Wedding Ambient Aerosol Sampling Inlet for an Intermediate Flow Rate (4 cfm) Sampler James B. Wedding," Mlchael A. Weigand, and Michael W. Llgotke Aerosol Science Laboratory, Colorado State University, Fort Collins, Colorado 80523
Ralph Baumgardner United States Environmental Protection Agency, Research Trlangle Park, North Carolina 277 11
rn An ambient air inlet for an intermediate flow rate sampler has been designed, fabricated, and performance tested. The device is presently designed to operate at 4 cfm and have an aerodynamic particle size of 10 pm as, is sociated with a 50% effectiveness value (D50)which commensurate with the anticipated changes in the National Ambient Air Quality Standards under consideration by the U.S. Environmental Protection Agency. However, the system can be easily modified to meet any DW value ultimately selected by the Administrator. It is cost effective and potentially more accurate to collect a greater amount of mass than is possible with the current dichotomous sampler in the same 24-h period. Determination of mass may then be performed on a five-place analytical balance. A suitable filter substrate can be employed such as Teflon or microquartz, which do not have an artifact problem as observed with the standard Hi-Vol glass fiber filter. The system is independent of all environmental Conditions, including windspeed and wind direction. The inlet employs a unique, omnidirectional cyclone fractionator. The nature of the cyclonic forces in conjunction with the tortuous pathway that the transmitted particles must follow to reach the exit plane of the inlet precludes the bouncing or reentrainment of particles experienced with all impaction-type devices.
Introduction The US.Environmental Protection Agency (EPA) under the auspice of the Clean Air Act Amendment of 1977 has reviewed the total suspended particulate matter (TSP) standard and will in all likelihood replace it with an alternative standard requiring the collection of a specific size fraction of atmospheric aerosol. The principal impetus for this revision are data that indicate that protection of public health may be better served by considering only those inhaled particles (IP)that penetrate to and deposit in the thoracic region of the respiratory tract. This has subsequently been referred to as thoracic particle (TP) deposition. This decision was reached based upon recent experimental data ( I ) , Proctor and Swift (5), and other scientific opinions and presented for comment to the Clean Air Scientific Advisory Committee (CASAC). While a recent EPA staff paper published by the Office of Air Quality Planning and Standards has proposed that the new primary standard be based on D50 = 10 pm, a study just 0013-936X/83/0917-0379$01.50/0
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completed by Swift and Proctor (5) indicated that a D50 6.0 pm with Do= 10 pm may be more justifiable to describe thoracic deposition. Additionally, proposed new primary average annual limits may be set somewhere in the range of 55-110 pg/min (arithmetic mean) with the 24-h limit set somewhere in the range of 150-350 pglmin. D m and Doare defined as the particle sizes associated with sampling effectivenessvalues of 50% and 0%,respectively, where effectiveness for a given particle size is defined as the ratio of concentration deposited on the collection substrate of the sampler to the total airborne concentration approaching the inlet. The fraction of particles reaching the thoracic fraction is shown in Figure 1,taken from a regression fit by Chan and Lippmann (I)on available experimental data on the respiratory tract. Note that this does not indicate actual deposition. Also shown in Figure 1is an effectivenesscurve for a reference or ideal inlet for assessing the true ambient particle concentration, defined by a lognormal effectiveness curve having'a D50 10 pm and a unity slope (ag= 1.0). The most reasonable definition for an ideal inlet is one whose performance characteristics allow collection of mass representative of the TP fraction. At present, utilizing the Chan-Lippmann data, the DW would be 10 pm. While these data may prove not to be the final word for TP deposition, they remain at this time the most widely accepted reference. The research reported herein consists of the development of the Wedding ambient aerosol, sampling inlet having a 50% cutpoint of 10 pm and a flow rate of 4 cfm. Wind tunnel testing was performed to determine the operational characteristics of the device at three different wind speeds, 2, 8, and 24 km/h, for a series of nominal aerodynamic particle sizes ranging in size from 1to 20 pm. Design Rationale and Inlet Description. The Wedding ambient aerosol sampling inlet for an intermediate flow rate sampler is illustrated in Figure 2. Design rationale is based upon the need for an ambient aerosol sampling inlet capable of collecting all particles of interest (IP) with the same D50,slope (gI6,gU), and shape of the collection effectiveness vs. aerodynamic diameter curveindependent of the sampling conditions. At the present time a need exists for an inlet with a D50 equal to 10 pm and a flow-controlled sampler operating at -4 cfm. There are three principal reasons. First, the new National Am-
0 1983 American Chemical Society
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Environ. Sci. Technol., Vol. 17, No. 7, 1983 379
~ P B I f o l m o n C Curve e for Wedding 4 c f m A m b ~ e n l Aerosol Sompler inlet
120,
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Figure 2. Wedding 4-cfm ambient aerosol sampler inlet.
