Anal. Chem. 1983,55,2083-2085
Passive !Sampler for Ambient Levels of Nitrogen Dioxide B a r r y C. Cadoff* and Jimmie Hodgeson' Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
A precise, high-rate passive sampler for NO, Is described. I t can be assembleid from a1 commerclally available devlce and can be used to rellably !sample low amlalent levels of NO,. Triethanolamine 11s used No collect the NO,, and the analysls method follows the tradltlonal Saltzman procedure. The device Is dlffuslon controlled and samples et a rate of approximately 110 mL/mln. I t has been evaluated at two levels of relative humldlty and exhilbns no Interference In the presence of a large excess of NO. Sampling has been conducted at NO, concentrations between 61 and 335 ppb with sampling perlods as short as 1 h.
Passive samplers offer many advantages over conventional pump-driven samplers foir the collection of gaseous pollutants in the atmosphere. These samplers can be made relatively small in size, are generally inexpensive, and do not require costly and often bulky sampling pumps. Passive samplers are used to measure time-average concentrations of gases and generally depend on mass transport of the gas by diffusion or permeation mechanisms. A variety of' devices have been designed for the sampling of inorganic and organic vapors and some are available commercially. Generally, these samplers have been devised for the industrial environment where concentrations of particular gases may be as much as 3 to 4 orders of magnitude higher than in nonindustrial ambient atmospheres. Our interest has been in developing a sampler for NO2 that could be used at ambient levels. Two NO2samplers have been described recently ( 1 , 2 ) that employ triethanolamine as the absorbing agent. These are used for industrial sampling but, due to their rathler low rate of sampling, many hours or days would be required to obtain an adequate sample a t low ambient levels. This report describes the development of a high-rate sampler, employing triethanolamine as the absorbing agent, that can obtain adequate samples of' NOz in a few hours or less. Several criteria were considered in the development of this passive sampler. These are (a) low cost, (b) reproducibility of results, (c) linearity of response as a function of NO2 exposure, (d) sampling rates that are adequate for short-term sampling (1h or less), and long-term sampling (24h or more). The sampler reported here has met these criteria in a laboratory evaluation.
EXPERIMENTAL SECTION Generation OP Standard NOz Test Mixtures. Known concentrations of N O p in air were generated by the gas-phase reaction (3) NO(excess) + O3 = NOz O2 (1) This is a rapid, bimolecular reaction which occurs at room temperature in which one 'molecule of NOz is produced for each molecule of NO and O3 consumed. The apparatus is shown schematically in F'igure 1. Compressed air flows through an air purifier to provide a supply of clean air. The air flow is kept constant by a differential flow controller and a needle valve such
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Present addrecis: Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803.
that any given flow from 0 to 5 L/min can be set. The flow rate is measured by a calibrated mass flowmeter; flows of approximately 5 L/min are used. The air is passed through a water saturator to produce air with a desired relative humidity. The relative humidity is controlled by regulating the pressure of the output air of the air purifier. Approximately 10% of the flow, is directed through a glass capillary into a quartz tube. Ozone is produced in the quartz tube by a low-pressure mercury lamp positioned about 5 cm from the quartz tube. Ozone concentrations can be varied by exposing a larger or smaller portion of the lamp by adjusting a sliding opaque sleeve which surrounds the lamp (4). The mixture of ozone and air then flows to a reaction chamber where it reacts with a known concentration of NO in Nz. The source of NO is an analyzed cylinder of NO in N2,at a concentration on the order of 100 ppm. Flow rates of NO are varied from 10 to 140 mL/min. The reaction chamber effluent is diluted with the remaining clean air flow, f 2 , and passes into the exposure chamber which is a bell jar, 15.5 cm i.d. and 26 cm high. Fly splitting the air flow in this manner, one makes available higher concentrations of reactants, thus ensuring a faster rate of reactioin. The passive samplers are placed within this chamber and the concentration of NOz is determined by measuring the ozone concentration before the addition of NO and again at the end (of the run when the flow of NO is stopped. An ultraviolet ozone monitor (Model 1003-AH,Dasibi Environmental Corp., Glendale, CA) is used in this scheme to measure O3 concentrations and WBS periodically calibrated by the NBS 3-m UV photometer as described by Wendt et al. (5). Concentrations of NO and NOz were monitored with a chemiluminescence detector, and relative humidity was monitored with a hygrometer. The sampling chamber was positioned atop a magnetic stirrer with a large Teflon-coated stirring bar placed in the chamber for stirring the gas mixture during a run. Fabrication of Passive Sampler for NOz. The passive sampler is constructed from a Nuclepore 47-mm aerosol holder (Catalog No. 430400, Nuclepore Corp, Pleasanton, CA). The base of the holder, which is normally attached to a sampling pump, is capped with a polyethylene cap. A Gelman Type A glass fiber filter is cut into a disk shape to fit the base of the aerosol holder. The filter disk is immersed in a solution of triethanolamine (TEA) in acetone (1:11by volume), removed, and placed on a paper towel to air-dry for a few minutes on each side. The dried disk is positioned atop the sampler base and the sampler is assembled, using a Nuclepore 47-mm polycarbonate membrane, 0.8 pm pore size (Catalog No. 111109). Figure 2 shows a schematic of the sampler and Figure 3 shows the assembled sampler. The assembled sampler can now be used to sample N O pin air either as an area monitor in a room or other location or as a personal monitor attached to the clothing of the wearer. If the sampler is not to be used immediately, it should be stored in an airtight container, such as a capped jar, until needed. After me, the time of sampling is recorded and the sampler is placed back in its airtight container. Analytical Procedure. The method described by Palmes et al. ( I ) was used for the analysis of NOz. This method employs a color-producing (Saltzman) reagent. The sampler is removed from its container and disassembled t o remove the membrane. The membrane can be reused. A disk of polyethylene, approximately 2 mil thick, is inserted in place of the membrane and the sampler is reassembled. The cap on the base is removed and approximately 3 mL of reagent is injected into the sampler with a hypodermic syringe. The color is allowed to develop for 30-45 min. The volume of solution is obtained from the weight difference before and after the addition of reagent and from the density of the reagent. The solution is withdrawn from the sampler with a syringe, the absorbance measured at 540 nm, and
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This article not subject to U.S.Copyright. Published 1983 by the American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 55. NO. 13, NOVEMBER 1983
2084
Table 1. Comparison ofAnalytical Results with Exposure to NO, under Variable Conditions
run no.
