Showing papers in "Aerosol Science and Technology in 1988"

Development of High Efficiency Virtual Impactors

Abstract: The authors briefly review the development of the virtual impactor which, as an inertial particle separator according to aerodynamic sizes, has played a unique role in particle sampling, concentration, classification, and generation. Its performance characteristics in size separation are predictable by theoretical model calculations. However, its behavior in terms of internal wall losses has thus far defied quantitative analysis, and its ultimate control has eluded most practitioners in virtual impactor design. Through experimentation, the authors identify the relevant parameters in a virtual impactor and indicate their relative sensitivity and acceptable ranges of variability. With the detailed illustration of specific high-efficiency virtual impactor design, which has a cutpoint of 2.5 μm and wall losses of under 1%, it is demonstrated the underlying principles cited are crucial to minimizing losses and may be generally applicable to future developments.

Design and Calibration of a Counterflow Virtual Impactor for Sampling of Atmospheric Fog and Cloud Droplets

Abstract: An instrument is described that samples cloud droplets by removing them from the surrounding air and small unactivated particles through inertial impaction. The sampled droplets are then evaporated, leaving behind the material dissolved or suspended in the droplets as residue particles or gases. The instrument is capable of sampling droplets as a function of their size; it has an adjustable cut size in the range between about 9 and 30 μm in diameter, rejects droplets and particles smaller than the cut size, and captures droplets larger than the cut size. Details of the instrumental design and construction are discussed, as well as a relative calibration of the collection efficiency. Results from the calibration experiments indicate that the counterflow virtual impactor probe behaves in accordance with theoretical predictions using Stoke's number calculations. A complete description of the calibration methodology is presented.

Characteristics of the Berner Impactor for Sampling Inorganic Ions

Abstract: The Berner cascade impactor was studied extensively to validate a method for measuring the size distributions of inorganic ionic chemicals in ambient air. Stage efficiencies were calibrated from 0.06- to 10-μm particle diameters; the monodisperse submicron test aerosol was solid ammonium fluorescein coated with oleic acid. Sampled alone, laboratory-generated liquid aerosol suffers blowoff from the last two stages, but this effect is absent with ambient aerosol. Stage cutoffs are sharp and wall losses small. A fluorocarbon grease compatible with acid and ion analysis was shown to prevent particle bounce effectively. Ammonium sulfate particles are sized properly at high relative humidity, ruling out possible growth by condensation during jet expansion. The volatilization loss of submicron ammonium nitrate particles is less than 10% under conditions that produce up to 95% loss from Teflon filters. On the basis of loading experiments, this result is ascribed mainly to the small surface-to-mass ratio of the de...

Unipolar Diffusion Charging of Ultrafine Aerosols

Abstract: Experimental data are reported for unipolar diffusion charging of NaCl and Ag aerosols in the 0.004- to 0.075-μm-diameter range. Monodisperse, uncharged particles were exposed to unipolar positive ions produced by a corona discharge. The nt product was varied between 3 × 106 and 1 × 107 (ions/cm3) (s), where n is the unipolar ion concentration and t is the charging time. The resulting aerosol charged fraction was measured by using a single-particle-counting condensation nucleus counter in conjunction with an electrostatic condenser. From the measured charged fraction and the known charging parameters, the combination coefficient between the neutral particles and the positive ions are obtained. Measurements are then compared with the available charging theories. It is found that the theory of Marlow and Brock best predicts charging rates in the ultrafine particle size range. Above 10 nm, the data approach the theory of Fuchs consistent with the experimental observation of Adachi et al. *Presented at Semina...

