A Newly Designed and Constructed Instrument for Coupled Infrared Extinction and Size Distribution Measurements of Aerosols

Although atmospheric particles are often non-spherical, Mie theory is commonly used to acquire aerosol optical depth, composition, and transport information from satellite retrievals. In the infrared (IR) region, the radiative effects of aerosols, usually modeled with Mie theory, are subtracted from satellite spectral data to determine key atmospheric and oceanic properties. To gain a better understanding of the infrared radiative effects of aerosols and the methods used to model them, an instrument has been designed to simultaneously measure infrared extinction spectra and particle size distributions obtained from a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). Infrared extinction spectra are simulated with Mie theory using the measured particle size distributions and available literature optical constants. As a result, the errors associated with using Mie theory to model the infrared extinction of mineral dust aerosol can be quantitatively examined. Initial results for this instrument are presented here. For ammonium sulfate, the Mie theory simulation is in good agreement with our measured extinction spectrum. This is in accordance with the nearly spherical shape of ammonium sulfate particles. However, for illite, an abundant clay mineral, there is poor agreement between the experimental spectrum and the Mie simulation. This result is attributed to particle shape effects.     

A methodology to estimate source-specific aerosol radiative forcing

This paper presents a novel approach to estimate source-specific radiative forcing by combining source apportionment results for particulate matter mass with satellite (moderate resolution imaging spectroradiometer (MODIS)) derived aerosol optical depth (AOD). Positive matrix factorization (PMF) was applied to particulate matter (PM) mass and its chemical constituents measured during a winter intensive study (December 2004) at Hisar, Haryana, India. The model resolved four factors including carbonate rich dust, combustion rich aerosol, secondary sulfate/nitrate, and an unidentified factor likely to be emission from polymer industries. Carbonate rich dust was the highest contributor to the measured PM mass closely followed by combustion rich aerosol with their average contributions accounting for 34.0% and 33.6%, respectively. Model apportioned species concentrations corresponding to each factor were then used to estimate factor specific optical and radiative properties, and radiative transfer calculations were performed for the shortwave regime. During the study period, although carbonate rich dust and combustion rich aerosol mass contributions were comparable, carbonate rich dust contributed to only 22% of top of the atmosphere forcing while combustion rich aerosol contributed nearly 56%. Overall, the results suggested that the aerosol radiative forcing was primarily governed by the aerosol optical and radiative properties, while the mass concentrations played a secondary role.     

Determination of non-spherical particle size distribution from chord length measurements. Part 1: Theoretical analysis

In this paper, extensive theoretical studies are described on two important issues in translating a chord length distribution (CLD) measured by FBRM instrument into its particle size distribution (PSD) including PSD-CLD and CLD-PSD translation models for general non-spherical particles. Analytical solutions to calculate the PSD-CLD models for spherical and ellipsoidal particles are developed. For non-spherical particles, a numerical method is given to calculate the PSD-CLD model. The iterative non-negative least squares (NNLS) method is proposed in the CLD-PSD model, because of its many advantages converting measured CLD into its PSD, such as insensitivity to measurement noise and particle shape. The effectiveness of the proposed methods is validated by extensive simulations.     

Single-scattering properties of tri-axial ellipsoidal mineral dust aerosols: A database for application to radiative transfer calculations

This paper presents a user-friendly database software package of the single-scattering properties of individual dust-like aerosol particles for application to radiative transfer calculations in a spectral region from ultraviolet (UV) to far-infrared (far-IR). To expand the degree of morphological freedom of the commonly used spheroidal and spherical models, tri-axial ellipsoids were assumed to be the overall shape of dust-like aerosol particles. A combination of four computational methods, including the Lorenz–Mie theory, the T-matrix method, the discrete dipole approximation, and an improved geometric optics method, was employed to compute the phase matrix, extinction efficiency and single-scattering albedo of ellipsoids with various aspect ratios and sizes. The scattering property database was developed for 42 particle shapes specified in terms of two aspect ratios, 69 refractive indices and 471 size parameters. Additionally, accompanying software, based on interpolation, was developed to provide the single-scattering properties for user-specified aspect ratios, refractive indices and size parameters. The software package allows for the derivation of the bulk optical properties for a given distribution of particle microphysical parameters (i.e., refractive index, size parameter and two aspect ratios). The array-oriented single-scattering property data sets are stored in the NetCDF format.     

