Document Ogbm29aJq47aRpgge40VZzXe

Environmental Science & Technology Evaluation of Bag Sequential Sampling Technique for Ambient Air Analysis John C. Polasak and Jarry A. Bullin Volume 12, Number 6 Pages 708-712 Copyright 1978 by the American Chemical Society and reprinted by permission of the copyright owner OLI 3052 The type of gas sampling bags described as "Snout" (U.S. Patent #3,907,164) and "Aluminized Polyester" are both exclusive products of Calibrated Instruments, Inc. Aluminized polyester bags are no longer available however since the manufacturer of the material (Minnesota Mining) has discontinued making same. Our current snout type bags have incorporated some of the properties of the aluminized polyester bags that are mentioned in the report and in our opinion, are suitable for all types of gas sampling. OLI 3053 Evaluation of Bag Sequential Sampling Technique for Ambient Air Analysis John C. Polasek and Jorry A. Bullin* Chemical Engineering Department, Texas A&M University, College Station, Tex. 77843 The bag sequential sampling technique for sampling am* bient air was evaluated by comparing results from bag se quential samplers and continuous analyzers. When placed side by side, carbon monoxide concentrations from the two methods did not correlate well. However, when the samples were drawn through a common header, the concentrations agreed to within 1 ppm for at least 90% of the data points for carbon monoxide concentrations between 1 and 8 ppm. Bags constructed of polyvinyl chloride; Tedlar; layers of polyester, polyvinyl chloride, aluminum foil, polymide, and polyethyl ene; and aluminized polyester were tested up to 100 h for their influence on sample integrity. Only the aluminized polyester bags were satisfactory for long-term storage. Certain pump and tubing materials were found to affect the sample. The bag sampling technique has good potential for collecting ambient air samples. With the increasing interest in pollution control, there is a much greater demand to monitor pollutant concentrations in ambient air. In most cases, the ambient concentrations are in the range of 0.01-10 ppm and are expensive as well as dif ficult to measure. In an attempt to overcome some of these problems, several new air monitoring techniques are being introduced. Before extensive economic and health decisions based on data from these new techniques can be made, expe rience dictates thorough evaluation programs. For example, decisions must be made regarding the con struction or major modification of roadways. Questions en countered include: Will the construction of a new roadway allow smoother traffic flow, thus reducing congestion on ex isting streets and the pollution load on the general area? Will the new roadway significantly affect the air quality in its im mediate vicinity? To answer these questions, experimental measurements must be made along existing roadways, and a capability to predict air pollution levels from roadways must be developed. Of course, the prediction capability or model can be no more accurate than the data it is based upon. Thus, it is imperative that data collecting techniques must be closely scrutinized. Most models for the dispersion of pollutants from roadways are based on experimental measurements of carbon monoxide along roadways. Carbon monoxide can be measured fairly accurately, and it is also stable. Pollutant Sampling Methods. There are three basic methods to acquire the data necessary for model development Intermittent Sequential Sampling (ISS) is the cheapest least elegant technique for data collection. A single monitoring instrument is attached to a multiport header through a series of solenoid activated valves. These valves are activated by a stepping switch that allows only one port to be open at any particular time. Sequential sampling allows several locations to be intermittently monitored with only one instrument The data obtained with this technique are rather pom for modeling purposes since no readings are simultaneous and sometimes the lag between subsequent readings at the same point is many minutes. According to probability theory, Papoulis (/), and Bendat and Piersol (2), a sampling frequency of at least twice the highest frequency of concentration variation is required to statistically reproduce the concentration. Other problems associated with the ISS technique include the long lines from the sampling points to the analyzing instrument. 