Generalized Correlations for Inertial Impaction of Particles on a Circular Cylinder

Abstract: The trajectories of spherical particles impacting a relatively larger circular cylinder in crossflow are calculated using finite difference methods to solve the equations of motion in a Lagrangian form. Drag forces on the particles are described using an empirical correlation for the drag coefficient (as a function of Reynolds number) and the solution for steady, inviscid, incompressible flow (potential flow) around a circular cylinder. Numerical integration of the particle equations of motion is started upstream of the cylindrical target, and calculations are carried out until the particle impacts the cylinder or bypasses it completely. The effects of particle interception are neglected. Results for target efficiency and angle of impingement compare favorably with previous numerical solutions at low-particle Reynolds numbers but are found to be more accurate for free-stream velocity Reynolds numbers (Re0) > 1. In addition, results for particle velocity, angle of impact, and particle concentration are pre...

Computer Modeling of Aerosol Filtration by Fibrous Filters

Abstract: An inline array of parallel circular cylinders, placed transverse to the flow, is used as a model for fibrous filters. The flow field within the array is obtained by solving the full Navier—Stokes equations with the assumption of periodic, fully developed flow. A control volume differencing scheme is used for this purpose. The flow field can be computed for both the viscous and laminar flow regimes. Predictions of pressure drop and particle collection due to interception and diffusion have been obtained using the inline array model, for packing densities varying from 0.029 to 0.136. The collection efficiency for deposition due to interception is directly calculated from the computed flow field. The deposition or particles due to diffusion is studied by numerically solving a separate transport equation for particle concentration, without making any boundary layer approximations. The results have been compared with the data from previous studies, both theoretical and experimental. It is shown that the resul...

A Study of the Effects of Anisokinetic Sampling

Abstract: The results of a numerical investigation of sampling bias through cylindrical probes are presented for both iso-and anisokinetic conditions for thin- and thick-walled probes. Laminar flow fields are calculated using a finite-difference solution of the Navier-Stokes equations; particle trajectories are then calculated with a fourth-order Runge-Kutta method to integrate the particle equation of motion. An ultra-Stokesian drag law and particle interception are included in the calculation. Based on a dimensional analysis of the problem and on the assumptions of the physical model, five dimensionless groups, Re, [Ubar]/U0, D/d, St, and ρ/(ρ p C) are identified that specify the aspiration coefficient, Ai=[Cbar]/Ca . Systematic investigations show that the tube Reynolds number, Re, and the slip-modified density ratio, ρ/(ρ p C), have only a minor influence in determining Ai . Calculations are made to determine the influence of the remaining groups—velocity ratio ([Ubar]/UB), diameter ratio (D/ d), and Stokes num...

Drag Force and Slip Correction of Aggregate Aerosols

Abstract: The drag force on aggregate particles of uniform spheres was measured in a Millikan apparatus as a function of Knudsen number. Our experiment was designed to study the effect of particle orientation on the slip correction factor of nonspherical particles. The velocities of charged particles in a gravitational field with and without an applied electrical field were measured. An electrical field strength of 2000 V/cm was used to align doublet and triplet particles. Results showed that an aggregate particle moved in random orientation while in the gravitational field. The same particle moved with its polar axis parallel to the electric field (doublets) or with its plane of centers parallel to the electrical field (triangular triplets). Using a nonlinear regression method, both the dynamic shape factor and slip correction factor could be determined separately from the data. The dynamic shape factors at different orientations were in good agreement with those obtained previously in a sedimentation tank. The sl...

Suppression of Particle Deposition to Surfaces by the Thermophoretic Force

Abstract: Product contamination by particles is of major concern in the electronics industry. It has become increasingly important because of the decreasing size of the product elements. In a study of the feasibility of using the thermophoretic effect to reduce particle contamination of surfaces, a simple asymptotic formula has been derived for the thickness of the dust-free space in the boundary layer over a flat plate with a flow normal to the surface. This simple formula compares well with the results of a numerical solution. For a temperature difference between the surface and the surrounding gas of 10–30° K and face velocities between 0.3 and 1.0 m/s, the thickness of the dust-free space is between 0.1 and 0.4 mm, much larger than the particle size (0.1 μm ≥ Dp < 0.5 μm). The results indicate that, under conditions representative of clean-room applications, thermophoresis can be effective in protecting surfaces against particle deposition.

