Development and Calibration of a Resin-Based Passive Sampling

Feb 20, 2003 - Development and Calibration of a Resin-Based Passive Sampling System for Monitoring Persistent Organic Pollutants in the Atmosphere ...
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Environ. Sci. Technol. 2003, 37, 1352-1359

Development and Calibration of a Resin-Based Passive Sampling System for Monitoring Persistent Organic Pollutants in the Atmosphere F R A N K W A N I A , * ,†,‡ L I S H E N , ‡ Y I N G D U A N L E I , †,‡ CAMILLA TEIXEIRA,§ AND D E R E K C . G . M U I R ‡,§ Division of Physical Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6, and National Water Research Institute, Environment Canada, Burlington, Ontario, Canada L7R 4A6

Responding to a growing need for inexpensive and simple monitoring of persistent organic pollutants (POPs) in the atmosphere, a passive air sampling technique based on the sorption of gaseous pollutants to the sampling resin XAD-2, a styrene-divinylbenzene copolymer, has been developed. A quantitative understanding of the uptake kinetics of the passive air samplers (PAS) was obtained through a combination of field calibration studies, controlled wind tunnel experiments, and flow field simulations. Fortytwo PAS were deployed for varying time periods up to 1 yr at three calibration stations in the Laurentian Great Lakes region and the Canadian High Arctic with ongoing conventional air sampling of organochlorine pesticides. The PAS take up quantifiable levels of POPs within a few weeks of deployment, and the amount of chemical collected increases steadily over a 1-yr sampling period. The uptake of POPs by the PAS is controlled by molecular diffusion and independent of wind velocity. The timeaveraged air concentrations of organochlorine pesticides derived from the PAS data are comparable with those from HiVol sampling. This study suggests that the XAD-2 resinbased PAS can be used to derive at least semiquantitative information on the vapor-phase concentrations of POPs in the atmosphere and are suitable for the measurements of long-term average concentrations at the levels occurring in remote regions.

Introduction Delivery of persistent organic pollutants (POPs) to agricultural and aquatic ecosystems, where they may bioaccumulate and pose risks to humans and other top predators, occurs usually through the atmosphere. Atmospheric transport occurs within the immediate vicinity of POP sources (1) as well as * Corresponding author e-mail: [email protected]; phone: (416)287-7225; fax: (416)287-7279. † University of Toronto at Scarborough. ‡ University of Toronto. § Environment Canada. 1352

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over continental distances (2, 3). It is thus of considerable importance to understand the spatial variability of atmospheric POP concentrations on a variety of scales, from local to global. Nevertheless, even the most extensive atmospheric monitoring campaigns for POPs, such as the Integrated Atmospheric Deposition Network (IADN) around the Laurentian Great Lakes, tend to be restricted to a fairly small number of sites, and the real spatial variability of POP concentrations in the atmosphere remains largely elusive. This is due to the complexity and expense of sampling trace amounts of POPs from the atmosphere using high-volume (HiVol) pumps, which currently constitute the state of the art of measuring POPs in the atmosphere. A passive air sampler (PAS) is a device that collects chemicals from the atmosphere without the help of a pump. Plant foliage is a natural passive sampling medium and has been used for the monitoring of POPs in the atmosphere (4, 5). Leaves and needles have a relatively large surface area, and their waxy cuticle has a high affinity for many organic pollutants. However, uptake capacity and kinetics vary with species, location, age, and season, greatly limiting their potential applications (6). In contrast to plants, man-made PAS first have to be deployed, but uniform construction facilitates comparability between different locations and time periods. Whereas such samplers have been routinely employed for the assessment of the workplace exposure of individuals and for the ambient monitoring of a variety of trace gases (7, 8), PAS are only recently being employed for the ambient monitoring of POPs (9-11). Specifically, semipermeable membrane devices (SPMDs) (12, 13) and polymer-coated fibers (14), glass disks (15), stir bars, and silicone tubing (16) have been employed for that purpose. The primary advantage of passive air sampling is its simplicity and low expense. Neither electricity nor highly qualified personnel are required for routine operation. PAS may be the only viable option for large-scale monitoring campaigns. They are also the method of choice in remote locations that are only accessible with difficulty and high expense and may lack a power supply. A passive sampler takes up chemicals from the atmosphere until an equilibrium between the atmospheric gas phase and the collection medium has been established. This suggests that two principal passive air sampling strategies are possible. If a PAS has achieved equilibrium with the atmospheric gas phase, the air concentration can be derived from the amount in the PAS and its uptake capacity. Accurate knowledge of the PAS uptake capacity for the chemical species of interest is therefore required. The value being obtained is the air concentration at the time of retrieval. The optimal equilibration sampler has a fast uptake rate and a very small uptake capacity so that equilibration is quickly established. A disadvantage of the equilibration sampling approach is that different chemicals will require different lengths of time to achieve sampler equilibration. Another disadvantage is the highly variable nature of the atmospheric environment. The equilibrium gas-phase concentrations are a constantly moving target as a result of temperature and concentration fluctuations. Equilibrium samplers are thus most suitable in situations that are less subject to periodic variations, such as the indoor environment. If the PAS is far from having reached equilibrium, it is feasible to derive air concentration from the amount in the PAS and its uptake kinetics. Accurate knowledge of the sampling rate, i.e., the amount of air being sampled per time unit, is required. The value being obtained is the average air concentration integrated over the entire deployment period. 10.1021/es026166c CCC: $25.00