AIRDOVNAMlC UIAMETER, rm
Figure 1. Thoracic fraction, Wedding inlet, ideal inlet curves.
bient Air Quality Standards may be based on the D50 = 10 pm effectiveness curve patterned principally from the thoracic fraction curve such as the one shown in Figure 1 or may be based on a different Dso value which the present inlet design can easily adapt to. Second, it is cost effective and more accurate to collect a greater amount of mass than is possible with the current dichotomous sampler in the same 24-h period. Mass determination may then be performed on a five-place balance. For example, using 40 pg/min as a reasonable ambient thoracic particulate level (or more generally inhalable particulate, IP) and the standard sampling period of 1440 min, a 4-cfm (0.113 m3/min) sampler will collect 0.0065 g of material, which allows the reasonable usage of a five-place analytical balance for mass analysis. Third, a suitable filter substrate can be employed such as Teflon or microquartz, which do not have the artifact problem observed with the standard Hi-Vol filter, the glass fiber filter (2, 3). The overall effectiveness curve for an inlet is the product of the fractionating device efficiency used to establish the desired D m and the efficiency of the inlet geometry, which both transports the aerosol to the fractionator and protecta the system from environmental factors. The choice of the fractionating device is practically limited to a conventional impactor, a virtual impactor, or a cyclone. The conventional impactor suffers from particle bounce problems, which may be temporarily reduced with an appropriate coating or angled collection surface to alter the particle trajectory. These fractionators will always require periodic maintenance at unpredictable intervals. The virtual impactor may be a candidate, but to date no workable prototypes have emerged. The cyclone, if designed to be omnidirectional, is the best candidate as the performance of the fractionation curves is comparable to that of an impactor and the human respiratory tract. The cyclone 380
Environ. Sci. Technol., Vol. 17, No. 7, 1983
has been shown to be unsusceptible to particle-bounce problems. It is for these reasons that the cyclone approach was adopted as the fractionating element for this proposed IPl? inlet. The inlet depicted in Figure 2 utilizes an omnidirectional cyclone allowing aerosol entry from any angle of approach. An angular impetus is imparted to the particle motion by evenly spaced entrance vanes of a two-dimensional nature. The body of the cyclone is cylindrical in cross section. As the particles enter the inlet they follow the fluid streamlines along the lower radius and enter the cyclone fractionator through the vane system. Particle removal is realized on the inner collection tube. The flow with unremoved particles then enters the middle tube, where the trajectory is altered and flow continues in upward ascension. An additional turn is made to alter the flow into a downward trajectory to allow the transmitted particles to ultimately deposit on the collection substrate for subsequent mass analysis. Experimental Measurements Effectiveness Curves. Inlet testing at a sampling rate of 4 cfm was performed in the closed-loop Aerosol Science Laboratory Wind Tunnel facility in Fort Collins, CO (Figure 3). The tunnel has a cross-sectional dimension of 1.22 m square at the test section. The longitudinal macroscale was -20 cm. The vertical macroscale was 2 cm. The tests utilized monodisperse aerosols with nominal aerodynamic diameters of 1-20 pm generated by a vibrating orifice atomizer operating in an inverted position. Particles employed in the study were made from an oleic acid-methanol mixture tagged with uranine. The aerosol was sized by calculation and microscopically. The spread factor, or ratio of measured to actual diameter for this size range on oil-phobic (coated with 3M fluorocarbon) glass slides, has been found to be 1.34. The equivalent spherical diameter was converted to aerodynamic diameter by using the percentages of pure oleic acid and uranine used in the droplet. Measurements with a Casella impactor have found doublets at the test section do not exceed 4% by
Table I. Wedding Ambient Aerosol Sampler Inlet:a Mass Distribution CY-
clone as-
DP
pm
WIND TUNNEL
8.39 10.40 12.48 14.44
TESTING F A C I L I T Y FOR AMBIENT A E R O S O L S A M P L E R S
r
TEST SECTION ~ I O E T A I L SHOWN ABOVE)
semscreen bly 1.5 0.9 2.4 5.5
inner tube
2.3 2.2 2.9 4.0
14.1 41.9 68.0 77.5
a 4 cfm, D,,= 10 pm;
-1
middle tube plug 0.8 0.6 0.5 0.5
middle outer tube tube 2.4 1.0 0.6 2.3 2.4 0.2 1.7 0.3
inlet filter 77.9 51.4 23.8 10.5
U, = 8 kph.