A Processor-Controlled Multisample Soot Photometer

Abstract: The technique of the integrating plate method for the measurement of aerosol light absorption has been expanded significantly in a multisample soot photometer. The instrument consists of a double-beam, single detector, digital measurements, and control system. Transmission changes due to particle absorption on the order of 10 −4 can be detected with the new instrument, corresponding to a few nanograms of elemental carbon per sample. The photometer has been calibrated with known amounts of a well-characterized reference soot. The usefulness of the instrument is demonstrated with field results of three areas of application: aerosol light absorption at a Swedish remote area station, soot in cloud water, and light-absorbing particles in an Arctic ice core.

Dynamic Shape Factor of a Plate-Like Particle

Abstract: Talc aerosol particles found in work environments have plate-like shapes. These particles are difficult to characterize by their physical dimensions or their aerodynamic behavior. This paper describes a detailed study of the physical and aerodynamic properties of a commercial-grade, respirable talc aerosol and establishes a relationship between the projected area diameter (D p) and the aerodynamic resistance diameter (D ar) The measured bulk density was 3.04 g/cm3. Electron micrographs showed that the particles were plate-like with a fairly constant thickness-to-diameter ratio of 0.096. The aerosol was aerodynamically separated in an aerosol centrifuge, and was collected flat on the substrate. The dynamic shape factor of the size-classified talc particle was calculated to be 1.88, which is in good agreement with the theory based on an oblate spheroid moving with its short axis parallel to the flow. The aerodynamic resistance diameter (D ar) was linearly related to the projected area diameter (D p) by the ...

Comparison of Different Numerical Schemes for Condensational Growth of Aerosols

Abstract: Three commonly used numerical schemes for condensational growth of aerosols are compared with analytical solutions. The sectional method is interpreted in light of the upwind differencing and the Smolarkiewicz method. Case studies for fast and slow condensation processes are used to demonstrate the performance of various numerical schemes.

Theoretical Analysis of the Performance of the TSI Aerodynamic Particle Sizer The Effect of Density on Response

Abstract: The flow field in the TSI aerodynamic particle sizer (APS) has been modeled numerically and particle trajectories have been calculated. The effects of varying physical parameters such as beam location, beam separation, and nozzle separation were studied. Values of these parameters were determined for a particular APS by comparing the calculated particle transit times with measured ones for polystyrene latex and polyvinyl toluene aerosols. This configuration was used to determine the effect of particle density on APS response and to generate correction factors permitting actual aerodynamic diameter to be determined from indicated aerodynamic diameter when particle density is known. Sensitivity studies show that the calculated correction factors will apply to any TSI APS manufactured and operated within the manufacturer's specifications.

Generation of Large, Solid, Monodisperse Calibration Aerosols

Abstract: During the calibration of four large-particle impactors developed at the University of Florida, techniques were developed for the generation of large, solid calibration aerosols. Using a slightly modified, vibrating-orifice aerosol generator, solid ammonium fluorescein particles of up to 70-μm aerodynamic diameter were successfully generated. When generated under the proper test conditions, the particles were found to be spherical and of uniform size. Although the described generation method requires considerable operator technique, it is felt that the method will be of value in situations where the use of liquid calibration aerosols may be inappropriate.

Agglomeration of chain-like combustion aerosols due to Brownian motion

Abstract: A dynamic Monte Carlo-type lattice model has been developed to simulate the agglomeration of nonspherical chain-like agglomerate combustion aerosols due to Brownian motion. Simulations are carried out in the free molecular and continuum regimes, for both initial mono-disperse and initial lognormally distributed aerosols, with and without source mechanisms, at number densities ranging from 1 × 109 cm−3 to 5 × 10,0 cm−3. Preservation of the chain-like structure of the agglomerate is accomplished throughout the simulation by describing the agglomerate as fractal, that is, scale-invariant, self-similar with a noninteger dimensionality. The clusters' diffusion coefficient is employed as the criterion that governs the probability for cluster movement. Simulation results indicate that cluster growth is more rapid in the free molecular regime than in the continuum, and the rate of agglomeration increases when the structure of the agglomerate is more fragmented (lower fractal dimension). Favorable agreement is obt...