conen of NO,, ppm
exposure, h
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.156 0.213 0.204 0.154 0.260 0.180 0.124 0.124 0.129 0.129 0.014 0.078 0.329 0.335 0.061 0.064
4.18 4.02 2.12 3.67 2.47 3.58 4.60 4.60 4.60 4.60 12.0 1.00 1.00 3.21 1.00 4.52
integrated exposure, ppm h
(W
% re1 av NO, found, cg std dev (A) (duplicates)
0.651 0.856 0.431 0.565 0.641 0.645 0.578 0.578 0.595 0.595
7.68 10.4 5.12 6.92 8.26 8.99 7.64 6.91 7.12 7.51 10.3 1.06 4.29 13.1 0.815 3.58
0.888
0.078 0.329 1.08 0.061 0.289
4.2 3.2 0.47 6.1 6.8 2.0 8.2 5.5 1.8 1.2 4.3 6.1 3.5 0.81 7.2 2.9
AIB‘
NO/NO,
11.8 12.1 11.9 12.3 12.9 13.9 13.2 12.0 12.0 12.6 11.6 13.6 13.0 12.1 13.4 12.4
4.2 2.0 3.9 1.7 12.1 18.4 5.0 5.0 4.8 4.8 7.6 7.6 6.3 2.4 5.5 9.6
% re1 humidity
20 20 60 60 60 60 60 60 20 20 20 60 20 60 20 20 ~~
Mean = 12.55. Re1 std dev = 5.6%. I.
FLOW VALVE CONTROLLER
MASS FLOWMETER
0,SOURCE
REACTION CHAMBER
MIXING CHAMBER
11 (NO
0” REGULATOR
HYGROMETER
CLEAN AIR SOURCE FLOWMETER
NO STD
m
e
1. Schemnc of Row system for ttw exposure of samplers. YEYBRINE SUPPORT
Flgua 2. Schemic of passive sampler.
the NO, mneentration determined by comparison with a standard curve obtained by using NaNO, solutions.
RESULTS AND DISCUSSION Generation of NO, Mixtures. It was found that the change in NO2 concentration at the beginning and end of a run varied between 0 and 3%, the larger differences being more frequent at the lower NO2 concentration levels. Stimng of the gas mixture inside the bell jar was necessary since smaller amounts of NO, were recovered by the sampler in an unstirred system. This effect, i.e., the need for a minimum air velocity, had been noted by Tompkins and Goldsmith (6). Steady-state conditions in the exposure chamber were disturbed momentarily by the intrusion of room air when the samplers were placed in the chamber. However, the concentration of the NO2, as determined by the chemiluminescence detector, was restored to its original concentrations within 3-4 min.
Figure 3. View of assembled sampler.