Table 11. Inlet Collection Characteristics Test Results OPTIONAL
,ATMOSPHERIC EXHAUST
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-
TEST
TEST COLLECTION IPAMMETEj (%I
1 1 I 1 1 1 1 ANGLE
Idegrewl
WIND SPEED 0 CHANGE FLOW RATE WIND SPEED DsoCHANGE l k r n l h 1 Gicronrl IClmI (kmlh 1 lm~cronrl
2
E X P E R I M E N T A L CONFIGURATION A E R O S O L SCIENCE L A B O R A T O R Y T E S T F A C I L I T Y
Flgure 3. Aerosol Science Laboratory Test Facility, Colorado State University.
number. Aerosol from the atomizer was injected through a 15.24-cm diameter tube containing a 85Kr charge neutralizer. The pipe diverged into an annular region between the two cones. Six pipes spaced around the ahnular region let the aerosol into the tunnel. This injection system produced a particle concentration profile across the test section that was found to be independent of windspeed over the range of 0.5-40 km/h. Variation of the conc&ntration across the width of the inlet never exceeded 10.29%. The inlet was tested at windspeeds of 2,8, and 24 kmjh. The velocities were measured upstream of the test section with a calibrated hot-wire anemometer (TSI temperature-compensated probe and TSI constant-temperature anemometer). The anemometer was calibrated by using a precision Pitot tube and micromanometer. To determine the inlet effectiveness,the aerosol concentration was sampled isokinetically before and after each test by using two sampling manifolds 0.90 m wide with six sampling nozzles each spaced at equal intervals (-12.5 cm) in the same horizontal plane and at the same location in the test section as the inlet opening. Each nozzle exhausted air to a 47-mm Gelman AE glass fiber filter. The IP inlet was mounted on a vertical tube approximately 1m long, connected to a General Metal Works filter holder on the bottom. The sampling effectiveness of the inlet for each particle size was determined by caleulating the ratio of the quantity of aerosol passed by the inlet and deposited on the filter to that collected by the isokinetic sampling manifold with appropriate corrections for differences in sampling volume and pressure drops as monitored in the lines. Collection substrates were washed in 50 mL of pure methanol and then diluted 1:l with distilled deionized water, which served as a buffer to stabilize fluorescence. A 4-mL aliquot of each sample solution was measured for uranine content with the aid of an Aminco fluorometer. Additionally, a series of tests were performed to determine the distribution of mass inside the cyclone fractionator. The inlet was tested for a long period of time (>1.5 h) utilizing monodispersed oleic acid particles tagged with uranine. Each collection surface of the inlet (Figure 2) was washed separately with methanol and analyzed for fluorescence as previously explained. Off-Mode Collection. For the off-mode test, the inlet was set in the wind tunnel and connected to the filter holder and pumping system, but not vaccum was applied.