 2003 American Chemical Society Published on Web 02/20/2003

FIGURE 1. Schematic uptake curves of a passive air sampler showing the impact of uptake capacity (expressed as a distribution coefficient between PAS and air KPAS/Air) and ambient air concentration CAir on the time required for equilibration. The sampler described in this study operates in the linear uptake region. The optimal linear uptake sampler has a very large uptake capacity and a fast uptake rate. In contrast to equilibration samplers, these types of PAS are also suitable for long-term exposure in the range of months to years. One of the disadvantages of the linear uptake samplers is the potential variability of the sampling rate in response to variable environmental conditions, most notably wind speed, and chemical parameters. The rate of uptake for POPs in a PAS is in most cases controlled by the thickness of the laminar boundary layer surrounding the sampling material. If the housing is designed in such a way that the thickness of this layer is largely independent of wind speed, the uptake rates should remain similar for different environmental conditions. Originally developed for sampling hydrophobic chemicals in aquatic systems (17-19), SPMDs find increasing use in monitoring POPs in the atmosphere (12, 13). SPMDs consist of triolein-filled polyethylene (PE) tubes, and organic chemicals are concentrated in the SPMD when they permeate through the membrane into the solvent system. Their potential uses, advantages, and disadvantages were reviewed by Ockenden et al. (20). SPMDs may not constitute the ideal passive air sampling device for POPs for a number of reasons. For the more volatile POPs, there are indications that the period of linear uptake is shorter than typical deployment periods (i.e., the SPMDs are approaching equilibrium) (12, 13). The PE and the triolein constitute separate storage compartments with variable capacities and additional transfer resistances, resulting in complex uptake kinetics greatly complicating data interpretation. Furthermore, the capacity and uptake kinetics of polymer materials such as PE are greatly influenced by temperature, and the behavior of PEbased samplers at subzero temperatures is not wellcharacterized. Finally, the cleanup of extracts from PE-based samplers requires size-exclusion chromatography to eliminate interferences from polymer fragments. In this paper, we describe a novel passive air sampling technique for POPs that is based on the sorbent resin XAD-2. This resin, a styrene-divinylbenzene copolymer, is used extensively for routine air monitoring of POPs using classical HiVol sampling techniques (e.g., refs 21-23). It has a very high and well-established capacity for POPs, its sorption properties are not affected by moisture, and its behavior at subzero temperatures is more predictable than that of PE and other polymers. In this study, wind tunnel and field calibration experiments were conducted to quantitatively characterize the uptake kinetics of an XAD-2-based PAS, and simplified computer-based flow simulations were performed to gain a qualitative understanding of the rate of air exchange in the PAS housings. The effects of wind velocity, temperature,

and relative humidity on the sampling rate and efficiency of these PAS over the sampling period are discussed. In addition, the time-averaged concentrations of selected POPs in the atmosphere derived from passive field sampling are compared with those obtained by HiVol sampling.