One question that arises is whether the gas phase reaction in which O3 is completely converted to NO, in dry atmospheres is valid under conditions where the relative humidity is 20% or 60%. Recent work has confirmed that the stoichiometry is only slightly affected (less than 1%)under these conditions
(7). Passive Sampler Results. In the design of a suitable substrate for the collection of NO2, it was found that TEA-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
impregnated glass fiber filter disks gave the most reproducible results. The large surface area of the filter offers numerous sites for reactilon to occur. Earlier attempts using stainless steel screens coated with TEA showed that NOz was not as efficiently trapped. Table I summarizes the data obtained from 16 runs with duplicate samplers over a range (0.06-0.33 ppm) of concentrations of NOz, at two levels of relative humidity, 20 and 60%, and a t varying ratios of NO/NOZ (-2-18). These data were obtained after subtracting 0.013 pg of NOz for the blank. The blank was determined by analyzing and averaging the results from four samplers that had not been exposed in the sampling chamber. Variable concentrations of NO and relative humidit:y appeared to have no i noticeable effect on NOz sampling. Furthermore, the sampler gave consistent results3 with concentrations of NOz ranging from 61 to 335 ppb anld exposure times as short as 1h. It is also noteworthy that the integrated exposure (ppm h) levels ranged over a factor of almost 20-fold. The average results and precision for duplilcate samplers run at the same time are also shown in Table I. A / B values obtained (pg of NOz found/(ppm h)) illustrate the consistency of results for all conditions. The relationship between A and B may also be expressed by tbe linear equation A := (11.98 f 0.37)B 0.22(&0.23) (2)
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where A is the pg of NOz analyzed and B is the integrated exposure (ppm h). The reported data were obtained by use of eight separale samplers and a single batch of 100 membrane filters. The rerwlts, therefore, include rmy imprecision in the samplers and membranes. Storage times of up to 1week with the unexposed sampler and the exposed sampler showed no effect on the analysis of NOz. Mechanism and Rate of Gas Transport. Passive samplers commonly employ either of two mechanisms, both dependent on concentration gradients, to achieve mass transport. For the first case, that of diffusion, the transport occurs across an "air-gap" and its rate depends on the concentration difference across that gap. Permeation depends on movement through a nonporous membrane in which the gaseous molecules dissolve in the membrane. Referring to Figure 2, it is seen that mass transport in the interior portion of the sampler occurs via diffusion. Similarly, the microporous membrane has been shown to be an air-gap membrane (8)and diffusion is thus the mechanism for transport through the membrane. Hence, this sampler is a diffusion-controlleddevice consisting of two diffusion elements and is governed by Fick's first law of diffusion. As a consequence of the diffusion law it would be possible to obtain eiither higher or lower sampling rates by changing the physical dimensions of the sampler (1,6). An importarit characteristic of diffusion-controlled samplers is that their sampling rate is only slightly affected by changes in ambient pressure and temperature. Changes in pressure, for example, have no effect on the collection efficiency and changes in temperature have an effect proportional to T1Iz, where T i s in K (1,6).
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The overall sampling rate may be calculated from the following relationship;
NO2 collected (pg) = CNOzX sampling rate
X time
(3)
where CNQ is the ambient NOz concentration. Using the results of run no. 10 of Table I as an example and converting ppm to pg/mL, at 25 "C and 760 torr, we found the sampling rate to be 112 mL/min. Graedel (9) reported an estimat,e of 37 ppb for the mean urban concentration of NOz. At this average level, a 2m.h sampling period would give a sample analogous to run no. 15 in Table I. This is by no means a lower limit of sample size for the passive sampler; the estimated lower limit, for analyses with the same precision, is about one-fifth of this amount, equivalent to an integrated exposure of 0.01 ppm h. Although this sampler has been tested in the laboratory, field trials under varying conditions will be needed before its effectiveness can be fully evaluated. Furthermore, this device gives promise of being a suitable sampler for other gaseous contaminants, provided appropriate absorbing substrates can be found.
ACKNOWLEDGMENT The authors wish to thank Arnold Bass for his calibration of the ozone monitor. Registry No. Nitrogen dioxide, 10102-44-0. LITERATURE CITED Paimes, E D.; Gunnison, A. F.; Dlmattio, J.; Tomczyk, C. Am. Ind. Hyg. Assoc. J . 1978, 37, 570. Kring, E. V.; Lautenberger, W. J.; Baker, W. 6.;Douglas, J. J.; Hoffman, R. A. Am. Ind. Hyg. Assoc. J . 1981, 4 2 , 373. Rehme, K. A.; Martin, 6.E.; Hodgeson, J. A. "Tentative Method for the Calibration of NO, NOp and O3Analyzers by Gas Phase Titration" EPA No. R2-73-246; Environmental Protection Agency: Research Triangle Park, NC, March 1974. Hodgeson, J. A.; Stevens, R. K.; Martin, 6.E. ISA Trans. 1972, 7 7 , 161. Wendt, J.; Kowaiski, J.; Bass, A.; Ellis, C.; Patapoff, M. NBS Spec. Publ. 1978 No. 520. Tompkins, F. C., Jr.; Goldsmith, R. L. Am. Ind. Hyg. Assoc. J . 1977, 38, 371. Fried, A.; Hodgeson, J. A. National Bureau of Standards, unpublished work, 1982. Ross, J. W.; Riseman, J. H.; Kruger, J. A. Pure Appl. Chem. 1973, 36,473. Graedei, T. E. "Chemical Compounds in the Environment"; Academic Press: New York, 1978; pp 16-17.
RECEIVED for review March 4, 1983. Resubmitted July 22, 1983. Accepted July 22, 1983. This work received financial support from the Environmental Protection Agency under Contract No. AD-13-F-2-535-0. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
