Environment International Vol. 3, pp. 259-264. Pergamon Press Ltd. 1980. Printed in Great Britain

Ambient Aerosol Sampling" Problems, Goals and Wind Tunnel Test Results of Hi-Volume Samplers*

James B. Wedding and T.C. Carney Civil Engineering Department, Colorado State University, Fort Collins, CO 80523, U.S.A. and R.K. Stevens Environmental Sciences Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, U.S.A.

The area of ambient air sampling and its inherent problems and current goals are discussed in general. In particular, recently completed tests of the collection effectiveness of the Rocky Flats Hi-Volume Sampler are compared to previously completed tests of the Standard EPA Hi-Volume Sampler for a variety of field realistic conditions. Collection effectiveness is defined as the ratio of the aerosol collected on the collection substrates of the sampler to that collected by an isokinetic sampling system.

The collection effectiveness of the Standard EPA Hi-Volume Sampler was determined as a function of particle size (5-50 #m) and sampler orientation (0 ° and 45 °) at a base condition wind speed of 4.6 m/s and 8% relative turbulence intensity. The results indicated a strong effect of orientation on collection effectiveness at a sampling rate of 1416 I/rain. Wind speed over the range of 1.5-4.6 m/s does not greatly influence the collection effectiveness of 15 ~m particles. Free stream turbulence levels of 1 and 8% relative intensity has no effect upon collection characteristics.

The collection effectiveness of the Rocky Flats Hi-Volume Sampler was determined as a function of particle size (1-34 ~tm), wind speed (1.52-12.19 m/s) and sampler orientation to the mean flow (0 °, 45 °, 180°). The results show the sampler, with an hdet flow rate of 880 1/min, has an inlet effectiveness that was a slight function of orientation angle for particles 1-10 #m with a larger effect seen for 20-34 ~an; a strong effect of velocity was seen up to 5 m/s where a further increase showed only a slight decrease in effectiveness.

The Microsorban-98 filter that is presently used in the Rocky Flats Sampler was tested for efficiency over the size range of particles from 0.01-1/tin and with three different face velocities using the sampler flow rates of 600, 800 and 1000 l/rain corresponding to pressure drops of 20-24 in. of water (3.74-4.49 cm HAG). The filter paper, which was of the fiber type, was found to be 99.9% efficient over the range of particle sizes and pressure drops tested.

Introduction

The problem

The capability of commercially available ambient aerosol samplers to successfully ingest particles greater than 5 ~m has become of increasing interest in the last few years. The purpose of this paper is to review the current problem of ambient air sampling and present goals as seen by EPA personnel. In addition, results of previous tests and recently completed tests will be given which evaluate the capabilities of two contemporary devices for effective collection of larger particles. As attempts continue to further exploit natural resources in search of alternate sources of energy, an increase in potential deleterious environmental impact is possible due to aerosolized particles arising from such operations

*Received in revised form 13 July 1979; received for publication 13 July 1979

as mining oil shale, coal gasification and fuel transpor- tation and utilization. The burning of high sulfur content fuels necessitating the use of scrubbers may increase the presence of larger sulfate carrying droplets which could cause property damage. Wind aerosolized dust exist from such anthropogenic activities as acci- dental or purposeful weapons explosions, the distri- bution of radioactive wastes, agricultural operations, construction, strip mining, demolitions, and aerial pesticide disseminations to mention some.

