A data library is developed containing the scattering, absorption, and polarization properties of ice particles in the spectral range from 0.2 to 100 μm. The properties are computed based on a combination of the Amsterdam discrete dipole approximation (ADDA), the T-matrix method, and the improved geometric optics method (IGOM). The electromagnetic edge effect is incorporated into the extinction and absorption efficiencies computed from the IGOM. A full set of single-scattering properties is provided by considering three-dimensional random orientations for 11 ice crystal habits: droxtals, prolate spheroids, oblate spheroids, solid and hollow columns, compact aggregates composed of eight solid columns, hexagonal plates, small spatial aggregates composed of 5 plates, large spatial aggregates composed of 10 plates, and solid and hollow bullet rosettes. The maximum dimension of each habit ranges from 2 to 10 000 μm in 189 discrete sizes. For each ice crystal habit, three surface roughness conditions (i.e., smooth, moderately roughened, and severely roughened) are considered to account for the surface texture of large particles in the IGOM applicable domain. The data library contains the extinction efficiency, single-scattering albedo, asymmetry parameter, six independent nonzero elements of the phase matrix (P11, P12, P22, P33, P43, and P44), particle projected area, and particle volume to provide the basic single-scattering properties for remote sensing applications and radiative transfer simulations involving ice clouds. Furthermore, a comparison of satellite observations and theoretical simulations for the polarization characteristics of ice clouds demonstrates that ice cloud optical models assuming severely roughened ice crystals significantly outperform their counterparts assuming smooth ice crystals.

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Soot particles strongly absorb sunlight and hence act as a short-lived warming agent. Atmospheric aging of soot particles changes their morphology and mixing state and consequently alter their optical properties. Here we collected soot particles at tunnel, urban, mountaintop, and background sites in Northern China and analyzed their mixing structures and morphology using transmission electron microscopy. Soot particles were further classified into three types: bare-like, partly coated, and embedded. Bare-like soot particles were dominant at the tunnel site, while most soot particles were partly coated or embedded type at other sites. Fractal dimensions (D f) of different types of soot particles ranged from 1.80 to 2.16 and were ordered as: bare-like < partly coated < embedded. Moreover, their average D f changed from 1.8 to 2.0 from the tunnel to the background site. We conclude that the D f can characterize the shape of soot aggregates reasonably well and its variation reflects soot aging processes. Compared with the reported D f of soot particles, we found that D f = 1.8 used in previous optical models primarily represents freshly emitted soot aggregates, rather than the ambient ones.

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Three terms, ''Waterman's T-matrix method'', ''extended boundary condition method (EBCM)'', and ''null field method'', have been interchangeable in the literature to indicate a method based on surface integral equations to calculate the T-matrix. Unlike the previous method, the invariant imbedding method (IIM) calculates the T-matrix by the use of a volume integral equation. In addition, the standard separation of variables method (SOV) can be applied to compute the T-matrix of a sphere centered at the origin of the coordinate system and having a maximal radius such that the sphere remains inscribed within a nonspherical particle. This study explores the feasibility of a numerical combination of the IIM and the SOV, hereafter referred to as the IIM þSOV method, for computing the single-scattering properties of nonspherical dielectric particles, which are, in general, inhomogeneous. The IIM þ SOV method is shown to be capable of solving light-scattering problems for large nonspherical particles where the standard EBCM fails to converge. The IIM þSOV method is flexible and applicable to inhomogeneous particles and aggregated nonspherical particles (overlapped circumscribed spheres) representing a challenge to the standard superposition T-matrix method. The IIM þ SOV computational program, developed in this study, is validated against EBCM simulated spheroid and cylinder cases with excellent numerical agreement (up to four decimal places). In addition, solutions for cylinders with large aspect ratios, inhomogeneous particles, and two-particle systems are compared with results from discrete dipole approximation (DDA) computations, and comparisons with the improved geometric-optics method (IGOM) are found to be quite encouraging.