Effects of particle nonsphericity and radiation polarization on retrieving dust properties from MODIS observations

The scattering and radiative properties of mineral dust aerosols at violet-to-blue (0.412, 0.441, and 0.470 μm) and red (0.650 μm) wavelengths are investigated. To account for the effect of particle nonsphericity on the optical properties of dust aerosols, in the present study, these particles are assumed to be spheroids. A combination of the T-matrix method and an improved geometric optics method is applied to the computation of the single-scattering properties of spheroidal particles with size parameters ranging from the Rayleigh to geometric optics regimes. For comparison, the Lorenz–Mie theory is employed to compute the optical properties of spherical dust particles that have the same volumes as their nonspherical counterparts. The differences between the phase functions of spheroidal and spherical particles lead to quite different lookup tables involved in retrieving dust aerosol properties. Moreover, the applicability of a hybrid approach based on the spheroid model for the phase function and the sphere model for the other phase matrix elements is also demonstrated. The present sensitivity study, employing the moderate resolution imaging spectroradiometer (MODIS) measurements and the fundamental principle of the Deep Blue algorithm, illustrates that neglecting the nonsphericity of dust particles usually leads to an underestimate of retrieved aerosol optical depth; although, depending on the scattering angle, an overestimate is noted in some cases. Furthermore, the effect of including full polarization treatment in forward radiative transfer simulation on dust property retrieval is also investigated. It is found that the effect of radiation polarization on the Deep Blue dust property retrieval is not negligible if the retrieval is based on two violet—blue channels centered at 0.412 and 0.470 μm.     

Modeling of the scattering and radiative properties of nonspherical dust-like aerosols

The optical and radiative properties of dust particles in solar and thermal infrared regions are investigated. Dust particles are assumed to be spheres and spheroids for a comparison aimed at understanding the nonsphericity effect of these particles on the radiation at the top of a dusty atmosphere. The classical Lorenz–Mie theory is employed to compute the optical properties of spherical dust particles. To compute the single-scattering properties of spheroidal dust particles, a combination of the T-matrix method and an approximate method is used in the present study. In the approximate method, applicable to large particles, the geometric optics method is applied to the computation of the scattering phase matrix. A combination of the solution from the geometric optics method and the contribution of the so-called edge effect is used to compute the extinction efficiency of a spheroidal particle whose absorption efficiency is computed by adding the so-called above- and below-edge effect (a term from the well-known complex angular momentum theory) to the geometric optics result. Numerical results show that the results from the T-matrix method and the present approximate approach converge at a size parameter of 50 for computing the integrated scattering properties (i.e., the extinction efficiency, single-scattering albedo, and asymmetry factor). Additionally, the phase functions computed from the two methods are quite similar for size parameters larger than 40 although some considerable differences may still be noticed for other phase matrix elements. Furthermore, the effect of surface roughness on the single-scattering properties of spheroidal particles is discussed. The present radiative transfer simulations illustrate the nonsphericity effect of dust particles is significant at short wavelengths, however, not at the thermal infrared wavelengths.     

Generation of Internally Mixed Insoluble and Soluble Aerosol Particles to Investigate the Impact of Atmospheric Aging and Heterogeneous Processing on the CCN Activity of Mineral Dust Aerosol

Heterogeneous reactions of trace gases with mineral dust aerosol not only impact the chemical balance of the atmosphere but also the physicochemical properties of the dust particle and the ability of the particle to act as a cloud condensation nuclei (CCN). Recent field studies have shown that carbonate minerals are preferentially associated with nitrates whereas aluminum silicates (i.e., clay minerals) are preferentially associated with sulfates. To better understand how this association can impact the climate effects of mineral dust particles, we have measured the CCN activity of a number of pure and internal mixtures of aerosols relevant to these recent field studies. The CCN activity of CaCO ₃ -Ca(NO ₃ ) ₂ aerosol, simulating the activity of mineral dust aerosol that has been partially processed by nitrogen oxides in the atmosphere, is significantly enhanced relative to CaCO₃ aerosol of the same diameter. Similar results are obtained for a clay mineral, kaolinite, internally mixed with (NH ₄ ) ₂ SO ₄ . For example, at 0.3% supersaturation, a 200 nm particle containing a soluble nitrate or sulfate component is 2 to 4 times more active than an unreacted particle. The results presented here show that when determining the contribution of mineral dust aerosol to the overall impact of the aerosol indirect effect on radiative forcing, changes in chemical composition due to atmospheric processing cannot be ignored.     