At present, there is still a much needed effort to be aimed at instrumentation development with the inlet capability to efficiently and accurately sample these larger particles as well as internal fractionation to separate the sample into respirable vs nonrespirable sizes. Measurements by Whitby et aL (1972) have shown that ambient aerosol has a distinct bimodal character in its volume distribution, with the minimum occurring for particles having diameters in the vicinity of 2 t~m. Chem- ical analysis of size fractionated aerosols (Stevens et aL,

259

260 J.B. Wedding, T.C. Carney and R.K. Stevens

1978) has shown sharp differences in the composition of the two modes: in general the small particle mode is acidic and are composed mostly of sulfates and carbon, with small amounts of lead, bromine and a variety of other elements at trace concentrations. In contrast the large particle mode is generally basic and composed of a variety of minerals including quartz, limestone, calcite, mica and clay, with trace quantities of tire dust, pollen and lead oxides. Increased inertia of the large particles presents a substantial challenge to the efficient internal transport preferred for any collection or fractionation apparatus. Characterization of suspended particles in the ambient atmosphere is primarily accomplished through the use of several thousand Standard Hi-Vol Samplers (1971) spread across the United States. These samplers have an effectiveness approaching 100~/0 for particles for which the gravitational and inertial forces are small (less than approximately 5#m): however, for larger-sized particles the efficiency is quite variable and depends upon not only particle size but also wind speed and direction (Wedding et al., 1977), and to a lesser degree upon air sampling rate. In field applications, characterization of aerosol concentration is difficult when the upper size limit of the sampler is variable. Also, since the Hi-Vol samples are not size segregated near the center of the distribution (e.g., fractionation of respirable/non-respirable particles) long term particle concentration trends may be misleading. Public Health Service studies (Spirtas and Levin, 1970) for the period of 1957 to 1966 which show that in urban regions the particle concentration had decreased whereas a corresponding increase had occurred in nonurban areas, may simply reflect a shift in particle size distribution in urban areas. It is possible that the concentration of respirable particles actually increased in urban areas during the period but, because of a decrease in the emissions of large particles due to the application of abatement technology, the overall concentration measured with the Hi-Vol approach would show a decrease. To enable interpretation of both historical data and data which is currently being accumulated with the aid of Hi-Vols, it is desirable to acquire additional understanding of the inlet particle transport character- istics of the system. Indeed, it would be advantageous for future sampling if an inlet were to be developed for the Hi-Vol which would minimize or preclude most of the inherent sampling biases. If this task were to be deemed unworthy then knowledge of the various biases that do exist for present samplers could be aquired so that some corrections may be made.

Analysis of data from most other ambient air samplers suffers from the same problem as the Hi-Vol

a lack of information on the inlet fractionation characteristics as functions of the environmental conditions. The resulting data, if inlet errors were corrected for, could be used not only for future judg- ments in aerosol sampler selection, but also for inter- pretation of previously acquired data. For example, Hi- Volume samplers have been used extensively for sampling in situations in which large wind-blown dust particles are present. The efficiency with which these larger particles are drawn into the samplers has recently been published (Wedding et al., 1977).

Recen t new approach

In 1972 , Environmental Sciences Research Laboratory (ESRL), U.S. Environmental Protection Agency (EPA) began to examine the application of con- ventional impactors to collect aerosols in two size ranges as suggested by the bimodal ambient aerosol distri- bution. Impactors that separate and collect aerosol particles depend upon the relative balance between inertial and aerodynamic forces. In the conventional impactor, an airstream turns abruptly as it approaches a flat plate. Particulates with the largest inertia tend to maintain a straight trajectory and impact on the plate while the viscous drag forces of the gas flow carry the smaller particles along the airflow streamlines. The particles collected on the plate would ideally consist of all sizes above a well defined cut-off diameter. Particles greater than 2 #m may bounce off the plate or shatter and are carried to a lower collection surface. Dzubay and Stevens (1975) have documented this problem and recommended coating the collection plates with grease to minimize particle bounce errors.

Because of the undesirability of contaminating the collected particles with the grease that is required for conventional impactors, the virtual impactor was found to be an appropriate alternative. In the virtual impaction collection method the larger particles travel into a slowly pumped void and collect on a filter, thus eliminating particle bounce and achieving a sharp separation between two particle sizes.

Although the single stage virtual impactor originally described by Hounam and Sherwood (1965) had obvious advantages over conventional impactors, large losses of particles within the device prevented its general use in air pollution monitoring programs.

