Laboratory Testing Method for PM-10 Samplers: Lowered Effectiveness from Particle Loading

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Laboratory Testing Method for PM-10 Samplers: Lowered Effectiveness from Particle Loading Walter John & Hwa-Chi Wang To cite this article: Walter John & Hwa-Chi Wang (1991) Laboratory Testing Method for PM-10 Samplers: Lowered Effectiveness from Particle Loading, Aerosol Science and Technology, 14:1, , DOI: / To link to this article: Published online: 08 Jun Submit your article to this journal Article views: 165 View related articles Citing articles: 8 View citing articles Full Terms & Conditions of access and use can be found at

2 Laboratory Testing Method for PM-10 Samplers: Lowered Effectiveness from Particle Loading Walter John* and Hwa-Chi wang$ Air and Industrial Hygiene Laboratory, California Department of Health Services, Berkeley, California Field tests have revealed large differences in the sampling efficiencies among PM-10 samplers which previously satisfied U.S. Environmental Protection Agency (EPA) performance requirements in wind tunnel testing. To investigate the causes resulting from processes internal to the samplers, a laboratory testing method was developed based on the introduction of aerosol to the sampler in a manner than approximates ambient sampling. The method was verified by showing that filter deposit uniformity and measurements of sam- pling effectiveness agree with field and wind tunnel data. The 50% cutpoints of a Wedding PM-10 sampler and an oiled Sierra Anderson 321A sampler loaded with deposits from the EPA Phoenix I11 field study were found to be lower than those for the clean samplers. Analysis of these data and data from Phoenix 111 shows that the sampling effectiveness of a PM-10 sampler with an oiled collection surface decreases with loading. INTRODUCTION On July 1, 1987, the U. S. Environmental Protection Agency (EPA) promulgated a new National Ambient Air Quality Standard for particulate matter which was designated PM-10, standing for particles sampled with a 50% cutpoint of 10 pm in aerodynamic diameter (U. S. Environmental Protection Agency, 1987). To be officially accepted, a PM-10 sampler must satisfy certain performance specifications as verified by explicit testing procedures, including measurements of sampling efficiency with liquid and solid particles in a wind tunnel. The intent was to allow flexibility in sampler design, but to ensure that sampling characteristics were closely the same. However, two candidate samplers, the Sierra Andersen model 321A (SA 321A) and the Wedding high-volume PM- 10 sampler (Wedding), which had passed the required tests, were found by *To whom correspondence should be adressed. 'present address: American Air Liquide, Chicago Research Center, Countryside, IL numerous researchers to give significantly different results in side-by-side sampling in ambient air, the SA 321A giving higher particle concentrations (Rodes et al., 1985; Langer, 1987; Mathai et al., 1988; Wedding et al., 1988). One of the most definitive ambient tests was the third in a series conducted at Phoenix, Arizona by the EPA (Purdue et al., 1986), referred to as Phoenix 111. In the presence of the high coarse particle concentrations at the site, the SA 321A averaged 58% higher mass concentrations than the Wedding sampler. Another SA321A was operated with an oiled impaction surface to reduce large-particle pass-through. This sampler averaged 15 % higher concentration than a "cleaned" Wedding sampler. Thus oiling did reduce the sampled concentration. However, during Phoenix I11 a loading effect was observed, i.e., a lowering of sampling efficiency as particles accumulated in the oiled samplers. A loading effect was anticipated for the Wedding sampler which contains an oil-soaked porous surface to retain the particles above the cutpoint. A Wed- Aerosol Science and Technology 14: (1991) O 1991 Elsevier Science Publishing Co., Inc.

