Showing papers in "Journal of Quantitative Spectroscopy & Radiative Transfer in 2006"

VLIDORT: A linearized pseudo-spherical vector discrete ordinate radiative transfer code for forward model and retrieval studies in multilayer multiple scattering media

Abstract: We describe a new vector discrete ordinate radiative transfer model with a full linearization facility. The VLIDORT model is designed to generate simultaneous output of Stokes vector light fields and their derivatives with respect to any atmospheric or surface property. We develop new implementations for the linearization of the vector radiative transfer solutions, and go on to show that the complete vector discrete ordinate solution is analytically differentiable for a stratified multilayer multiply scattering atmospheric medium. VLIDORT will generate all output at arbitrary viewing geometry and optical depth. The model has the ability to deal with attenuation of solar and line-of-sight paths in a curved atmosphere, and includes an exact treatment of the single scatter computation. VLIDORT also contains a linearized treatment for non-Lambertian surfaces. A number of performance enhancements have been implemented, including a facility for multiple solar zenith angle output. The model has been benchmarked against established results in the literature.

HELIOS-CR – A 1-D radiation-magnetohydrodynamics code with inline atomic kinetics modeling

Abstract: HELIOS-CR is a user-oriented 1D radiation-magnetohydrodynamics code to simulate the dynamic evolution of laser-produced plasmas and z-pinch plasmas. It includes an in-line collisional-radiative (CR) model for computing non-LTE atomic level populations at each time step of the hydrodynamics simulation. HELIOS-CR has been designed for ease of use, and is well-suited for experimentalists, as well as graduate and undergraduate student researchers. The energy equations employed include models for laser energy deposition, radiation from external sources, and high-current discharges. Radiative transport can be calculated using either a multi-frequency flux-limited diffusion model, or a multi-frequency, multi-angle short characteristics model. HELIOS-CR supports the use of SESAME equation of state (EOS) tables, PROPACEOS EOS/multi-group opacity data tables, and non-LTE plasma properties computed using the inline CR modeling. Time-, space-, and frequency-dependent results from HELIOS-CR calculations are readily displayed with the HydroPLOT graphics tool. In addition, the results of HELIOS simulations can be post-processed using the SPECT3D Imaging and Spectral Analysis Suite to generate images and spectra that can be directly compared with experimental measurements. The HELIOS-CR package runs on Windows, Linux, and Mac OSX platforms, and includes online documentation. We will discuss the major features of HELIOS-CR, and present example results from simulations.

Einstein A-coefficients and statistical weights for molecular absorption transitions in the HITRAN database

Abstract: This paper describes the calculation of the statistical weights and the Einstein A-coefficients for the 39 molecules and their associated isotopologues/isotopomers currently present in the line-by-line portion of the HITRAN database. Calculation of the Einstein A-coefficients was carried out using the HITRAN line intensities and the necessary statistical weights. The Einstein A-coefficient and the statistical weights of the upper and lower levels of the transition were added in the new format of the line parameters for the most recent edition of the HITRAN database.

Purgatorio—a new implementation of the Inferno algorithm

Abstract: An overview of P urgatorio , a new implementation of the I nferno [Liberman, Phys Rev B 1979;20:4981–9] equation of state model, is presented. The new algorithm emphasizes a novel subdivision scheme for automatically resolving the structure of the continuum density of states, circumventing limitations of the pseudo- R matrix algorithm previously utilized.

Light scattering by complex ice-analogue crystals

Abstract: Angle-dependent light-scattering measurements on single ice analogues crystals are described. Phase functions and degree of linear polarization are measured for electrodynamically levitated crystals. A procedure for randomizing particle orientation during levitation is demonstrated. The dependence of scattering on the shape, complexity and surface roughness of the crystals is examined. The phase functions from complex crystals with smooth surfaces show little dependence on shape. There is close agreement between the measured functions and the analytic phase function for ice clouds. However, rosettes with rough surfaces have qualitatively different phase functions, with raised side and back scattering. The asymmetry parameter is typically about 0.8±0.04 and 0.63±0.05 for smooth and rough crystals, respectively. The 22° halo peak is present for smooth rosettes and aggregates but absent for rough rosettes. Two-dimensional scattering patterns from several crystals in fixed orientations are also shown. The results suggest that it may be possible to use such patterns to discriminate not only between crystals of different shape but also to obtain some information on surface properties.