In 1973, ESRL, EPA funded Environmental Research Corporation (ERC), St. Paul, MN to develop an efficient virtual impactor termed the dichotomous sampler. It was named the dichotomous sampler (Dzubay and Stevens, 1975) because the system collected particles into two size ranges. As part of this program ERC designed several inlets to minimize effects of wind speed and direction on particle sampling effi- ciences (Stevens and Dzubay, 1975). Most of the ERC designs were an improvement over conventional inlet types but all sampled particles > 15 #m with an effi- ciency which decreased markedly with increasing wind speed.

Wedding et al. (1977), under a grant from EPA developed an improved inlet to the dichotomous sampler, that reduced the effects of wind speeds on particulate collection of particles > 15 /zm in aero- dynamic diameter. However, even with this design the collection efficiency did drop significantly with wind speeds greater than 20 km/h. Work is continuing in several laboratories to develop an inlet which will mini- mize changes in sampling efficiency with changing wind speeds.

In 1976 Beckman Instruments began the development of a low cost automated version of the dichotomous sampler based on the design of Loo et ai, (1976). The sampler incorporates a novel differential flow control design, a microprocessor that controls the automated

Ambient aerosol sampling 261

changer, and uses an improved virtual impactor design by Loo (1979). This impactor has particle losses of no more than 6% at 2.5 ~m and almost zero losses out to 20 /~m. This sampler is currently being used in several ESRL field studies.

The present paper will address the experimental verification of Hi-Volume Sampling effectiveness by giving key results of prior tests conducted by Wedding et ai. (1977) at the Aerosol Science Laboratory at Colorado State University along with a study just completed on the Rocky Flats Hi-Volume Sampler for comparison.

Efficiency o f the Microsorban-98 membrane filter*

While numerous papers appear in the literature on the subject of filtration efficiency, a complete study experi- mentally measured filtration efficiency of fiber or mem- brane filters over a wide range of particle sizes and pressure drops is noticeably lacking.

As some of the particles present in the atmosphere may be of submicron size, the question of filtration effi- ciency must be addressed. Thus, an experimental evalu- ation of the efficiency of the Microsorban-98 filter was performed as no recent data on the presently manufac- tured filter was available.

Experimental procedure

Wind tunnel tests

The test results to be presented were based upon studies conducted in the Environmental Wind Tunnel Facility at Colorado State University shown schemati- cally in Fig. 1. The tunnel has a 3.65 m wide test section with a roof that can be adjusted up to 2.44 m in height - - a feature which allows the tests to be conducted with a zero pressure gradient in the direction of flow. The present series of tests was performed with a 1.83 m ceiling height sufficient to preclude blockage effects which require less than 5% obstruction in the tunnel

A ~ f ~ro$ot Genero lors

NeuttOliler - Sp l i t fer Plote$

I r Somple

LI L! 0 /U", U ~ ~ U ~ / A r ~ Flow / // / , 01sperse M ! ~ '

Injec~qon Pump Pipes

S+dt View

Fig. 1. Colorado State University environmental wind tunnel experi- ment configuration for injection and sampling of particle cloud and

sample test section location. •

*Delbag-Luftfilter GmbH, 1 Berlin S1 (Halensee), 4 Dtisseldorf- Heerdt West Germany. There is at present a study underway at the University of Minnesota

under the direction of Dr. B.Y.H. Liu aimed at developing a hand- book on filtration efficiency.

cross sectional area (Maskell, 1965). Previous tests involving the General Metal Works Standard EPA Hi- Volume Sampler (Wedding et al., 1977) and the just completed study of the Rocky Flats Hi-Volume sampler were conducted under a variety of field realistic fluid flow conditions using monodisperse aerosol (5-50/zm for the previous tests and 1-34/~m for the Rocky Flats sampler), variable flow velocities and at different inlet orientations to the mean flow. Referring to Fig. l , the aerosol was generated using the vibrating orifice type atomizer (Wedding et al., 1978) operating in an inverted manner. The particles were injected into the tunnel through two 15 cm diameter tubes positioned in the roof of the tunnel.