Theory The passive sampling system that we are developing is a diffusive sampling device that consists of a packed solid sampling medium with a high affinity for POPs and functions by physical sorption of vapor-phase POPs onto the sampling medium. The mass transfer of POPs between the atmosphere and the sampling medium XAD-2 can be described by Fick’s first law:

dm/dt ) kA(CAir - CSurface)

(1)

where dm (pg) is the mass of a POP collected by the sampling medium during time interval dt (day), CAir (pg m-3) is the atmospheric concentration of the POP, CSurface (pg m-3) is the concentration of the POP immediately adjacent to the surface of the sampling medium, k (m day-1) is a mass transfer coefficient, and A (m2) an interfacial transfer area. If CAir and ambient temperature remain constant over the sampling period, the amount of POP accumulated in the PAS will steadily increase until equilibrium is achieved. Figure 1 shows schematically the three distinct phases of chemical uptake in a PAS:linear uptake, curvilinear region, and equilibration phase. The passive sampling technique that we are presently developing is operating in the linear uptake phase. During that phase, the rate of desorption can be considered to be very slow relative to the rate of uptake. CSurface can thus be considered to be effectively 0. Under this condition, the sampler concentration of a POP can be related to its atmospheric concentration and exposure time:

CPAS ) CAirRt

(2)

where CPAS (pg sampler-1) is the sampler concentration; CAir is the time-averaged atmospheric concentration over the deployment period t (day); and R (m3 day-1 sampler-1) is the sampling rate of the system, which combines k and A from eq 1 in one term. In linear uptake samplers, a very large uptake capacity of the sampling medium is required to ensure that equilibrium between the sampling phase and the gas phase will not be reached during a sampling period. When recommending on the selection of sampling media, Cao (24) proposed that a sorbent is satisfactory for passive sampling if the retention VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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volume for the sampled chemicals exceeds 0.1 m3 g-1. The uptake capacity of XAD-2 for organic chemicals has previously been characterized (25, 26). Specifically, the retention volume for hexachlorobenzene (HCB) was determined to be 104 m3 g-1 at 25 °C (26). At lower temperatures and for POPs less volatile than HCB, the retention volumes will be even larger than 104 m3 g-1, suggesting that the capacity of XAD-2 for most POPs is clearly sufficient for it to serve as sampling medium in linear uptake passive samplers. If the retention volume for a particular substance has not previously been determined, it can be reliably estimated using equations based on vapor pressure, boiling point, or linear solvation energy parameters (25, 26). Because of the reversible nature of the sorption process, the interaction between the sampled chemical and the sampling medium will affect the sampling efficiency. Underhill defined the sampling efficiency of a passive sampling system as the ratio of the sampling rate to the initial (and also maximum possible) sampling rate (27). Ideally, the sampling rate would remain at a maximum throughout the sampling period. In actuality, the sampling rate will be reduced as soon as sampling starts because of the buildup of the sampled chemical at the sampling surface. If the sorption is characterized by a linear isotherm, the sampling efficiency drops below 90% after the sampling medium reaches 10% equilibrium (27). The sorption isotherm of HCB onto XAD-2 at very low, environmentally relevant concentrations was observed to be linear (26). If the sampling rate R is assumed to be 1 m3 day-1 sampler-1, a sampler is assumed to contain 20 g of XAD-2, and the atmospheric concentration of HCB is assumed to be 40 pg m-3, then XAD-2 would reach less than 0.2% equilibrium at 20 °C over a 1-yr sampling period. This suggests that an XAD-2-based passive sampler will remain highly efficient for POPs over long-term sampling periods. Temperature is an environmental factor possibly affecting the performance of a PAS. Whereas the sorption of gaseous POPs onto XAD-2 decreases greatly at higher temperatures (26), the calculation above reveals that the capacity of XAD-2 even at high environmental temperatures will remain sufficiently large to ensure that equilibrium is not approached. However, the rate of diffusion to the sampler material may be affected by temperature, which will be discussed in more detail below. Another factor with potential impact on the characteristics of a PAS is relative humidity. XAD-2 is a hydrophobic sorbent, and most POPs are halogenated hydrocarbons with no hydrophilic substituents. Water is thus not expected to influence the sorption of POPs to XAD-2 (28), and the uptake of POPs in XAD-2-based passive samplers should be independent of the relative humidity in the atmosphere.