Optical properties of plasmas based on an average-atom model

Abstract: Optical properties of plasmas, including dielectric constants, indices of refraction, and absorption coefficients, are determined from an average-atom point of view. Linear response of an average atom to a harmonic electric field leads to an average-atom version of the Kubo–Greenwood formula, which is used to calculate the frequency-dependent electric conductivity of the plasma. The frequency-dependent dielectric function is determined from the conductivity using Kramers–Kronig dispersion relations. The index of refraction and absorption coefficient of the plasma are subsequently obtained from the dielectric function. Comparing the present results with the free-electron model helps one understand anomalies observed recently in space and time resolved interferograms of Al plasmas produced by 13.9- and 14.7-nm X-ray lasers.

The new Los Alamos opacity code ATOMIC

Abstract: In this work we report on developments of ATOMIC, a Los Alamos code designed to compute opacities under both LTE and non-LTE conditions for a large range of elements. We report here, for the first time, data associated with the newly generated Rosseland opacity table for oxygen. In addition, the various equation-of-state packages, absorption spectra generation packages, and parallel techniques used in the generation of this table will be discussed.

Light scattering modeling of small feldspar aerosol particles using polyhedral prisms and spheroids

Abstract: The use of simplified particle shapes for modeling scattering by irregularly shaped mineral-dust particles is studied using polyhedral prisms and spheroids as model particles. Simulated phase matrices averaged over shape and size distributions at wavelength 633 nm are compared with a laboratory-measured phase matrix of feldspar particles with known size distribution with effective radius of 1.0 μ m . When an equi-probable shape distribution is assumed, prisms and oblate spheroids agree with measurements to a similar degree, whereas prolate spheroids perform markedly better. Both spheroids and prisms perform much better than spheres. When an automatic fitting method is applied for finding optimal shape distributions, it is found that the most elongated spheroids are most important for good fits, whereas nearly-spherical spheroids are generally of very little importance. The phase matrices for the different polyhedral prisms, on the other hand, are found to be similar, thus their shape-averaged phase matrices are insensitive to the shape distribution assumed. For spheroids, a simple parameterization for the shape distribution, where weights increase with increasing departure from spherical shape, is proposed and tested. This parameterization improves the fit of most phase matrix elements attained with an equi-probable shape distribution, and it performs particularly well for reproducing the measured phase function.

A simplified model to predict the effects of aggregation on the absorption properties of soot particles

Abstract: An exact electrostatics formulation for sphere clusters is used to predict the Rayleigh-limit radiative absorption properties of soot aggregates In the near to mid IR wavelengths, it is shown that aggregation can result in absorption cross sections that are significantly larger than that predicted by an independent-sphere (Rayleigh–Gans) model The relative increase in absorption increases with the number of spheres in the aggregate, and reaches an asymptote for aggregates containing 100 – 200 spheres A simplified correlation is developed to predict the aggregate absorption cross section as a function of number of spheres and refractive index Implications of the effects of aggregation on absorption and emission of thermal radiation by soot in flame and atmospheric environments are discussed

The vibrational levels of methane obtained from analyses of high-resolution spectra

Abstract: Methane and its tetrahedral isotopologues are spherical-top molecules whose high-resolution rovibrational spectra can only be analyzed in detail, thanks to sophisticated symmetry-adapted tensorial models. However, the effective Hamiltonian parameters of such models do not give direct access to the positions of the vibrational sublevels. In this paper, we present a calculation of the vibrational level positions for 12CH4, 13CH4, 12CD4 and 13CD4 performed using the effective Hamiltonian parameters obtained through recent analyses. We also include the results of a re-analysis of the octad system of 12CH4 performed with a higher order of the development which slightly improves the previous work on this polyad.

