A Numerical Study of the Performance of an Aerosol Sampler with a Curved,Blunt, Multi-Orificed Inlet

The purpose of this study was to numerically simulate the performance of an aerosol sampler with a curved, blunt, multi-orificed inlet in order to understand the sampling characteristics of the first prototype of the button personal inhalable aerosol sampler ("button sampler"). Because the button sampler inlet design is too complicated to apply a three-dimensional model, an axisymmetric two-dimensional model was created to be similar in geometry and to simulate the major features of the airflow through the sampler when facing the wind. Particle trajectories were calculated in a variety of wind velocities and were categorized into 5 groups based on their interactions with the curved surface of the sampling plane. Empirical sampling efficiencies of the button sampler for 3 particle sizes were used to adjust the calculated sampling efficiencies in an attempt to improve the accuracy of the two-dimensional axisymmetric model in accounting for interactions between particles and the surface of the inlet of the button sampler. Sampling efficiencies for other particle sizes were then predicted. The results showed that sampling efficiency decreased with increasing particle size up to approximately 40 w m and then remained virtually unchanged at about 35% up to 100 w m. Although the efficiencies were lower than the American Conference of Governmental Industrial Hygienists' (ACGIH) inhalability curve for larger particles, the pattern of the predicted sampling efficiency was quite similar to the ACGIH inhalability curve. Sampling efficiencies for liquid aerosol particles larger than 15 w m were predicted to be noticeably lower than those for solid particles. The results also showed that the multi-orificed curved surface played an important role in establishing a pressure drop with desired flow alignment inside the sampler, thus greatly reducing the wind effect and significantly improving the uniformity of particle deposition on the filter. The less uniform deposition found at high wind velocity can be improved by increasing the sampling flow rate.     

Evaluation of a High Volume Aerosol Concentrator

A virtual impactor sampler, which is designed to concentrate aerosols from a 1000 L/min ambient air sample into a 1 L/min exhaust airflow stream, was tested with near monodisperse aerosols in aerosol wind tunnels to characterize sampling performance. New methodology is introduced to correct results for the presence of doublet and satellite aerosol particles that can be present in the particle size distribution from a vibrating jet atomizer. Aerosol penetration from the free stream near the sampler inlet to the outlet of the device has a peak value of 78% at a particle size of 3.9 w m AD. Sampling effectiveness, which is the mean penetration over the size range of 2.5 to 10 w m AD, is 48%. There are 4 virtual impaction stages in the sampler, and examination of the regional losses shows that most of the aerosol deposition occurs on surfaces of the last 2 stages. The ideal power expenditure of the sampler (excluding electrical and frictional losses in the motor and bearing losses in the blower) is 58 watts as compared to the actual power consumption of 320 watts.     

Ultrafine Particle Sampling with the UNC Passive Aerosol Sampler

A model is presented to describe the collection of ultrafine particles by the UNC passive aerosol sampler. In this model, particle deposition velocity is calculated as a function of particle size, shape and other properties, as well as a function of sampler geometry. To validate the model, deposition velocities were measured for ultrafine particles between 15 and 90 nm in diameter. Passive aerosol samplers were placed in a 1 m ³ test chamber and exposed to an ultrafine aerosol of ammonium fluorescein. SEM images of particles collected by the samplers were taken at 125 kX magnification. Experimental values of deposition velocity were then determined using data from these images and from concurrent measurements of particle concentration and size distribution taken with an SMPS. Deposition velocities from the model and from the experiments were compared and found to agree well. These results suggest that the deposition velocity model presented here can be used to extend the use of the UNC passive aerosol sampler into the ultrafine particle size region.     

