Development and Evaluation of a Small Active Ozone Sampler

active ozone sampler that utilizes a single etched glass denuder as the collection substrate. The denuder is coated with a solution containing the nit...
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Environ. Sci. Technol. 1997, 31, 2326-2330

Development and Evaluation of a Small Active Ozone Sampler ALISON S. GEYH,* JACK M. WOLFSON, AND PETROS KOUTRAKIS Department of Environmental Health, Harvard School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 02115 JAMES D. MULIK U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711 EDWARD L. AVOL Department of Preventive Medicine, University of Southern California, 1540 Alcazar Street, Los Angeles, California 90033

Current methods for monitoring ambient and microenvironmental ozone include continuous measurement instruments, such as UV photometric and chemiluminescence monitors, open-path diffusion optical absorption spectrometer, and passive sampling devices. This paper introduces a new small active ozone sampler that utilizes a single etched glass denuder as the collection substrate. The denuder is coated with a solution containing the nitrite ion that reacts with ozone to produce nitrate. It is attached to a small personal pump driven by a 9-V battery. When compared with UV photometric measurements, the active sampler demonstrated very good accuracy (active sampler/UV photometer ) 0.94-1.00) and precision (%P ) (4.16.5%) under laboratory and ambient conditions for sampling period from 1 to 60 h. The Harvard active ozone sampler performed as well as the Timed Exposure Diffusion sampler, the only other active ozone sampler currently in use. Sampler performance was found to be insensitive to variations in relative humidity (active sampler/UV photometer ≈ 1.00 at 20-80% RH) except at very low relative humidity (active/UV photometer ) 0.81 at 12%). A low limit of detection of 10 ppb‚h allows for sampling at very low concentrations.

Introduction In many regions of the United States and around the world, the presence of ambient ozone at elevated levels has elicited concern in the public health community. Ozone, a product of photochemical processes involving nitrogen oxides and volatile organic compounds, is a respiratory irritant that has been shown to cause decrements in lung function (1). Although much is known about the effect ozone has on respiratory health, a “no-effects” threshold has not been established (2). This is in part due to the fact that actual personal exposure has not been well characterized, making it difficult to determine individual risk. Current models developed to estimate personal exposure are dependent on ozone concentration data collected at area monitoring stations. Outdoor exposure is estimated using ambient data, * Corresponding author fax: [email protected].

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with some models assuming concentrations to be uniform within a 15-km radius of the measurement location (3). Indoor ozone, which may be as high as 80% of that found outdoors, can contribute significantly to total personal exposure since people spend a major fraction of their time inside (4). Models estimate indoor exposure using such information as ambient ozone data, air exchange rates, ozone decay rates, presence of air conditioning, types of ventilation, presence of copying machines or electrostatic precipitators, and typical indoor/outdoor ozone concentration ratios (3, 5-7). However, it has been difficult validating such models because of the lack of personal exposure data. Obtaining such data has been challenging because the necessary personal sampling tools have not been available. Recent work in the area of ozone sampling methods has resulted in the development of several passive ozone samplers that are inexpensive and easy to use (8-10). Major drawbacks to several of these methods are that they suffer from a positive interference by NO2, an important atmospheric oxidant and very high limits of detection. Koutrakis (11) developed a passive ozone sampler that employs the ozone/nitrite reaction

NO2- + O3 f NO3- + O2

(1)