70S Environmental Science 8 Technology Bag sequential sampling can yield an improved quality of data at a modest increase in cost if it is performed properly. Again there is only one analytical instrument, but instead of piping the sample to the instrument directly, it is pumped into a series of air-tight bags at each sample point. Thus, when filled properly, each bag can represent the averaged concen tration for a given period. The bags can then be analyzed at leisure. Currently, there are two manufacturers of bag se quential samplers. One of the primary problems with the bag sequential sampling technique is the interaction of the sample with the materials of construction, that is, the tubing, pump, and storage bag. Some materials will adsorb one or more of the pollutants, carbon monoxide, hydrocarbons and nitrogen oxides, while others will desorb these compounds. In addition, the ability of the bag sequential sampler to collect a repre sentative sample must be verified. Continuous monitoring yields a great improvement in data quality and quantity; however, the costs also increase greatly. In this method, an analyzing instrument is located at each sampling point For the highest quality data, the instruments can be interfaced to a digital computer or data logger which can record the instrument readings simultaneously and at any desired frequency. For carbon monoxide, the two most prac tical instrument types for continuous monitoring are elec trochemical and chromatographic instrumenta. The major problems with this approach are the expense and adaptability of some instruments to operate in the ambient atmosphere. Previous Work. The previous work to evaluate the bag sequential sampling technique has been concentrated on sample deterioration in the bags. Ranzieri et al. (3) reported sample deterioration testa for bags made of mylar and alu minized polyester. The bagB were filled with certified zero gas and with various span gases at concentrations up to 92 ppm. The samples in the bags were analyzed by nondispersive in frared (NDIR) analyzers after assorted time spans. The alu minized polyester bags did not affect the samples for the re ported total tasting time of 4-6 h. The mylar material was shown to be completely unacceptable. Noll et al. (4) describe the comparison of a set of bag se quential samplers with an intermittent sequential sampling system. Five points were checked in a test at an actual roadway site. All bags were analyzed by an electrochemical analyzer, and some were reanalyzed by a nondispersive infrared ana lyzer. The bags were made of aluminized polyester and were used to take 15-min samples. The ISS system consisted of an NDIR tied to aluminum foil jacketed tygon lines. No signifi cant differences in the concentration were noted. However, due to the nature of the teat, variations of less than 20% were not considered significant. Clemena (5) compared the sampling capabilities of two sequential samplers produced by different manufacturers. One contained aluminized polyeater bags and piston-type pumps while the other contained polyvinyl chloride (PVC) bags and rubber diaphragm pumps. At a roadway site, the samplers were placed side by side along with a gas chroma tograph with a flame ionization detector. The sample con centrations from the systems using aluminized polyester bags and piston pumps were all within 12% of the chromatograph readings, lie concentrations from the system using PVC bags and rubber diaphragm pumps were about 50% below the chromatograph reading at carbon monoxide levels of about 20 ppm and about 20% high at concentrations of about 5 ppm. OLI 3054 From additional tests, Clemens concluded that the rubber t diaphragm pump and tubing in the sampler were also ab sorbing or adsorbing the carbon monoxide from the sample. In a limited test, Moe (6) checked sample deterioration in Tedlar and PVC bags. The bags were filled with a carbon monoxide span gas in the lab and analyzed periodically for up to 24 h with a nondispersive infrared analyzer. With 45.0 ppm span gas, the concentration in the PVC bags deviated a max imum of only 6% over the 24-h period. However, with 7.5 ppm span gas the rnaiimum deviation was about 50% high, and the average deviation from all'of the time intervals was 16% high. Due to leaks discovered in several of the Tedlar bags, the re sults were considered inconclusive. Current Work. From the above discussion, it is apparent that the validity of the bag sequential sampling technique has not been established. The current work describes a complete experimental analysis of the bag sequential sampling tech nique. Tests of sample deterioration with various bag and sampler construction materials are also discussed. Experimental Methods and Discussion of Results To evaluate