The spatial consistency of the aerosol concentration was determined prior to and subsequent to each test using two isokinetic sampling manifolds positioned in two parallel horizontal planes, six inches apart vertically and spanning the sampler inlet. Each manifold was approximately 90 cm in width with the six isokinetic sampling nozzles and filters spaced at equal intervals ('x,15 cm) on the same plane. The sampling effectiveness of the instruments was then determined by comparing the quantity of aerosol deposited on the collection sub- strates of a particular sampler to that detected by the isokinetic sampling system with appropriate corrections for differences in sampling volumes.

Figure 2 illustrates the Rocky Flats sampler as tested and the Standard Hi-Volume sampler with inlet flow rates, dimensions, and angle orientation conventions noted. Note that all the Rocky Flats samplers at present are attached to power poles in the field so that sampling effectiveness of this sampler was determined with a portion of the pole in place in the wind tunnel.

a = , a o ,

HIGH VOLUME SAMPLER ROCKY FLATS SAMPLER SAMPLING RATE :141~fpm SAMPLING RATE , 880 ~pm

Fig. 2. Standard hi-volume sampler and Rockwell International sampler with approach flow orientation angle (ct).

Sample analysis

The particles used in the studies were formed from the atomization of an oleic acid solution tagged with uranine dye, the latter used for increasing mass sensi- tivity through fluoroscopic analysis. Analysis was per- formed by soaking in pure ethanol the collection sub- strates (filters) from the particular sampler being tested.

262 J.B. Wedding, T.C. Carney and R.K. Stevens

The resulting solution was then diluted l : l with distilled water. One drop of 1 N NaOH was added to each 4 ml aliquot sample solution analyzed to stabilize fluores- cence. These aliquots were quantified in terms of fluor- escent content with the aid of a calibrated Turner Model 111 fluorometer.

Filtration efficiency tests

In the absence of electrostatic effects, filtration effi- ciency of filters varies with particles size and flow rate. For a given particle size and flow rate, a ratio was ob- tained of the number of particles retained by the filter after passage of a known volume of gas to the original number of particles in the same volume of gas before passage through the filter. Efficiency is defined as one minus the ratio of the downstream to upstream number concentration of the particle size of interest.

The filtration efficiency of the Microsorban-98 filter was determined by means of the experimental apparatus shown in Fig. 3. Measurements were made at flow rates of 15.6, 20.7 and 25.9 l/min through a filter section held in a commercial 47 mm .filter holder. These flow rates resulted in filter face velocities of 27, 36 and 45 cm/s equivalent to those occurring at flow rates of 600, 800 and 1000 l/min used in the Rocky Flats Hi-Volume sampler. The experimental apparatus consisted of a sub- micron atomizer (Liu and Lee, 1975), a charge neutrali- zation and drying chamber, a filter holder in parallel with a bypass line, a vacuum pump and flow metering equipment, and an Electrical Aerosol Analyzer (Thermo-Systems, Inc. Model 3030). The Electrical Aerosol Analyzer (EAA) utilized has the ability to accu- rately resolve a size distribution consisting of particles between 0.01 and 1 #m into 8 logarithmically equal in- crements. This resolution was sufficient for the scope of this study.

The filtration efficiency of the Microsorban-98 filter was determined as follows: a polydisperse aerosol was formed by atomization of a NaCl solution in the syringe pump driven atomizer. The atomizer was operated at a pressure of 50.8 N/cm ~ (35 psig), resulting in an aerosol flow rate of 4.61 l/min. Excess electrical charge on the aerosol, a consequence of the atomization process, was

neutralized with the aid of the 10 mC Kr-85 source and the freshly generated aerosol was dried in a 12.7 cm (5 in.), 61 cm (24 in.) long diameter tube containing silica gel. The upstream number concentration was measured by closing valve 2, opening valve 1, and with the EAA flow rate set, adjusting valve 3 until the flow meter indi- cated the difference between the desired flow rate and the aerosol flow rate. After a time sufficient for the drying and charge neutralization chamber to reach a steady state concentration, the EAA output was re- corded at the 9 pertinent internally programmed high voltage settings.