to determine atmospheric ozone concentrations. This method is specific for ozone. Interferences from ambient oxidants including NO2, H2O2, HONO, SO2, and PAN have been shown to be negligible for this system (12). The limit of detection is 100-200 ppb‚h or 4 to 8 ppb for a 24-h sample. However, because these samplers are passive by design and so depend on diffusion for sample collection, variations in air velocity across the collection face can cause fluctuations in the effective sampling rate that are difficult to quantify. When this sampler is deployed for stationary microenvironmental sampling under a polyvinyl chloride cap, the collection rate is stable, and ozone concentrations measured are both accurate and precise (13, 14). Similar results are found when this sampler is used for indoor ozone sampling (15). However, variations in air velocity become problematic when this device is used without any air flow control as is the case for personal sampling (16, 17). Currently, methods development efforts have been directed toward producing an ozone sampling device that includes a flow controlling mechanism. One such active sampler is the Timed Exposure Diffusion (TED) ozone sampler. This system, which is the size of a toolbox, was developed for monitoring microenvironmental ozone and has been used extensively for studies supported by the California Air Resources Board (18). The TED sampler incorporates the Harvard passive ozone sampler, exposing it in a controlled flow cell to minimize effects of variations in face velocity. A second active sampler is introduced in this paper. It was developed in response to the need for a small, lightweight, flow-controlled device that can be used for microenvironmental ozone sampling. This paper presents and discusses the performance of the Harvard active ozone sampler (AS). This sampler consists of a single tube diffusion denuder attached to a small, light-weight personal sampling pump. Ozone collection is accomplished using the same chemistry as employed by the Harvard passive ozone sampler. It allows for the specific measurement and collection of ozone while avoiding the problems inherent in the passive methods.

Experimental Section Sampler Development. The AS employs a hollow tube diffusion denuder similar to that used previously for collecting ammonia (19). The inner wall of the tube is coated with a

S0013-936X(96)00945-5 CCC: $14.00

 1997 American Chemical Society

FIGURE 1. Prototype configuration of the Harvard active ozone sampler (AS) I is the inlet of the sampling tube; P is the PAS 500 personal sampling pump; ST is the etched glass sampling tube. nitrite-based reagent similar to that previously used in the passive ozone sampler (11). To minimize sampler size, the tube length was set to equal that of the battery-operated sampling pump (10 cm). Tests were performed to determine the optimum flow and tube inner diameter (i.d.) with a target collection efficiency (CE) for ozone of at least 98%. Initial tests were conducted to determine whether it was possible to use theoretical calculations to predict CE (20). These tests were conducted under laminar flow conditions, achieved by preceding the coated collection tubes with uncoated tubes of appropriate length. Collection tubes with inner diameters ranging from 0.5 to 1.3 cm were tested at a flow of 120 cm3/min, the maximum stable flow for the personal pump. In general, the measured CE was much lower than predicted by theory, especially under conditions of low relative humidity (RH), and so it was concluded that the coated walls were not a perfect sink for ozone (21). Since simple theory did not predict CE for this system, the determination of optimum flow and tube diameter was found experimentally. Using the in-line laminar flow system and reducing the flow to 65 cm3/min, 10 cm tubes with an i.d. of 1.3 cm achieved the 98% target CE with ozone concentration at 55 ppb and approximately 15% RH. The sampler collection efficiency was tested at 200 ppb ozone for 22 h. Analysis of the air exiting the tube showed the sampling tube to be collecting 95% of the available ozone over the entire period. Analysis of the remaining nitrite showed that 40% had been consumed. Validation Studies. The performance of the Harvard active ozone sampler was evaluated under controlled laboratory and field conditions. Sampler Description. Harvard Active Ozone Sampler (AS). The prototype active sampler configuration is shown in Figure 1. It consists of a sampling tube made of a single etched borosilicate tube (1.3 cm i.d. × 10 cm L) attached to a small, light-weight (4 oz) pump (PAS-500 personal air sampler, Spectrex Corp., Redwood City, CA) using 1/8-in. silicone rubber tubing. The pump operates at 65 cm3/min on a 9-V alkaline battery for approximately 20 h or on a 9-V lithium