Experimental Section Sampler Design. The PAS consists of a resin-filled container placed in a protective sampling shelter with an opening at the bottom (Figure 2). The sampling container is a long thin cylinder made of a fine stainless steel mesh held in shape by two end caps. The mesh cylinder minimizes potential contamination and adsorptive wall loss. The upper end cap has a loop attached to it. The entire cylinder is filled with XAD-2 resin as sampling medium. The shelter with an opening at the bottom is designed to minimize the effect of wind velocity and to prevent exposure of the sampling medium to precipitation and large aerosol particles subject to gravitational settling. It consists of a lid and a bottom part, which fit snugly into each other, and is made of stainless steel that is sturdy and able to withstand severe weather conditions. The cylinder is hanging on a carabine hook that is attached on the inside of the lid. This arrangement allows the repeated opening and closing of the shelter and the 1354

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FIGURE 2. Design and dimensions of the XAD-2-based passive air sampling system for persistent organic pollutants. transfer of the cylinder into a shipping container without the operator ever touching the resin-filled container. Spokes fitted into the steel shelter prevent the dangling and accidental unhooking of the cylinder. Air exchange in the shelter is through the bottom opening and a number of small holes in the top of the shelter. A grid with a very wide mesh is fitted into the bottom opening to keep larger animals away from the sampling cylinder while maintaining sufficient air exchange. A double-lid design prevents rain or snowmelt water from entering through the holes on the top. In the field, the entire sampler is attached to an existing structure or a pole at a height of 1.5 m above the ground. For transport, the sampling cylinder is placed into a Teflon tube and sealed by Teflon tape-wrapped rubber stoppers. Preparation of PAS. To reduce contamination, the cleaning of the sampling resin and the assembly of the sorbent container were conducted in a clean room with a carbonand HEPA-filtered air supply at the National Water Research Institute (NWRI). The XAD-2 resin (20/60 mesh, 350 m2/g surface area, 9 nm pore diameter, Supelco) was rinsed with Milli-Q water and Soxhlet extracted three times for 4 days each using in turn methanol, acetonitrile, and dichloromethane. After being washed with sodium hydroxide to remove potential acidic interferents, dichloromethane, and methanol, the XAD-2 was stored in methanol. Approximately 60 mL of wet resin (XAD-2 in methanol) was added to a precleaned stainless steel mesh container plugged with a small amount of clean glass wool at the bottom and covered with glass wool on the top. The column was then transferred to a big amber glass jar, dried by nitrogen, and sealed in the shipping containers until use. Wind Tunnel Experiments. Within the linear uptake region of a PAS, the rate of chemical uptake is independent of the capacity of the sampling medium, and the dependence of the sampling rate on the wind speed should be largely independent of the chemical nature of the sampling resin and the sampled compound. Accordingly, it should be possible to evaluate kinetic aspects of the performance of the PAS by recording the uptake of water in a hygroscopic material with the same physical characteristics, namely, the same mesh size, as the sampling resin. This approach was used to investigate the effect of wind velocity on the uptake kinetics. A sampling cylinder was filled with dried silica gel (35/60 mesh, Aldrich Chemical Co., Milwaukee, WI) and placed in a sampler housing that was mounted in a wind tunnel capable of producing wind velocities between 4 and 20 m s-1. All experiments were conducted in the wind speed