3 94 W. John and H.-C. Wang /-- ding sampler that was partially cleaned between runs averaged 16% higher mass con- centrations than an uncleaned Wedding sampler. It was also found that an oiled SA - E 321A that was operated only during the last - half of the study averaged 6% more mass ZONE concentration than the oiled SA 321A operated over the entire study. This suggested - that a loading effect was also present in the - oiled SA 321A. Indeed, such a loading ef- fect in an oiled SA321A was reported earlier by McFarland and Ortiz (1985). The oiling of sampler surfaces, designed to prevent large-particle pass-through, apparently results in the buildup of particle deposits which influence sampling efficiency. The field tests revealed sampler problems which were unsuspected on the basis of the wind tunnel tests prescribed by the EPA regulations. However, field tests do not supply detailed mechanistic explanations of the observed effects and field tests are very expensive. The present work was undertaken to develop a laboratory method capable of detecting anomalous sampling effects owing to processes internal to the sampler. The method was validated and then used to investigate the particle loading effect. The loading effect is only one example of effects that can change sampler performance. Other effects leading to oversampling have been investigated with the laboratory method and will be reported separately. METHODOLOGY A typical PM-10 sampler consists of an inlet, a size-fractionator, a filter, and a flow controller. The inlet is designed to capture a representative sample of the ambient aerosol independent of wind speed and wind direction. A size-fractionator is then used to segregate the particles aerodynamically, with an efficiency ideally following that of a specified lung deposition function. The problems associated with the inlet aspiration efficiency have to be studied using wind tunnel or field testing. However, the problems associated TUREWLENT FIGURE 1. Schematic drawing of the flow pattern for a 15 km/h wind approaching a SA 321A sampler. The shaded portion enters the sampler. with size-fractionators can be investigated using laboratory aerosol techniques, if the test aerosol can be introduced into the sampler in a manner roughly approximating that in ambient air. Figure 1 illustrates schematically the flow pattern of a 15 km/h wind (the U. S. national average) approaching a SA 321A PM- 10 Hivol. The sampled air parcel is symmetric about the centerline and supplies 68 m3/h to the sampler. The width of the sampled air parcel is about one-third of the sampler diameter. The height of the air parcel is roughly that of the slit in the inlet. Most of the sampled particles come from upwind; only a small portion is expected to enter from the sides or the back. This flow pattern is emulated by introducing the aerosol from a flexible duct (7.6 cm ID) with a flow velocity of 15 km/h as shown in Figure 2. The duct is connected to a collar around the circumference of the sampler inlet slit. The transition between the duct and the sampler inlet is smooth and has a constant cross-sectional area so that the average flow velocity is maintained at 15 km/h. A removable test section was installed before the transition section to determine the concentration of the challenge aerosol using isokinetic sampling. A stainless steel filter holder (Figure 2) supported at the center of the test section, is used to collect the particle sample on a 12-mm filter.

4 Testing Methods for PM- 10 Samplers FIGURE 2. Laboratory arrangement for simulating ambient sampling. The complete experimental setup for the first series of tests is shown in Figure 3. Monodisperse aerosol was generated by a vibrating orifice aerosol generator (TSI 3050), neutralized by a Kr-85 source, and transported to a manifold. In most cases, solid ammonium fluorescein spheres (Vanderpool and Rubow, 1988) were used and the collected mass, extracted by ammonium hydroxide solution, was quantitated by fluorometry. Three PM- 10 samplers, including a Sierra Andersen SA-321A, a Wedding sampler, a Sierra Andersen SA-246B dichotomous sampler (SA-Dichot), and an audit filter could be operated in parallel. These samplers were arranged as shown in Figure 3 so that the flow was symmetric. Filtered air of about 100 L/min was introduced at the horizontal section of the piping to reduce sedimentation loss of coarse particles. The flow was induced by the blower or vacuum pump associated with the samplers. The total sampling flow rate, when all four samplers are operated, is 138 m3/h, which gives a velocity close to 15 km/h in the vertical pipe. The makeup air was drawn from an air-conditioned room through glass wool. Blank samples which were taken indicated no interference with fluorescence analysis from room air. h-8v Aerosol Generat or Neutralizer Filtered Air n Wedding 68 ~)/hr I I / M 'hr Filter I ' I Samplers 7 Glass Wool SIDE VIEW I TOP VIEW Room Air FIGURE 3. Laboratory arrangement for simultaneous testing of several PM- 10 samplers.

5 W. John and H.-C. Wang / WEDDING FIGURE 4. Distribution of filter deposits by quadrant. Arrows indicate the direction aerosol entered sampler. Filter Deposition Distribution Since the laboratory aerosol is introduced into one side of the sampler inlet with a velocity of 15 krn/h, it is necessary to investigate whether the directional feed of aerosols affects the measurements. Any nonuniformity should be greater for larger particles. However, the sampling effectiveness of the PM-10 inlets decreases with increasing particle size. The 15-pm aerodynamic diameter particles were used as a compromise. The 20 x 25-cm glass fiber filters were cut into four equal pieces for analysis by fluorometer. The mass fraction collected on each quadrant is shown in Figure 4. The aerosol was introduced from the third quadrant for the Wedding sampler and from the fourth quadrant for the SA 321A. There was a slight tendency for the near quadrant to have a higher mass fraction than the far quadrant. However, the coefficients of variation between the four quadrants are within 2% for both samplers, indicating almost no feed direction effect even for 15-pm particles. The directional feed of aerosols is successfully smoothed out through the inlet. In ambient sampling, wind direction changes during the sampling period would further smooth out any uneven deposition. There- FIGURE 5. Locations of 25-mm disks cut from the PM-10 sampler filters for determination of deposition uniformity. fore, the use of any quadrant for chemical analysis would be justified. To investigate the detailed deposition distribution on the filter, mm disks were cut from each filter arranged in the three groups shown in Figure 5: center (C), edge (E), and middle (M), which is intermediate between C and E. Table 1 shows the results as a function of particle diameter for the SA 321A and the Wedding sampler. The first column for each sampler indicates the coefficient of variation based on all 13 disks, the second column gives the coefficient of variation for the four middle disks, the third column gives the average mass ratio between the middle disks and the center disk, and the fourth column gives the average mass ratio between the edge disks and the center disk. For both samplers, the overall coefficient of variation is highest at 15 pm where, TABLE 1. Uniformitv of Filter Devosits SA-321A Wedding PM- 10 Aerodynamic CV, CV, E CV,CV, M E diameter all M Z 4 all M 4 4 (w) (%) (%) (%) (%) The locations of C, M, and E are shown in Figure 5. CV, coefficient of variation.