battery for approximately 60 h. The ends of the sampling tube are sealed with polyethylene caps. The inlet to the sampling tube is made of 5/32 in. i.d. PFA Teflon tubing. A small particle filter is placed between the sampling tube and the pump to keep the pump clean. The sampling tube is encased in a polyvinyl chloride plastic tube to both protect against breakage and guard against possible photochemical reactions. The capacity of the AS, calculated at 20% consumption of available nitrite, is approximately 7000 ppb‚h. Passive Ozone Sampler (PS). The Harvard passive ozone sampler has been described elsewhere (11). To control sampler face velocity during experiments, the passive devices were placed in TED samplers. Timed Exposure Diffusion (TED) Sampler. The TED ozone sampler was developed for unattended microenvironmental sampling. Sampler components are housed in a plastic toolbox. The sampler consists of a 8 × 8 × 40 cm PTFE Teflon channel through which air is drawn by a small fan mounted at the channel exit. Air velocity is set between 0.5 and 2 m/s, the optimal flow range for the PS based on previous work (11). Air enters the sampler through 25 1/4 in. i.d. × 2 in. L PTFE Teflon tubes creating laminar flow conditions. A Teflon shield protects the inlet from rain and strong winds. A movable Teflon plate is located above the midpoint of the channel. Two PS, loaded at one end with coated filters, are mounted in the plate and secured with o-rings. When activated for sampling, a servo motor moves the plate over the flow channel placing the samplers into the sampling position. At the end of the sampling, the plate retracts preventing further exposure. Experiments carried out in an environmental exposure chamber showed the effective collection rate of the TED sampler to be 21.3 ( 3.0 cm3/min. Sampler Preparation. The nitrite-containing solution used to coat the surface of the filters and sampling tubes has been described previously (11). All water used for solution preparation was ultrapure (Millipore Milli-Q). Reagent-grade potassium carbonate (Mallinkrodt), glycerol, and methanol (Fisher Scientific) were purchased and used without further purification. Sodium nitrite (Fisher Scientific) was crystallized twice from 2:3 ethanol/ultrapure water and dried overnight under vacuum. Crystals were dissolved in ultrapure water and tested for the presence of nitrate ion by ion chromatography. Crystals were considered usable if nitrate content was 0.05% or less by weight. Before coating, the etched borosilicate tubes used in the AS were cleaned by sonication in 0.05 N KOH and then in ultrapure water. Tubes were then rinsed twice with ultrapure water and once with ethanol and allowed to dry in room air. Tubes were coated by capping one end of the tube, introducing 2.5 mL of nitrite-containing coating solution, capping the second end, and then gently rocking and turning the tube several times. Excess coating solution was poured off, and the tubes were dried on a manifold under a stream of filtered air. After drying, tubes were assembled for sampling and stored in a dark refrigerator until used. All coating and assembly work was carried out in a glovebox purged with filtered air. Preparation of the PS has been described previously (11). Sampler Testing. AS flow rates were adjusted to 65 ( 5 cm3/min. Before and after each exposure period, pumps flows were measured using a soap bubble meter (mini-Buck Calibrator model M-5, Orlando, FL). For each exposure test, one to four AS and/or PS were reserved as experiment blanks. All samplers were stored and/or shipped at approximately -20 °C. Counter-Top Chamber Exposure Tests. Laboratory exposure testing was carried out on a counter-top in a rectangular glass chamber (60 × 30 × 30 cm, L × W × H) with an interior volume of approximately 54 L. Ozonated air flow through the chamber was set at about 6 L/min, resulting in approximately 7 air exchanges/h. Two small fans were posi-

