Environ. Sci. Technol. 1989, 23, 1487-1492
Ottawa, Ontario, Canada, 1983; INFO-0096. Sextro, R. G.; Moed, B. A.; Nazaroff, W. W.; Revzan, K. L.; Nero, A. V. In Radon and I t s Decay Products: Occurrences, Properties, and Health Effects;Hopke, P., Ed.; ACS Symposium Series 331; American Chemical Society: Washington DC, 1987; Chapter 2. Nazaroff, W. W.; Moed, B. A.; Sextro, R. G. In Radon and Its Decay Products Indoors; Nazaroff, W. W., Nero, A. V., Eds.; Wiley: New York, 1988; Chapter 2. Wood, J. A. Porter, M. L. J.Air Pollut. Control Assoc. 1987, 37,609-615. Hodgson, A. T.; Garbesi, K.; Sextro, R. G.; Daisey, J. M. J. Air. Pollut. Control Assoc., in press. Turk, B. H.; Prill, R. J.;Fisk, W. J.; Grimsrud, D. T.; Moed, B. A.; Sextro,R. G. Radon and Remedial Action in Spokane River Valley Homes; Lawrence Berkley Laboratory: Berkeley, CA, 1987; Vol. 1, LBL-23430. Turk, B. H.; Prill, R. J.; Sextro, R. G.; Harrison, J. In Proceedings of the 1988 Symposium on Radon and Radon Reduction Technology; Denver, CO; U.S. Environmental
Protection Agency: Denver, CO, 1988; Paper No. VI-3. Turk, B. H.; Prill, R. J.; Fisk, W. J.; Grimsrud, D. T.; Moed, B. A.; Sextro, R. G. In Proceedings of the 79th Annual Meeting of the Air Pollution Control Association; Minneapolis, MN; Air Pollution Control Association: Pittsburgh, PA, 1986; Paper No. 86-43.2. Nazaroff, W. W.; Lewis, S. R.; Doyle, S. M.; Moed, B. A.; Nero, A. V. Enuiron. Sci. Technol. 1987, 21, 459-466. Nazaroff, W. W.; Doyle, S. M. Health Phys. 1985, 48, 265-281. Nazaroff, W. W.; Feustel, H.; Nero, A. V.; Revzan, K. L.; Grimsrud, D. T.; Essling, M. A.; Toohey, R. E. Atmos. Enuiron. 1985, 19, 31-46. Mowris, R. J. Analytical and Numerical Models for Estimating the Effect of Exhaust Ventilation on Radon Entry in Houses with Basements or Crawl Spaces; Lawrence Berkeley Laboratory: Berkeley, CA, 1986; LBL-22067.
(14) Loureiro, C. 0. Simulation of the Steady-state Transport of Radon from Soil into Houses with Basements under Constant Negative Pressure: Lawrence Berkeley Labo-
ratory: Berkeley, CA, 1987; LBL-24378. (15) Mowris, R. J.; Fisk, W. J. Health Phys. 1988,54,491-501. (16) United States EnvironmentalProtection Agency A Citizen's Guide to Radon: W h a t Is I t and W h a t Do W e Do About It?; United States Environmental Protection Agency:
Washington, DC, August 1986; OPA-86-004.
(17) United States Environmental Protection Agency Radon Reduction Methods: A Home Owners Guide; United States
Environmental Protection Agency: Washington, DC, August 1986; OPA-86-005. (18) Marynowski,J. M. Measurement and Reduction Methods of Cinder Block Wall Permeabilities; Center for Energy and Environmental Studies: Princeton University, Princeton, NJ, 1988; Working Paper No. 99. (19) Harris, B. B.; Ruppersberger, J. S.; Walton, M. Presented at 1988 Symposium on Radon and Radon Reduction Technology; Denver, CO; U.S. Environmental Protection Agency, Denver, CO, October 1988. (20) Garbesi, K. Experiments and Modeling of the Soil-Gas Transport of Volatile Organic Compounds into a Residential Basement; Lawrence Berkeley Laboratory: Berkeley, CA, 1988; LBL-25519 Rev. (21) Hillel, D. Fundamentals of Soil Physics; Academic Press: New York, 1980; Chapter 11. (22) Conner, J. J.; Brebbia, C. A. Finite Element Techniques for Fluid Flow; Newnes-Butterworths: Boston, MA, 1976. Received for review March 16,1989. Accepted August 17,1989. This work was supported by the Occidental Chemical Corp. under Agreement BG 86024 and by the Assistant Secretary for Conservation and Renewable Energy, Office Building and Community S y s t e m , Building Systems Division of the US.Department of Energy under Contract DE-ACOS- 76SF00098.