Comparing plasma population kinetics codes: Review of the NLTE-3 Kinetics Workshop

Abstract: Comparison of plasma population kinetics codes, which has become an important tool for their testing and verification, was the subject of the third Non-LTE Code Comparison Workshop held at the National Institute of Standards and Technology in December 2003. The motivations for the workshop are presented here, together with its technical organization. The cases studied range from carbon to gold, and include both steady-state and time-dependent calculations. The set of results represents the current capabilities in kinetics modeling, and illustrates the effectiveness of this workshop approach to code comparisons. It was found that good agreement between codes and with experiments is observed near closed shells, but significant differences exist otherwise, or when density effects are important. On a more general level, we discuss how to meaningfully compare kinetics data. We conclude with plans for the next workshop and for a kinetics database.

Discrete ordinate method with a new and a simple quadrature scheme

Abstract: Evaluation of the radiative component in heat-transfer problems is often difficult and expensive. To address this problem, in the recent past, attention has been focused on improving the performance of various approximate methods. Computational efficiency of any method depends to a great extent on the quadrature schemes that are used to compute the source term and heat flux. The discrete ordinate method (DOM) is one of the oldest and still the most widely used methods. To make this method computationally more attractive, various types of quadrature schemes have been suggested over the years. In the present work, a new quadrature scheme has been suggested. The new scheme is a simple one and does not involve complicated mathematics for determination of direction cosines and weights. It satisfies all the required moments. To test the suitability of the new scheme, four benchmark problems were considered. In all cases, the proposed quadrature scheme was found to give accurate results.

Discrete transfer method applied to radiative transfer in a variable refractive index semitransparent medium

Abstract: Application of the discrete transfer method (DTM) has been extended to the analysis of radiative heat transfer in a variable refractive index participating medium. To validate the DTM formulation, radiative heat transfer in an absorbing, emitting and isotropically scattering planar medium was considered. The participating medium was assumed to be in radiative equilibrium. For both constant and variable refractive indices of the medium, the DTM results were compared with those available in the literature. The DTM was found to provide accurate results.

Effect of detailed cell structure on light scattering distribution: FDTD study of a B-cell with 3D structure constructed from confocal images

Abstract: Human B-cells play an important role in the immune system, and because of their relatively simple structures with a nearly spherically shaped cell membrane and a large nucleus, they provide a good case to study on how the details of cell structure affect light scattering properties. A finite-difference-time-domain (FDTD) method is used to calculate angle-resolved light scattering distributions from a B-cell. Published FDTD simulations to date have used a smooth shape with a certain degree of symmetry to approximate the actual cell shape. In contrast, for this work, the shapes of the cell and its nucleus were determined from confocal microscopy measurements. An automated procedure was developed to construct a realistic three-dimensional structure of a B-cell from a stack of two-dimensional confocal images. The angle-resolved Mueller matrix elements of the B-cell were calculated and averaged for 30 different angles of incidence using a parallel FDTD code. These results were compared with those from a homogeneous and a coated sphere. Scattering from the two sphere models and the B-cell were very similar for scattering angles less than 5°, and the coated sphere and B-cell agreed well for scattering angles up to 20°. However, at larger angles, the scattering from the B-cell was a much smoother function of angle than scattering from either sphere model. Additionally, the homogeneous sphere results were the most similar to the B-cell results for most angles between 120° and 150°, and at angles greater than 150°, the B-cell scattered more light than either of the spheres. These results yield strong evidence that accurate modeling of light scattering by biological cells requires not only the high accuracy of the employed numerical method but the realistic cellular structure as input information as well.