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tioned in the center of the chamber to ensure concentration homogeneity. Tubing from the ozone source was placed between the fans. A humidifier, consisting of a cheeseclothcovered wire frame connected to an elevated water reservoir, was placed in front of one fan. Humidity in the chamber was controlled by raising or lowering the reservoir. Water in the reservoir was room temperature. Temperature in the chamber ranged between 75 and 85 °C. Relative humidity and temperature were monitored using a thermohygrometer (Model RH411 Omega Engineering, Inc., Stamford, CN). To produce ozone, room air was pumped through potassium permanganate-coated alumina, activated charcoal, and silica gel. The purified air was then passed into a UV photometric ozone calibrator (Model 49PS, Thermo Environmental Instruments, Franklin, MA) modified for these tests and employed as both ozone generator and analyzer. Concentration stability was (4 ppb. Calibrator accuracy was verified using a transfer standard UV photometer (Model 1003PC, Dasibi Environmental Corp., Glendale, CA). The chamber was allowed to equilibrate at the target ozone concentration before the start of each experiment. Analog outputs of both ozone concentration and chamber RH were recorded on a stripchart. AS performance was tested in the counter-top chamber at approximately 10, 20, 50, and 80% RH with the ozone concentration set to 20 ppb. AS performance was then evaluated at 20, 50, 100, and 200 ppb ozone with RH ranging between 40% and 60%. Exposure times lasted between 3 and 4 h. A dynamic blank exposure test was also conducted at 0 ppb at approximately 50% RH to ensure that no surface reactions other than reactions with ozone were responsible for nitrate production. Each experiment involved four replicate samples and was repeated three times. At the beginning of each test, samplers were inserted through holes in the top of the chamber and attached to pumps. In order to avoid battery use during testing, small 115-V AC pumps (Model VP0125, MEDO USA, Wood Dale, IL) were employed. Small metering valves were used to regulate flow set at approximately 65 cm3/min. At the end of each test, samplers were withdrawn from the chamber, capped, and stored in a refrigerator prior to chemical analysis. Environmental Exposure Chamber Tests. In the roomsize environmental exposure chamber, performance comparison tests were conducted between TED samplers and AS at 25, 100, and 300 ppb ozone. Sampler performance was evaluated at total ozone exposures of 25, 75, 150, 300, 600, and 900 ppb‚h. For these tests, chamber ozone exposure levels were selected to correspond to ozone levels possibly encountered during ambient sampling. Each test involved three replicate samples and was repeated twice. Ozone was generated by irradiating a stream of purified air using mercury vapor discharge grid lamps (BHK, Inc., Monrovia, CA). Purified air was obtained by passing ambient air through an air purification unit consisting of permanganate-coated alumina (Purafil, Inc., Chamblee, GA), activated charcoal, and a high-efficiency particulate filter. Chamber concentrations were measured using dual ultraviolet photometers (Model 1003 PC, Dasibi Instruments, Glendale, CA) attached to a single glass sampling manifold. Calibration of the instruments was performed using a pen-ray ozone lamp and a transfer standard ultraviolet photometer calibrated at the South Coast Air Quality Management District reference laboratory. Ozone concentrations were set by manually adjusting the lamp voltage. Specific ozone concentrations of 25, 150, and 300 ppb were achieved after 25-35 min of ozone generation. Relative humidity was indirectly regulated by the chamber air conditioning system, which kept relative humidity at approximately 50%. Directly before each experiment, TED samplers were loaded with two single filter PS. Before ozone generation was initiated, TED samplers were placed in the exposure

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chamber in three rows of four units each, equidistant apart. After the target ozone concentration was achieved, the chamber operator entered the chamber and activated each sampler. AS were also placed in the chamber at this time. AS sampling was initiated by connecting the sampling tube to a running personal pump. At the end of the sampling period, the chamber operator entered the chamber and deactivated all samplers. AS sampling was terminated by disconnecting the sampling tube from the personal pump. TED sampling was concluded by switching the sampler to the OFF position. Ambient Air Exposure Tests. Nine multiple-hour exposure tests of AS performance under ambient conditions were conducted in North Carolina at the U.S. Environmental Protection Agency Research Triangle Park monitoring station between February and April 1995. During this time period, ozone concentrations ranged between 0 and 86 ppb. Duplicate samplers were deployed under aluminum rain covers and secured to tripods located approximately 12 ft above ground level. Samplers were placed 2 ft apart and 2 ft from a continuous monitor inlet. Continuous measurements were made using a UV photometric ozone analyzer (Model 49 Thermo Environmental Instruments, Franklin, MA). Exposure times ranged from 7 to 60 h. Before each exposure, pump flows were set to 65 ( 6 cm3/min. Flow rates were measured both before and after sampling. Sampling tubes identified as field blanks were handled in the same manner as those used for testing. For each set of exposures, one sampling tube was placed under the aluminum rain cover and left at ambient temperature for the entire sampling period. Four 1-h ambient exposure tests of AS performance were conducted in southern California on August 30, 1995. Four sets of four replicate samplers were exposed for approximately 1 h between 1 and 5:30 PM PDT. The temperature during sampling ranged between 87 and 95 °F. AS inlets were placed approximately 1 m from the ground, 1 m apart, and 1 m from the inlet to a continuous monitor. Continuous measurements were made using a UV photometric ozone analyzer (Model 1003 PC, Dasibi Instruments, Glendale, CA). During this period, ambient ozone concentrations ranged from approximately 60 to 80 ppb. Sampler Handling and Background Nitrate. Storing and shipping AS sampling tubes at approximately -20 °C was found to control production of background nitrate. Both the background levels and variation of nitrate were found to increase when AS sampling tubes were left at room or ambient temperature for extended periods of time. This increased the limit of detection by as much as a factor of 5. Sample Analysis. All AS were disassembled in a purgedair glovebox. Sampling tubes were extracted with 5 mL of ultrapure water. PS filters from samplers were placed in clean 8-mL polyethylene vials, extracted with 5 mL of ultrapure water, and sonicated for 15 min. Extracts from both systems were poured into an 8-mL polyethylene vial. Extracts were analyzed for nitrate ion concentration by ion chromatography using a Model 2000i Dionex ion chromatograph equipped with an Ionpac AS4A analytical column and conductivity detector. Ozone concentrations, CO3 (ppb) were determined from the following equation:

CO 3 )

NVR SKMWNO3T

(2)

where C is the integrated ozone concentration (ppb); N is the corrected nitrate concentration (sample - average blank, µg/ mL); V is the extraction volume (mL); MWNO3 is the molecular weight of the nitrate ion (µg/µmol); R is the conversion factor 106 (cm3/m3); S is the collection rate (cm3/min), equal to the temperature and pressure-corrected average measured pump flow for all AS samples and to 21.3 cm3/min for all TED

FIGURE 2. AS performance counter-top exposure chamber experiments. Each data point is the mean of four replicate samples ( 1 SD. Tests at each concentration were conducted three times. Relative humidity ranged from 40% to 60% with temperature approximately 25 °C. The solid line is the best-fit regression line. Best-fit first-order regression based on all points: intercept ) 3.9 ( 1.4, p-value ) 0.0066; slope ) 0.94 ( 0.01; r 2 ) 0.99.

FIGURE 3. Relative humidity effects on AS performance. Chamber ozone concentration was 20 ppb for all tests. Each data point is the mean of four replicate samples ( 1 SD. Each test was conducted once.

samples; K is the conversion constant 0.0409 µg/(ppb‚m3) determined at 298 K and 1 atm; and T is the exposure time (min). Limits of detection (LOD) were determined using eq 3:

LODO3 )

3σNO3VR SKMWNO3

(3)

where LOD is the limit of detection (ppb‚h); σNO3 is the standard deviation of average nitrate concentration from test blanks (µg/mL); and S is 65 cm3/min (nominal flow) for AS samples.

Results and Discussion Counter-Top Chamber Tests. The response of the AS (Figure 2) was linear over the range of concentrations measured, slope ) 0.94 ( 0.01; r2 ) 0.99 with the AS under reporting the ozone concentration by 6%. The intercept was small, but statistically significant (intercept ) 3.9 ( 1.4, p-value ) 0.0066). This is most likely the result of calibrating the ozone analyzer at only two calibration points, 0 and 400 ppb. AS and UV photometric measurements were in good agreement (AS/UV photometer ≈ 1.0; Figure 3) between 21% and 79% RH. At very low RH (12%) the average AS/UV photometer ) 0.81. This reduced efficiency may be due to a partial loss of molecular water from the sampling tube coating, reducing the amount of dissolved nitrite available for the reaction with ozone that occurs in the aqueous phase. Environmental Exposure Chamber Tests. Figure 4 summarizes the comparative performance of the AS and TED samplers. AS and TED samplers responded similarly over the range of ozone concentrations tested, slope ) 1.00 ( 0.07; r2 ) 0.94. The intercept was statistically insignificant (intercept ) -16.7 ( 19.7, p-value ) 0.4). Ambient Air Exposure Tests. The response of the AS under ambient exposure conditions (Figure 5) was again linear over the range of concentrations measured, slope ) 1.01 ( 0.04; r2 ) 0.98. The intercept was small and statistically insignificant (intercept ) -0.09 ( 0.59, p-value ) 0.88). Results of the 1-h sampling tests are shown in Table 1. With the exception of