range of 5-15 m s-1 and were measured using a Pitot tube connected to an inclined water manometer. As air was drawn through the inlet, the weight gain of the container as a function of time was determined gravimetrically. The relative humidity and air temperature were measured with a micropsychrometer and a thermometer placed in the wind tunnel. Measurements at variable wind velocities were conducted within a single day to minimize the confounding effects of changes in temperature and relative humidity. Field Calibration. To test the XAD-2 PAS over a wide range of environmental conditions, three locations were selected in the vicinity of IADN and Canadian Northern Contaminants Program (NCP) monitoring sites with ongoing conventional air sampling of POPs. Two of them were located in the Great Lakes region at Point Petre (Lake Ontario, 43°50′ N, 77°09′ W) and Burnt Island (Lake Huron, 45°48′ N, 82°57′ W), whereas the third site was located in the Canadian High Arctic, Alert on Ellesmere Island (82°31′ N, 62°17′ W). At each of these stations, 14 passive samplers were installed close to each other at the same time. Deployment at Point Petre, Burnt Island, and Alert took place on May 12, June 1, and August 24 2000, respectively. Duplicate samplers were retrieved after exactly 1, 2, 4, 6, 8, 10, and 12 months and returned in Teflon shipping containers to NWRI by courier. The samplers were stored frozen until analysis. Two additional passive samplers were collocated with a HiVol air sampler at the Canadian Wildlife Service (CWS) field station near Long Point on Lake Erie (42°35.4′ N, 80°26.79′ N). Whereas the PAS were exposed from May 31 to December 13, 2000, active air sampling was carried out every 2 weeks from May 30 to September 20, 2000, at the CWS field station using a HiVol air sampler (TE-5000, Tisch Environmental Inc., Cleves, OH). Sampling media were polyurethane foam plugs (PUF) and a GF/A glass fiber filter to trap particulate matter. PUFs were pre-extracted in a Soxhlet apparatus with hexane under clean room conditions and dried with highpurity nitrogen prior to use. GF/A filters were preheated at 450 °C for 16 h prior to use. The sampler was operated for 16 h, yielding air volumes of 100-116 m3. Extraction and Analysis. The XAD-2 from the sampling container was transferred to an elution column and extracted with 250 mL of methanol followed by 350 mL of dichloromethane. Methanol in the combined eluent was removed by 250 mL of 3% sodium chloride, and the extracts were concentrated on a rotary evaporator. The solvent was then exchanged to isooctane, and the volume of the extracts was further reduced by nitrogen evaporation. The samples were fractionated on a silica gel (70/230 mesh, high purity, Supelco) column, eluting sequentially with 65 mL of hexane as fraction A and 80 mL of a 1:1 mixture of hexane:dichloromethane as fraction B. To each fraction, isooctane was added, and the sample was concentrated to 1.0 mL with nitrogen. All solvents used in this work were pesticide grade. The extract fractions were analyzed for the chlorobenzenes and several organochlorinated pesticides (OCP) including dieldrin, endosulfan, the hexachlorocyclohexanes (HCHs), chlordane-related compounds, DDT, and its break-down products DDE and DDD. A Hewlett-Packard (HP) 5890 gas chromatograph (GC) equipped with dual electron capture detectors (ECDs) and HP-5 (30 m × 0.25 mm × 0.25 µm) and HP-1 (30m × 0.25 mm × 0.25 µm) capillary columns was employed. Helium was used as carrier gas at a flow rate of 1 mL min-1. The GC-ECD was operated under the following conditions: injector temperature, 220 °C; detector temperature, 350 °C; temperature program, 80 °C for 2 min, 4 °C min-1 to 140 °C, 2 °C min-1 to 250 °C, and 5 °C min-1 to 280 °C. Quality Assurance. Routine quality assessment procedures established in the laboratory were followed in this study. Procedural blanks and resin blanks were run together with

FIGURE 3. Uptake rate of water in silica gel as a function of wind speed as determined using a passive air sampling system (such as shown in Figure 2) installed in a wind tunnel. the exposed samples by performing the entire extraction, cleanup, and analytical procedures to estimate the background contamination originally present in the XAD-2 resin and potentially introduced during extraction and cleanup. In addition, a couple of samplers, referred to as field blanks, were brought to each of the three sites, stored there without ever opening the Teflon containers, and returned with the exposed samplers. These field blanks served to assess possible contamination caused by shipping, handling, and storage. For most POPs of interest, the resin blanks and field blanks were higher than the procedural blanks. All results were blankcorrected using the averages of 13 resin blanks and 8 field blanks. These resin blanks and field blanks were also used to calculate the method detection limits (MDLs) defined as the average blank value plus three times the standard deviation. These MDLs are given in a table in the Supporting Information. δ-Hexachlorocyclohexane (δ-HCH) and polychlorinated biphenyl PCB-166 had been added to the XAD-2 columns in the laboratory prior to shipping. A suitable number of XAD-2 columns was further spiked with 1,3,5-tribromobenzene (1,3,5-TBB) and PCB-204 prior to extraction. Standards of δ-HCH, 135-TBB, PCB-166 and PCB-204 were obtained from Ultra Scientific. Average recoveries of these four standards were 60% for δ-HCH, 63% for 135-TBB, 88% for PCB-166, and 100% for PCB-204. The reported data were not corrected for recovery.