Light scattering by single erythrocyte: Comparison of different methods

Abstract: In this paper we investigate the capabilities of different light scattering programs for light scattering simulation of the single human red blood cell, also known as erythrocyte. Knowledge of the scattering properties can help to solve the inverse problem of classifying erythrocytes according to size and shape using measured scattering diagrams. We compare the different programs by presenting the corresponding scattering diagrams. Then we give an overview of computation times and point out the different characteristics of the methods.

Optical characterization of Mn2+, Ni2+ and Co2+ ions doped zinc lead borate glasses

Abstract: This paper reports on the development and optical characterization of heavy metal oxide (HMO)-based transparent glasses in the chemical composition of 15PbO–40B2O3–(45−x) ZnO−x TM2+ (=Mn2+ or Ni2+ or Co2+) (where x = 02 , 05 mol%) For these glasses both absorption and emission spectra have been measured, in order to understand their optical performances The XRD profiles have confirmed their glassy nature and the FTIR spectral features have been analyzed From the emission spectra, a bright green emission (538 nm) from Mn2+-glasses, an intense red emission (670 nm) from Ni2+ and from Co2+ (625 nm) glasses have been noticed very clearly Based on the UV-absorption spectra of these materials, both direct and indirect bond gaps have been computed Apart from the spectral analysis, different physical properties of these glasses have also been carried out Due to the presence of both PbO and ZnO, these glasses are found to be good moisture-resistant optical systems Both optical and physical properties have been found to be more encouraging towards their use as novel luminescent optical materials

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

Abstract: 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– 0.2 μ m and the largest spheroids effective radius by 20 μ m , 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.

Effect of ice crystal shape and effective size on snow bidirectional reflectance

Abstract: We tested the applicability of three rigorous radiative transfer computational approaches, namely, the discrete ordinates radiative transfer (DISORT) method, the adding–doubling approach, and an efficient computational technique based on Ambartsumian's nonlinear integral equation for computing the bidirectional reflectance of a semi-infinite layer. It was found that each of these three models, in a combination with the truncation of the forward peak of the bulk scattering phase functions of ice particles, can be used to simulate the bidirectional reflectance of a semi-infinite snow layer with appropriate accuracy. Furthermore, we investigate the sensitivity of the bidirectional reflectance of a homogeneous and optically infinite snow layer to ice crystal habit and effective particle size. It is shown that the bidirectional reflectance is not sensitive to the particle effective size in the visible spectrum. The sensitivity of the bidirectional reflectance in the near-infrared spectrum to the particle effective size increases with the increase of the incident wavelength. The sensitivity of the bidirectional reflectance to the effective particle size and shape is attributed fundamentally to the sensitivity of the single-scattering properties to particle size and shape. For a specific ice crystal habit, the truncated phase function used in the radiative transfer computations is not sensitive to particle effective size. Thus, the single-scattering albedo is primarily responsible for the sensitivity of the bidirectional reflectance to particle size, particularly, at a near-infrared wavelength.

A study of ion-dynamics and correlation effects for spectral line broadening in plasma : K-shell lines

Abstract: A method for the calculation of spectral line broadening in plasma has been developed and implemented. The method is based on a numerical simulation of the motion of the interacting plasma particles (both ions and electrons) and the use of the resulting time-dependent field to obtain the evolution of the radiator system. The Fourier transform of the resulting radiator time-dependent dipole function then gives the spectral line shape. This approach thus naturally accounts for all frequency regions of the plasma-particle fields and for the effects of the particle interactions on the fields. Due to a rather general approach used for solving the Schrodinger equation, the method is applicable to line-shape calculations of isolated and overlapping spectral lines involving both dipole-allowed and dipole-forbidden radiative transitions. In addition, line shapes under a simultaneous influence of externally-applied (constant or time-dependent) electric and magnetic fields can be calculated in a self-consistent manner, and polarization properties of the emitted light, caused by such external fields, can be investigated. Part of the method capabilities is demonstrated. Results presented are for spectral lines of H- and He-like ions of C, Si, and Ar in non-magnetized plasmas. It is found that ion dynamics contributes to the line broadening significantly, in several cases exceeding the electron impact widths by a few times. Also, the interactions between the radiator and the perturbers cause a significant reduction in the line widths, even for overall-weakly-coupled plasmas; a relation between this effect and the radiator–perturber coupling is made.