Development of Variable Flow Rate Isokinetic Sampling System for 0.5–15-μm Aerodynamic Diameter Particles

Isokinetic sampling is required when evaluating the aerodynamic sizes of particles released from dry powder inhalers (DPI) under simulated breathing condition since anisokinetic sampling may lead to significant sampling error for coarse particles. We propose an isokinetic measuring system for aerosol particles from a stream in a narrow conduit of variable flow rates (variable flow rate aerosol sampler, VFAS) combined with Aerodynamic Particle sizer® APS^TM spectrometer (model 3321, TSI Inc.). The VFAS was capable of generating variable sampling flow rates by adjusting the flow resistance of makeup air to produce constant flow rate of aerosol to the APS. The penetrations through the VFAS-APS system were measured using monodisperse particles with a size range of 0.7–15 μm by applying a rectangular flow rate–time pattern of sampling air, and we found that the VFAS-APS system can measure the number concentration of particles with the particle detection efficiency (particle penetration through the system) of almost unity. The VFAS-APS system may be a powerful tool to measure the size and concentration of powder released by the DPI in the size range of 0.5–15 μm.

Copyright 2012 American Association for Aerosol Research     

Inlet characteristics of bioaerosol samplers

The inlet sampling characteristics of several commercial bioaerosol samplers operating in indoor and outdoor environments have been analyzed by use of available and newly developed equations for sampling efficiency. With a focus on the physical aspects of sampling efficiency, the aspiration and transmission efficiencies have been calculated for the bioaerosol particle size range 1–30 μm, which represents single bacteria, bacteria aggregates, bacteria carrying particles, fungal spores, yeast, and pollen. Under certain sampling conditions, the bioaerosol concentration was found to be significantly over- or underestimated. At wind velocities between 0 and 500 cm s⁻¹, calculations show that the AGI-30 would sample 1–10 μm particles with an inlet sampling efficiency of 20–100%. The entrance efficiency of the 6-stage Andersen viable sampler is 90–150% when sampling isoaxially with respect to horizontal aerosol flows, and 8–100% when oriented vertically at a right angle to the horizontal aerosol flow. For the Burkard portable air sampler, an even wider range of deviation may occur. The bioaerosol samplers used for large particles such as pollen are even less accurate: e.g. 10 times the ambient concentration of Lycopodium spores has been calculated to be aspirated by the Lanzoni sampler when operated at 0.5 1 min⁻¹ facing the wind at wind velocity of about 500 cm s⁻¹.

The actual bioaerosol concentration can be calculated from the measured data by use of the indicated procedures. The sampling efficiency graphs presented can be used to bracket the sampling conditions that enable the investigator to avoid or minimize significant sampling biases for each sampler. The findings can also be used for the design of new samplers or for improving commercially available samplers.     

Overall Performance Evaluation of Aerosol Number Samplers

At present, there is neither an officially accepted size-selective fiber (aerosol number) sampler, nor are there established performance criteria. In this work, a prototype preclassifier (multihole impactor) was used to connect a conventional asbestos sampler so that the aerosol penetration test and particle counting process could be performed. The bias, as a function of particle size, was defined as the difference between the measured penetration curve and the target ISO/ACGIH/CEN respirable convention. The imprecision was the standard error with reference to the mean aerosol penetration curve. A statistical term, one standard error shift (OSES) was used in a previous study to combine the sampling bias and imprecision. The bias and imprecision could be for aerosol number, aerosol mass, or even surface area. In this work, an additional step was taken by introducing another statistical term, maximum sampling shift (MSS), to further combine the OSES with the counting imprecision. For the surrogate sampler tested, the particle counting imprecision increased with increasing particle diameter and decreased with increasing geometric standard deviation. The particle counting imprecision was comparable with the OSES, and the resultant MSS map was actually the summation of imprecision and OSES.     