Characterization of packing structure of tape cast with non-spherical natural graphite particles

A three-dimensional microstructure of green and pressed tapes cast with graphite particles of non-spherical shape were examined quantitatively on the basis of the distribution of void sizes among the packed particles. The distributions measured over the cross-sections of the tape in three directions were expressed by the theoretical ones deduced for non-spherical particles. Particle shape was characterized by shape indices defined by Fourier analysis of particle outlines measured over the corresponding cross-sections of the green tape. The relationships were totally established between the limiting packing density, characterizing the void size distribution at the same section voidage, and the shape index of particle over the section. As a result, the normalized median void diameter as well as the limiting packing density of the pressed tape was found to increase with the particle shape index, corresponding to wider void size distribution. Therefore, based on developed correlations, the optimization of packing microstructure of the cast tape can be expected to result in high performance battery by using shape-modified graphite particles.     

A note on the aerodynamic diameter and the mobility of non-spherical aerosol particles

The aerodynamic diameter of a non-spherical aerosol particle is primarily related to the final settling velocity of the particle. The aerodynamic diameter is not obtained directly from mobility measurements by formally calculating a sphere diameter from the mobility equation for a spherical particle. Instead, a correction factor involving the dynamic shape factor of the non-spherical particle must be applied.     

CALIBRATION OF A WHITE-LIGHT/90° OPTICAL PARTICLE COUNTER FOR “AERODYNAMIC” SIZE MEASUREMENTS—EXPERIMENTS AND CALCULATIONS FOR SPHERICAL PARTICLES AND QUARTZ DUST

This paper describes the calibration of an optical particle counter with the help of an aerodynamic particle sizer. The calibration method and influencing errors are discussed. The transfer characteristic of the optical particle counter is determined with monodisperse fractions of polystyrene latex spheres and verified with spherical particles (glycerine) of known refractive index. In calibration experiments with three optical particle counters of the same type the method proved to be transferable to different devices. The described method was validated for nonspherical particles (quartz). The results from calibration by means of an aerodynamic particle sizer are in perfect agreement with the results from calibration by means of sampling cyclones. The enhancement of scattered light intensity due to irregular particle shape is demonstrated by the comparison of experimental quartz calibrations with calculations for spherical quartz particles.     

Determination of non-spherical particle size distribution from chord length measurements. Part 2: Experimental validation

In this paper, the theory on the translation of a measured chord length distribution (CLD) into its particle size distribution (PSD), which was developed in the first part of this study [Li and Wilkinson, 2005. Determination of non-spherical particle size distribution from chord length measurements. Part 1: theoretical analysis. Chemical Engineering Science 60, 3251-3265], has been validated using experimental results. CLDs were measured using the Lasentec focused beam reflectance measurement (FBRM) with three different materials, spherical ceramic beads and non-spherical plasma aluminium and zinc dust particles. Meanwhile, the particle shape and PSD of each material were also investigated by image analysis (IA). Comparison of the retrieved PSDs with the measured PSDs by IA shows that the PSD can be retrieved from a measured CLD successfully using the proposed iterative nonnegative least squares (NNLS) method based on the PSD-CLD model.     

Discrete particle simulation of gas fluidization of ellipsoidal particles

Fluidization is widely used in industries and has been extensively studied, either experimentally or theoretically, in the past decades. In recent years, a coupled simulation approach of discrete element method (DEM) and computational fluid dynamics (CFD) has been successfully developed to study the gas–solid flow and heat transfer in fluidization at a particle scale. However, to date, such studies mainly deal with spherical particles. The effect of particle shape on fluidization is recognized but not properly quantified. In this paper, the CFD–DEM approach is extended to consider the fluidization of ellipsoidal particles. In the simulation, particles used are either oblate or prolate, with aspect ratios varying from very flat (aspect ratio=0.25) to elongated (aspect ratio=3.5), representing cylinder-type and disk-type shaped particles, respectively. The commonly used correlations to determine the fluid drag force acting on a non-spherical particle are compared first. Then the model is verified in terms of solid flow patterns. The effect of aspect ratio on the flow pattern, the relationship between pressure drop and gas superficial velocity, and microscopic parameters such as coordination number, particle orientation and force structure are investigated. It is shown that particle shape affects bed permeability and the minimum fluidization velocity significantly. The coordination number generally increases with aspect ratio deviating from 1.0. The analysis of particle orientations shows that the bed structures for ellipsoids are not random as that for spheres. Oblate particles prefer facing upward or downward while prolate particles prefer horizontal orientation. Spheres have the largest particle–particle contact force and fluid drag force under the comparable conditions. With aspect ratio deviating from 1.0, particle–particle interaction and fluid drag become relatively weak. The proposed model shows a promising method in examining the effect of particle shape on different flow behaviour in gas fluidization.     