the bag sequential sampling technique, a series of experimental tests were designed. These tests included Comparison between bag samplers and continuous ana lyzers placed side by side Comparison between bag samplers and continuous ana lyzers sampling through a common header Sample deterioration with time for various bag materi als Sample deterioration by sample passing through the bag sampler tubing and pump. The bag samplers used in these tests consisted of 24 rubber diaphragm pumps similar to aquarium air pumps, with each pump connected to a 1.5-L bag by gum rubber tubing. The sequencing was controlled by an electronic timer programmed to run each pump for 15 min, then advance to the next. The sampler internals were sheltered in a plastic barrel about 2 ft in diameter and 4 ft high. The sample intake ports for the pumps were distributed uniformly around the barrel at a 3-ft height. The continuous analyzers were of the electrochemical type (Ecolyzers) with a range of 0-50 ppm carbon monoxide, a span drift of 1% of full scale per 24 h, and a zero drift of 0.5 ppm/24 h. The continuous analyzers were read simultaneously every 10 s by a minicomputer, and the readings were logged onto magnetic cassette tape. To ensure good accuracy, the instruments were zeroed and calibrated every 2 h. The in strument readings were corrected by assuming a linear span drift between calibrations. No correction was made for zero drift which was quite small. A gas chromatograph was also used to analyze the bag samples. The principle of operation of both the gas chroma tograph and electrochemical analyzers is discussed in Amer ican Society for Testing and Materials (7) and Benchley et al. (8). Bag Sampler and Continuous Analyzer Side by Side. In this test, bag samplers with PVC bags and continuous electrochemical analyzers were placed side by side at various distances ranging from 15 to 150 ft from a 10-lane freeway. The intakes for the continuous analyzers were 4 ft above ground. The pump intakes for the bag samplers were 1 ft below the continuous analyzer intakes and between 6 in. and 3 ft away laterally, depending on which pump was running. Each day of the tests, the bag samplers were operated until all 24 bags were filled (6 h). At the end of the day, the bags were collected and analyzed by one of the electrochemical analyzers which was rezeroed and recalibrated after each 24 bags. Figure 1. Comparison of bag samplers with PVC bags to nearby con tinuous monitors Typical results from these tests are shown in Figure 1. Less than 30% of the points fall within 1 ppm of the 45* line of perfect agreement, and the points were almost uniformly distributed. This indicates large random deviations which completely override any systematic deviations that may be present. The poor correlation was believed to be caused by the asymmetry of the sample ports on the sampler. Since almost no correlation was found, further tests were undertaken to ensure that the continuous analyzers were correct as well as to evaluate the bag samplers. Bag Sampler and Continuous Analyzer with Common Intake Header. A sample header was built to deliver a com mon air sample to two bag samplers and two continuous monitors at the same time. The header consisted of 1-in.diameter polyethylene tubing. The sample entered through a 2-ft-long sample intake, and then was split to serve each of the bag samplers. The sample intakes for the bag samplers which were distributed around the barrel were connected to the 1-in. polyethylene header. The sample ports for the con tinuous analyzers were placed in the 2-ft header intake section. The air was pulled through the header at a high rate by two vacuum pumps downstream of the bag samplers. Both vacu um pumps were adjusted to ensure a near uniform air flow through each branch of the header. To make the bag samplers completely independent of the continuous analyzers, a gas chromatograph was used to analyze the bags for these tests. Each bag was disconnected from its pump and sealed within 15 min of pump shut down to prevent any sample loss back through the pump. Tests were conducted with PVC bags and Tedlar bags in the samplers. Two continuous analyzers were attached to the common header to test the accuracy and reliability of the continuous measurements. The continuous analyzers were read every 10 s by the minicomputer. A representative comparison of the 10-s values as a function of time for the two analyzers is shown in Figure 2. This figure also shows that the carbon monoxide concentration along roadways varies widely and at frequencies up to 2 cydes/min. A representative