A Shrouded Aerosol Sampling Probet Andrew R. McFarland,' Carlos A. Ortlz, Murray E. Moore, Robert E. DeOtte, Jr., and Srlram Somasundaram Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843
A new device-a shrouded probe-has
been developed for sampling aerosol particles from moving airstreams. In its design, a 30-mm-diameter sampling probe is located concentrically within a 105-mm-diameter, cylindrically shaped shroud. The flow rate through the sampling probe is a constant value of 170 L/min. The dynamic pressure of the external airstream forces flow through the region between the shroud and the internal probe. The velocity of air in the shroud is 0.40 that of the free stream over a wide range of free stream velocities (2-14 m/s). The wall losses of 10-pm particles in the shrouded probe operated at 170 L/min in a 14 m/s airstream are 13% as compared with 39% for an isokinetic probe. Wind tunnel experiments with 10-pm-diameter particles over the range of free stream velocities of 2.0-14 m/s and a flow rate of 170 L/min show the transmission ratio for the shrouded probe to be within the range of 0.93-1.11.
A = C,/Co The aspiration coefficient can be represented functionally as
A = f(St, Re, Uo/V,, a, 0 )
(2)
where St is the Stokes number ( C p ,2UO/9pd),and Re is the probe Reynolds number ( p ,d/p). C is the Cunningham correction for slip flow (approximately unity for particles that exhibit anisokinetic sampling effects), p , is the particle density, D, is the particle diameter, U, is the free stream velocity, p is the fluid viscosity, d is the diameter of the probe inlet, p is the fluid density, V is the spatial mean velocity at the probe inlet plane, a n 8 a and
8
Introduction
The standard approach to obtain a representative sample from a moving gas stream in a flow duct is with an 'Aerosol Technology Laboratory Publication 5897/01/09@3/ ARM. 0013-936X/89/0923-1487$01.50/0
isokinetic probe (1-3). Considerable research has characterized the effects of sampling with isokinetic probes under conditions that are either anisokinetic or in which the probe is aligned at off-axis angles, e.g., probe at nonzero pitch or yaw angles relative to the moving airstream (4-8). Two parameters describe the efficacy of a sampling probe: the aspiration coefficient, A , and the transmission ratio, T. The aspiration coefficient gives the ratio of the spatial mean concentration a t the probe inlet, C,, to the concentration in the free stream, C,, i.e.:
0 1989 American Chemical Society
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1487
0 are the angles of orientation of the probe to the free stream velocity vector (pitch and yaw). In this study it is assumed that the axis of the probe is parallel to the free stream velocity vector; thus, cy and B are not variables. The Reynolds number is of secondary importance in that its influence causes only minor modifications to the shape of the flow field in the vicinity of the probe. Thus, the only parameters of consequence are St and Uo/Vp. If the Stokes number is written in terms of the flow rate, Q, it becomes
St =
~CPJ),ZQ(UO/Vp) 93rpd3
(3)
Empirical relationships have been developed for A, one of which was recently given by Vincent et al. ( 5 ) . For a probe with the axis parallel to the velocity vector, their expression is 1.05 St A = 1 + ( R - 1) (4) 1 + 1.05 St where R = Uo/Vp. In general, the anisokinetic effects increase with increasing Stokes number. From eq 3, note that the value of St is affected by the inverse cube of the inlet diameter. If, for a given value of R, the inlet size can be selected at the discretion of the designer, anisokinetic effects can be reduced by employing a larger inlet diameter and a correspondingly larger flow rate. The transmission ratio, T , the second parameter used to characterize the performance of a probe, is the ratio of the aerosol concentration transmitted through the probe to the free stream concentration. The transmission ratio and aspiration coefficient are related by T = A F,, (5)
+
where F,, is the fraction of aerosol deposited on internal surfaces of the probe (wall losses). For probes to collect integral samples as in EPA method 5 applications (3), probe wall losses can be recovered in the subsequent analysis approach, and the aspiration coefficient is the more important parameter. In situations where the aerosol must be continuously monitored, T is more important. One critical example of such an application is the continuous monitoring of radioactivity in nuclear facilities. Signals from the monitors trigger alarms, which not only alert personnel but also actuate safety control sequences. Wall losses should be minimized with any type of continuous monitor. With isokinetic probes, the wall losses can be substantial. For example, in testing of PM-10 inlets in wind tunnels, isokinetic probes were used to characterize the aerosol concentration in the free stream (9). Monodisperse aerosol particles are drawn into the probe and collected on fibrous filters at the back of the probe. It is not unusual in sampling 10 pm aerodynamic diameter aerosol particles for one-third of the sampled aerosol to be deposited on the internal walls of the probe and two-thirds on the actual sampling filter. Our observations have been that proportionally less material is inadvertently deposited on internal walls of the larger diameter isokinetic probes. In conjunction with the design of aerosol sampling apparatus for monitoring the emissions from the U S . DOE Waste Isolation Pilot Plant (WIPP) at Carlsbad, NM, we reexamined the need for isokinetic sampling and produced an alternative scheme that has merit for selected applications. The WIPP site, which will be a test storage facility for nuclear wastes, has underground operations involving mining and vehicular traffic movement in a salt stratum. 1488
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Mine ventilation air exhausted to the surface through a 4.3-m-diameter shaft normally contains salt aerosol particles at concentrations of = 1 mg/m3 with mass median aerodynamic diameters of approximately 5 pm (10). The ventilation air must be continuously sampled with the sampled streams flowing to continuous air monitors (CAM) and filter air samplers (FAS). The CAM samplers have built-in radioactive counters to signal alarms and provide ventilation-air control functions in case of a release of radioactive aerosol from the underground waste storage area. The FAS units will collect cumulative samples to be analyzed for aerosol particle mass and concentration of radionuclides. Possible nominal flow rates through the ventilation shaft are 200, 100, 70, and 30 m3/s; the latter flow rate would be employed if the CAM samplers signaled the presence of radioactivity and caused diversion of the exhaust airstream through a bank of HEPA filters. This broad range of flow rates leads to a range of velocities in the 4.3-m shaft of 2-14 m/s. Under emergency conditions, the velocity in the HEPA filter exhaust duct would be 11 m/s. The sampling criterion recommended by the State of New Mexico is that at least 50% of 10-pmparticles shall reach the CAM and FAS samplers under all exhaust-air flow rate conditions (11). The choice of a particle size of 10 km coincides with the EPA division between thoracic and nonthoracic aerosols (12). Because of the uncertainty of the size distribution of aerosols in an accidental release of radioactive material (through an underground fire or other such causes), it is important that larger sized particles be effectively transported to the CAM and FAS samplers. DOE had planned to use a rake comprised of approximately 6-mm-diameter isokinetic probes to sample a flow rate of 170 L/m from the air stream. A review by the State of New Mexico Environmental Evaluation Group (EEG), Westinghouse Waste Isolation Division, and the US. DOE WIPP Project Office resulted in the recommendation that the sampling system be redesigned because of the likelihood that the transmigsion of 10-pm particles would be unsatisfactory (13). Our approach to solving the problem of sampling over a broad range of free stream velocities was to develop a shrouded probe. Shrouds have been employed by aerodynamiciststo negate angle-of-attack effects in pitot tubes. The shrouded and vented Kiel-type probe can be operated at angles of f63' relative to the free stream and still provide values of the dynamic pressure that are within f l % of the true value (14). In a combined aerosol and aerodynamic application, Torgeson and Stern (15) utilized a shroud to negate angle-of-attack effects and to reduce the probe inlet velocity in an aircraft-borne sampler for the collection of radioactive particles at the 500-mbar level. They noted that during extended flights of a WB-50 aircraft, the angle of attack of the aircraft changed with fuel load. The first stage of their sampler was an inertial impactor with the requirement that the velocity in the impactor jet be approximately half that of the free stream (flight) velocity. The shroud achieved this reduction. They controlled the probe flow rate so that the probe operated isokinetically relative to the velocity in the shroud. McFarland and Ortiz (16) developed a shrouded-inlet, helicopter-borne, aerosol sampler for the collection of fly ash in the vicinity of the stack of a coal-fired power plant. Here also, the shroud was used primarily to negate angle-of-attack effects. This study extends the shrouding principle to develop a probe that has higher values of aerosol transmission, T, than an isokinetic probe. The shrouded probe operates at a constant flow rate in free streams where the velocity
STREAM FREE Lo
170 L l m n FLOW TO F A S 0 CAM SAMPLERS
Figure 1. Shrouded probe designed for continuous monitoring of the exhaust air at the WIPP site. Sampling flow rate, 170 L/min.
