Habibi, K., Jacobs, E. S., Kunz, W. G., Pastell, D. L., “Characterization and Control of Gaseous and Particulate Exhaust Emissions from Vehicles,” presented a t the Air Pollut. Contr. Ass., West Coast Section, Fifth Technical Meeting, San Francisco. Calif. Reoort available from E . I. DuPont DeNemours and Co., Wilmiigton, Del., October 1970. Hirschler, D. A , , Gilbert, L. F., Lamb, F. W., Kiebylski, L. M., Ind. Eng. Chern., 49, 1131-42 (1957). Hirschler, D. A., Gilbert, L. F., Arch. Enuiron. Health. 8 . 297-313 (1964). Lee, R. E . . Patterson, R. K., Crider, W. L., Wagman, J., Atrnos. Environ , 5,225-37 (1971). Macfarlane, J. J., Holderness, F. H . , Whitcher, F. S. E., Cornbust Flame, 8,215-27 (1964). McKee, H . C., McMahon, W. A . , J. Air Pollut Contr. Ass., 10, 456-62 (1960). Moran, J. B., Manary, 0. J., “Effect of Fuel Additives on the Chemical and Physical Characteristics of Particulate Emissions in Automotive Exhaust,” Interim Reut. to the Nat. Air Pollut. Contr. Ass., submitted by the Dow. Chemical Co.. Midland, Mich., July 1970. Moran, J. B., Manary, 0. J., Fay, R. R., Baldwin, M. J., “Development of Partic;late Emission Control Techniques for Spark Ignition Engines, Final Rept. to the Environmental Protection Agency, submitted by The Dow Chemical Co., Midland, Mich., July 1971. ~
~~
~
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Mueller, P. K., Helwig, H. L., Alcocer, A. E . , Gorg, W. K., Jones, E . E., Symposium on Air Pollution Measurement Methods, American Societv for Testing and Materials, Special Tech? Publ. No. 352, pp60-77 (1962).Ninomiva, J . S., Bergman, W., Simpson, B. H., “Automotive Particulate Emissions,” presented 2 the Second International Clean Air Congress, Int. Union of Air Pollut. Prevention Ass., Washington, D.C. report available from Automotive Emissions Office, Ford Motor Co., Dearborn, Mich., December 1970. Sampson, R. E., Springer, G. S., “Effects of Temperature and Fuel Lead Content on Particulate Formation in Spark Ignition Engine Exhaust,” Fluid Dynamics Laboratory, Publication No. 72-1. DeDartment of Mechanical Engineerine. The Universitv of Michiga;, Ann Arbor, Mich., 1972. Schalla, R. L., Hibbard, R. R., NACA Report No. 1300, Chap. IX, pp 346-455 (1957). Springer, G. S., “Engine Emissions,” G. S. Springer and D. J. Patterson, Eds., Chap. V, Plenum Press, New York, N.Y., 1972. Ter Haar, G. L., Lenane, D. L., H u , J . N., Brandt, M., J . Air Pollut. Contr. Ass., 22,39-46 (1972). Watson, H. H., Ind. Hyg. Quart., 3 , 2 1 4 (1954). I
Y
Received for review May 19, 1972. Accepted October 17, 1972. This work u a s supported by the Environmental Protection Agency under Grant No. AP-01012-01.