An Aerosol Wind Tunnel for Evaluation of Massive-Flow Air Samplers and Calibration of Snow White Sampler

Massive-flow air samplers are being deployed around the world to collect aerosol samples for analysis of radioactivity as a result of nuclear tests and nuclear accidents. An aerosol wind tunnel capable of an 1100 m³ min^?1 flow rate was built at Lovelace Respiratory Research Institute (LRRI) to test the sampling efficiency of these samplers. This aerosol wind tunnel uses a stationary air blender to enhance mixing, and therefore it achieves the required uniform distribution of wind speed and aerosol concentration in the test section. The test section of the wind tunnel has a cross section that is 4.3 m × 3.7 m. The aerosol wind tunnel was tested for performance in terms of distribution of wind speed, turbulent intensity, SF₆ tracer gas concentration, and aerosol concentration. Test criteria consistent with U.S. Environmental Protection Agency (EPA) and American National Standards Institute (ANSI) standards were adopted as the guidelines for the aerosol wind tunnel. Additional criteria for aerosol wind tunnel were also recommended. Initial test of the aerosol wind tunnel showed that the wind tunnel could be operated in a wind speed range of 2 to 24 km h^?1. Within this range, the distribution of wind speed SF⁶ trace gas concentration and aerosol concentration in two-thirds of the central area of the test section showed coefficient of variances (COVs) of less than 10% for the range of wind speeds. This met the stringent guidelines for aerosol wind tunnel performance set by EPA and ANSI standards.

The LRRI wind tunnel was used to evaluate the collection efficiency of the sampling head of massive-volume air samplers, including the Snow White sampler. The sampler was tested in this aerosol wind tunnel for particles between 2 and 20 μm. The sampling flow rates were 500 and 700 m³ h^?1 for the tested wind speeds of 2.2 and 6.6 m S^?1, respectively. The results showed that sampling efficiency was influenced by both sampling flow rate and wind speed. The sampling efficiency decreased with an increase in particle size of between 2 and 20 μm. The sampling efficiency also decreased as the wind speed was increased from 2.2 to 6.6 m S^?1.     

Improved Aerosol Collection by Combined Impaction and Centrifugal Motion

ABSTRACT

A new principle for collecting airborne particles, including microorganisms, has been introduced by injecting the particles into a swirling airflow from where they are removed onto a collection surface. A dry surface, a surface coated with an adhesive substance or a surface wetted by a liquid swirled onto the collection surface from a reservoir below can be used in the new collection method. The swirling air motion and aerosol injection into it are achieved by drawing the airborne particles through nozzles that are directed at an angle toward the collection surface. This principle has been incorporated into a new sampler that has been named “Swirling Aerosol Collector” (SAC; commercially available as the “BioSampler” from SKC Inc., Eighty Four, PA). The physical performance of the SAC has been evaluated against the widely used AGI-30 impinger by measuring the particle concentrations upstream and downstream of each sampler with an aerodynamic particle sizer. Tests with monodisperse polystyrene latex (PSL) particles ranging from 0.3 to 2.0 μm have shown that the SAC has better collection efficiency than the AGI-30 when the same collection liquid is used. A conventional impinger maintains constant collection efficiency for a relatively short sampling period, as the liquid evaporates quickly due to the violent bubbling of the liquid. In contrast to conventional impingers, the SAC can be used with nonevaporating liquids that are considerably more viscous than the liquids used in the impingers. Thus, the SAC can sample over any period of time. The new aerosol sampler produces minimal or no reaerosolization of particles collected in the liquid in contrast to significant reaerosolization in a conventional impinger. Since the SAC projects the aerosol particles toward the collection surface where they are removed from the swirling flow, it avoids or significantly reduces particle bounce from the collection surface even when the surface is dry.     

A theoretical study of aerosol sampling by an idealized spherical sampler in calm air

The performance of an idealized spherical sampler operating in calm air for an inlet arbitrarily oriented relative to the gravity force is studied theoretically. Under potential flow assumption the air velocity field is obtained by using a model of a finite-size sink on a sphere. The particle motion equations are solved to find the limiting trajectory surface and to calculate the aspiration efficiency. The singular points of the motion equations as a function of settling velocity of particles and the sampler orientation angle are investigated. The connection between the pattern of typical zones of particle trajectories around the sampler and the location of the singular points is illustrated. The effects of partial sampling from zones without particles and of particle screening are discussed. The results of parametrical investigations of the dependence of the aspiration efficiency on the Stokes number and their analysis are presented. In the case of vertically upwards orientation of the sampler the proposed mathematical model gives fair agreement with experimental data from the work by Su and Vincent (Abstracts of sixth international aerosol conference, Taipei, Taiwan, 2002a, pp. 639–640).     