The overall operation and theory of the instrument is given by Liu and Pui (1975). The instrument's reprodu- cibility and reliability have been well documented. The size distribution downstream of the filter was determined in like manner with valve 2 opened and valve 1 closed. The pressure drop across the filter was monitored during measurement of the downstream number concentration. For the pressure drops encountered in this experiment, direct use was made of the data without a need to correct for volume expansion across the filter as the correction was less than 6%. The filter efficiency of each size range increment was deter- mined by ratioing the corresponding values of the downstream to the upstream number concentration measurements and subtracting this number from 1.

Results and discussion

Sampler effectiveness

The reduced data for the Standard Hi-Volume sampler and the Rocky Flats sampler are presented in Table 1 for comparison purposes. The Rocky Flats sampler effectiveness was determined at four different particle sizes (1, 10, 20 and 34 #m), three different approach flow orientations and velocities from 1.52- 12.2 m/s. Presented in Table 1 is data at ~,5 m/s along with the Standard Hi-Volume which was tested previ- ously (Wedding et al., 1977) at particle sizes ranging from 5-50 #m, and orientation angles of 0 ° and 45 °. Note that the inlet flow rates of the two samplers differ

FLOW METER

DILUTION A I R J 1 PRESSURE I ORY GAGE

~ S U B M I R E G U L

INFUSION PUMP

LIQUID RESERVOIR

VACUUM PUMP

SAMPLING CHAMBER

KRYPTON - 85 I VALVE CHARGE NEUTRALIZER FILTER J

TSI 30

Fig. 3. Experimental system for measuring filter efficiency.

Ambient aerosol sampling

Table 1. Comparison of sampling effectiveness (%) of Hi-Volume samplers at "~,5 m/s . 1: Standard Hi-Volume;

2: Rockwell Internat ional Hi-Volume

263

Particle diameter 0Lm)

lZ-51 10 z 15L20 ~ 301-34 z 501 Sampler orientat ion

Sampler 0 45 180 0 45 180 0 45 180 0 45 180 0 45 180

1 97 100 . . . . 35 55 - - 18 41 - - 7 34 - - 2 100 100 100 75 72 72 27 42 47 20 30 23 - - - - - -

Sampling rates { 1 = 14151/m

2 = 8801/min

appreciably (1416 and 880 1/min for the Standard Hi- Volume and the Rocky Flats sampler, respectively).

Considering the Rocky Flats sampler, orientation angle is not important for particles ~ 10 #m and the 45 ° orientation is more efficient than 0 ° for the larger par- ticles. The 180 ° orientation places the power-support pole directly upstream so initially one may presuppose that the 180 ° position would have a lower effectiveness than the 0 °. This doesn't prove to be the case as the 180 ° direction is more effective than the 0 ° direction, the latter orientation acting more as a pure impaction surface. The effect of the pole is seen to increase the inlet velocity somewhat (15-20070 determined by hot wire anemometer velocity measurements of the entire inlet) as well as create a negative pressure region that serves to cause the 20 #m particles to enter the inlet more effectively than at 0 °. At the 34 #m particle size, the effectiveness is about the same at 20-23070. The 45 ° direction proves to be the most effective overall inlet orientation.

The Standard EPA Hi-Vol is even more efficient at 45 ° than the Rocky Flats Hi-Vol but proves to be more sensitive to orientation. The flow around the Standard Hi-Volume inlet is extremely unsound aerodynamically with the sharp corners causing the separated flow pattern to deviate greatly with angle. Note also that the inlet flow rate is substantially greater so that the flow area effectively altered by the inlet velocity is greater than the Rocky Flats sampler and thus one may expect a greater capability of the Standard Hi-Vol to alter the trajectories of large particles.

