Effect Of Particle Non-sphericity Research Articles

Retrieval of dust aerosol optical and microphysical properties from ground-based Sun-sky radiometer measurements in approximation of randomly oriented spheroids

Deserts currently covering more than one tenth of the land surface gradually spread out over more and more areas due to desertification. Mineral dust as one of the most widespread aerosol types plays increasingly important role in atmospheric processes. An accurate accounting for physical and chemical characteristics of desert dust, particles shape in particular, in methods for aerosol retrievals from remote sensing data, is needed. The neglect of dust particle non-sphericity may lead to errors in quantifying the aerosol radiative and regional climatic effects.Earlier we developed a software package Sun-Sky Measurements for Aerosol ReTrieval (SSMART) to retrieve aerosol optical and microphysical characteristics from spectral ground-based Sun and sky radiance measurements. The single scattering albedo and scattering phase function were retrieved in two ways: (i) directly from the almucantar measurement data without any assumptions about the shape of aerosol particles and (ii) through the calculation on the basis of the Mie theory using the retrieved particle size distribution function and complex refractive index. However, there is experimental evidence that in modeling the optical properties of desert dust, the effects of particle non-sphericity should be taken into consideration.In this paper, we propose an improved version of the SSMART algorithm, in which the inverse light scattering problem is solved using the model of a mixture of randomly oriented polydisperse spheroids. On the basis of closed numerical experiments, the accuracy of aerosol retrievals is investigated in error-free conditions and in the presence of measurement errors. The results of the algorithm applying to almucantar scans obtained at Dakar AERONET site during desert dust events in 2012 are discussed. The retrieved aerosol optical and microphysical characteristics are compared with those obtained by the AERONET inversion algorithm.

Simple technique to evaluate effects of nonsphericity and orientation on particle size distribution retrieval

This paper reports the formulation of an algorithm based on the anomalous diffraction theory to infer the particle size distribution (PSD) from theoretical perspective, taking the effects of particle nonsphericity and orientation into account. Preliminary analysis of input parameters sensitivity of the analytical retrieval formulas of the different PSDs, it is found that there exist simple scale transformation relations between the retrieved and the true PSD for spheroids and triangular prisms if inappropriate shape and orientation assumption are made, but scale transformation relation is invalid for cubes, because the particle modes occur in retrieval procedure. Key words: Particle size distribution, extinction spectrum, inversion, atmospheric particles.

Application of randomly oriented spheroids for retrieval of dust particle parameters from multiwavelength lidar measurements

Multiwavelength (MW) Raman lidars have demonstrated their potential to profile particle parameters; however, until now, the physical models used in retrieval algorithms for processing MW lidar data have been predominantly based on the Mie theory. This approach is applicable to the modeling of light scattering by spherically symmetric particles only and does not adequately reproduce the scattering by generally nonspherical desert dust particles. Here we present an algorithm based on a model of randomly oriented spheroids for the inversion of multiwavelength lidar data. The aerosols are modeled as a mixture of two aerosol components: one composed only of spherical and the second composed of nonspherical particles. The nonspherical component is an ensemble of randomly oriented spheroids with size‐independent shape distribution. This approach has been integrated into an algorithm retrieving aerosol properties from the observations with a Raman lidar based on a tripled Nd:YAG laser. Such a lidar provides three backscattering coefficients, two extinction coefficients, and the particle depolarization ratio at a single or multiple wavelengths. Simulations were performed for a bimodal particle size distribution typical of desert dust particles. The uncertainty of the retrieved particle surface, volume concentration, and effective radius for 10% measurement errors is estimated to be below 30%. We show that if the effect of particle nonsphericity is not accounted for, the errors in the retrieved aerosol parameters increase notably. The algorithm was tested with experimental data from a Saharan dust outbreak episode, measured with the BASIL multiwavelength Raman lidar in August 2007. The vertical profiles of particle parameters as well as the particle size distributions at different heights were retrieved. It was shown that the algorithm developed provided substantially reasonable results consistent with the available independent information about the observed aerosol event.

The Receptor-Mediated Endocytosis of Nonspherical Particles

Enveloped viruses and nanosized biomimetic particles for drug and gene delivery enter target cells mainly through receptor-mediated endocytosis. A few models have been presented to elucidate the mechanics of particle engulfment by the cell membrane, showing how size and surface chemico-physical properties favor or oppose internalization. In this work, the effect of particle nonsphericity is addressed considering elliptical cylindrical particles with aspect ratio Γ. Using a continuum energetic approach, three different conditions have been identified: for sufficiently small Γ, the particle is not even wrapped by the cell membrane; for sufficiently large Γ, the particle is partially wrapped (“frustrated endocytosis”); and for intermediate values of Γ, the particle is fully wrapped and eventually internalized. Given the pleomorphism of viruses and the broad spectrum of shapes for nanosized biomimetic particles, the results presented may be of interest to virologists, pharmacologists, toxicologists, and nanotechnologists.