Comparative studies of the reflectance and degree of linear polarization of particulate surfaces and independently scattering particles

Abstract: We compare measurements of the phase-angle dependencies of the intensity and degree of linear polarization of particles in air and particulate surfaces. The samples were measured at two spectral bands centered near 0.63 and 0.45 μm. The surfaces were measured with the new photometer/polarimeter at the Astronomical Institute of Kharkov National University. The scattering measurements of the particles in air were carried out with the equipment currently located at the University of Amsterdam. We study a suite of samples of natural mineral particles of different sizes all in the micrometer range, i.e. comparable with the wavelengths. The samples are characterized by a variety of particle shapes and albedos. The samples have been studied in several works and in this paper we include new SEM microphotographs of particles and spectra of powders in a wide spectral range, 0.3–50 μm, using the RELAB equipment of Brown University. We made measurements of particulate surfaces in a phase-angle range, 2–60° that is significantly wider than that of our previous studies. We confirm our earlier results that the negative polarization of the surfaces may be a remnant of the negative polarization of the single scattering by the particles that constitute the surfaces. We also find differences in the spectral behavior of the polarization degree of particles in air and particulate surfaces at large phase angles.

A complete study of the line intensities of four bands of CO2 around 1.6 and 2.0 μm: A comparison between Fourier transform and diode laser measurements

Abstract: Atmospheric carbon dioxide is a key specie for the Earth climate. Two spectral windows at 1.6 μm and 2.0 μm are of particular interest for the in situ and remote monitoring of carbon dioxide from satellite, balloon or airborne platforms using infrared absorption spectroscopy. A precise knowledge of the line strengths is a prerequisite for an accurate concentration retrieval. In this paper, we have revisited in the laboratory the (3001)III←(0000) and (3001)II ← (0000) bands of CO2 near 1.6 μm and the (2001)III ← (0000) and (2001)II ← (0000) bands near 2.0 μm by implementing both a high-resolution Connes-type Fourier-transform spectrometer and a tunable diode laser spectrometer equipped with several telecommunication-type semiconductor laser devices. Approximately 200 (respectively 18) transitions of CO2 have been carefully investigated in spectra recorded with the FT spectrometer (respectively with the tunable diode laser spectrometer). The intensity measurements achieved with both instruments are thoroughly compared to previous instrumental determinations, ab-initio calculations and available atmospheric molecular database.

K-shell radiation physics in low- to moderate-atomic-number z-pinch plasmas on the Z accelerator

Abstract: Dense z-pinches produced by 100 ns implosions of wire arrays or gas puffs produce substantial soft X-ray power. One class of z-pinch radiation sources includes low- to moderate-atomic-number K-shell radiators, such as aluminum and iron. These loads are designed for 1–10 keV K-shell X-ray generation, and offer opportunities for crystal spectroscopy that can reveal fundamental properties of the plasma when studied using plasma spectroscopic modeling. Typically these plasmas are characterized by ion densities of ∼ 10 20 cm - 3 , diameters of 1–5 mm, electron temperatures up to several keV, and a range of opacities of the K-shell lines. Measurements from wire arrays on Sandia's 20 MA Z accelerator are presented along with collisional radiative and hydrodynamic simulations. The impact of opacity and 3D structure on non-LTE, non-diffusive radiation transport and X-ray production is discussed.