NOTES
Dependence of Hi-Vol Measurementson Airflow Rate Arnold L. Cohen Office of Research and Monitoring, National Environmental Research Center, Environmental Protection Agency, Cincinnati, Ohio 45268
Comparisons of particulate concentration data obtained from Hi-Vol air pollution samplers, operating a t different airflow rates, can lead to errors. A difference of only 1% was found between hi-vol samplers operated a t 50 and 60 cfm; however, hi-vols operated a t 40 and 60 cfm exhibited a 3.8% difference. Loss of submicron aerosol particles through the filter pores a t the higher face velocities account for the discrepancies. The test data reported here point out the necessity to control the conditions under which air samples are collected in the various localities. H
The high volume (hi-vol) air sampler is widely used by air pollution control agencies and industry in the U.S., Canada, Japan, and other countries to determine the concentration of total suspended particulate matter. The sampler is operated for a 24-hr period by drawing air a t the rate of 40-60 cfm through an 8 x 10-in. glass fiber filter capable of removing nearly 100% of all particulates 0.3 in diam or greater (Pate and Tabor, 1962). Although most hi-vol samplers in use are of approximately the same configuration-i.e., gabled roof design, plywood or aluminum construction, comparable air inlet area-the samplers are not operated in a uniform fashion. Air pollution control agencies may sample a t an initial flow rate that can vary from 35 cfm to 65 cfm. Since the particle capture velocity a t the sampler air inlet is a direct function of airflow rate, the question arises as to the comparability of particulate concentration data collected from samplers not operating at the same flow rate. 60
Environmental Science & Technology
A study was conducted a t the Environmental Toxicology Laboratory of the National Environmental Research Center, Cincinnati, to determine the effect of airflow rate on particulate concentration measurements made with hivol samplers. Four gabled roof hi-vol samplers (GMW Model 2000) were operated on the third-floor roof of the Laidlaw Avenue Laboratory in conjunction with a National Air Surveillance Network (NASN) cascade impactor sampler (Lee and Flesch, 1969) for determining the size distribution of suspended particulate matter. Samples were collected from June 17 to July 22, 1971. The hi-vols were operated for 24-hr sampling periods a t flow rates that varied from 30 to 60 cfm. The flow rates on each sampler were changed periodically to eliminate possible effects inherent in an individual sampler. The initial flow rate was set on each hi-vol using a variable transformer, and, at the end of the 24-hr sampling period, the flow rate was again determined with a calibrated rotameter according to prescribed EPA procedures (U.S. EPA, 1971). The average flow rate for the 24-hr period was determined from the initial and final readings. In general, the decrease in flow rate owing to particulate loading on the filters did not exceed 3 cfm. The results of these tests are presented in Table I. The data indicate a trend toward lower concentration measurements with increasing airflow rate. The particulate concentrations over the entire seven-day sampling period varied from approximately 100 to 200 wg/m3, thereby providing a wide aerosol range for evaluation. This range is in general agreement with NASN results for 1970 where the total suspended particulate concentration varied from 45
Table I . Concentration Measurements from Hi-Vol Samplers Operated at Different Flow Rates Cascade impactor 50cfm pg/m3 60 cfm p g / m 3 YO part < 1 pdiam Day 30cfr-r pg m3 40cfm pg m 3 1 2 3 4 5 6 7
112 168 200 138 132 159 141
Mean
150
104 160 191 129 121 141 134 140
110 161 199 132 123 148 137 144
to 265 pg/m3. The geometric average concentration was 101 pg/m3 ( S A S X , 1972). Similarly, the particle size data given in Table I is within the range found in Cincinnati in 1970 by the YASN (Lee and Goranson, 1971). The mass median diameter of the aerosol ranged from 0.58 to 1.03 p compared to the range of 0.26 to 1.33 p found in 1970. These data show that the aerosol sampled in the study reported here was typical; it was comparable to that previously determined for Cincinnati and represented a broad range of particle sizes ordinarily found in ambient air. A number of previous studies have shown that a high degree of precision can be obtained with concurrently operated hi-vols. Lee et al. (1971) found that 95% of the data from over 430 paired hi-vols operating in Great Britain were within 5% of their operating mate; similarly. Clements (1972) found that hi-vols in the U.S. S A S S were operating with an average precision of 4.5%. An average precision of 3 . 