can range from 2 to 14 m/s. The combination of aerosol deceleration within the shroud and the corresponding use of a large-size probe causes the wall losses to be less than those of isokinetic probes with the same flow rate.
Shrouded Probe Design In development of the shrouded probe, several designs were investigated. The configuration considered best for the WIPP site application, and for which results are reported herein, is shown in Figure 1. The sampling rate of the probe was set at 170 L/min to accommodate the flow requirements of three FAS and CAM samplers that would be supplied aerosol by a single probe. The inlet diameter of the probe is 30.0 mm, which provides a velocity at the probe inlet plane, V,, of 4 m/s. The probe body is gradually expanded (3.5O half-angle for the inside surface and 7O half-angle for the outside surface) for a transition to a 51-mm-diameteraerosol transport line. The inside and outside diameters of the shroud are 102 and 108 mm, respectively. The inlet is located 153 mm from the shroud entrance plane. The waistline on the outside surface of the probe is expanded to provide a blockage of the cross-sectional area between the shroud and the probe; for the case shown, the open area between the waistline of the probe and the shroud is 30% of the total shroud cross-sectional area. This blockage controls air flow through the shroud. The shroud is 406 mm long. Principle of Operation The shrouding about the probe serves as a flow decelerator; through adjustment of the open area between the probe waistline and the shroud, the seIection of the velocity ratio, Us/ U,, is at the discretion of the designer. Here, Us is the velocity of decelerated air in the region of the shroud sampled by the probe. Experimentally, Uswas typically measured at a distance of 50 mm upstream of the probe inlet. In early experiments in an aerodynamicwind tunnel, we noted the velocity of air in the gap between the probe waistline and the shroud was approximately the same as the free stream velocity, Uo;hence, we could easily set the velocity Us.For the present design, the ratio of Us/Uohas been chosen to be 0.40. Under conditions in which the free stream velocity is large, the shroud both allows a lower velocity sample to be presented to the inlet and permits a larger probe diameter to be employed. Although the velocity in the shroud is anisokinetic relative to the free stream velocity, the large diameter of the shroud reduces the magnitude of the inertial enrichment or depletion of aerosol concentration that is the usual consequence of anisokinetic sampling. The model of Vincent et al. (5) has been used to predict the performance of the shrouded probe for 10-pm-diameter particles. In Figure 2, values of A are shown for the shroud
061 0
'
'
'
FREE
I
5
'
'
'
I
STREAM VELOCITY,
10
a
'
'
'
I
4 , m/s
Figure 2. Aspiration coefficients of the shroud, probe, and combination of the two. Calculated from the model of Vincent et al. (5) for a sampling rate of 170 L/min and a particle size of 10-pm aerodynamic diameter.
alone (As),for the probe alone ( A ), and for the combination of the shroud and probe Here, A, = C,/Co, A, = C,/Cs, and A, = Cp/Co;where C, is the spatial mean concentration at the shroud inlet plane. It is assumed that there are no aerosol losses in the shroud between the inlet plane and the probe entrance plane, and that the concentration of aerosol at any cross section in the shroud is uniform. For these conditions, A, = ASAP. Note from Figure 2 that over the range of velocities of 2.0-14 m/s, the aspiration ratio for the probe and shroud combined, A,, is bounded by 1.01 and 1.13, whereas the aspiration coefficient for the probe alone, A,, is bounded by 1.00 and 1.02.
(1,).