Results and Discussion Effect of Wind Velocity on the PAS Uptake Kinetics. Figure 3 shows the water uptake of silica-filled PAS at different wind velocities in the wind tunnel. The amount of water in the samplers increased linearly with time. No significant change in the rate of water uptake was observed in the investigated wind speed range of 5-15 m s-1. From the slopes of the water uptake curves in Figure 3 and the measured relative humidity, the sampling rate R of the PAS for water can be derived. The average value of 10 experiments conducted at 12-15 °C was 3.5 ( 0.2 m3 day-1 sampler-1. A simplified two-dimensional fluid mechanics simulation of the passive sampler housing performed with ALGOR helps to better understand the effect of wind speed on the sampling rate. Figure 4 displays the result of a simulation, which assumes that wind blows from the left with a speed of 5 m s-1. The air flow cannot directly enter the sampler shelter but instead swirls around it, and the back flow sneaks into the housing. This simulated flow pattern explains why the VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Two-dimensional computer simulation of the air flow around a passive air sampling shelter using ALGOR. The air flow is assumed to come from the right with a velocity of 5 m s-1. wind has little effect on the sampling rate. By revealing that there is no direct air flow through the shelter, the simulation further suggests that the sampling rate is restricted by the limited air exchange in the sampler housing. This implies that, for this sampler design, the mass transfer of chemical vapors to the sampling resin is mostly controlled by molecular diffusion. Field Calibration Experiments. Most of the investigated POPs were detectable in the passive samplers, even after field deployment as short as 1 month. High levels of some OCPs in selected blanks from Point Petre indicated that at least some of the 14 samplers deployed at that station had been contaminated. These data from Point Petre were therefore not included in the data interpretation. The amounts of R-HCH, γ-HCH, and HCB collected by replicate samplers after variable deployment times at Burnt Island are shown in Figure 5. No significant differences are observed between the amounts quantified in replicate samplers. This was also case for the other POPs measured in the PAS from Burnt Island and Alert. The mean normalized difference between duplicates was 8% for all POPs detected above the

MDLs at these stations (n ) 80), indicating good reproducibility of XAD-2 PAS for collecting vapor-phase POPs from the atmosphere. For the calibration only those compounds were selected that were present above the MDLs in all samples (i.e., including those that were exposed for only 1 month) and for which atmospheric concentrations could be derived from the results of the IADN and NCP. This was the case for HCB and several of the OCPs. Three OCPs (R-HCH, γ-HCH, and R-endosulfan) at Alert, four OCPs (R-HCH, γ-HCH, dieldrin, and trans-nonachlor) at Burnt Island, one OCPs at Point Petre (R-HCH), and most of the chlorobenzenes at all three stations were found in all of the samplers above their MDLs. Uptake curves for those pesticides and HCB are presented in Figure 6. The XAD-2 PAS clearly show uptake within 1 month of deployment, and steady uptake is maintained throughout the 1-yr sampling period. Some of the uptake curves indicate higher slopes at the beginning and end of the sampling period and lower slopes in the middle. As sampling started in early summer, such behavior would be expected if air concentrations of the POPs are higher in spring and summer than in winter. Such seasonal air concentration changes are commonly observed for many of the investigated compounds. R-HCH and HCB are the most volatile of the investigated POPs and would be expected to reach equilibrium first. The fact that no leveling off in the uptake curves of these compounds was observed suggests that the XAD-2 indeed has a sufficiently large capacity to prevent the PAS from entering the curvilinear region. This is consistent with the predictions based on laboratory-derived retention volumes presented in the Theory section. If the time-averaged atmospheric concentrations during the deployment period are known, sampling rates (R) of PAS under field conditions can be derived using eq 2 and the slopes of the linear regressions through the uptake curves shown in Figure 6. Although IADN and NCP performed HiVol sampling for POPs at Burnt Island, Point Petre, and Alert during the study period in 2000/2001, the air concentrations for this time period have not yet been reported. To nevertheless estimate field sampling rates, we predicted timeaveraged air concentrations at these sites for selected POPs by extrapolating published data for the years of 1993-1998. If a chemical had not shown a temporal trend during those years, the average of the annual mean air concentrations was used as an estimate of the air concentration during the PAS deployment period. If a decrease in the levels of the

FIGURE 5. Comparable amounts of r-HCH, γ-HCH, and HCB collected by replicate samplers after variable deployment times at Burnt Island indicate good reproducibility. 1356