Sampling Errors in Cylindrical Nozzles

This paper reviews publications on aerosol aspiration by axisymmetric tubes, a widely used form of practical sampler. Axisymmetric tubes are widely used, as a rule, in stack sampling and sometimes in other areas of aerosol sampling as well (e.g., workplaces, ambient atmosphere). Numerous reports on aspiration coefficients for particles sampled from disperse flows contain two contradictory viewpoints on the sampling efficiency at suction velocities exceeding that of wind: although some authors claim that the sample representativeness worsens, others maintain that it is improved. Aerosol aspiration from calm or weakly turbulent air has not been investigated fully, despite the fact that the problem of determining sampling errors under such conditions is important in relation to occupational hygiene and environmental monitoring. Along with the analysis of the results published by other investigators (Davies et al., Vincent et al., etc.), this paper contains the axisymmetric sampler aspiration data obtained by us during the last 5-year period.

Experimental evidence is given for the secondary aspiration of particles after their bounce or blow-off, not only from the front face of the sampling tube but also from its external side surface. This effect is responsible for the qualitative discrepancy between the aspiration coefficient values obtained by different methods. The sampling conditions, for which aspiration distortions can be compensated for by using the inertial aspiration coefficient calculated from conventional theory, have been determined for axisymmetric samplers. The aspiration coefficient dependences on the anisokinetic coefficient, Stokes number, sampler wall thickness, and yaw angle have been analyzed for the aerosol sampling from steady-state flows. Possibilities of using these dependences to estimate errors in sampling aerosols from flows with the wind vector fluctuating in direction and magnitude are discussed. The poorly predictable secondary aspiration and flow turbulence effects observed with thick-walled samplers are shown to invariably influence the aspiration coefficient, making correction for sampling errors extremely difficult.

The inertial aspiration coefficient values measured for low-velocity wind and calm air have been analyzed. These results point to the not-so-obvious dependence of this coefficient on the sampling conditions. Experimental data are included, which make it possible to determine aspiration distortions at the orifices of samplers used with commercial aerosol analyzers.     

Development of a Cyclone-Based Aerosol Sampler with Recirculating Liquid Film: Theory and Experiment

This article describes the theoretical considerations, design criteria, and experimental performance of a cyclone-based, liquid-film, bioaerosol sampler. Different from conventional cyclones, this novel sampler draws air tangentially into the bottom of a swirling cyclone, creating a negative pressure differential which causes continuous suction of sorption liquid from its reservoir into the cyclone. The liquid swirls with the air vortex and rises spirally along the sampler wall in the form of a thin film. In the presence of an excess pressure differential, the liquid goes over the upper edge of the cyclone (overflow mode) and flows back to the bottom of the sampler. As a result, there is a continuous circulation of the sorption liquid in the sampler, which enhances the efficacy of capturing viable aerosol particles from incoming air. In this study, mathematical models using simplified Navier-Stokes equations are developed to describe the behavior of the airflow, the formation of the liquid film, and the precipitation process of the aerosol particles. Numerical solutions are presented as an approximation to these complex air and liquid flow streams in the whirlwind cyclone. Based on the theoretical assessment, practical design criteria for a novel sampler were formulated and a series of prototype samplers were fabricated and evaluated. In this report, experimental findings concerning the thickness of the air vortex, the pressure profile in the cyclone, and the apex height of the liquid film are presented. The results are in good agreement with theoretical prediction. However, the theory seems to overestimate the capturing efficiency for particles around the cutoff size (in the study, 1–2 μm) when comparing with data obtained from the experiments.     