Test Apparatus And Procedures Aerodynamic testing was performed in a wind tunnel with a 600 X 600 mm test section and a velocity capability of 7-45 m/s. The shrouded probe was strut-mounted for the testing, and a flow of 170 L/min was drawn through the probe. Detailed mapping of the velocity profiles within the shroud was performed with a fast-response, hot-wire anemometer (TSI IFA l00/200 System). Qualitative mapping of the external streamlines was done with smoke produced by forcing pressurized air through a canister that contained a cigar. Smoke was discharged into the tunnel through a 3-mm-diameter wand. Aerosol testing was conducted in two wind tunnels designed for aerosol-related work. The tunnels are special in that the achievement of uniform aerosol profiles over the center two-thirds of the wind tunnel was the principal design goal. Discussions of aerosol wind tunnels have been presented elsewhere (9,17).One tunnel with a 600 X 600 mm cross section is capable of operation over the range of 0.5-7 m/s. The other tunnel has a circular cross section of 860-mm diameter for the range of velocities of 4-20 m/s. Test aerosol was generated with a vibrating jet atomizer (18) from a solution of oleic acid (a nonvolatile oil) dissolved in EtOH. An analytical tracer, sodium fluorescein, was added to the solution in the ratio of 10% tracer to oleic acid (m/v). Particle sizing was performed microscopically with samples deposited on glass slides treated with an agent to render them nonwetting to the oleic acid (Chemical FC-721, 3M Co., St. Paul, MN). Droplet size was calculated by the factor of Olan-Figueroaet al. (19),which accounts for the flattening of the droplets on the slide. The resulting droplet size was converted to aerodynamic size by use of the droplet density, which was based on the weighted mean of the densities of oil and sodium fluorescein. During testing, the shrouded probe (test probe) was operated for a period of time sufficient to collect a readily analyzable sample of aerosol. An isokinetic probe was Environ. Sci. Technol., Voi. 23, No. 12, 1989
1489
I
VELOCITY
I
--
L.
-100I
I
operated either in parallel with the probe or sequential to the test probe. Both the test probe and the isokinetic probe were equipped with glass fiber filters for aerosol particle collection. Flow rate through the probes was monitored with calibrated rotameters, and the readings were corrected for air density differences due to pressure drop across the filters and flow lines. At the completion of the particle collection process, the sodium fluorescein was eluted from the filters with EtOH, and the internal surfaces of the probes were washed with EtOH to remove particles inadvertently deposited on the walls. The EtOH/sodium fluorescein solutions were then diluted with equal amounts of distilled water and mixed, and 4-mL aliquots were removed for analysis in a fluorometer (Model 450, Sequoia-Turner Corp., Mountain View, CA). One drop of 1N NaOH was added to each aliquot to stabilize fluorescence.
Results Aerodynamic Testing. The velocity profiles upstream and within the shroud are illustrated in Figure 3. These data, taken at a free stream velocity of 15 m/s, show that the profile at a distance of 100 mm downstream from the shroud entrance plane is quite flat and that the reduction ratio, U,/Uo, is 0.40 on the probe axis ( r = 0, where r is the radial distance). Other experiments, conducted at velocities of 4.3-27 m/s show U,/Uo to be 0.43 and 0.38, respectively. Measurements of turbulent intensity showed the free stream intensity to be 1.3'30,whereas the intensity inside the shroud, at a distance of 100 mm downstream from the shroud entrance, was approximately 2% over the range of free stream velocities of 4.3-27 m/s. Aerosol Testing. The results of wind tunnel testing to determine the transmission ratios, T, and the aspiration coefficients, A , are shown in Figure 4. All the data were taken with a particle size of 10-wm aerodynamic diameter and with a sampling rate through the probe of 170 L/min. Also shown for comparison are the transmission ratios for a set of isokinetic probes. These isokinetic probes were geometrically similar to the probe shown in Figure 1 and were operated at flow rates of approximately 170 L/min. Note from this figure, that the values of T for the shrouded probe range from 0.93 to 1.11 over the velocity range of 2-14 m/s. The aspiration coefficient of the shrouded probe varies from 0.97 to 1.26 over the given range of velocities. The values of T for the isokinetic probes decrease mono1490
SCALE
:H
15 m/r
up
Environ. Sci. Technol., Vol. 23, No. 12, 1989
---I I
I
I
q
2 0,
0 z
Ln
z
15
I RATIO T
A
0
V ---
SHROUDED P R O B E
-c ISOKINETIC
NOZZLE
. 'e
0
5
FREE STREAM
..