6 Testing Methods for PM-10 Samplers however, the variation over the middle disks remains relatively small. These results reflect a difference between groups of disks as indicated by the ratios in columns 3 and 4. For the SA 321A, the center disk collects the highest mass whereas the middle disks collect the least. The difference increases with particle size. For the Wedding sampler, the edge disks have about the same mass as the center disk, whereas the middle disks collect less. These variations are probably related to the flow arrangements in the samplers following the particle separation stage. Similar deposition patterns in ambient sampling were noted by Burton and Hayward (1988). Overall the results indicate that, for chemistry of particles smaller than 5 pm, a piece of the loaded filter can be randomly cut without significant bias. For larger particles, best results can be obtained by cutting a quarter of the filter along the vertical and horizontal axes. Sampling Effectiveness To further evaluate the laboratory testing arrangement, sampling effectiveness measurements were made with monodisperse ammonium fluorescein particles on clean samplers. The effectiveness for the SA 321A, the Wedding sampler, and the SA-Dichot were determined simultaneously. The reference concentration for each sampler was individually determined by an isokinetic probe. The effectiveness is the ratio of the detected mass per unit flow rate between the sampler and the reference filter. Nine particle sizes were used, ranging from 3 to 20 pm. Four 5-min runs were taken for each size. To compare with the results in the literature, it is necessary to have a standard procedure for the determinations of D,, (particle diameter corresponding to 50% effectiveness), steepness (S), and skewness (K). Since the separation efficiency 7 as a function of the particle aerodynamic diameter D,, is best described by an S-shaped curve, the fitting function (Hasan and Dzubay, 1987) can be used: The three parameters, DS0, S, and K, can be determined for a set of efficiency data using a least squares fit. A subroutine E04FDF using the gradient search technique from the NAG Fortran Workstation Library (Numerical Algorithms Group, Downers Grove, 11.) was adopted for multiparameter nonlinear regression. Applying this fitting procedure to data on the effectiveness (E) of the PM-10 inlets, where E=l-4, (3) the D,,, S, and K were determined and listed in Table 2. Also included in Table 2 are the calibration results from previous wind tunnel testing (McFarland and Ortiz, 1983; Ranade and Woods, 1986; Woods et al., 1986). The values of D,, and S were in good agreement with the wind tunnel results, while K, the least sensitive fitting parameter, disagreed in some cases. The close correspondence between the present effectiveness results and those from wind tunnels plus the lack of directional effect on the filter deposit show that the present technique of piping the aerosol into the inlet realsitically simulates ambient sampling. The technique can be used to investigate all aspects of sampler performance except, of course, inlet aspiration efficiency. EFFECTS OF AMBIENT LOADING Decrease in Cutpoint Two samplers used in the Phoenix III test were acquired by our laboratory to investigate the loading effect. The loaded Wedding sampler was used throughout the field test