These Hi-Volume samplers tested have a separation flow region whose effect on particle trajectories is com- plicated - - more so by the presence of the power pole in the case of the Rocky Flats sampler. The particle trajectories are altered by the presence of the free shear layer that accompanies separated regions. In the lower particle size ranges ( ~ 10 #m) more diffusion controlled behavior tends to cause the cloud concentration to become nearly homogenous throughout the separated region around the sampler (with more particles prob- ably entering the inlet from the downstream side). As the particle behavior becomes more inertially governed, the particles are deflected less around the sampler and begin to penetrate closer to the sampIer body itself. There will tend to be more losses by impaction on the sampler exterior and fewer particles will successfully negotiate the two sharp turns required to enter the inlet. Thus, as particle size increases, a t radeoff on the inlet effectiveness is seen to exist between increased inertial

losses versus the increased penetration of the particles to that area near the inlet where there is sufficient aero- dynamic force created by the inlet velocity to alter the particle trajectory to allow initial entry into the inlet. To conclude, only the upstream portion at 0 ° orientation of a prismatic-like bluff body will serve more nearly like an impaction surface so that one cannot expect the inlet effectiveness curves to be directly predictable by inertial mechanism along (i.e., decay proportional to the square of the particle size). One may even see a tendency for the effectiveness to increase for a time for certain flow geo- metries as the particle size increases due to the greater ability of the particle to cross the fluid streamlines.

Figure 4 is a reproduction of part of a figure that appeared in Wedding et al. (1977) with the present data from the Rocky Flats Hi-Volume added in for compari- son purposes. A complete set of results for the sampling effectiveness of the Rocky Flats Hi-Volume sampler for effectiveness at all combinations of particle size, approach velocity and orientation are given by Wedding and Carney (1978).

~00

80 ==

6 0

>

~o 4 0

ao

0 0

~ TEST VELOCITY • 5 m I sec

El, II STANDARD Hi-VOLUME SAMPLER ([3--45*,1--0 =') O, ~., V ROCKY FLATS SAMPLER

0--0". ~--45", V*-180")

".0p

- ~ : . ; , ~ - - - o .

iio i I 0 i 20 3 40 PARTICLE DIAMETER , F.m

-41

I 5 0

Fig. 4. Sampling effectiveness of the Standard Hi-Volume sampler compared to the Rocky Flats sampler vs particle diameter for different

orientations to the mean flow.

Filter efficiency

Table 2 contains data for a representative test of the filtration efficiency results of the Microsorban-98 fiber filter. Particle sizes from 0.01-1 t~m were tested at a pressure drop of "x,4.5 cm Hg. Upstream concentrations (particles/cm') are given for the indicated size incre- ments and a cumulative percent by number calculation is shown. Note that while the cumulative percentages in column 3 reach values near 100 for the larger size incre- ments used, there are still sufficient particles present for

264 J.B. Wedding, T.C. Carney and R.K. Stevens

Table 2. Typical set of data showing particle sizes employed and concentrations measured during the efficiency tests of the Microsorban 98 fiber filter tested at pressure drop 4.5 cm Hg

1 2 3 4 5 6 Panicle sizes Number concen- Cumulative ¢/0 Number concen- Minimum detectableMaximum detectable

(~m) tration upstream of less than stated size tration downstream concentration filtration efficiency the filter of the filter

(panicles/cm 3) (particles/cm 3) (panicles/cm~) * (%) 0.0100

9.740 x 10 s - - 417 99.9 0.0178 25.97

1.597 x 106 - - 164 99.9 0.0316 68.54

4.538 X 10 s - - 87 99.9 0.0562 80.64

4.275 x 105 - - 45 99.9 0.100 92.04

2.262 x 10 s ~ 24 99.9 0.1780 98,06

6.238 x 104 ~ 12 99.9 0.3160 99.73

8.965 x 10 a - - 6 99.9 0.562 99.97

1.23 x 10 a - - 3 99.8 1.000 100,00

*This number is derived from the background noise of the device. Also this instrument detects an average concentration and does not count single particles.