comparison of the 15-min averages from the two analyzers is shown in Figure 3. Based on a comparison of 101 flfteen-min averages, the standard error between the two continuous analyzers was 0.28 ppm, and the standard deviation of the error was 0.22 ppm. The values from the Tedlar bags were consistently low for both sampling days. OLI 3055 Volume 12. Number 6, Jus 1978 70S Additional taata with two continuous analyzers and two bog samplers both containing PVC bags, sampling through a common header were performed (Figure 4). The results showed that all points were within a 1 ppm limit except a few in the 5-6 ppm range where the bag samples gave low values. Based on a comparison of 98 fifteen-min averages between the continuous analyzer and the bag sampler, the standard error was 0.41 ppm, and the standard deviation of the error was 0.39 ppm. To check the consistency between the two bog samplers, the results from the samplers were plotted against each other as shown in Figure 5. This plot shows that Bag Sampler No. 2 gave.higher values than Sampler No. 1 about 80% of the time. From a comparison of 44 fifteen-min average* between the two bag samplers, the standard error was 0.54 ppm, and the standard deviation of the error was 0.55 ppm. Semple Deterioration with Bag Materials. The influ ence of bag material on sample deterioration was tested by filling bap made from different materials with calibration gas containing carbon monoxide, nonmethane hydrocarbons, and methane. The samples in the hap were analyzed immediately after filling end after 24,48. and 100 h bye gee chromatograph. The bag types were polyvinyl chloride (PVC), Tedlar, "Snout", and "aluminized polyeater". The "snout" bap were constructed of layers of polyester, polyvinyl chloride, alumi num foil, polymide, end polyethylene whereas the "aluminized polyester" bap were constructed of layer* of polyester and aluminum. The snout bap were purged six times with zero air before being filled with span gas. The purge gas wss left in the hap for several hours during each purge. All bap were stored in a controlled environment between analyses. A summary of TIME COST Figure 2. Comparison of two continuous monitors on common header as a function of time Figure 4. Comparison of bag sampler with PVC bags end continuous monitor on common header Figure 3. Comparison of two continuous monitors on common head- header 710 Environmental Science 4 Technology OLI 3056 1 th result* hots the carbon monoxide tests is shown in Table rubber connecting tubing was tasted. PVC begs were filled I for the four bag types, and s summary from the nonmethane with nominal 10 ppm span gss and attached to the pump in hydrocarbon tests is shown in Table 11. The results showed takes on a sampler. The calibration gss flowed through the that: tubing and diaphragm pump by action of the diaphragm PVC bags are good only for short-term (leas than 10-15 pump. The gas was collected in other PVC bags and then an h) sample storage for both carbon monoxide and nonmethane alysed by a gaa chromatograph. The results are shown in Table hydrocarbons. III. The pumps and tubing contributed significantly to the Tedlar bags are poor for sample storage of ambient air system error, causing drifts aa larp as 0.6 ppm in the observed concentration level*. carbon monoxide values. Snout bagB are excellent for long-term sample storage of carbon monoxide. However, for nonmethane hydrocarbons, Summary and Conclusions the snout bags are completely unacceptable. Extensive testa have been performed to evaluate the bag Aluminized polyester bags are excellent for long-term sequential sampling technique for sampling ambient air. sample storage of both carbon monoxide and nonmethane When placed side by aide, carbon monoxide concentrations hydrocarbons. The concentration variations for both gases from bag samplers and continuous electrochemical analyzers were within the accuracy of the chromatograph at all times did not correlate well. However, when the samples to the bag in the 100-h tests. samplers and continuous analyzers were drawn through a All of the bag materials tested except the aluminized common header, the concentrations agreed to within 1 ppm polyester appear to give off nonmethane hydrocarbons during for at least 90% of the data points. the first 24-48 h resulting in an increase in the concentration. To determine the influence of bag materials on samples, This is followed by a long-term decay where the hydrocarbons calibration gaa was placed in the begs and then analyzed after are apparently reabsorbed or readsorbed by the bag materi 0,24,48, and 100 