FIGURE 4. AS performance vs TED sampler performance. Each data point is the ratio of three averaged measurements for each method. Each test was conducted twice. The solid line is the best-fit regression line. Best-fit first-order regression based on all points: intercept b ) -16.7 ( 19.7; p-value ) 0.42; slope ) 1.00 ( 0.07; r 2 ) 0.94.

TABLE 1. 1-H Ambient Exposure Resultsa run no. exposure time (min) AS ( SD (ppb) UV ( SD (ppb) AS/UV 1 2 3 4

60 59 59 60

60 ( 4 65 ( 5 58 ( 5 60 ( 2

59 ( 6 74 ( 9 63 ( 5 72 ( 11b

1.02 0.88 0.92 0.83

a Each AS measurement is the mean of four replicate samples; each test was conducted once. Each reported UV photometric continuous measurement is the mean of 16 3-min averages. Average ratio of first 3 h ) 0.94. b Continuous measurement reported for run 4 includes an atypical +20 ppb change in ozone concentration that occurred 25 min into the sampling hour.

the last hour, during which the performance of the UV photometer became unstable, ozone concentrations mea-

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be considered a reasonable alternative to that method when the use of a continuous instrument is not feasible and timed sampling is not required. The calculated LOD for the active sampler, approximately 10 ppb‚h, is much lower than those found for other small portable samplers. This along with good measurement precision allows for very short-term sampling at relatively low concentrations.

Acknowledgments We would like to thank Steve Ferguson (HSPH) for engineering design and support and Karen Anderson (Rancho Los Amigos Research and Educational Institute) for work on the 1-h exposure experiments. We acknowledge the Research Division of the California Air Resources Board for providing salary funding for E.L.A. and for the use of the TED samplers. Although the research herein has been primarily funded by the United States Environmental Protection Agency under Cooperative CR 822816-01 to the Harvard School of Public Health, the manuscript does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. FIGURE 5. AS performance in multiple hour ambient exposure tests. Each data point is the mean of two replicate samples ( 1 SD. Each test was conducted once. The solid line is the best-fit regression line. Best-fit first-order regression based on all points: intercept ) -0.09 ( 0.57; p-value ) 0.88; slope ) 1.01 ( 0.04; r 2 ) 0.98. sured by the AS were 94 ( 4% of the concentration reported by the continuous instrument. Precision. Percent precision (%P) was calculated according to the root mean squared error (RMSE) method, which allows for comparisons of differences between more than two collocated samplers (22):

RMSE )

∑d

x

2

ij

N

(4)

where dij ) xij - mj, for n ) 1 ... i collocated samples, mj ) the mean of i samples at the jth concentration, and N is the total number of samples collected. The %P is estimated by

%P )

RMSE × 100 M

where

M)

∑m

j

N

Active Sampler Precision. The %P estimate for the AS using sets of quadruplicate samples collected during each countertop chamber test was (4.1% or (0.8 ppb at 20 ppb. The %P estimate from duplicate samples collected during the multiplehour ambient exposure tests was (6.5% or 1.3 ppb at 20 ppb. Sampler Handling and the Limit of Detection (LOD). Ensuring that AS sampling tubes were stored and shipped at approximately -20 °C resulted in an average LOD of 10 ppb‚h. This very low LOD along with the good precision allows for sample collection over very short sampling periods when ozone concentrations are low. The Harvard active ozone sampler has demonstrated good accuracy (AS/UV photometer ) 0.94-1.00) and precision (%P ) 4.1-6.5%) under both laboratory and ambient exposure conditions at ozone concentrations of 20-200 ppb and exposures times of 1-60 h. Sampler performance has been demonstrated to be insensitive to relative humidity except at very low (12%) RH, which is very seldom encountered during routine ambient sampling. The Harvard active ozone sampler performed as well as the Timed Exposure Diffusion sampler, the only other active ozone sampler currently in use, and can