. ..
-
-s-
-0-
..
- - ..-
. ..
-----+
10
15
V E L O C I T Y , Uor m/s
Figure 4. Transmission ratio, T , and aspiration ratio, A , for the shrouded probe. Particle size, 1Gpm aerodynamlc diameter; sampling flow rate, 170 Llmin. For comparison, the values of T for isokinetic probes with flow rates of = 170 L/min are also shown.
tonically from 0.97 to 0.63 as the velocity is increased from 2 to 14 m/s. The variations of T and A with particle size at a free stream velocity of 14 m/s are shown in Figure 5. For the shrouded probe, Tis between 1.00 and 1.09 over the range of sizes from 1-to 15-pm aerodynamic diameter. At 15 wm, the aspiration coefficient is 1.38; however, wall losses cause the value of T to be 1.09. By comparison, at 15 hm, the isokinetic probe has a transmission ratio of approximately 50 yo.
Two isokinetic probe designs were tested to determine if the angle of divergence inside the probe affects the aerosol transmission. Although both designs are similar to the probe shown in Figure 1, one had a 3.5O half-angle and the other an 11.5" half-angle in the flow deceleration section (diffuser). The 3 . 5 O angle of expansion is considered optimal for a diffuser in that flow separation is not significant in the deceleration process (20); with an 11.5O angle, the flow separates and causes the level of turbulence to increase. This separation potentially could cause an increase in the aerosol losses, but the results showed essentially the same internal losses for both expansion angles,
20,
I RATIO
z
0
[ Z R E A K
-
LINE)
SHRoUD PROBE
FLOW
SHROUDED PROBE
z n -
z t + 0
.
of the flow do not undergo an abrupt change in direction during the deceleration process; i.e., the streaklines are relatively straight for the part of the flow that is ultimately sampled by the probe. Inertial enrichment or depletion is associated primarily with regions of sharp streamline curvature, which suggests the shrouded probe should not greatly bias the samples. Also, the observations showed the streaklines to remain intact upon entering the shroud, which means there are no anomalies such as flow separation at the shroud entrance.
I-
a
$ E 8
Flgure 7. Streakline patterns observed from smoke tests with a transparent shroud. The streaklines in the core region do not undergo abrupt changes of direction, which tends to mlnimize inertial enrichment or depletion.
I
051
l
4
I-
z
12-
2
.
w
EXPERIMENTAL R E S U L T S
-,
.
n-
-- _ -- - - Q -
U
g
10-
0
2
-
0
2
__.v-V
.y7 ._..._. v...... ~
A t FROM T H E MODEL OF VINCENT et a1
08-
FREE S T R E A M VELOCITY, Uo, m/s Figure 6. Comparison of experimentally determined values of the aspiration coefficient, A , with the total aspiration coefficient, A t , predicted from the model of Vincent et al. (5). Sampling flow rate, 170 L/min; particle size, 10-pm aerodynamic diameter.
suggesting that the losses are associated mainly with the intake region of the probe. One further experiment was conducted to demonstrate the utility of the shrouding principle. The probe inside the shroud has a velocity at the inlet of 4 m/s when operated at a flow rate of 170 L/min. The probe was removed from the shroud and tested for wall losses by using it as an isokinetic probe in a free stream at a velocity of 4 m/s. It was then replaced in the shroud and again tested for wall losses in a 4 m/s free stream. The results showed the wall losses of the unshrouded probe were 7.5% and those of the shrouded probe were 4.6%. The values of the total aspiration coefficient, A,, from the model of Vincent et al. ( 5 ) are compared with experimental data in Figure 6. Note that at the higher velocities, the model underestimates the inertial enrichment. For example, at the flow rate of 170 L/min and a free stream velocity of 14 m/s, the model and experimental values of A are 1.26 and 1.13, respectively. The difference may be because of variations in aerosol concentration across the shroud cross section due to the nature of the process of deceleration of the free stream; in this case, the Vincent model would not be completely adequate in describing the aspiration coefficient. Smoke was used to qualitatively map the streaklines of the flow field. A clear plastic shroud was fabricated to permit visualization within the device. The flow appeared as shown in Figure 7. Note that the streaklines in the core