7 98 W. John and H.-C. Wang TABLE 2. Effectiveness Parameters for PM-10 Samplers Cutpoint, Steepness, Skewness, Sampler, condition Dm S K Reference SA dichotomous Clean This work Clean McFarland and Ortu (1983) Wedding Hivol PMlO Clean This work Clean Ranade and Woods (1986) Loaded This work Loaded Woods et al. (1986) SA 321A Clean This work Clean O Ranade and Woods (1986) Loadeda This work he he SA 321A was not as heavily loaded as the Wedding sampler without cleaning and was heavily loaded. The SA 321A was oiled and used in the second half of the test period; it was therefore less loaded than the Wedding sampler. Effectiveness measurements were made on these two loaded samplers and then repeated after cleaning the samplers. The fitting parameters for the effectiveness data are listed in Table 2. The fitted curves for the Wedding sampler are plotted in Figure 6. The FIGURE 6. Measured effectiveness of a clean Wedding PM-10 sampler and one dirty from the Phoenix I11 field experiment. The lines are fits of the function in Eq. 1 to the present data, which refer to the sampler's fractionator only. largest percent change in the effectiveness occurs in the vicinity of the cutpoint. Both the Wedding and Sierra Andersen samplers showed a decrease in D,, with ambient loading, which suggests that this may be a universal phenomenon for an oiled collection surface. A straightforward mechanism can be suggested to account for the effect: the oiled surface has a cutoff determined by a critical trajectory for that particle which just misses the surface. When the particle deposit projects upward from the surface, it intercepts particles that previously missed the surface. This removes more of the large particles, shifting the cutpoint downward. ~1 Clean Wedding. Ranadc and Woods 0 Clean Weddmg, Thls work Loaded Weddm~, Thls work Loading Effect at Phoenix The data from Phoenix I11 can be analyzed to obtain the rate of loading of the oiled samplers in ambient air. For this purpose, the oiled samplers will be compared to two dichotomous samplers operated at Phoenix 111. These two samplers, the SA Dichot and the Wedding dichotomous sampler (manufactured by General Metal Works), both of which are unoiled, agreed closely during the

8 Testing Methods for PM-10 Samplers field test, with an average PM-10 concentration ratio, SA/Wedding = 0.98 k PM- 10 concentrations from the dichotomous samplers were consistently between higher values from the SA 321A and lower values from the Wedding sampler. The oiled SA 321A agreed much better with the dichotomous samplers than did the unoiled SA 321A. Likewise, the cleaned Wedding sampler agreed better with the dichotomous samplers than did the loaded Wedding sampler. These facts strongly suggest that the dichotomous samplers obtained more accurate concentrations than the unoiled SA 32 1A or the loaded Wedding sampler. Since the dichotomous samplers were unoiled, any loading effects should be much smaller than in oiled samplers. Therefore, without implying that the dichotomous samplers should be considered reference samplers, the oiled sampler data will be compared to the average of the values from the two dichotomous samplers. In Figure 7a, the fractional difference between the concentrations from the oiled SA 321A and the dichotomous samplers is plotted vs. the cumulative mass in the sampler. The latter is calculated from the difference between the mass sampled with a collocated high-volume sampler and the PM-10 mass from the oiled SA 321A. Assuming that the high-volume sampler collects essentially all particles, the difference is the mass that stays in the sampler and is therefore a measure of the loading. A similar plot is shown in Figure 7 b for the Wedding sampler. For the oiled SA 321A, the slope of the regression line is % /g and for the Wedding sampler the slope is % /g. Therefore, the decrease of effectiveness with loading is virtually identical for the two samplers; the agreement is well within the considerable scatter of the data. The agreement in spite of the quite different fractionator geometries can perhaps be explained by the fact that both samplers have the same basic sampling effectiveness curve. The agreement also supports the use of the CUMULATIVE TSP-SA0.g CUMULATIVE TSP-WED, g FIGURE 7. (a) Fractional difference between the mass concentrations sampled by an oiled SA 321A and the dichotomous samplers at Phoenix I11 vs. the cumulative difference between TSP and the oiled SA 321A. The regression line has a slope of % /g. (b) Plot similar to a for the Wedding PM-10 sampler. The slope of the regression line is -2.23%/g. mass difference TSP - (PM-10) as the measure of loading. For example, if cumulative SA 321A mass is used as the independent variable, the slope for the SA 321A is % /g whereas the corresponding slope for the Wedding against cumulative Wedding mass is - 4.4% /g, two apparently different loading rates. This difference results from the lower 50% cutpoint of the Wedding sampler, which also produces the negative intercept in Figure 7b. It should also be mentioned that the loading rates may vary somewhat at different sites. It is likely that the lowered concentration obtained with a loaded sampler results from