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Literature Cited (1) Lippmann, M. J. Exp. Anal. Environ. Epid. 1993, 3, 103-129. (2) Larsen, R. I.; McDonnell, W. F.; Horstman, D. H. J. Air Waste Manage. Assoc. 1991, 45, 455-459. (3) Johnson, T.; Capel, J.; McCoy, M. IT Air Quality Services for the Office of Air Quality Planning and Standard; U.S. Environmental Protection Agency: Washington, DC, 1996. (4) U.S. Environmental Protection Agency. Exposure factors handbook; Office of Health and Environmental Assessment: Washington, DC, 1989; EPA/600/8-89/043. (5) Allen, R. J.; Wadden, R. A.; Ross, E. D. Am. Ind. Hyg. Assoc. 1978, 39, 466-471. (6) Allen, R. J.; Wadden, R. A. Environ. Res. 1982, 27, 136-149. (7) Davies, T. D.; Ramer, B.; Kaspyzok, G.; Delaney, A. C. J. Air Waste Manage. Assoc. 1984, 31, 135-137. (8) Hangrtner, M.; Kirchner, M.; Werner, H. Analyst 1996, 121, 12691272. (9) Grosjean, D.; Hisham, M. J. Air Waste Manage. Assoc. 1992, 42, 169-173. (10) Kanno, S.; Yanagisawa, Y. Environ. Sci. Technol. 1992, 26, 744749. (11) Koutrakis, P.; Wolfson, J. M.; Bunyaviroch, A.; Froehlich, S. E.; Hirano, K.; Mulik, J. D. Anal. Chem. 1993, 65, 209-214. (12) Lurmann, F. W.; Roberts, P. T.; Main, H.; Hering, S. V.; Avol, E. L.; Colome, S. Phase II Report Appendix A: Exposure Assessment Methodology; Prepared for the California Air Resources Board; 1994; Section 5, p 11. (13) Liu, L.-J. S.; Koutrakis, P.; Leech, J.; Broder, I. J. Air Waste Manage. Assoc. 1995, 45, 223-234. (14) Mulik, J. D.; McClenny, W. A.; Williams, D. D. Presented at the 1995 EPA/AWMA International Symposium: Measurement of Toxic and Related Air Pollutants, May 16-18. (15) Liou, L.-J. S.; Olson, M. P., III; Allen, G. A.; Koutrakis, P.; McDonnell, W. F.; Gerrity, T. R. Environ. Sci. Technol. 1994, 28, 915-923. (16) Liu, L.-J. S.; Koutrakis, P.; Suh, H. S.; Mulik, J. D.; Burton, R. M. Environ. Health Perspect. 1993, 101, 318-324. (17) Brauer, M.; Brook, J. R. J. Air Waste Manage. Assoc. 1995, 45, 529-537. (18) Lurmann, F. W.; Roberts, P. T.; Main, H.; Hering, S. V.; Avol, E. L.; Colome, S. Phase II Report Appendix A: Exposure Assessment Methodology; Prepared for the California Air Resources Board; 1994; Section 5, p 14. (19) Ferm, M. Atmos. Environ. 1979, 13, 1385-1393. (20) Gormley, P.; Kennedy, M. Proc. R. Ir. Acad. 1949, 52A, 163-169. (21) Reiss, R.; Ryan, P. B.; Koutrakis, P. Environ. Sci. Technol. 1994, 28, 504-513. (22) Harvard School of Public Health. Quality Assurance Project Plan for Washington, D.C. Metropolitan Aerosol Acidity Characterization Study; Report prepared for U.S. Environmental Protection Agency, 1994.

Received for review November 6, 1996. Revised manuscript received March 15, 1997. Accepted March 17, 1997.X ES960945+ X

Abstract published in Advance ACS Abstracts, June 15, 1997.