Discussion and Conclusions An aerodynamic shroud about an aerosol sampling probe offers an advantage over isokinetic probes for some applications. First, the wall losses in the shrouded probe are less than those in the isokinetic probes. In particular, at a free stream velocity of 14 m/s, the wall losses in a 170 L/min isokinetic probe were 39% for 10-pm-diameter aerosol particles. For the shrouded probe operated at the same flow rate, but with the velocity in the shroud decelerated to 0.40 that of the free steam velocity, the wall losses were only 13%. Second, it is possible to design a shrouded probe to provide an approximatelyrepresentative sample of 10-pm particles over a range of free stream velocities while sampling at a constant flow rate. In the design developed for continuous monitoring at the WIPP site, the transmission of 10 pm aerodynamic diameter aerosol particles varied between 0.93 and 1.11 over the free stream velocity range of 2-14 m/s with a sampling rate of 170 L/min. The applications of this work should be of value in two general areas involving sampling of aerosol particles from flow ducts. Because the wall losses are less for a shrouded probe, in situations where continuous monitoring is needed, a more reliable sample could be presented to the monitor with a properly designed shrouded probe than with an isokinetic probe. Also, accommodations are unnecessary to account for reasonable variations in the velocity of the free stream. For size-selective sampling, such as characterizing PM-10 concentrations in flow ducts, the shrouded probe offers a distinct advantage. A PM-10 system needs to have a fractionator (e.g., a cyclone) in the sampled flow line, and the flow rate must be constant for the fractionator to maintain a constant cut point. Currently, this is done by exhaust gas recirculation and a variable flow rate through the isokinetic probe; however, the approach could be simplified with a shrouded probe and the sample flow rate held constant. Particles with sizes greater than 10 pm, for which there would be inertial enrichment in the shrouded probe, would be stripped by the PM-10 fractionator. The design optimization for a shrouded probe involves selecting the proper flow deceleration values (Us/U,J for a given sampling rate and the anticipated upper limit of particle diameter. A model such as that of Vincent et al. (5)permits selection of a probe diameter that will provide the best characteristics for the application. The selected probe design should be checked with particle sizes other Environ. Sci. Technol., Vol. 23, No. 12, 1989
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Environ. Sci. Technol. 1989, 23, 1492-1499
(11) Rodgers, J. C. Exhaust Stack Monitoring Issues at the
than that used in the basic calculations. Acknowledgments We thank Mr. Kevin J. Shenk of Westinghouse Waste Isolation Division for his advice and suggestions.
Literature Cited Guide t o Sampling Airborne Radioactive Materials i n Nuclear Facilities; American National Standards Institute, Inc.: 1430 Broadway, New York, 1969; ANSI Standard
N13.1-1969. Determining the Concentration of Particulate Matter i n a Gas Stream; The American Society of Mechanical En-
gineers: 345 East 47th St., New York, 1980;ANSI/ASME Power Test Code 38-1980. Standards of Performance for New StationarySources. U S . Environmental Protection Agency Fed. Regist. 1971, 36, No. 237, 40 CFR Part 60. Okazaki, K.; Wiener, R. W.; Willeke, K. Environ. Sci. Technol. 1987,21, 183. Vincent, J. H.; Stevens, D. C.; Mark, D.; Marshall, M.; Smith, T. A. J . Aerosol Sci. 1986, 17, 211. Belyaev, S. P.; Levin, L. M. J . Aerosol Sci. 1974, 5 , 325. Sehmel, G. A. Am. Znd. Hyg. Assoc. J . 1970,31, 758. Durham, M. D.; Lundgren. J. Aerosol Sci. 1980,11, 179. McFarland, A. R.; Ortiz, C. A,; Bertch, R. W., Jr. J . Air Pollut. Control Assoc. 1984, 34, 544. Newton, G. J.; Shenk; K. J.; Su, Y.-F.; Yeh, H.-C.; Hoover, M. D.; Boecker, B. B. Aerosol Studies for Evaluation of “Zsokinetic” Sampling at the Waste Isolation Pilot Plant: Phase Z, Measurements in the 14-ft Diameter Exhaust Shaft; Lovelace Inhalation Toxicology Research Institute:
Albuquerque, NM, 1987.