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POPs had been observed during that time interval, air concentrations during the PAS sampling period were estimated assuming the applicability of first-order half-lives (29; H. Hung, Meteorological Service of Canada, personal communication). Table 1 summarizes the sampling rates for individual chemicals obtained from the field calibration experiments. The standard deviation of the R values in Table 1 include the uncertainty of the slopes in Figure 6 as well as an estimated uncertainty in the predicted air concentrations. R for different POPs at the same station were comparable, but R varied between location. The average sampling rates are 0.52 ( 0.11, 0.97 ( 0.46, and 1.97 ( 0.40 m3 day-1 sampler-1 at Alert, Burnt Island, and Point Petre, respectively. The data suggest that the sampling rates for POPs onto XAD-2 are much lower than those obtained for water onto silica in the wind tunnel experiments. These results become understandable when it is assumed that molecular diffusion is controlling the rate of vapor uptake in the PAS and when the influence of molecular size and temperature on molecular diffusion coefficients is taken into account. If molecular diffusion is controlling the rate of uptake, the sampling rate (R) is related to the diffusion coefficients in air (DAir; mm2 s-1) through the following equation:

R ) hDAir

FIGURE 6. Increase of the concentrations of several POPs measured in passive air samplers deployed for up to 1 yr at the three monitoring sites: Alert (top), Burnt Island (middle), and Point Petre (bottom). The error bars refer to the standard deviation of the concentrations measured in duplicate samplers.

(3)

where h is a coefficient of proportionality and should depend only on the geometry of the sampler. DAir is temperaturedependent, which is typically expressed by a proportionality to T 1.75 or T 1.5 (30, 31). The mean temperatures during the 1-yr field calibration experiments were 8.7 °C at Point Petre (May 12, 2000-May 12, 2001), 7.3 °C at Burnt Island (June 1, 2000-June 1, 2001), and -17 °C at Alert (August 24, 2000August 24, 2001) (C. Audette and H. Hung, Meteorological Service of Canada, personal communication). DAir for selected POPs at these temperatures was estimated from molecular mass and the sum of structural volume increments using the Fuller, Schettler, and Giddings (FSG) equation (30, 31) and are listed in Table 1. DAir of various POPs is very close at a given temperature (approximately 4 mm2 s-1) but is considerably lower than DAir of water, which is 25.6 mm2 s-1 at the average temperature of the wind tunnel experiments (13 °C). This is likely the reason for the large discrepancy between the wind tunnel and field-derived sampling rates. Water molecules are much smaller and diffuse faster to the sampler resin than the relatively bulky POP molecules. Molecular diffusion of water in air is approximately six times faster than that of the POPs, and the sampling rate in the wind tunnel is similarly higher than the field-derived uptake rates for the POPs. This indicates a reasonable agreement considering that the R for water was determined over a time range of minutes in a wind tunnel, whereas the R for the POPs was derived from year-long field experiments. The differences in the sampling rates observed between the three sampling locations are more difficult to explain. The results of the wind tunnel experiments exclude the possibility that differences in wind speed are responsible. The three stations also do not differ sufficiently in terms of their wind speeds and exposure to wind to account for the observed differences. Differences in temperatures may be partly responsible for the different R measured at the three sampling sites, as higher temperatures at the temperate stations will accelerate the diffusion of vapor-phase POPs to the sampling medium relative to the Arctic station. However, the difference in DAir caused by the temperature difference is smaller than the observed difference in field-derived sampling rates (Table 1). There is also the possibility that thermally induced convection in the sampling shelter is VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Field-Derived Sampling Rates (R) and Estimated Molecular Diffusion Coefficients in Air (DAir) at Average Temperatures of Alert, Burnt Island, and Point Petre for Selected Persistent Organic Pollutants Alert

DAir (mm2 s-1)

0.59 ( 0.06 0.42 ( 0.12 0.63 ( 0.38 0.43 ( 0.07

4.40 4.10 4.10 3.58 3.48 3.29

(m3 day-1 PAS-1) HCB R-HCH γ-HCH R-endosulfan dieldrin trans-nonachlor

Burnt Island

R

1358

9

DAir (mm2 s-1)

1.12 ( 0.11 1.25 ( 0.37 1.49 ( 0.43

5.15 4.80 4.80 4.19 4.08 3.86

(m3 day-1 PAS-1)