Energy transfer and infrared-to-visible upconversion luminescence of Er3+/Yb3+-codoped halide modified tellurite glasses

Abstract: We report on the energy transfer and frequency upconversion spectroscopic properties of Er3+-doped and Er3+/Yb3+-codoped TeO2-ZnO-Na2O-PbCl2 halide modified tellurite glasses upon excitation with 808 and 978 nm laser diode. Three intense emissions centered at around 529, 546 and 657 nm, alongwith a very weak blue emission at 4 10 nm have clearly been observed for the Er3+/Yb3+-codoped halide modified tellurite glasses upon excitation at 978 nm and the involved mechanisms are explained. The quadratic dependence of fluorescence on excitation laser power confirms the fact that the two-photon contribute to the infrared to green-red upconversion emissions. And the blue upconversion at 410 nm involved a sequential three-photon absorption process. (c) 2005 Elsevier Ltd. All rights reserved.

Optical characterization of Cu2+ ion-doped zinc lead borate glasses

Abstract: We have developed a new series of zinc lead borate (ZLB) glasses by varying ZnO content, to enhance UV transmission, in the chemical composition of xZnO–15PbO–(85−x)B2O3, where x = 0 , 5, 10, 15, 20, 25, 30, 35, 40 and 45 mol% ZnO. From the measurement of UV absorption spectra both the direct and indirect band gaps have been evaluated. Also different physical properties of a reference glass of 45ZnO–15PbO–40B2O3 have been studied. From the measurement of refractive indices at six different wavelengths, Cauchy's constants ( A = 1.578743209 ; B = 131832.33 nm 2 and C = - 0.77756 × 1 0 10 nm 4 ) have been computed and a satisfactory correlation has been achieved between the theoretical and the experimental results. Absorption spectra of Cu2+(45−x)ZnO–15PbO–40B2O3 (where x=0.1, 0.2, 0.5 and 1.0 mol%) have shown two absorption bands at 428 nm (2B1g→2Eg) and 777 nm (2B1g→2B2g). Emission spectra of (1.0 mol%) Cu2+:ZLB have revealed two emission transitions at 400 and 493 nm with excitations at 288 and 316 nm.

On the reflection and polarisation properties of ice cloud

Abstract: A number of cirrus ice crystal scattering models are tested using measurements of total reflectance and polarised reflectance obtained from the space-based polarisation and directionality of Earth's reflectances (POLDER) instrument. In this paper, 1 day of global POLDER data is utilised taken from the 25 June 2003 to test the assumed ice crystal models. The POLDER instrument is able to test the validity of various ice crystal models since it can measure the total reflectance and polarised reflectance at up to 14 different viewing directions almost simultaneously between the scattering angles of about 60–180°. It is found that ice crystal models that are randomised (in this case the randomisation element is through distortion) from some pristine ice crystal geometry best fit simultaneous measurements of total and polarised reflectance. The optimal distortion parameter that best describes the POLDER measurements is found to be 0.40, which has been applied to a randomly oriented six-branched bullet-rosette and randomly oriented chain-like aggregate. Moreover, distorted ice crystals that have undergone significant distortions beyond 0.40 may fit the total reflectance measurements but not the polarisation measurements. Therefore, total reflectance measurements by themselves do not provide sufficient information to constrain assumed complex/distorted ice crystal models.

Finite element method for radiation heat transfer in multi-dimensional graded index medium

Abstract: In graded index medium, ray goes along a curved path determined by Fermat principle, and curved ray-tracing is very difficult and complex. To avoid the complicated and time-consuming computation of curved ray trajectories, a finite element method based on discrete ordinate equation is developed to solve the radiative transfer problem in a multi-dimensional semitransparent graded index medium. Two particular test problems of radiative transfer are taken as examples to verify this finite element method. The predicted dimensionless net radiative heat fluxes are determined by the proposed method and compared with the results obtained by finite volume method. The results show that the finite element method presented in this paper has a good accuracy in solving the multi-dimensional radiative transfer problem in semitransparent graded index medium.