Waste Isolation Pilot Plant. Report No. EEG-37; Environmental Evaluation Group, Health and Environment Department, State of New Mexico, Santa Fe, NM, 1987. (12) Ambient Qu&Y Standards for p a r t i c ~ t Matter, e u.s, Environmental Protection Agency 40 Fed. Regist. 52,1987, 24634-24750, CFR Parts 50-53and 58. (13) Environmental Evaluation Group Peer Review Meeting ~
on the WZPP Stack Monitoring System, November 14, 1986; Environmental Evaluation Group: P.O. Box 968,
Santa Fe, NM, 1986.
(14) Owen, E.; Parkhurst, R. C. The Measurement of Air Flow;
Pergamon Press: Oxford, U.K., 1977. (15) Torgeson, W. L.; Stern, S. C. J . Appl. Meteorol. 1966, 5, 205. (16) McFarland, A. R.; Ortiz, C. A. A helicoptor-borne sampler for plume fly ash. Aerosol Technology Laboratory Report 5276/01/07/88/ARM; Department of Mechanical Engineering, Texas A&M University, College Station, TX. (17) Ortiz, C. A.; McFarland, A. R. J. Air. Pollut. Control Assoc. 1985, 35, 183. (18) Berglund, R. N.; Liu, B. Y. H. Environ. Sci. Technol. 1973, 7, 147.
(19) Olan-Figueroa,E.; McFarland, A. R.; Ortiz, C. A. Am. Znd. Hyg. Assoc. J . 1982, 43, 395. (20) Olson, R. M. Essentials of Fluid Mechanics; Harper & Row: New York, 1980. Received for review September 19,1988. Accepted July 31,1989. This research was funded by the US.Department of Energy, W I P P Project Office under Contract DE-AC04-86AL31950 to Westinghouse Waste Isolation Division. Westinghouse, i n turn, funded us under Subcontract No. 94- WLM-26907-SD.
Mechanisms of Atmospheric Release of Chlorobenzene and Toluene from a Storage Lagoon under Ambient Conditions Richard A. Wadden’ School of Public Health, University of Illinois at Chicago, Chicago, Illinois
Linda R. Berrafato-Trlemer Exxon Biomedical Sciences, Exxon Corporation, East Millstone, New Jersey
Release rates of toluene and chlorobenzene were determined from an aqueous pool in the open air. Simultaneous measurements of air and water concentrations as well as meteorological conditions were made in the atmosphere during 1124-h periods from September through December 1983 in Chicago. For periods of 2 7 h after organic addition, a two-film model with the liquid phase controlling was found to be appropriate for describing emissions, with aqueous-air mass-transfer coefficients of 12.7 X lo4 m/s for toluene and 2.6 X lo* m/s for chlorobenzene. However, the mechanism of release was found to be different for shorter periods. Emission of toluene in the first 1/2 h was well-described by assuming evaporation of an organic film overlying the aqueous phase. The release of chlorobenzene followed a three-phase model, which included a submerged chlorobenzene-rich organic phase. The patterns of emissions implied by these findings differ significantly from those expected with only a twofilm model. Waste lagoons or surface impoundments are commonly used to collect liquid wastes for storage, treatment, or disposal. Wastes designated as “hazardous” under the 1492
Environ. Sci. Technol., Vol. 23, No. 12, 1989
Resource Conservation and Recovery Act are also treated in this fashion, with over 20 million tons disposed in surface impoundments in 1981 alone ( I ) . Airborne release of chemicals from liquid waste lagoons is a potential public health concern and measurable emissions have been reported from a number of studies (2-6). Because field measurements will always be limited by time and resources, it is desirable to have reliable methods for predicting the emission rates of hazardous materials. Although a number of theoretical models are available to estimate evaporative release rates (6-15), there have been only a few two-phase validation studies under ambient field conditions to support their use (6, IO). In this research, a data set appropriate for validation of theoretical emission models was collected and evaluated with respect to mass transfer to the atmosphere. The release of toluene and chlorobenzene from aqueous solutions was measured in an outdoor setting. These chemicals are representative of organics found in industrial and municipal waste waters ( 5 , 6 ) . Meteorological variables and lagoon temperature and depth were measured in addition to liquid and air chemical concentrations. Data were collected over 1124-h intervals from September through December under varying
0013-936X/89/0923-1492$01.50/0
0 1989 American Chemical Society