9 100 W. John and H.-C. Wang a lowering of the cutpoint. To test this hypothesis, the mass predicted to be sampled by the loaded Wedding sampler and a dichotomous sampler were calculated. For each sampler, the measured sampling effectiveness (parameters in Table 2) was multiplied by a lognormal ambient particle size distribution and the product integrated over all particle diameters. For Phoenix, the mode diameter was estimated to be about 13 pm and the geometric standard deviation about 2 (Rodes et al., 1985). The fractional difference between the loaded Wedding and the dichotomous sampler is predicted to be - 28 %, which compares favorably with the data near the end of the field test (Figure 7b). The large loading effect at Phoenix was due to the extreme conditions with high concentrations of coarse aerosol. The above data on the loading effect can be used to predict the performance of an oiled sampler under more normal conditions. For example, if the ambient TSP concentration were 100 pg/m3, typical for an urban area, and the PM-lO/TSP were 0.5, also typical, then the - 2.2%/g rate predicts a loss of 10% of the effectiveness in 57 days. Scheduled sampler cleaning can limit the loss to an acceptable amount. Such maintenance is now recommended by the manufacturers of PM-10 samplers. SUMMARY AND CONCLUSIONS A laboratory methodology has been developed to investigate anomalies in sampler performance resulting from processes internal to the sampler. A wind tunnel is not used, but the test aerosol is introduced into the sampler in a manner approximating that in ambient air. That conditions are realistic inside the sampler is indicated by two results. The first is that the filter deposits are relatively uniform. Moreover, those nonuniformities which are observed for the SA 321A and the Wedding PM-10 sampler are similar to those seen on filters taken during the Phoenix 111 field study and appear to be related to the flow geometries within the samplers. The second is that measured sampler effectiveness parameters are close to those obtained in wind tunnels, indicating that conditions at the fractionator are realistic. The present methodology can therefore be used to investigate processes within the sampler. It is not intended for nor is it capable of investigating inlet aspiration effects. The 50% cutpoints of a Wedding PM-10 sampler and an oiled SA 321A sampler, both loaded with particles from operation during Phoenix 111, were found to be lower than those for the clean samplers. The observed shift of the effectiveness curve with loading is consistent with sampler performance during Phoenix 111. It is shown that the effectiveness of an oiled SA 321A sampler decreased with particle loading at the same rate as did the Wedding sampler, when the loading is expressed in terms of the cumulative mass above the cutoff, i.e., the cumulative difference between TSP and PM- 10. This formulation rather than the simple PM-10 loading is necessary because of the unusually high coarse-to-fine ratio prevailing at Phoenix 111. It is concluded that decreasing effectiveness with loading is a characteristic of oiled samplers. The magnitude of the effect, % /g, permits control of the resulting error by periodic sampler cleaning. We thank Dr. Wolfgang Winklmayr for assistance with the multiparameter regression analysis. Helpful discussions were held with Larry J. Purdue and Kenneth Rehme. This research was partially supported by EPA Cooperative Agreement CR REFERENCES Burton, R. M., and Hayward, S. B. (1988). Paper presented at 1988 Annual Meeting of the American Association for Aerosol Research, Chapel Hill, NC. Hasan, H, and Dzubay, T. G. (1987). Aerosol Sci. Technol. 6: Langer, G. (1987). Report RFP-4122, UC-38 Engineering and Equipment, DOE/TIC-4500 (Rev. 73). Rockwell International, Golden, Col.

10 Testing Methods for PM-10 Samplers 101 Mathai, C. V., Belknap, K. L., and Hicks, D. B. (1988). In Transactions, APCAIEPA International Specialty Conference, PM-10: Implementation of Standards, San Francisco, Calif., February 1988, TR-13. Air Pollution Control Association, Pittsburgh, Pa., pp McFarland, A. R., and Ortiz, C. A. (1983). Air Quality Laboratory Report 4716/Ol/ll/83/ARM. Texas A& M University, College Station, Tex. McFarland, A. R., and Ortiz, C. A, (1985). Air Quality Laboratory Report 47 16/02/06/85/ARM. Texas A& M University, College Station, Tex. Purdue, L. J., Rodes, C. E., Rehme, K. A,, Holland, D. M., and Bond, A. E. (1986). J. Air Pollut. Control Assoc. 36: Ranade, M. B., and Woods, M. (1986). EPA Wind Tunnel Test Report No. 16, May Research Triangle Institute, Research Triangle Park, NC. Rodes, C. E. Holland, D. M., Purdue, L. J., and Rehme, K. A. (1985). J. Air Pollut. Control Assoc. 35: US. Environmental Protection Agency. (1987). Federal Register 52: Vanderpool, R. W., and Rubow, K. L. (1988). Aerosol Sci. Technol. 9: Wedding, J. B., Weigand, M. A,, Kim, Y. J., and Lodge, J. P. Jr. (1988). In Transactions, APCA/EPA International Specialty Conference, PM-10: Implementation of Standards, San Francisco, Calif., February 1988, TR-13. Air Pollution Control Association, Pittsburgh, Pa., pp Woods, M. C., Chen, F. L., and Ranade, M. B. (1986). EPA Wind Tunnel Test Report No. 19, July Research Triangle Institute, Research Triangle Park, N.C. Received June 26, 1989; accepted June 11, 1990.