h. The results showed that polyvinyl chloride al. bags are satisfactory for sample storage up to about 15 h, Sample Deterioration Due to Pump and Tubing. The whereas Tedlar bags are, in general, unsatisfactory for sample effect of bag sampler rubber diaphragm-type pumps and gum storage. Bags consisting of layers of polyester, polyvinyl Table I. Carbon Monoxide Semple Deterioration with Time In Beg* of Various Materials Bee melerlai No. of bags tested Concn of calibration gas used to fill bags, ppm 0 h after filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 24 h after filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 48 h after filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 100 h after filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 eve 10 9.0 8.9 -0.12 0.056 8.5 -0.50 0.306 8.4 -0.63 0.497 7.9 -1.2 1.5 TeWer 10 9.0 8.5 -0.5 0.352 7.5 -1.5 4.2 6.8 -2.2 7.7 5.2 -3.8 17.8 * Consist* of layers of polyester, polyvinyl ctVoride. aksnSun. potymUe, aid polyethylene. Wteet* 5 8.2 8.2 0 0.012 8.0 -0.18 0.048 8.3 0.10 0.010 8.0 -0.18 0.088 potyMtar 3 8.2 8.0 -0.17 0.030 7.9 -0.30 0.097 8.2 -0.03 0.010 7.7 -0.50 0.25 Table II. Nonmethane Hydrocarbons Semple Deterioration with Tima In Saga of Various Materials Tra ----*-* No of bags tested Concn of calibration gaa used to fid bags, ppm 0 h after filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 24 h after Ailing, av ppm Av deviation, ppm Av sqd deviation, ppm2 48 h after filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 100 h attar filling, av ppm Av deviation, ppm Av sqd deviation, ppm2 eve 10 7.3 7.4 0.02 0.010 8.0 0.80 0.390 7.5 0.07 0.065 8.8 -0.80 0.880 TadUr 10 7.3 7.5 0.08 0.172 8.6 1.15 2.52 8.7 1.3 3.83 8.3 0.90 4.0 * Consists of layers of polyester, pofyvtryyt creoride. afesnbesn. putynSOs. snd polyethylene. OLI 3057 bee* 5 9.5 13.3 3.8 15.8 15.5 5.9 39.1 15.7 '6.2 41.9 15.4 5.9 38.4 >yelsr 3 9.5 9.6 0.1 0.016 9.8 0.30 0.090 9.6 0.10 0.017 9.4 -0.10 0.017 Volume 12, Ntsntoer 0. Jute 1978 711 Table HI. Sample Data*loratlon Dua to Pump and Tubing Material*1 Gas coocn to pump, ppm Five-min av exit coocn Pump No. 1, ppm Pimp No. 2, ppm Pimp No. 3. ppm Pump No. 4, ppm 10.3 10.3 9.9 10.0 10.5 10.0 9.5 9.1 9.1 9.8 * Rubber dtaptvagm pimp and gwn nObar tubing. 9.0 8.7 8.4 8.4 8.9 chloride, aluminum foil, polymide, and polyethylene are ex cellent for long-term storage of carbon monoxide, but com pletely unacceptable for nonmethane hydrocarbons. Alumi nized polyester bags consisting of a layer of polyester on both sides of an aluminum layer are excellent for long-term storage of both carbon monoxide and nonmethane hydrocarbons. Tests also show that rubber diaphragm pumps and gum rubber tubing can have a significant (0.6 out of 10 ppm) im pact on ambient air samples. In conclusion, the bag sequential sampling technique is a good method to collect samples of ambient air economically when used with the proper precautions. Literature Cited (1) Pspoulis, A_, "Probability, Random Variables, and Stochastic Processes", McGraw-Hill, New York, N.Y., 1966. (2) Bendat, J. S., Pieraal, A G., "Measurement and Analyiis of Random Data", Wiley, New York, I960. (3) Ranzieri, A J., Bemii. G. R., Shirley, E. C., "Air Pollution and Roadway Location, Design, and Operation", California Div. at Transportation Rep. No. A-DOT-TL-7O0O-75-15, Sacramento, Calif., 1975. (4) Noll, K. E,, Miller, T.L., Rainey, R.H., May, R.C., "Final Report on the Air Monitoring Program to Determine the Impact of High ways on Ambient Air Quality", Dept, of Civil Engineering, Uni versity of Tenneasee, Knoxville, Tenn., 1975. (5) Clemena, G. C, "An Evaluation of Automatic Sequential Air Sampling Systems", presented at the Annual Meeting of the Transportation Research Board, Washington, D.C., Jan. 1976. (6) Moe, R. D., Texas State Dept, of Highways and Public Trans portation, Austin, Tex., private communication, 1976. (7) American Society for Testing and Materials, "Instrumentation for Monitoring Air Quality", ASTM Publ. No. STP 555, Philadel phia, Pa., 1974. (8) Benchley, D. L., Turley, C. D.. Yarmac, R. F., "Industrial Source Sampling", Ann Arbor Science, Ann Arbor, Mich., 1973. Received for review June 23,1977, Accepted January 4,1978. Work sponsored by the Texas State Department of Highways and Public Transportation in cooperation with the U.S. Department of Transportation, Federal Highway Administration. The contents of this paper reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration, nor does this paper constitute a standard, specification, or regulation. 712 Environmental Science A Technology OLI 3058