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.     

A New Aircraft Inlet for Sampling Interstitial Aerosol: Design Methodology,Modeling, and Wind Tunnel Tests

The design of a new aerosol sampler, called the blunt-body aerosol sampler (BASE), to sample interstitial particles inside clouds while avoiding the problem of cloud droplet shatter artifacts is introduced. The primary design feature of the inlet is a blunt body that houses an aerosol inlet toward its aft end. The housing is designed to be blunt enough to deflect large cloud particles traveling around the body while being streamlined enough to maintain an attached boundary layer under aircraft flow conditions. The attached flow requirement ensures that shatter particles formed from the impaction of cloud droplets on the blunt body are retained close to the surface of the body. A region of large particle shadow is, thus, created in the aft of the blunt-body housing, where an aerosol inlet can sample interstitial particles in the absence of cloud particles. Computational fluid dynamics (CFD) simulations are used to optimize the shape of the blunt body, and the final sampler design is predicted to sample particles smaller than 2 μm from the freestream while being uninfluenced by cloud droplet shatter particles of the same size. Wind tunnel tests were performed on a prototype model to confirm the attached nature of the boundary layer flow around the blunt body and to establish the size-dependent behavior of shatter particles in the vicinity of the housing. While the experiments provide initial validation of the interstitial inlet design concept, some discrepancies were observed between the wind tunnel tests and CFD predictions, suggesting a need for improvements in simulations, inlet design, and/or test methodology. Initial analyses of field data obtained from the first aircraft deployment of BASE confirm that sampling of shatter-free interstitial aerosol is possible with the inlet, but full performance characterization of BASE will require significant additional aircraft-based experiments under a range of cloud conditions.

Copyright 2013 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.     

Large Particle Penetration During PM10 Sampling

The objective of the present study was to characterize the performance of a federal reference method (FRM) PM₁₀ size-selective inlet using analysis methods designed to minimize uncertainty in measured sampling efficiencies for large particles such as those most often emitted from agricultural operations. The performance of an FRM PM₁₀ inlet was characterized in a wind tunnel at a wind speed of 8 km/h. Data were also collected for 20 and 25 μm particles at wind speeds of 2 and 24 km/h. Results of the present sampler evaluation compared well with those of previous studies for a similar inlet near the cutpoint, and the sampler passed the criteria required for certification as a FRM sampler when tested at 8 km/h. Sampling effectiveness values for particles with nominal diameters of 20 and 25 μm exceeded 3% for 8 and 24 km/h wind speeds in the present study and were statistically higher than both the “ideal” PM₁₀ sampler (as defined in 40 CFR 53) and the ISO (1995) standard definition of thoracic particles (p < 0.05) for 25 μm particles leading to the potential for significant sampling bias relative to the “ideal” PM₁₀ sampler when measuring large aerosols.

Copyright 2014 American Association for Aerosol Research     

A Computational Fluid Dynamics Study of Particle Penetration through an Omni-Directional Aerosol Inlet

Computational fluid dynamics (CFD) was used to study aerosol penetration through the entrance section of a bell-shaped omni-directional ambient aerosol sampling inlet. The entrance section did not include either an insect screen or a large-particle pre-separator. Simulations of the flow field were carried out for wind speeds of 2, 8, and 24 km/h and a fixed exhaust flow rate of 100 L/min; and, particle tracking was performed for 2 to 20 μ m aerodynamic diameter particles. Penetration calculated from CFD simulations was in excellent agreement with experimental results from previous studies with the root mean square relative error between simulation and experimental data being 3.8%. CFD results showed that the most significant regional particle deposition occurred on the upwind side of a curved flow passage between two concentric axisymmetric shells of the inlet housing and that deposition at the leading edges of the shells and within the exhaust tube was far less significant. At a wind speed of 2 km/h, penetration was affected by gravitational settling, e.g., penetration of 20 μ m particles was 71.9% when gravity was included and 80.4% without gravity. At higher wind speeds gravity had little effect. An empirical equation was developed to relate aerosol penetration to a Stokes number, a gravitational settling parameter, and a velocity ratio. Good fits of the correlation curves to experimental data and numerical results were obtained.     