statistically significant tests. There were no particles detected downstream of the Microsorban-98 filter for any of the three face velocity conditions noted earlier. The limit of detectability for particle size of the instru- ment ultimately is 0.0032 and 0.01/zm was used in these tests. The minimum detectable concentration is also given in Table 2. The minimum penetration is deter- mined by the ratio of values given in column 5 to those given in column 2 and the maximum filtration efficiency is presented in column 6. Column 4 indicates that no particles were detected passing through the filter for any size ranges. The minimum detectable concentration pre- sented in column 5 is based upon the background noise of the instrument. Thus, the collection efficiencies as shown in column 6 are conservative estimates and will be higher in most cases. Note that the figure of 99.8°70 in column 6 for the 0.562-1.00/~m particle size does not indicate that the efficiency is anticipated to be lower for these larger particles but merely reflects the lower upstream concentration of 1.236 x 103 particles/cm 3 approaching the filter.

References

Dzubay, T.G. and Stevens, R.K. (1975) Ambient air analysis with dichotomous sampler and X-ray fluorescence spectrometer. Environ. Sci. Technol. 9, 633-668. Dzubay, T.B., Hines, L.E. and Stevens, R.K. (1976) Particle bounce errors in cascade impactors. Atmos. Environ. 10, 299-334. Houman, R.E. and Sherwood, R.J. (1965) The cascade centripeter: a device for determining the concentration and size distribution of aerosols. Am. ind. Hyg. Ass. J. 26, 122-131.

Liu, B.Y.H. and Lee, K.W. (1975) An aerosol generator of high stability. Am. ind. Hyg. Ass. J. December, 861-865. Liu, B.Y.H. and Pui, D.Y.H. (1975) On the performance of the elec- trical aerosol analyzer. J. Aerosol Sci. 6, 249-264. Maskell, E.G. (1965) A theory of the blockage effects on bluff bodies and stalled wings in a closed wind tunnel. REM no. 2400, Aeronauti- cal Research Council, London. McFarland, A.R., Wedding, J.B. and Cermak, J.E. (1977) Wind tunnel evaluation of a modified andersen impactor and an all weather sampler inlet. Environ. Sci. Technoi. 11, 535-539. Spirtas, R. and Levin, H,J. (1970) Characteristics of particulate patterns. AP-61, U.S. Department of Health, Education and Welfare, Public Health Service, National Air Pollution Control Adminis- tration. Stevens, R.K. and Dzubay, T.G. (1975) Recent developments in air particulate monitoring. IEEE Trans. Nucl. Sci., NS-22, 849-855. Stevens, R.K., Dzubay, T.G., Russwurm, G. and Rickel, D. (1978) Sampling and analysis of atmospheric sulfates and related species. Atmos. Environ. 12, 5-68. U.S. Environmental Protection Agency (1971) National primary and secondary ambient air quality standards. Fed. Regist., 36 (84), 8186, April 30. Wedding, J.B., McFarland, A.R. and Cermak, J.E. (1977) Large particle collection characteristics of ambient aerosol samples. Envi- ron. Sci. Technol. 11, 387-390. Wedding, J.B. (1975) Operational characteristics of the vibrating ori- fice aerosol generator. Environ. Sci. Technol. 9, 637-674. Wedding, J.B., Carney, T.C. and Montgomery, M.E. (1978) Vibra- ting orifice aerosol generator: extending the capability to 200 #m dia- meter particles. To be submitted to Environ. Sci. Technol. September, 1978. Wedding, J.B. and Carney, T.C. (1978) Determination of sampling effectiveness of Rocky Flats high-volume sample. Final Report, Atomic International Division, Rockwell International, Rocky Flats Laboratory, Golden, Colorado 80401, June 1978. Whitby, K.T. Husar, R.B. and Liu, B.Y.H. (1972) J. Colloid Interface Sci. 39, 177.