Evaluation of DMA Size Selection of Dry Dispersed Mineral Dust Particles

Mineral dust particles play a significant role in the Earth's radiative balance via direct interaction with solar radiation and indirectly through their ability to initiate cloud formation. Many field and laboratory studies utilize a differential mobility analyzer (DMA) for particle size selection. Here we evaluate the use of a DMA to size-segregate dry dispersed mineral dust particles. We examine the post-DMA size distribution using four different techniques: a scanning mobility particle sizer (SMPS) for mobility sizing, an optical particle sizer (OPS) for optical sizing, the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument for vacuum aerodynamic sizing, and electron microscopy (EM) for geometric sizing. While the SMPS measured a narrow mobility size distribution at the DMA-selected diameter, the OPS, PALMS, and EM in most cases showed broader distributions and a smaller mode size than that selected by the DMA. These techniques also observed super-micrometer particles, often extending beyond the upper size limit of a typical SMPS scan. Complicating analysis, particle shape factor (χ) was observed to be a function of mobility size, ranging from 1.3 at 500 nm to 3.1 at 1000 nm. We conclude that mobility size selection of mineral dust particles using a DMA most often does not yield particles of the desired physical size or surface area. We suggest that attempts to size-select from a broad distribution of non-spherical particles require an independent measurement downstream of the DMA to verify the actual selected size.

Copyright 2015 American Association for Aerosol Research     

Calibration Correction of an Active Scattering Spectrometer Probe to Account for Refractive Index of Stratospheric Aerosols: Comparison of Results with Inertial Impaction

Real-time particle size spectra are being acquired on our research aircraft with relative ease and speed by techniques that make use of the real-time interaction of laser light with aerosols and cloud droplets. The results are, however, sometimes ambiguous, because the optical “signatures” of the particles depend on their refractive indices in addition to physical dimensions. The calibration supplied by the manufacturer is based on instrument response to a specific test aerosol, e.g., latex spheres (refractive index = 1.59). Such a calibration is strictly valid only for sample aerosols of refractive index and shape similar to the test aerosol. Whenever the sample aerosol differs from the test aerosol, a calibration correction is in order. Of concern here is the use of an active scattering spectrometer probe (ASAS-X), to measure sulfuric acid aerosols on high-flying U-2 and ER-2 research aircraft. Correcting the calibration of the ASAS-X for dilute sulfuric acid droplets (refractive index = 1.44) that predominate the stratospheric aerosol changes the inferred sizes by up to 32% per size interval from that determined from the nominal calibration. This results in an average increase in particle surface area and volume of 42 ± 10% and 71 ± 19%, respectively. The calibration correction of the optical spectrometer probe for stratospheric aerosol is validated by independent and simultaneous sampling of the particles with impactors. Sizing and counting of particles on microphotographs of scanning electron microscope images give results on total particle surface areas and volumes. After the calibration correction, the optical spectrometer data (averaged over four size distributions) agree with the impactor results (similarly averaged) to within a few percent. We conclude that the optical properties, or chemical makeup, of the sample aerosol must be known for accurate size analysis by optical aerosol spectrometers.     

Microparticle characterization using digital holography

Digital holography is an effective 3D imaging technique, with the potential to be used for particle size measurements. A digital hologram can provide reconstructions of volume samples focused at different depths, overcoming the focusing problems encountered by other imaging based techniques. Several particle analysis methods discussed in the literature consider spherical particles only. With the object sphericity assumption in place, analysis of the holographic data can be significantly simplified. However, there are applications, such as particle analysis and crystallization monitoring, where non-spherical particles are often encountered. This paper discusses the processing of digital holograms for particle size and shape measurement for both spherical and arbitrarily shaped particles. An automated algorithm for identification of particles from recorded hologram and subsequent size and shape measurement is described. Experimental results using holograms of spherical and non-spherical particles demonstrate the performance of the proposed measuring algorithm.     