NUMERICAL PREDICTION OF THE PERFORMANCE OF A MANIFOLD SAMPLER WITH A CIRCULAR SLIT INLET IN TURBULENT FLOW

The performance of a bioaerosol manifold sampler with a circular slit inlet in a turbulent flow field was modeled using a 3-D numerical approach. The standard κ–ε turbulence model was used for simulating the mean turbulent flow, and the Lagrangian approach was used for predicting the particle trajectories. The ratios of wind velocities to sampler inlet velocities were from 0.5 to 3.5. Calculations were conducted for particle sizes of 2, 8, 15,and26 μm. The agreement between numerical and empirical sampling efficiencies was good. It was found that lower sampling efficiencies at high R values were associated with increased positive pitch of the velocity vectors generated at the inlet slit. Unbalanced sampling velocities between the upstream and downstream arcs were found only at high R values. At an inlet velocity of 0.8 m/s, sampling efficiencies for 15 μm particles decreased about 24% as R was increased from 0.5 to 3.5. A similar effect was observed at an inlet velocity of 0.4 m/s. Turbulence decreased sampling efficiency and was related to the sum of the magnitudes of the wind and sampling velocity vectors.     

Aspiration Efficiency of IOM-Like Personal Aerosol Samplers from Experiments with a New Rapid Data Acquisition System

New automated instrumentation for the rapid acquisition of aerosol sampler aspiration efficiency data has been applied to an investigation of a range of personal aerosol samplers of the type developed during the 1980s at the Institute of Occupational Medicine (IOM) in Edinburgh, Scotland, U.K. The experimental research was carried out in a small wind tunnel, and the relation of the results for IOM-like samplers to full-scale life-size personal aerosol sampling scenarios—like those encountered in occupational aerosol exposure assessment—was investigated by reference to the scaling laws that have been developed based on familiar aerosol mechanics as they apply to the physics of aerosol sampling. In the small-scale experimental study, the IOM-like sampler was mounted centrally on a rectangular bluff body, simulating the wearing of the sampler on the body (e.g., as by a worker in an industrial setting). Scaling with respect to the corresponding, more-realistic full-scale system for a corresponding full-scale windspeed of 1.0 m/s was achieved by varying the inlet diameter, the windspeed and the sampling flowrate. The results for windspeeds in the scaled experiments of 1.5 m/s and lower were found to differ significantly from those for windspeeds of 2.0 m/s and higher. In particular, the measured aspiration efficiency values for the lower windspeeds were markedly higher than—and clearly not consistent with—the higher windspeed group of results. It is considered likely that such divergence may be associated with a characteristic of the small wind tunnel in which the experiments were conducted. However, the scaling laws developed were found to work well for windspeeds in the scaled experiments of 1.5 m/s and higher. The results confirm that the performance of the IOM personal inhalable aerosol sampler is in quite good general agreement with the inhalability criterion.     

Use of Numerical Calculations to Simulate the Sampling Efficiency Performance of a Personal Aerosol Sampler

Numerical calculations were conducted to simulate air and particle behavior near and into the inlet of an aerosol sampler in order to determine sampling efficiency performance. This was done with the pre-verified commercial computational fluid dynamics (CFD) software package, FLUENT (Fluent, Inc., Lebanon, NH, US). Air flow behavior was calculated for steady-state conditions approaching and flowing into 3D geometries of an aerosol sampler free in the air that was similar in dimension to two commercial samplers, namely the Gesamtstaubprobenahme sampler (GSP) and the conical inhalable sampler (CIS). Particle trajectories were calculated in a Lagrangian reference frame on the resulting velocity fields. Based on the particle trajectories, sampling efficiencies were calculated and compared to those reported in the literature for a CIS aerosol sampler. They were found to have similar overall trends for particle sizes up to 21 μ m. Using a correction factor, agreement was observed to be very good for smaller particles, but less so for larger particles.     