A numerical study of the aspiration efficiency of a thin-walled sampler facing the wind for high velocity ratios

In this work the fluid flow and particle trajectories for a thin-walled sampling probe facing the wind are considered. The flow around the sampler and into the sampler has been determined numerically and the paths of the particles in the flow are then traced and the efficiency of the sampler investigated.A variety of operating conditions have been considered, in particular large values of the velocity ratio, R, which is equal to the ratio of the undisturbed free stream air velocity to the average sampling velocity. The situation of large R values is becoming increasingly important as samplers are developed with low flow rates. Previous experimental results have shown that the empirical model developed for sampling in moving air does not accurately predict the efficiency of samplers operating at these high values of R. The numerical results show that the aspiration efficiency for high R is not significantly affected by gravitational effects for the majority of cases of interest but it is dependent upon the magnitude of the reversal of the flow within the sampling tube.     

Development of an Automatic Beta Gauge Particulate Sampler with Filter Cassette Mechanism

A beta gauge particulate sampler for measuring the aerosol mass concentration in the ambient air is described. The instrument is automatically calibrated with two self-calibration mass standards during each sampling period, while it samples particles continuously with minimum sampling dead-time loss. Key design features of the instrument based on the attenuation of beta radiation include filter cassette mechanism, auto-calibration system, low sampling dead-time, high sensitivity, and straightforward audit procedures. The instrument consists of three main components: PM 10 inlet, mechanical filter movement system, and control and data processing system. The mechanical filter movement system includes particle collection system with filter cassette magazine, g -ray measuring module and particle sampling module, auto-calibration system, and flow control system. The control and data processing system performs filter cassette movement control, sampling pump control, and data analysis. The instrument has been tested in the field to compare the measurement results with those by gravimetric mass measurement. The developed beta gauge instrument has been proved to be an efficient measuring guage for the ambient particulate mass determination.     

Experimental measurements of aspiration efficiency for idealized spherical aerosol samplers in calm air

In a recent previous paper (Su & Vincent, J. Aerosol Sci. 33 (2002) 103) we described a new method by which to investigate the relationships between aspiration efficiency, particle inertia, gravitational effect and sampling orientation for aerosol sampling in perfectly calm air. All previous experimental work to elucidate the basic nature of aerosol sampling in calm air described has been carried out for thin-walled tubes, and none has yet been reported in relation to blunt samplers. To begin to fill this important gap, the present paper describes the application of our new method towards acquiring new measurements of aspiration efficiency for simple blunt samplers.Experiments were carried out to determine the aspiration efficiencies of simple, idealized, spherical blunt samplers for a range of sampling scenarios, for two sizes of blunt samplers and different sampling inlet diameters, and for upwards and downwards sampling scenarios (and hence a range of governing dimensionless physical quantities). It was shown that aspiration efficiency decreased both with increasing inertia (as represented by the Stokes’ number) and with increasing gravitational effect (as represented by the ratio of particle settling velocity to the air velocity at the sampler inlet). The results enabled qualitative physical explanation of the difference between what was observed for upwards and downwards-facing sampling, respectively, in terms of (a) the role of the sampler body in deflecting the air flow in the region close to the body of the sampler (in turn influencing the performance of the sampler), and (b) the interception of particles in the downwards-facing scenario falling within the ‘shadow’ projected upwards by the sampler body.The significant contribution of this work has been the acquisition of a definitive set of new experimental data. These data will be valuable in the future development of understanding of the physics underlying aerosol sampler performance. Such knowledge will be of practical value because (a) blunt samplers are generally the most representative of the types of instruments used in practical occupational, and (b) calm or slowly moving air is characteristic of many indoor situations.     