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The tests were conducted within f0.5 pm of the D50, 20-pm liquid particles and -2-pm solid particles, at 2 and 24 km/h for a period of >4 h. Angle of Attack. For the angle of attack test, the inlet is set in the wind tunnel and tested at &20° orientation as shown in Table 11. The tests were conducted with aerosols of nominal aerodynamic diameters of 5-13 pm. Flow-Rate Dependency. For the flow-rate dependency test, the inlet was operated at 4.4 and 3.6 cfm. The tests were conducted with aerosols of nominal aerodynamic diameters of 5-13 pm. Tests were performed by using potassium biphthalate (KHP) aerosols with nominal aerodynamic diameters of -2 X Dm These solid particles were well-known for their especially ''bouncy'! nature.
Results and Discussion The results of the performance testing of the Wedding ambient aerosol sampling inlet are shown in Figure 4. Each plotted point represents the average value of at least eight data points taken on different days. Additionally, error bars and Dw values are tabulated along with 616 and us4values to provide indication of the slope of the collection curve. 616 is defined as the ratio D16/D50 and defined as the ratio D50/Ds4. The D50 values for the windspeeds of 2, 8, and 24 km/h are 9.8, 9.8, and 9.5, respectively. Thus, it is seen that the Dm shift is -0.3 pm over the range of windspeeds tested. The three curves at all three windspeeds are essentially parallel to each other with nearly identical 616 and crs4 values as noted in Figure 4. Note that the performance curves are thus nearly lognormal, which means that 616 = us4= ug: The fact that the inlet performance is independent of wndspeed results from the unique inlet housing design and sharp cut characteristics of the cyclone fractionator. Additionally one should note that the inlet for an intermediate flow-rate sampler or any other IP sampler must have small ugvalues Envlron. Sci. Technol., Vol. 17, No. 7, 1983
381
110
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WEDDING 4 c f m AMBIENT AEROSOL SAMPLER INLET O C T O B E R , 1981
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Figure 4. Performance curves for 4-cfm Inlet.
(steep fractionation curve), be windspeed independent to avoid errors in the coarse-fraction mass collected, and be windspeed insensitive. Quantitative assessment of the influence of inlet performance characteristics on collected mass and how this collected mass relates to the actual ambient mass distribution, as well as particles penetrating to the thoracic region of the respiratory tract, was recently investigated in a paper by Wedding and Carney (4). A regression analysis of available deposition data taken from Chan and Lippmann (1)was used to represent particle penetration to the thoracic region of the respiratory tract. This regression curve along with the 8 km/h effectiveness curve for the Wedding ambient aerosol sampling inlet is shown in Figure 1. Note the dramatic similarity between the TP curve and the Wedding inlet performance curve. In the Wedding paper (4), a quantitative mathematical analysis of the mass collected by the Wedding ambient aerosol sampling inlet as well as the TP fraction was undertaken. The results of that analysis show that the total mass collected for a typical urban distribution by the Wedding ambient aerosol sampling inlet and the TP fraction are essentially identical at 45.15and 44.49 pg, respectively (4). If a fine-fraction mass was collected, no sampling bias would be found between the mass collected by the Wedding inlet or the TP fraction. If a coarse-fraction mass were collected, the Wedding inlet would collect almost exactly the same amount of mass as would be collected in the thoracic region of the respiratory tract. Even if different curves were ultimately selected to represent TP, the performance of the inlet could be modified to collect mass that adequately monitored TP deposition, thus providing for the margin of safety required in the Clean Air Act. Table I presents mass distribution data for particles deposited within the cyclone fractionator. Note that es382 Environ. Sci. Technol., Vol. 17, No. 7, 1983