Evaluation of total and inhalable samplers for the collection of carbon nanotube and carbon nanofiber aerosols

A growing number of carbon nanotubes and nanofibers (CNT/F) exposure and epidemiologic studies have utilized 25- and 37-mm open-faced cassettes (OFCs) to assess the inhalable aerosol fraction. It has been previously established that the 37-mm OFC undersamples particles >20?µm in diameter, but the size-selective characteristics of the 25-mm OFC have not yet been fully evaluated. This article describes an experimental study conducted to determine if the 25- and 37-mm OFCs performed with relative equivalence to a reference inhalable aerosol sampler when challenged with CNT/F particles. Side-by-side paired samples were collected within a small Venturi chamber using a 25-mm styrene OFC, 37-mm styrene OFC, 25-mm aluminum OFC, and button inhalable aerosol sampler. Three types of CNT/F materials and an Arizona road dust were used as challenge aerosols for the various sampler configurations. Repeated experiments were conducted for each sampler configuration and material. The OFC samplers operated at flow rates of 2 and 5?L/min. Results showed that the 25-mm OFC performed comparably to the button sampler when challenged with CNT/F aerosols, which was demonstrated in five of the six experimental scenarios with an average error of 21%. Overall, the results of this study indicate that the sampling efficiency of the 25- and 37-mm OFCs adequately followed the ISO/ACGIH/CEN inhalable sampling convention when challenged with CNT/F aerosols. Past exposure and epidemiologic studies that used these OFC samplers can directly compare their results to studies that have used other validated inhalable aerosol samplers.     

PM 2.5 Semivolatile Organic Material at Riverside,California: Implications for the PM 2.5 Federal Reference Method Sampler

Particulate semivolatile organic compounds can be lost from particles on a filter during sample collection and storage, resulting in a negative artifact. Gas-phase organic compounds can adsorb on a quartz filter to cause a positive artifact. A sampler (Particle Concentrator-Brigham Young University Organic Sampling System: PC-BOSS) has been developed that uses a cyclone and virtual impactor (particle concentrator) inlet to provide a concentrated stream of 0.1-2.5 w m particles. The concentrator is followed by a BOSS diffusion denuder to remove interfering gas-phase compounds and filter packs to collect particles, including any semivolatile species lost from the particles during sampling. The sampler can be used for the determination of both fine particulate nitrate and semivolatile organic material without significant "positive" or "negative" sampling artifacts. The sampler has been evaluated at Riverside, CA. The collection efficiency of the particle concentrator was stable, being 65% - 2% and 61% - 1% for particulate sulfate and soot, respectively. Results obtained with the PC-BOSS for the determination of PM 2.5 organic material including semivolatile components agreed with results from a BOSS, but not with filter pack results. The precision of the PC-BOSS results for particulate organic material was - 8%. An average of 50% of the particulate organic material was lost from the particles during sampling for all samplers used. As a result of the loss of semivolatile organic material and nitrate, the PM 2.5 Federal Reference Method sampler underdetermined PM 2.5 by an average of 34% with the under measurement varying from negligible to 27 w g/m 3 , averaging 8.9 w g/m 3 .     

Design and Test of 2.5 μ m Cutoff Size Inlet Based on a Particle Cup Impactor Configuration

Based on the particle cup impactor configuration, an inlet for sampling fine particles smaller than 2.5 w m in diameter was designed for operation at a flow rate of 25 l/min. To determine the optimal dimensions of the particle cup impactor applicable to the PM 2.5 inlet, calibration experiments were carried out at wind velocities of 2 and 24 km/h in a wind tunnel. It was noted that the particle cup impactor having an impaction nozzle diameter of 3.2 mm and the nozzle-to-cup spacing of 3.6 mm yielded a sharp size cutoff. Supplementary experiments were conducted on sampling performance with the inlet having the optimally selected configuration at a near-zero wind speed in the test chamber. Results of the tests showed the inlet had a cutoff size of 2.43 w m in aerodynamic diameter, at 25 l/min, and that particles larger than 2.5 w m were trapped in the cup. Additional experiments covering a flow rate between 10 and 40 l/min with particle sizes between 0.8 and 4.3 w m were conducted in the test chamber. A field test was performed to examine the PM 2.5 inlet in real situations. The performance indicated that the inlet design met the basic requirements of fine-particle sampling.     