Classification of Individual Particles Based on Computer-Controlled Scanning Electron Microscopy Data

Abstract: Computer-controlled scanning electron microscopy (CCSEM) is known as a powerful tool for measuring individual particle characterization, including various size parameters and the major elemental composition in a short analysis time. To exploit CCSEM as a receptor-modeling technique in a source apportionment study, it is important to define the membership of each particle in a well-defined particle class. Various clustering methods were examined to obtain possible members of homogeneous particle classes. An expert system was then used to build a universal classification rule based on examples of the homogeneous particle classes. These methods were extensively explored and tested using data from a study of El Paso, TX.

Indoor Outdoor Aerosol Measurements for Two Residential Buildings in New Jersey

Abstract: Indoor /outdoor aerosols were studied in two residential homes in New Jersey by making hourly measurements of the concentration and particle size for 2 week intervals in the wintertime. Indoor particle concentrations ranged from 104 to 107 cm−3 and were highly dependent on household activities, including cooking, cigarette smoking, and use of a fireplace and a kerosene space heater. Besides producing large amounts of particles, the kerosene space heater also produced concentrations of carbon dioxide, up to 3000 ppm, and carbon monoxide, up to 9 ppm. The ratio of indoor/outdoor aerosol concentrations varied from 0.2 to 41 (correlation coefficients < 0.3), consistent with the low air infiltration rate measured (0.3–0.5 air changes/h). The results indicate that indoor particles were mainly from indoor origins. In addition, radon progeny particle-size distributions were calculated by applying the attachment theory to the measured aerosol size distribution. This mode was found to range from 0.04 to 0.1 μm, a r...

Monitoring Particulate Emissions from Combustions by Photoemission

Abstract: Aerosol photoemission has been applied to monitor and control combustion in an oil stove. It is shown that the signal of the aerosol photoemission sensor is closely correlated to carbon monoxide and polyaromatic hydrocarbon concentrations. The influence of temperature, soot concentration, and dilution on the aerosol photoemission measurement is discussed. The proper operation of the sensor is demonstrated by its use for combustion control over an extended period of time.

Wind-trajectory method for determining compositions of particles from major air pollution sources

Abstract: Compositions of particles from several types of air pollution sources can be determined by a wind-trajectory method applied to a large data set on particle compositions obtained in St. Louis by other workers, who operated ten dichotomous samplers for 2 years and analyzed all samples by X-ray fluorescence. Samples heavily influenced by emissions from specific sources are identified by searching the data for unusually large concentrations of each element. Mean wind directions during the identified sampling periods often cluster about angles pointing to dominant sources of the elements. Trajectories for a given element obtained at different stations often converge near the dominant source of the element. Compositions of the particles released are determined from linear regressions of concentrations of each element in the samples making up the trajectories versus those of prominent elements from the source. Compositions of particles released by an iron works, an iron-and-steel complex, a pigment plant, a muni...

Source Apportionment of the El Paso Aerosol by Particle Class Balance Analysis

Abstract: A particle class balance analysis has been applied to ambient particulate samples from El Paso, Texas. Profiles of particle class mass fractions from 20 sources were previously developed using advanced pattern recognition methods. These source profiles show very low collinearity and were thus applied to apportioning the mass fractions of the particle class observed in ambient TSP samples. For TSP samples taken near the sources, excellent fits to the data were obtained. Sampling sites farther removed from the sampled sources showed much poorer quality fits. The results suggest that the large number of measured particle classes in the source profiles provides for greater resolution of sources than is generally possible with conventional hulk analysis of particle samples.