Force model considerations for glued-sphere discrete element method simulations

Despite knowing that particle shape plays a significant role in the dynamics of powder flow, most discrete element method (DEM) simulations utilize spherical particles. The reasons for using spheres are that (a) the contact detection scheme for spherical particles is simple, and (b) the contact force models for contacting spheres are well known (e.g. a Hertzian contact).Several schemes for modeling non-spherical particles have been proposed including those that involve polyhedra, ellipsoids, sphero-cylinders, and superquadrics. Perhaps the most common approach for modeling non-spherical particles, however, is using “glued spheres,” in which irregular particle shapes are produced by rigidly connecting individual, and possibly overlapping, spheres. The advantage of the glued-spheres approach is that even for complex particle shapes the simple spherical contact detection algorithm may be retained.Recent publications have focused on how approximating a given particle shape using a glued-sphere geometry affects the rebound of colliding particles [e.g. Price, M., Murariu, V., Morrison, G., 2007. Sphere clump generation and trajectory comparison for real particles. In: Fourth International Conference on Discrete Element Methods (DEM), Brisbane, Australia; Kruggel-Emden, H., Rickelt, S., Wirtz, S., Scherer, V., 2008. A study on the validity of the multi-sphere discrete element method, Powder Technology 188 (2), 153-165]. These investigations have focused on the errors introduced by approximating the geometry of the true particle shape. What has not been investigated, however, is how the spherical particle derived force models used in glued-sphere particle geometries influence the response of particle collisions. This paper demonstrates that in instances where more than a single component sphere in a glued-sphere model is involved in a contact, a modified force model must be used to produce an accurate force-deflection response.     

An Experimental Method for Three-Dimensional Dynamic Contact Angle Analysis

Abstract

Droplet dynamics analysis concerns the measurements of droplet volume, cap and base areas and contact angles, as they change in time to study evaporation, wettability, adhesion and other surface phenomena and properties. In a typical procedure, the two-dimensional measurements are based on a series of images recorded at successive stages of the experiment from a single view. Only a few basic dimensions of sessile droplets are commonly measured from such images, while many other quantities of interest are derived utilizing geometrical relationships. The reliability of these calculations is limited by the necessary assumption that the droplet shape can be approximated as a spherical cap. In reality, the sessile droplet shapes are influenced by gravity, liquid surface tension, local surface anisotropy and microstructure, which often produce non-spherical cap shapes.

This paper describes an experimental methodology for determination of key parameters, such as volume and contact angle for dynamic sessile droplets that can be approximated either by spherical or ellipsoidal cap geometries. In this method, images collected simultaneously from three cameras positioned orthogonally to each other are used to record the dynamic behavior of non-spherical droplets. Droplet shape is approximated as an ellipsoid of arbitrary orientation with respect to the cameras, which allows determination of volume and contact angle along the base perimeter. A major advantage of this method is that the dynamic parameters of droplets on anisotropic surfaces can be determined even when the orientation of the axes changes throughout the droplet lifetime. The method is illustrated with experimental results for a spherical and an ellipsoidal droplet.     

Chemical and microphysical properties of wind-blown dust near an actively retreating glacier in Yukon,Canada

Abstract

Airborne mineral aerosols emitted in high-latitude regions can impact radiative forcing, biogeochemical cycling of metals, and local air quality. The impact of dust emissions in these regions may change rapidly, as warming temperatures can increase mineral dust production and source regions. As there exists little research on mineral dust emissions in high-latitude regions, we have performed the first study of the physico-chemical properties of mineral dust emitted from a sub-Arctic proglacial dust source, using a method tailored to the remote conditions of the Canadian North. Soil and aerosol samples (PM₁₀ and deposited mineral dust) were collected in May 2018 near the Ä’äy Chù (Slims River), a site exhibiting strong dust emissions. WHO air quality thresholds were exceeded at several receptor sites near the dust source, indicating a negative impact on local air quality. Notably, temporally averaged particle size distributions of PM₁₀ were very fine as compared to those measured at more well-characterized, low-latitude dust sources. In addition, mineralogy and elemental composition of ambient PM₁₀ were characterized; PM₁₀ elemental composition was enriched in trace elements as compared to dust deposition, bulk soil samples, and the fine soil fractions (d?<?53?µm). Finally, through a comparison of the elemental composition of PM₁₀, dust deposition, and both fine and bulk soil fractions, as well as of meteorological factors measured during our campaign, we propose that the primary mechanisms for dust emissions from the Ä’äy Chù Valley are the rupture of clay coatings on particles and/or the release of resident fine particulate matter.

Copyright © 2019 American Association for Aerosol Research     

Radiative heat transfer in circulating fluidized bed furnaces

Three methods of estimating the effective emissivity of a gas-particle suspension are compared and the radiative heat transfer coefficient of an isothermal suspension is defined. Heat flux measurements obtained from circulating fluidized bed combustors are examined. Radiation from a particle suspension with core temperature dominates the radiative heat transfer in the upper part of the furnace, where the particle density is low and no substantial particle boundary layers are formed. Over the lower parts of the heat transfer surfaces, where significant thermal and particle boundary layers are present, the radiative heat flux is dominated by emission from the relatively low temperature particle layer in the vicinity of the heat receiving surface.