New PM10 Inlet Design and Evaluation

ABSTRACT

A new PM₁₀ inlet for a beta-gauge sampler was designed based on the particle cup impactor concept. The performance of the inlet was evaluated at near-zero wind velocity in the test chamber and at the wind velocities of 2 and 8 km/hr in the wind tunnel. The performance indicated that particles with aerodynamic diameters of 10 μm or larger were collected in the particle cup and the inlet proved to meet the basic requirement of PM₁₀ sampling.     

Development of polydisperse aerosol generation and measurement procedures for wind tunnel evaluation of size-selective aerosol samplers

Accurate development and evaluation of inlets for representatively collecting ambient particulate matter typically involves the use of monodisperse particles in aerosol wind tunnels. However, the resource requirements of using monodisperse aerosols for inlet evaluation creates the need for more rapid and less-expensive techniques to enable determination of size-selective performance in aerosol wind tunnels. The goal of recent wind tunnel research at the U.S. EPA was to develop and validate the use of polydisperse aerosols, which provide more rapid, less resource-intensive test results, which still meet data quality requirements necessary for developing and evaluating ambient aerosol inlets. This goal was successfully achieved through comprehensive efforts regarding polydisperse aerosol generation, dispersion, collection, extraction, and analysis over a wide range of aerodynamic particle sizes. Using proper experimental techniques, a sampler’s complete size-selective efficiency curve can be estimated with polydisperse aerosols in a single test, as opposed to the use of monodisperse aerosols, which require conducting multiple tests using several different particle sizes. While this polydisperse aerosol technique is not proposed as a regulatory substitute for use of monodisperse aerosols, the use of polydisperse aerosols is advantageous during an inlet’s development where variables of sampling flow rate and inlet geometry are often iteratively evaluated before a final inlet design can be successfully achieved. Complete Standard Operating Procedures for the generation, collection, and analysis of polydisperse calibration aerosols are available from EPA as downloadable files. The described experimental methods will be of value to other researchers during the development of ambient sampling inlets and size-selective evaluation of the inlets in aerosol wind tunnels.

© 2018 American Association for Aerosol Research     

Method and test system for evaluation of bioaerosol samplers

A method and test system have been developed for the laboratory evaluation of the performance of bioaerosol samplers. The method differentiates between the overall physical sampling efficiency (which reflects the inlet and collection efficiencies) and the biological sampling efficiency (which reflects the survival of the test microorganisms during the sampling process). The number concentrations of laboratory-generated bioaerosol particles are measured with an aerosol size spectrometer up- and downstream of the bioaerosol sampler being tested. In a bioaerosol impactor, which was specially designed for testing microbiological aspects of bioaerosol sampling, the inlet and collection efficiencies are differentiated by measuring downstream of the collection surface location with and without the collection surface in place. The number of recovered particles is counted as microcolonies with a microscope after sampling the bioaerosol particles into agar and culturing them. The total recovery of these bioaerosol particles is determined as a ratio of the number of viable microorganisms recovered to the number of bioaerosol particles present in the air sampling volume upstream from the sampler. This total recovery is a measure of the ratio of culturable to non-culturable bacteria present in the air. By measuring physical and microbiological aspects simultaneously, information is gained on aspects of bioaerosol sampling that cannot be determined by either of these branches of science alone. This is exemplified by tests on the influence of relative humidity and desiccation time on colony count.