Diffusion of Fibers in a Tubular Flow

Abstract: Based on an ellipsoidal particle model, the equivalent diameter for the slip correction, diffusion coefficient, and diffusion diameter of fibers were obtained from the adjusted sphere method of Dahneke. The diffusion coefficient calculated for polydisperse crocidolite fibers compared favorably with available experimental data. Deposition of fibers in a tubular flow was then calculated with the use of the derived diffusion coefficient and applied to the human lung airways. The effect of velocity shear on particle orientation was also considered. It was found that the velocity shear had only a small effect on deposition. For a given fiber size, deposition increased in the lung distally, but at the same fiber diameter, the efficiency decreased with increasing aspect ratio.

Assessment of Aerosol Extinction to Backscatter Ratio Measurements Made at 694.3 nm in Tucson, Arizona

Abstract: The aerosol extinction-to-backscatter ratio, S a, is a key parameter in interpreting scattering measurements made with lidar. Whereas solution techniques for solving the lidar equation generally assume some constraining relation for S a (i.e., such as S a is constant with range), few measurements of S a have been made to establish the statistics and properties of this parameter. Measurements of S a, for a wavelength 694.3 mm, obtained from slant-path lidar observations made in Tucson, Arizona, from May 1979 to June 1982 yielded values of S a between ∼ 5 and 100 with the majority concentrated between ∼ 10 and 45. The weighted mean (weighted by inverse variances of S a) of all S a values and the arithmetic mean of the main grouping of S a values (10 < S a < 45) both equaled approximately 25. Interpretation of the S a measurements in terms of various size distributions and refractive index values representative of atmospheric aerosols indicated the following: 1) S a values in the 10 to 20 range are indicativ...

Target-Transformation Factor Analysis of Airborne Particulate Samples Selected by Wind-Trajectory Analysis

Abstract: In a companion paper, Rheingrover and Gordon showed that samples significantly influenced by single point sources could be identified by examining concentration and meteorological values in the data base for the Regional Air Pollution Study of St. Louis, MO. They examined the relationships between measured elemental concentrations by simple linear regression analysis to deduce compositions of particles emitted by the identified sources. To obtain more complete source profiles, the same sets of sample data were subjected to target-transformation factor analysis. Results of target-transformation factor analysis are compared with those of other approaches. As expected, target-transformation factor analysis extracts concentrations for more elements than do linear regressions of the same data sets. Also, the application of target-transformation factor analysis to groups of samples previously selected by wind-trajectory analysis yields more satisfactory results than the application of target-transformation fact...

Generation of monodisperse aerosols in the 0.1-1.0-μm diameter range using a mobility classification-inertial impaction technique

Abstract: A new technique to generate monodisperse aerosols in the 0.1–1.0-μm diameter range has been developed and tested. This technique involves the use of a differential mobility analyzer to classify “equal mobility” aerosol particles and a micro-orifice impactor to remove undesirable, multiply charged particles of larger size. The system is capable of reducing the multiply charged particles to 6% or less of the primary-size, singly charged particles. Details of the selection of the system components and experimental procedures are provided.

An Empirical Model for the Aspiration Efficiencies of Blunt Aerosol Samplers Orientated at an Angle to the Oncoming Flow

Abstract: In the previous Chapters all work has been conducted in two dimensions, this is an idealised situation and in order to model the problem of blunt body sampling more realistically it is necessary to consider the problem in three dimensions. Also, as samplers do not always face the oncoming flow the situation when they are orientated at an angle to it must also be investigated. Vincent (1987) developed a model which included these features but found that when comparing the results obtained with existing experimental data for the human head orientated with respect to the flow, that agreement was not good, as commented on in Chapter 2. It was shown in Chapter 3 that Vincents expression for s for a spherical sampler orientated at an angle to the flow, is not verified analytically. In modelling the human head Vincent (1987) assumed that it approximated to a sphere and hence in obtaining results assumed a value of s which may have led to the discrepancies. Therefore, in this Chapter an attempt has been made to extend the model developed by Vincent (1987), using the same approach, to include any previous analytical or numerical knowledge of the flow field.

