特性的影响将会受到复折射率实部的制约。     

The method to determine the optimum refractive index parameter in the laser diffraction and scattering method

It is the most important and difficult problem to decide the refractive index of sample particles in particle size distribution measurements using the laser diffraction and scattering method. In this study, the method to decide both particle size distribution and refractive index of particle is investigated. The new method to determine the refractive index of particles is applied for several sample powders (polystyrene latex, glass beads, etc.) and the usefulness of this new method is confirmed.     

Integral light-scattering and absorption characteristics of large, nonspherical particles

We obtain and analyze simple analytical formulas for asymmetry parameters and absorption cross sections of large, nonspherical particles. The formulas are based on the asymptotic properties of these characteristics at strong and weak absorption of radiation inside particles. The absorption cross section depends on parameter phi, which determines the value of the light-absorption cross section for weakly absorbing particles. It is larger for nonspherical scatterers. The asymmetry parameter depends on two parameters. The first is the asymmetry parameter g(0) of a nonspherical, transparent particle with the same shape as an absorbing one. The second parameter, beta, determines the strength of the influence of light absorption on the value of the asymmetry parameter. Parameter beta is larger for nonspherical particles. One can find these three parameters (phi, g(0), and beta) using a ray-tracing code (RTC) for nonabsorbing and weakly absorbing particles. The RTC can then be used to check the accuracy of the equations at any absorption for hexagonal cylinders and spheroids. It is found that the error of computing the absorption cross section and 1 - g (g is the asymmetry parameter) is less than 20% at the refractive index of particles n = 1.333. Values for asymmetry parameters of large, nonabsorbing, spheroidal particles with different aspect ratios are tabulated for the first time to our knowledge. They do not depend on the size of particles and can serve as an independent check of the accuracy of T-matrix codes for large parameters.