0 7 ~was found in a collaborative test of hivol samplers sponsored by EP4 (Faoro. 1972). In the study reported here, concurrently operated hi-vols at these low flow rates exhibited an average precision of 1.3% (Table 11). Consequently. intercomparisons of the data given in Table I can be based on this precision-i.e.. comparative concentration values in excess of 1.3% may be considered significant. The data were subjected to the Friedman Two-Way Analysis of Variance test (Siegel. 1956). The hypothesis tested was that the mean ranks of the concentration data for each flow rate shown in Table I are equal. The conclusion of this test was to reject the equality of the mean ranks a t the 0.001 level of significance, proving that the differences due to variable flow on the mass loading of particulate matter are significant. This nonparametric test was used to avoid making the following questionable assumptions: Observations are independently drawn from normally distributed populations. The populations have the same variance. The effects are additive. The data clearly show the errors inherent in comparing hi-vol data collected at different flow rates. A difference Table I I. Concentration Measurements from Hi-Vol Samplers Operating at Same Flow Rate on a Given Day Flow rate Hi-vol samplers, p g / m 3 cfm 60 50 40 30
Mean
160 156 141 102 139.8
160 147 140 100 136.8
160 152 140 98 137.5
Mass median diam
p
105
151 192 125 121 142 136 139
49 52 59 67
1 03 0 78 0 69 0 58
slightly less than 1% can be expected between hi-vol samplers operated a t 50-60 cfm. The difference increases appreciably (to 3.8%) when data from a hi-vol operated a t 40 cfm is compared to a system operating a t 60 cfm. However. a 60-cfm sampler exhibited a 7.4% lower concentration than a 30-cfm sampler. It is postulated that this effect lies in the loss of submicron aerosol through the filter pores at the higher face velocities. Although the glass fiber filters are reported to be extremely efficient for 0.3-p diam particles. it is likely that high flow rates sweep away a larger proportion of particles in the lower submicron range than at lower flow rates, probably through reentrainnient in the airstream. Large particles would be expected to remain within the filter through mechanical entrapment in the filter. This is especially plausible in view of the predominantly submicron size of urban particles and may suggest that more accurate concentration measurements can be achieved by operating hi-vol samplers a t comparatively low flow rates. It is recognized however, that other factors such as careless filter weighing, improper sampler operation, and other errors may contribute to sampling inaccuracies despite the flow rate. Acknowledgment Grateful acknowledgment is given to Robert E. Lee, Jr.. for his valuable suggestions and to Gerald Akland for his statistical assistance. Literature Cited Clements, H.. Report on duplicate hi-vol samplers operated in the N A S S at 50 sites. unpublished data, 1972. Faoro, R.. U.S. Environmental Protection Agency. Research Triangle Park. S . C . . unpublished test results, 1972. Lee. R. E.. J r . . Caldwell, J. S., Morgan, G. G., “The Evaluation of Methods for Measuring Suspended Particulates in Air,” presented at the 162nd Sational Meeting. ACS, Washington. D.C., September 12-17,1971. Lee. R. E., Jr., Flesch, J . P . , “A Gravimetric Method for Determining the Size Distribution of Particulates Suspended in Air,” presented at the annual meeting of the Air Pollution Control Association, Yew York, N.Y., June 22-26, 1969. Lee. R . E.. Jr.. Goranson, S.. “The S A S S Cascade Impactor Network: First Year Operation,” presented at the 162nd S a tional Meeting. ACS, Washington. D.C., September 12-17. 1971.
160 152 139 100 137.8
S A S Y , National Air Surveillance Setwork data compilation for 1970. Environmental Protection Agency, National Environmental Research Center. Research Triangle Park. X.C., unpublished data. 1972. Pate, J . B.. Tabor, E. C.Amer. Irzd. Hyg. Ass. J . , 23, 144 (1962). Siegel, S.. “Non-Parametric Statistics for the Behavior Sciences.” McGraw-Hill, S e w York. N.Y., 1956. U.S. Environmental Protection Agency. “Xational Primary and Secondary Ambient Air Quality Standards.” Fed. Regist.. 36 (84), 8190 (April 30. 1971).
=Precision 1 3% Receiced for recieu M a y 30, 1972. Accepted August 24, 1972.
Volume 7, Number
1, January 1973
61