Aerosol Penetration through Silica Gel Tubes

Silica gel is commonly used by industrial hygienists to collect gases and vapors in the work place, in particular air contaminants with high polarity. The collected air pollutants are then treated and analyzed to identify their type and to determine the concentration using various methods and instrumentations. In addition to collection of gaseous pollutants, the silica gel tubes are also used for acid mist collection according to the listed official analytical methods (e.g., NIOSH method 7903 and OSHA method ID-165SG). However, the filtration characteristics of silica gel tubes have not been thoroughly investigated. A constant output aerosol generator and an ultrasonic atomizing nozzle were used to generate submicrometer-sized and micrometer-sized aerosol particles, respectively. A scanning mobility particle sizer and an aerodynamic particle sizer were used to measure particles smaller and larger than 0.6 w m, respectively. Potassium sodium tartrate and dioctylphthalate were used as the solid and liquid test agents, respectively. Two types of SKC silica gel tubes (Cat No. 226-10 and 226-10-03) were examined for aerosol penetration, air resistance, and loading characteristics. The results show that the aerosol penetration through the silica gel tubes could be as high as 80% at the penetration maximum (or collection minimum) under the normal sampling flow of 0.5 L/min, well within the inertial impaction dominated region. Two glass wool plugs and one urethane plug between sorbent sections and at the back end of the SKC 226-10 contributed about 22% of the total air resistance, and the remaining 78% of the air resistance was caused by the silica gel. When the filtration efficiency by these separators was deduced, the aerosol penetration at the most penetrating size was as high as 90%. The aerosol penetration increased and the penetration curve shifted to a smaller particle size as the sampling flow increased. However, this increase in aerosol penetration of particles smaller than the penetration maximum reached a maximum and then decreased as the sampling flow was increased beyond 1.5 L/min (equivalent filtration velocity of 93 cm/s), a clear evidence of inertial impaction surpassing the diffusion deposition. As a result, the use of silica gel tubes for acid mist collection may not be appropriate if the behavior of the complete aerosol size distribution is not considered as part of the assessment of these devices.     

Hygroscopic Properties of Pasadena,California Aerosol

The hygroscopic behavior of Pasadena, CA aerosol was continuously measured from August 15 to September 15, 1999 using a tandem differential mobility analyzer. Two dry particle sizes were sampled, 50 nm and 150 nm in diameter; humidification of the dry aerosol was carried out at 89% relative humidity. Complex growth patterns were observed for both size modes, with aerosol distributions splitting from a single mode at times to more than 6 modes. Diurnal profiles for the observed multiple peaks were noted, with the greatest number of measurable growth modes being found during the late night and predawn hours for 50 nm particles. For 150 nm particles, more modes were present during the afternoon hours, with the humidified aerosol becoming bimodal during the late night/early morning hours. Growth factors, defined as the ratio of humidified particle diameter (at 89%) to dry diameter, were determined for modes with significant number concentrations. Average growth factors over the sampling period for the 2 particle sizes ranged from 1.0 to 1.6. Hygroscopic growth increased in the latter half of the sampling period when forest fires were present. In short, treating this complex urban aerosol as a combination of "less" and "more" hygroscopic fractions is an oversimplification.     

The Effects of Bluntness and Orientation on Two-Dimensional Samplers in Calm Air

This article uses numerical simulation to investigate the effect of sampler bluntness, particle size, and sampler orientation on aspiration efficiency in calm air. The procedure is to first numerically solve the velocity field around the sampler in calm air and then to trace the particle trajectories and calculate the the aspiration efficiency. Two samplers are studied: a two-dimensional parallel plate and a two-dimensional blunt cylinder. The variation of aspiration efficiency with particle size shows two minima between two asymptotic values. When the samplers are facing upward, the asymptotic values are 1 for very small particles and the ratio of particle settling velocity to suction velocity for very large particles. At other orientations, the horizontal-facing and the downward-facing, the asymptotic value for large particles is 0. The sampler bluntness has an important effect in the region of particle size where there is competition between the particle inertia and the fluid drag force (i.e., 5 μm < d > 100 μm in our case). A blunt sampler always has higher aspiration efficiency than does a sharp-edged sampler in this region of particle sizes. For very small particles and very large particles, the aspiration efficiencies approach asymptotic values and the sampler bluntness has little effect. The results also show that the sampler orientation affects the predicted aspiration efficiency     