Measurement of carbon ionization balance in high-temperature plasma mixtures by temporally resolved X-ray scattering

Abstract: We have measured carbon ionization balance in a multi-component plasma in the high-temperature, up to fully ionized, regime by spectrally resolved X-ray scattering. In particular, the measurements have been performed in an underdense ( n e ≈ 10 21 cm - 3 ) 0.35- μ m laser-produced plasma, containing a mixture of C, H with Al and Ar impurities, by using time-resolved back-scattered spectra from a 9.0 keV Zn He- α X-ray probe detected with a high-efficiency graphite Bragg crystal coupled to a framing camera. Measured values for the plasma temperature and carbon ionization state as well as impurity concentrations were obtained by fitting the Doppler-broadened and Compton-shifted scattered spectra at various times after the plasma heating with a modified X-ray form factor that includes the full effects of cross-correlation between different species. These data test collisional-radiative and radiation hydrodynamics modeling from cold ( T e ≲ 5 eV) to fully ionized carbon ( T e ∼ 280 eV).

An empirical line-by-line model for the infrared solar transmittance spectrum from 700 to 5000 cm 1

Abstract: An empirical line-by-line model for the infrared solar transmittance spectrum is presented. The model can be incorporated into radiative transfer codes to allow fast calculation of all relevant emission and absorption features in the solar spectrum in the mid-infrared region from 700 to 5000 cm - 1 . The transmittance is modelled as a function of the diameter of the field-of-view centered on the solar disk: the line broadening due to solar rotation as well as center-to-limb variations in strength and width are taken into account for stronger lines. Applications of the model presented here are in the fields of terrestrial remote sensing in the mid-infrared spectral region when the sun is used as radiation source or scattered solar radiation contributes to the measured signal and in the fields of atmospheric radiative transfer algorithms which compute the propagation of infrared solar radiation in the terrestrial atmosphere.

Light scattering by small feldspar particles simulated using the Gaussian random sphere geometry

Abstract: The single-scattering properties of Gaussian random spheres are calculated using the discrete dipole approximation. The ensemble of model particles is assumed to be representative for a feldspar dust sample that is characteristic for weakly absorbing irregularly shaped mineral aerosol. The morphology of Gaussian random spheres is modeled based on a statistical shape analysis using microscope images of the dust sample. The size distribution of the dust sample is based on a particle sizing experiment. The refractive index of feldspar is estimated using literature values. All input parameters used in the light scattering simulations are thus obtained in an objective way based on the true properties of the mineral sample. The orientation-averaged and ensemble-averaged scattering matrices and cross sections of the Gaussian random spheres are compared with light scattering simulations using spheroidal shape models which have been shown to be applicable to the feldspar sample. The Gaussian random sphere model and the spheroidal shape model are assessed using the measured scattering matrix of the feldspar dust sample as a reference. Generally, the spheroidal model with strongly elongated prolate and strongly flattened oblate shapes agrees better with the measurement than the Gaussian random sphere model. In contrast, some features that are characteristic for light scattering by truly irregular mineral dust particles are rendered best by the Gaussian random sphere model; these features include the flat shape of the phase function and a minimum in the scattering matrix element F-22/F-11 as a function of the scattering angle. (c) 2005 Elsevier Ltd. All rights reserved.

The effects of the choice of the k-interval number on radiative calculations

Abstract: The selection of the number of k-interval is a foundation to correlated k-distribution method and the problem of how to do it still remains unsettled It is pointed out by numerical computation in this work that choosing the number of k-interval is a major factor affecting accuracy and speed in radiative calculation To increase the number of k-interval is an efficient method to improve the accuracy However, it is found by this study that there exists a saturation of the accuracy to an increase of the number The optimal rules on the number of k-interval choosing are proposed in the paper Then, five versions on atmospheric absorption by gases appropriate for GCMs are given according to them