A discrete random medium is an object in the form of a finite volume of a vacuum or a homogeneous material medium filled with quasi-randomly and quasi-uniformly distributed discrete macroscopic impurities called small particles. Such objects are ubiquitous in natural and artificial environments. They are often characterized by analyzing theoretically the results of laboratory, in situ, or remote-sensing measurements of the scattering of light and other electromagnetic radiation. Electromagnetic scattering and absorption by particles can also affect the energy budget of a discrete random medium and hence various ambient physical and chemical processes. In either case electromagnetic scattering must be modeled in terms of appropriate optical observables, i.e., quadratic or bilinear forms in the field that quantify the reading of a relevant optical instrument or the electromagnetic energy budget. It is generally believed that time-harmonic Maxwell's equations can accurately describe elastic electromagnetic scattering by macroscopic particulate media that change in time much more slowly than the incident electromagnetic field. However, direct solutions of these equations for discrete random media had been impracticable until quite recently. This has led to a widespread use of various phenomenological approaches in situations when their very applicability can be questioned. Recently, however, a new branch of physical optics has emerged wherein electromagnetic scattering by discrete and discretely heterogeneous random media is modeled directly by using analytical or numerically exact computer solutions of the Maxwell equations. Therefore, the main objective of this Report is to formulate the general theoretical framework of electromagnetic scattering by discrete random media rooted in the MaxwellLorentz electromagnetics and discuss its immediate analytical and numerical consequences. Starting from the microscopic Maxwell-Lorentz equations, we trace the development of the firstprinciples formalism enabling accurate calculations of monochromatic and quasi-monochromatic scattering by static and randomly varying multiparticle groups. We illustrate how this general framework can be coupled with state-of-the-art computer solvers of the Maxwell equations and applied to direct modeling of electromagnetic scattering by representative random multi-particle groups with arbitrary packing densities. This first-principles modeling yields general physical insights unavailable with phenomenological approaches. We discuss how the first-order-scattering approximation, the radiative transfer theory, and the theory of weak localization of electromagnetic waves can be derived as immediate corollaries of the Maxwell equations for very specific and well-defined kinds of particulate medium. These recent developments confirm the mesoscopic origin of the radiative transfer, weak localization, and effective-medium regimes and help evaluate the numerical accuracy of widely used approximate modeling methodologies.

.[1] The effects of particle fields including bubbles on the optical volume scattering function (VSF) were investigated in the surf zone off Scripps Pier as part of an ongoing effort to better understand the underlying dynamics in the VSF in the subsurface ocean. VSFs were measured at 20 Hz at angles spanning 10°-170°in 10°increments with a device called the Multiangle Scattering Optical Tool (MASCOT). Modification of the phase function was observed in passing suspended sediment plumes, wave-injected bubble plumes, and combinations of these particle populations relative to the background. Phase function enhancement in the 60°-80°range was observed in association with bubble plumes, consistent with theoretical predictions. VSFs were inverted to infer size distributions and composition using a least squares minimization fitting procedure applied to a library of phase functions, each representing a lognormally distributed subpopulation with refractive index and coating, where applicable. Phase functions representative of nonspherical mineral particle subpopulations were computed using discrete dipole approximation (DDA) and improved geometric optics method (IGOM) techniques for randomly oriented, asymmetric hexahedra. Phase functions for coated bubbles were computed with the Lorenz-Mie theory. Inversion results exhibited stable solutions that qualitatively agreed with concurrent acoustical measurements of bubbles, aggregate particle size distribution expectations, and anecdotal videography evidence from the field. Although a comparable inversion with a library that assumed spherical shaped particles alone provided less stable results with some incorrectly assigned subpopulations, several dominant subpopulation trends were consistent with the results obtained using nonspherical representations of mineral particles.Citation: Twardowski, M., X. Zhang, S. Vagle, J. Sullivan, S. Freeman, H. Czerski, Y. You, L. Bi, and G. Kattawar (2012), The optical volume scattering function in a surf zone inverted to derive sediment and bubble particle subpopulations,