Numerical Simulation of Scavenging of Small Particles by Charged Droplets

The trajectories of fine aerosol particles in the vicinity of a free falling collector droplet and their deposition on it were investigated numerically by solving the equations of motion of the particle and the droplet in quiescent air. The droplet was assumed to be charged to one half of the Rayleigh limit. The Coulomb, image, Stokes, inertial, and gravitational forces acting upon the particle near the droplet were taken into consideration in the equations of motion. The equations of the droplet motion were also incorporated into the set of equations including the Coulomb and image forces on the droplet due to the particle charge. The flow field in the vicinity of the droplet was determined by numerical solution of the Navier-Stokes equations. The equations of particle motion were solved in threedimensional (3-D) space by the Runge-Kutta method of the fourth order. The collection efficiency of the particles on the droplet was determined by searching the limiting trajectory within the entire space. The results for particles charged to 10 elementary charges of the same and opposite polarity as the droplet, as well as the electrically neutral ones, were compared. The assumption on the charge of the particle was rather arbitrary. It was assumed that particles are not intentionally charged but only possess a charge generated by tribocharging due to random contacts and were independent of the particle size. Charging the collector causes the Coulomb forces between these 2 species to improve particle deposition on the droplet and in this way the aerosol is removed from the gas. For the aerosol particles charged to the same polarity as the collector, the collection efficiency is still higher than for uncharged particles due to the action of the image forces. In this case, the collection efficiency increases for smaller droplets and for particles with increasing diameter.     

An Ion Generator for Neutralizing Concentrated Aerosols

An ion generator was developed to neutralize concentrated streams of large, highly charged particles in a low-velocity wind tunnel. The aerosol stream tested consisted of 30 mu m aluminum oxide particles (aerodynamic diameter 52 mu m) at a flow rate of 9.6 m³/h (160 L min) and a mass concentration of 43 g/m³. The average number of excess charges per particle was 240,000 (positive), which corresponds to a neutralizing current requirement of 0.11 mu A. Neutralization to < +/- 10,000 charges per particle was necessary to prevent electrostatic sampling artifacts. Neutralization with radioactive sources would have required an impractically large source. The ion generator, constructed from 21 and 32 mm PVC pipe, has 4 peripheral radial electrodes of 0.5 mm tungsten wire and a 2.0 mm diameter central electrode. The aerosol flowed through the ion generator along its axis. The ion generator was powered by an adjustable (0-8.5 kV) power supply. Performance of the ion generator was monitored with an isokinetic Faraday-cup sampler connected to a Keithley Model 6512 electrometer capable of 0.1 fA resolution. The sampler used a stainless steel 47 mm filter holder as the Faraday cup. The cup was insulated with Teflon inside a 90 mm diameter stainless steel enclosure with a 21 mm diameter inlet. This setup gave near real-time measure ment of the charge state of the aerosol in the wind tunnel. By adjusting the ion generator power supply, particle charge could be reduced to < 2% of its original charge. Ion generator output was sufficiently stable to maintain the particle charge within +/- 2% of the original charge over a 1 h period. These reduced charge levels are comparable to charge levels found on workplace aerosols.     

Large Particle Size Distribution in Five U.S. Cities and the Effect on a New Ambient Participate Matter Standard (PM10)

A mobile aerosol-sampling system was used to determine the large particle ambient aerosol size distribution (up to approximately 100 μm particle diameter) in five cities across the United States: Birmingham, Alabama; Research Triangle Park, North Carolina; Philadelphia, Pennsylvania; Phoenix, Arizona; and Riverside, California. A mobile wide range aerosol classifier (WRAC) developed at the University of Florida was used. The study shows that any measurement of ambient particulate matter with a size-fractionating inlet sampler will be influenced by the ambient particle size distribution.

Mass distribution measurements determined by the WRAC were compared with mass measurements obtained simultaneously using TSP Hi-Vol and 15 μm cut-size inhalable particulate network samplers. Aerosol size-classification results showed the presence of a large particle mass mode at all sites sampled. The position and magnitude of the large particle mode varied and was not a simple function of concentration. The percentage of the total aerosol mass collected by the present EPA reference method high-volume air sampler varied from about 85 to 95%. The percentage of total aerosol mass less than 10 μm varied from about 50 to 90%, depending on the sampling location and sampling condition.