Particle Shape Models Research Articles

Coupled infrared extinction spectra and size distribution measurements for several non-clay components of mineral dust aerosol (quartz, calcite, and dolomite)

Simultaneous size distributions and Fourier transform infrared (FTIR) extinction spectra have been measured for several representative components of mineral dust aerosol (quartz, calcite, and dolomite) in the fine particle size mode ( D=0.1–1 μm). Optical constants drawn from the published literature have been used in combination with the experimentally determined size distributions to simulate the extinction spectra. In general, Mie theory does not accurately reproduce the peak position or band shape for the prominent IR resonance features in the 800–1600 cm −1 spectral range. The resonance peaks in the Mie simulation are consistently blue shifted relative to the experimental spectra by ∼20–50 cm −1. Spectral simulations, derived from a simple Rayleigh-based analytic theory for a “continuous distribution of ellipsoids” particle shape model, better reproduce the experimental spectra, despite the fact that the Rayleigh approximation is not strictly satisfied in these experiments. These results differ from our previous studies of particle shape effects in silicate clay mineral dust aerosols where a disk-shaped model for the particles was found to be more appropriate.

Preliminary results of a non-spherical aerosol method for the retrieval of the atmospheric aerosol optical properties

There is experimental evidence that the non-sphericity of certain atmospheric particles can cause scattering properties different from those predicted by standard Mie theory. Numerous studies indicate the need to consider the presence of non-spherical particles in modeling the optical properties of atmospheric aerosols. On the other hand, natural aerosols show a great variety of shapes, making difficult a realistic choice of a particle shape (or shape mixture) model. In this paper, we test a parameterization of the particle shape in the retrieval of size distribution, phase function, single scattering albedo and asymmetry parameter from direct and sky-radiance measurements. For this purpose we have substituted the Kernel based on the Mie theory included in the model SKYRAD.PACK by one derived for non-spherical particles. The method is applied under different atmospheric conditions, including Saharan dust outbreak, polluted and local mineral episodes. We compare the results with those obtained by the well known spheroids algorithm used in the AERONET network.

Atomic force and scanning electron microscopy of atmospheric particles

Atomic force microscopy (AFM) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) have been used for both morphological and elemental mass analysis study of atmospheric particles. As part of the geometrical particle analysis, and in addition to the traditional height profile measurement of individual particles, AFM was used to measure the volume relative to the projection area for each particle separately, providing a particle shape model. The element identification was done by the EDS analysis, and the element mass content was calculated based on laboratory calibration with particles of known composition. The SEM-EDS mass measurements from two samples collected at 150 and 500 m above the surface of the Mediterranean Sea were found to be similar to mass calculations derived from the AFM volume measurements. The AFM results show that the volume of most of the aerosols that were identified as soluble marine sulfate and nitrate aerosol particles can be better estimated using cylindrical shapes than spherical or conical geometry.

Influence of particle shape models on T-matrix light scattering simulation

Simulation of light scattering gets more and more important; topics of interest are for example characterization of the shape of particles, detection of airborne fibers, determination of spherical and non-spherical particles, etc. To develop advanced methods for optical particle characterization corresponding light scattering theories and simulation programs are needed. For these kinds of simulations adequate input data representing the particle shape is important. If a specific particle shape is obtained from some reconstruction method the model will only be an approximation of the real shape. So the quality of this input particle shape has influence on the quality of the obtained results.In our research we concentrate on the development of the null-field method with discrete sources for light scattering simulation. With this method the T-matrix of complex structured particles can be computed. This paper gives an overview of solving light scattering problems with the null-field method with discrete sources and shows the influence of the accuracy of the particle shape model on the computed scattering results.

Sensitivity of cirrus cloud albedo, bidirectional reflectance and optical thickness retrieval accuracy to ice particle shape

We examine the sensitivity of cirrus cloud albedo and bidirectional reflection function to particle shape using the phase functions of liquid water spheres, regular hexagonal ice crystals, and random‐fractal ice particles calculated at a nonabsorbing visible wavelength of 0.63 μm. Accurate multiple‐scattering calculations for plane‐parallel clouds show that hexagonal ice crystal clouds have systematically larger planetary and global albedos than liquid water clouds of the same optical thickness. There is accumulating evidence that the idealized phase function of regular hexagonal crystals, which causes pronounced halos, is not necessarily the best representation of the range of reflectance characteristics of the majority of ice clouds. A more typical representation of the scattering phase function for ice clouds that are composed of a complex set of crystal shapes and sizes may be obtained using a model of randomly shaped irregular particles. Even larger cloud albedos are obtained for the random‐fractal particle model because of its smaller asymmetry parameter. Our computations also show that a larger planetary albedo does not always imply a larger reflectance and that the relative brightness of ice versus liquid water clouds is highly scattering‐geometry dependent. Use of the wrong particle shape model (crystal instead of water droplet and vice versa) in retrieving cloud optical thickness from bidirectional reflectance measurements can result in an underestimation or overestimation of the true optical thickness by a factor that can exceed 3. At some scattering geometries, use of the wrong model can give an unrealistically large optical thickness or no solution at all. Overall, bidirectional reflectance differences between random‐fractal and regular hexagonal particle shapes are significantly smaller than those between either ice crystal and liquid water spheres, except at the back scattering direction.

TEM/STEM characterization of industrial type supported Pd catalysts: Effect of an in situ hydrogen activation

TEM is routinely used to control the metallic dispersion of industrial supported metal catalysts, since in complement to the chemisorptive methods it provides unique local information about particle size distribution, i.e. about the quality of the metal dispersion. An evaluation by TEM of the mean particle size can be useful to quantify the metal dispersion assuming a simple model for the particle shape, e.g. spherical or cubic. However, except for the cases where chemisorption fails (e.g. poisonning), this evaluation is mainly considered as a mean of checking the representativity of the statistical distribution of local information in comparison with chemisorption results. Discrepancies are often observed between the two techniques and many reasons can be advanced for these such as statistics, particle shape models, electron beam induced changes, chemisorption assumptions (cf coverage), etc... In fact, we consider that in a number of cases one of the primary limitations is likely to be due to the in situ problem in TEM, which is inherent to most techniques involving an ex situ characterization.