sentially all of the mass entering the cyclone fractionator is deposited in the inner tube and that there are virtually no losses in the vane structure or middle or outer tubes. This is important as then the middle tube may act as a reservoir to store collected particles. Table I1 presents results of the off-mode, angle of attack, and flow-rate dependency tests. Off-Mode Collection. Results of the off-mode collection tests revealed no detectable mass on the collection filter for all particle sizes and approach flow velocities. Note that while this test is important, it is not as critical as testing with solid particles that can bounce, thus increasing the probability of transmission to the filter. For the present inlet design discussed and tested herein, it is believed to be impossible for a particle or droplet of any material under any off-mode conditions to penetrate the fractionator and reach the collection substrate. However, this is not true of inlets whose entrance plane and geometry forces the particles to enter parallel to the horizontal plane and turn downward toward the filter substrate. The vertical entry tube is susceptible to the entry of rain droplets, debris, and solid particles, with the higher probability of these material penetrating to the collection substrate under off-mode conditions. Angle of Attack. The angle of attack tests are important as often the mean winds approaching the inlet entrance plane are not parallel to the horizontal plane and often are gusting and swirling with vector directions that may exceed 45". So that tests on the potential effect of this phenomenon can be conducted, the angle of approach flow must exceed 14", as this is the magnitude of the angle of attack wherein flow separation occurs on prismatic bodies (Robertson et al., (6)). Tests conducted at angles less than 14" will in all likelihood reveal no change of sampling effectiveness and are of little value. No shift in D50 value was seen at f20" for windspeeds of 8 and 24 km/h. A shift of -0.4 pm was observed at 2 km/h for both f20". Thus, the effect of angle of attack is inconsequential with respect to mass collected. Flow-Rate Dependency. The flow-rate dependency tests are necessary for determining compliance with the allowable limits stated in the Federal Reference Method for the sampler flow-rate stability. The inlet Ds0 change values given in Table I1 are relative to the original D50 values bbtained at 4 cfm. There was no change in the D m value observed at 4.4 cfm for all three windspeeds. A +0.4-pm shift in Dm was observed at 3.6 cfm for all three windspeeds-again inconsequential in terms of collected mass. Tests using KHP particles of 20-pm aerodynamic diameter revealed effectiveness values indiscernable from the oleic acid droplets. However, when an inlet system is utilized under field conditions, it is possible for once-deposited particles to become reentrained and pass through to the collection substrate. To preclude this occurrence, the sight of principal particle deposition was lined with an oil-impregnated material; thus, the surface becomes a perfect absorber (see Figure 2). The oil effectively preclud'es the reentrainment of deposited particles, is selfregenerating, and will not reach the filter even in vapor form. N
Conclusions and Recommendations (1)An acceptable IP inlet with D m e 10 pm to operate at a flow rate of 4 cfm has been designed, fabricated, and performance tested. The impetus for its development was provided by a need for cost-effective mass analysis, the five-place balance (4), and the option of filter substrate material. It is believed to represent a completely viable
Environ. sci. Technol. 1983, 17, 383-388
candidate for use by state and local agencies in the EPA IP network or by industry to meet monitoring needs. The unit has been found to have small c16 values to ensure windspeed independency. Computer analysis has shown that, for an urban mass distribution the inlet transmits 45.15 pg as compared to that collected in the thoracic region of the respiratory tract, 44.49 pg (4). (2) The unit was tested under off-mode (no flow rate) conditions and did not collect any detectable mass. (3) The unit was operated at &20° orientation to the mean flow. The effect on D50 was negligible. (4) The unit was operated at two different sampling rates in addition to the 4 cfm standard flow rate. At both 3.6 and 4.4 cfm, there was no significant change in the effectiveness values. (5) The inlet can be operated in the field for an indefinite period of time without any maintenance whatsoever. (6) The inlet collects virtually all mass in the inner tube as designed. (7) At this point in time, a D50 value has not been officially decided upon. This inlet design can, however, be modified to other realistic D50 values such as in the 6.07.5-pm range. (8) Consideration of possible particle reentrainment in the case of a “dirty” inlet has given rise to the concept of
a perfect absorber surface for the primary site of particle deposition. (9) A 40-cfm inlet and sampler have also been completed at this time. Both the herein described 4-cfm inlet and sampler and the 40-cfm unit are available through General Metal Works, Village of Cleves, OH 45002.