The modified chemical vapor deposition process in a concentric annulus: an extension for focused high-rate deposition

Abstract: A considerable percentage of the increasing production of glass fibers for telecommunication is based on the modified chemical vapor deposition (MCVD) process. The efficiency of this process is strongly hampered by the restricted and incomplete deposition of the expensive basic materials and the large taper effect. The introduction of an annular MCVD process yields not only complete and focused deposition but also an increased production rate. To investigate the conditions for optimal performance, the governing partial differential equations were solved. By identifying the characteristic parameters and their influence on the process, optimal design conditions were found for specific combinations of those parameters.

Generation and Characterization of Respirable Beryllium Oxide Aerosols for Toxicity Studies

Abstract: Methods are described to produce respirable particles of 7Be-labeled beryllium oxide at temperature treatments of up to 1000°C for use in toxicity studies. Solutions of beryllium oxalate and suspensions of beryllium hydroxide were nebulized to produce polydisperse particles with activity median aerodynamic diameters in the range of 0.5–0.7 μm and geometric standard deviations between 2.7 and 3.0. Conversion to beryllium oxide was incomplete when the nebulized particles were passed through a tube furnace at temperatures up to 1000° C. Complete conversion was achieved by calcining samples of the collected particles in a muffle furnace for 16 hours at 500 or 1000°C. Particles produced by nebulizing and heat-treating the beryllium chloride or oxalate solutions were hollow shells, but particles nebulized from beryllium hydroxide suspensions were aggregates morphologically similar to industrially produced beryllium oxide particles.

Asymptotic behavior of aerosol growth in the free molecule regime

Abstract: Although the asymptotic behavior of aerosol growth by condensation/evaporation and by coalescence has been well established independently, no complete theory is available for such simultaneous processes. In this paper, through the use of a supercomputer, it is shown that asymptotic limit-size distributions exist for mass conserved systems when condensation/evaporation and coalescence occur simultaneously in the free molecule regime. Unlike the independent growth process either by condensation/evaporation or by coalescence, “crossover” or change from one dominating growth process to another growth process takes place continuously. It is also demonstrated that substantial savings in central processing unit time on a supercomputer can be achieved by restructuring the numerical algorithm and by vectorizing the inner DO loops.

Calibration of the Aerodynamic Particle Sizer 3310 (APS-3310) with Polystyrene Latex Monodisperse Spheres and Oleic Acid Monodisperse Particles

Abstract: We have calibrated the new, extended-range aerodynamic particle sizer (model APS-3310, TSI, Inc.) with polystyrene latex monodisperse spheres and compared the response to that of oleic acid monodisperse particles from 2.5 to 38.7 μm in diameter. The results compare well with previous findings and cover a larger size range permitted by the new instrument.

Aerosol formation during water ingress into the core of a pebble bed high-temperature reactor

Abstract: In order to give an estimation of the safety of nuclear power plants, it is necessary to characterize the aerosol that develops in the event of a serious accident. Aerosol formation in a graphite-moderated high-temperature pebble bed reactor is possible when water or steam intrudes into the helium primary circuit. Because of the great temperature gradient between water droplets and the pebble surface, the evaporation of water is determined by the thin steam layer between the droplet and hot surface. The thin steam layer reduces the heat flux from the pebble to the water droplet. This effect is the well-known Leidenfrost phenomenon. The steam discharges from the steam layer at a high velocity, and is able to dislodge particles from the pebbles. The chemical reaction of the hot pebble graphite with steam results in a gasification by producing a gas mixture of H2O, He, CO, C02, and CH4. In order to investigate the process of aerosol formation a test device was set up containing 1500 spherical graphite elemen...