Aerosol retrievals from AVHRR radiances: effects of particle nonsphericity and absorption and an updated long-term global climatology of aerosol properties

The paper describes and discusses long-term global retrievals of aerosol properties from channel-1 and -2 Advanced Very High-Resolution Radiometer (AVHRR) radiances. We reconfirm the previously reached conclusion that the nonsphericity of dust-like and dry sea salt aerosols can lead to very large errors in the retrieved optical thickness if one mistakenly applies the scattering model for spherical particles. Comparisons of single-scattering albedo and A ̊ ngström exponent values retrieved from the AVHRR data and those measured in situ at Sable Island indicate that the currently adopted value 0.003 can be a reasonable choice for the imaginary part of the aerosol refractive index in the global satellite retrievals. Several unexpected features in the long-term satellite record indicate a serious problem with post-launch calibration of channel-2 radiances from the NOAA-11 spacecraft. We solve this problem by using a simple re-calibration procedure removing the observed artifacts and derive a global climatology of aerosol optical thickness and size over the oceans for the period extending from July 1983 to December 1999. The global monthly mean optical thickness and A ̊ ngström exponent of tropospheric aerosols show no significant trends over the entire period and oscillate around the average values 0.145 and 0.75, respectively. The Northern Hemisphere mean optical thickness systematically exceeds that averaged over the Southern Hemisphere. The AVHRR retrieval results during the period affected by the Mt. Pinatubo eruption are consistent with the retrievals of the stratospheric aerosol optical thickness based on Stratospheric Aerosol and Gas Experiment Data (SAGE). Time series of the aerosol optical thickness and A ̊ ngström exponent derived for four separate geographic regions exhibit varying degrees of seasonal variability controlled by local meteorological events and/or anthropogenic activities.

Effect of particle non-sphericity on satellite monitoring of drifting volcanic ash clouds

The NASA total ozone mapping spectrometer (TOMS) instruments aboard the Nimbus-7, Meteor3, ADEOS, and Earth Probe satellites have produced a unique data set of global S0 2 volcanic emissions since 1978. Besides S0 2, a new technique has been developed which uses the measured spectral contrast of the backscattered radiances in the 0.34–0.38 μm spectral interval (where gaseous absorption is negligible) in conjunction with radiative transfer models to retrieve properties of ash clouds including the optical depth and effective particle radius, R eff. Using the T-matrix method for computing the scattering properties of randomly oriented spheroids, we have tested the sensitivity of the TOMS volcanic ash retrievals to particle shape. For the case of the August 19, 1992 Mt. Spurr ash cloud and the TOMS observational geometry, modeling the ash as spherical particles causes the TOMS-retrieved R eff to be underestimated for R eff >0.1 μ m by as much as 30%. On the other hand, the optical depth will be overestimated by as much as 25%. In terms of the total mass of the cloud, the compensating errors in R eff and optical depth tend to produce a 5–20% underestimation.

Light scattering by polydispersions of randomly oriented spheroids with sizes comparable to wavelengths of observation

We report the results of an extensive study of the scattering of light by size and size-shape distributions of randomly oriented prolate and oblate spheroids with the index of refraction 1.5 + 0.02i typical of some mineral terrestrial aerosols. The scattering calculations have been carried out with Waterman's T-matrix approach, as developed recently by Mishchenko [J. Opt. Soc. Am. A 8, 871 (1991); Appl. Opt. 32, 4562 (1993)]. Our main interest is in light scattering by polydisperse models of nonspherical particles because averaging over sizes provides more realistic modeling of natural ensembles of scattering particles and washes out the interference structure and ripple typical of monodisperse scattering patterns, thus enabling us to derive meaningful conclusions about the effects of particle nonsphericity on light scattering. Following Hansen and Travis [Space Sci. Rev. 16, 527 (1974)], we show that scattering properties of most physically plausible size distributions of randomly orientednonspherical part cles depend primarily on the effective equivalent-sphere radius and effective variance of the distribution, the actual shape of the distribution having a minor influence. To minimize the computational burden, we have adopted a computationally convenient power law distribution of particle equivalent-sphere radii n(r) α r(-3),r(1) ≤ r≤r(2). The effective variance of the size distribution is fixed at 0.1, and the effective size parameter continuously varies from 0 to 15. We present results of computer calculations for 24 prolate and oblate spheroidal shapes with aspect ratios from 1.1 to 2.2. The elements of the scattering matrix for the whole range of size parameters and scattering angles are displayed in the form of contour plots. Computational results are compared with analogous calculations for surface-equivalent spheres, and the effects of particle shape on light scattering are discussed in detail.

Determination of vertical profiles of aerosol size spectra from aircraft radiative flux measurements: 2. The effect of particle nonsphericity

The effect of particle nonsphericity upon the retrieval of panicle size distributions, computed from aircraft radiative flux measurements, has been examined for the data of September 4, 1974. The particle inversion procedure is identical to that described by Kondratyev et al. (1981), involving quadramodal particle size spectra. Nonspherical particle attenuation coefficients and phase functions were calculated according to the semi‐empirical approach developed by Pollack and Cuzzi. It was found that variations of the phase function by nonspherical panicles affected the retrieval of particle size spectra to a far greater degree than did variations of panicle scattering efficiency. Nonspherical panicles generally have smaller values of asymmetry factor than do their spherical counterparts. The inferred size spectra of nonspherical panicles show decreased values both of large and small panicle number densities along with a broadening of the large particle mode to include particles of larger sizes.