Literature Cited (1) Chan, T.L.; Lippmann, H. Am. Ind. Hyg. Assoc. J . 1980, 41,399-409. (2) Lee, R. E.,Jr.; Wagman, J. Am. Ind. Hyg. Assoc. J . 1966, 27,266-271. (3) Spicer, C. W.; Schumacher, P. M.;Kouyomjian, J. A. EPA Report No. EPA-600/2-78-06, 1978. (4) Wedding, J. B.;Carney, T. C. Atmos. Environ., in press. (5) Swift, D. L.; Proctor, D. F. Atmos. Environ. 1982,16,2279. (6) Robertson, J. M.;Wedding, J. B.;Peterka, J. A.;Cermak, J. E. J . Ind. Aerodyn. 1977,2, 345-359.
Received for review January 11, 1982. Revised manuscript received July 6,1982.Accepted January 3,1983. This work was performed under the auspices of the United States Environmental Protection Agency Cooperative Agreement No. 808011, with Ralph Baumgardner as Project Officer. This support is gratefully acknowledged.
Abastumani Forest Aerosol Experiment (1979): Comparison to Other Nonurban Halocarbons and Nitrous Oxide Measurements Dagmar R. Cronn,* W. Lee Bamesberger,+ and Valentln M. Koropalod Air Pollution Research Section, Department of Chemical Engineering, Washington State University, Pullman, Washington 99164, and Institute for Applied Physics. Moscow. USSR
w A joint U.S./USSR experiment conducted in July 1979 in the Caucasus Mountains of the USSR studied the relative contributions of biogenic emissions and long-range transport of anthropogenic pollutants to the aerosol burden of a remote atmosphere. To document any regional pollution buildup or long-range anthropogenic pollutant transport to the site, the levels of nitrous oxide and the (anthropogenic) trace gases CF2C12(F-12),CFC13 (F-ll), CH3CC13,and CCh were compared to a rural site in eastern Washington state. Diurnal patterns observed for each compound in the USSR and CC14 at the US. site were consistent with the local micrometeorology and the position of known local sources. Transport of anthropogenic gases to the study site was observed, but significant regional pollution buildup did not occur. The site was well chosen to meet the objectives of the study since it was shown to be relatively remote from anthropogenic influence with respect to the trace gases.
Introduction A cooperative field project was conducted at the Abastumani Geophysical Observatory in the Georgia Republic, USSR, during July 1979. The Abastumani Forest Aerosol Experiment (AFAEX) 1979 had participants from the United States including the Environmental Protection Agency (Environmental Sciences Research Laboratory, Research Triangle Park, NC, the Department of Civil Engineering, University of Washington, Seattle, WA, and t Washington State University.
Institute for Applied Physics. 0013-936X/83/0917-0383$01.50/0
the Washington State University (WSU) Air Pollution Research Section of the College of Engineering, Pullman, WA. Numerous Soviet laboratories also participated in the field study including the Main Geophysical Observatory, Leningrad, the Institutes of Atmospheric Physics and Applied Geophysics, Moscow, and the Institute of Physics, Vilnuis, Lithuania. The principal goal was to study the relative importance of biogenic emissions to the aerosol burden in remote atmospheres. WSU was responsible for collecting aerosol samples for individual organic component analysis by high-resolution mass spectrometry, for collecting aerosol samples for elemental analysis by X-ray fluorescence, and for ascertaining the extent to which anthropogenic emissions impacted the study site. This latter objective, to provide information on any long-range transport of anthropogenically polluted air masses to the study site, was met by operation of a continuous, automatic gas chromatograph with electron capture detection for analysis of the trace gases CFzClz (fluorocarbon-12 (F-12)), CFC13 (fluorocarbon-11 (F-ll)), CH3CC13 (l,l,l-trichloroethane),CCll (carbon tetrachloride), and N20 (nitrous oxide). The results of the organic analyses are reported in the following paper in this issue. Similar uses have been made of trace gases such as F-11, F-12, and CCll as indicators of transport of urban air masses ( I d ) . These trace gases are a good choice for characterizing anthropogenic contamination of the atmosphere because they are ubiquitous emissions of urban industrial areas and are chemically stable (lifetimes in years). The data collected during the Abastumani field study were interpreted relative to comparable data collected
0 1983 American Chemical Soclety
Envlron. Sci. Technol., Vol. 17,No. 7, 1983 383