The newly-developed system can be used to test any bioaerosol sampler. A special single-stage impactor was designed, built and used to study how different sampling and analysis variables affect the total recovery of bioaerosol particles. The designed impactor was calibrated using PSL particles. Its inlet sampling efficiency was found to be within the range of 96–99.5%, depending on the sampling conditions and particle size, if the latter is less than 8 μm (this range represents single bacteria, bacterial agglomerates, and fungi). The collection efficiency was found to be about 100% when collecting PSL particles larger than 0.7 μm in diameter at 201 min⁻¹ or higher air flows.

The total recovery of microorganisms measured under these conditions is characterized only by the “survivability” of microorganisms during their sampling. It was found that relative humidity had a pronounced effect on total Pseudomonas fluorescens recovery. Experimental data also showed that the sampling time may be limited due to bacterial desiccation and subsequent loss in viability of collected microorganisms.     

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.     

A wind tunnel evaluation of the physical sampling efficiencies of three bioaerosol samplers

The physical sampling efficiency of three commonly used samplers for bioaerosols has been determined under controlled conditions in a wind tunnel. Non-biological, monodisperse test aerosols of aerodynamic diameters up to 23 μm were used in a range of wind speeds up to 5 m s⁻¹. The performance of each sampler type was different. For the Andersen Microbial Sampler and the Casella Slit Sampler, sampling efficiency dropped both with increasing wind speed and particle size, while for the Aerojet General Glass Cyclone, performance was generally independent of windspeed and particle size. This work is part of a larger study to determine both the physical and the biological sampling efficiencies of currently used samplers for bioaerosols. The results highlight the importance of understanding the performance of aerosol monitoring equipment, if results obtained in the field are to be interpreted correctly.     

Characteristics of traffic-induced fugitive dust from unpaved roads

The characteristics of fugitive dust emitted from vehicles traveling on unpaved dirt roads were measured using a suite of instruments including a real-time fugitive dust sampler. The fugitive dust sampler is formed from a combination of a large particle inlet and an optical particle spectrometer that reports particle sizes from 6 to 75 µm. The large particle inlet permits the sampling of particles up to 75 µm with only a moderate dependence of sampling efficiency on wind-speed. Measurements made with the sampler showed that particles as large as ～50 µm were suspended from vehicular movement on the dirt roads, with the mode of the fugitive dust particle number size distribution ～2 µm, while the mass distribution mode was ～7 µm. A comparison of the fugitive dust sampler measurements with those made using standard PM instruments showed that the conventional instruments have a wind-direction bias that can result in under-sampling of large particles. The current measurements suggest that particles suspended from dirt roadways are of importance for local air quality within the near-road environment.

Copyright © 2017 American Association for Aerosol Research     

Novel Instrument to Separate Large Inhalable Particles

Large inhalable particles are present in the workplace, yet few instruments exist to count and size such particles in situ. Inhalable-aerosol exposure can be evaluated using mass-based samplers such as the IOM or Button sampler, but these devices do not provide information on particle size distributions. Size-resolved samplers such as cascade impactors or the Aerodynamic Particle Sizer are limited to particle sizes <20 μm due to difficulties with particle aspiration and transmission losses. This work describes the development of two samplers capable of measuring the concentration and size distribution of airborne particles from 20 to 100 μm in aerodynamic diameter. One device is based on the principles of an upflow elutriator, whereas the other eliminates the potentially adverse effects of an upward-facing jet to separate particles from a quiescent airstream. Analytical models and computational fluid dynamics simulations were used to predict the performance of the two samplers. Sampling efficiencies of these devices were tested in a calm-air chamber with polydisperse, fluorescent microspheres (10–100 μm). Epifluorescent microscopy of settled dust was used to determine reference particle counts and sizes. Both devices are capable of size-selective sampling; however, the second sampler produced higher sampling efficiencies and sharper cut points compared to the simpler elutriator design. Experimental sampling efficiencies for both samplers showed good agreement with computational and analytical solutions. This work suggests that these devices can size-segregate inhalable aerosols in quiescent environments.

Copyright © 2015 American Association for Aerosol Research     

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.     

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.