increasing the sampling rate at the temperate locations, but the effect is likely to be minor. By having established that the samplers are far from equilibrium with the atmospheric gas phase, even at the highest encountered ambient temperatures, we can further exclude the possibility that the influence of temperature on the sorption capacity of the XAD resin is responsible. In any case, the uptake capacity is lower at higher temperatures, and in contrast to the observations, R would be expected to be smaller at warmer sampling stations if equilibrium would indeed be approached. There is however also the possibility of an indirect effect of temperature. Different sampling efficiencies have been observed for XAD-2 vs PUF in HiVol air samplers (32). During warmer temperatures, PUF was shown to have lower collection efficiency for naphthalene than XAD-2. Colder temperatures improved the sampling efficiency of the PUF. This difference in efficiency was not observed with higher molecular weight PAHs. Breakthrough in the PUF during warmer temperatures was given as the cause for the difference (32). If the HiVol PUF air samplers used to calibrate the PAS in this study have the same temperature-dependent sampling efficiency, this could have led to the difference in calculated PAS sampling rates. If CAir was underestimated by the HiVol air sampler in the warmer climate (Burnt Island and Point Petre), this would cause an increase in the calculated R relative to those derived for Alert, and this was indeed observed. Higher molecular compounds may not be subject to this effect, which would explain why dieldrin and transnonachlor have lower R values at Burnt Island than HCB and the HCHs (Table 1). Another explanation for the discrepancies between the sampling rates from the three sites is the uncertainty in the extrapolated mean air concentrations. The extrapolations over 3 yr based on a record of 5 yr of measurements could easily have errors on the scale of the discrepancies between the R from the three sampling sites. Additional field calibration experiments are required to shed further light on the possible variability of uptake rates R. Such experiments should preferably use XAD-2 as the sampling resin in the HiVol samplers to eliminate the possibility of breakthrough artifacts. The field study suggests that, at typical concentration levels of POPs, minimum deployment periods for the PAS are in the range of several months. For example, the collection of 100 m3 of air, a typical volume for the HiVol sampling of POPs, would require an exposure duration of around 3 month. With shorter exposure periods, the amounts accumulated in the PAS are too close to the MDL to be quantified with confidence. This is not necessarily a disadvantage because the determination of time-averaged atmospheric concentrations requires longer sampling periods to compensate for within-day and day-to-day variations. Deployment periods could be reduced substantially if the design of the sampling shelter is changed to increase air flow and thus the sampling rates. However that would come at the cost of a sampling rate that is dependent on wind speed, which compromises quantitative interpretation. ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 7, 2003

Point Petre

R

0.40 ( 0.12 0.58 ( 0.17

R

DAir (mm2 s-1)

1.68 ( 0.18 2.25 ( 0.67

5.20 4.85 4.85 4.23 4.11 3.89

(m3 day-1 PAS-1)

TABLE 2. Comparison of Annual Mean Air Concentrations (CAir) (pg m-3) Derived from PAS Data at Alert (August 2000-August 2001), Burnt Island (June 2000-June 2001), and Point Petre (May 2000-May 2001) with Averages of CAir in 2000 and 2001 Predicted from NCP and IADN Results for Selected Persistent Organic Pollutants Alert

HCB R-HCH γ-HCH R-endosulfan cis-chlordane trans-chlordane trans-nonachlor heptachlor ep. oxychlordane dieldrin endrin o,p′-DDT o,p′-DDD

this study

NCP

62 33 5.4 7.2 0.7 0.1 0.1 0.2

61 41 4.4 9.2 0.4 0.3 0.5 0.6

Burnt Island this study

Point Petre

IADN

this study

IADN

36 20 11 9.8 0.6 0.5

30 14 6.2 22 2.0 1.7

26 13 8.8 25 1.8 2.4

34 14 8.0 77 2.6 1.9

1.0 3.6 2.5 0.1 0.4

1.2 8.4 0.3 1.6 0.1

1.7 3.2

1.0 8.1

Evaluation of Passive Air Sampling Data. To evaluate the capability of the samplers to quantitatively determine volumetric air concentrations of POPs, we compared PASderived air concentrations with data from the IADN network and NCP monitoring sites. Volumetric air concentrations were calculated using the average sampling rate (R) at each calibration site in combination with the amounts of POPs measured in the replicate samplers that had been exposed for a full year at Burnt Island, Point Petre, and Alert. For comparison, we predicted the air concentrations during the PAS deployment period from the IADN and NCP data for previous years using the same procedures as mentioned above. The agreement between the PAS-derived air concentrations and those derived from HiVol sampling is good (Table 2, Figure 7). For the more abundant contaminants, PAS-derived concentration data are typically within (30% of the IADN and NCP results, except for γ-HCH at Burnt Island (70%) and R-endosulfan at Burnt Island (-60%) and Point Petre (-70%). PAS-derived data for compounds with air concentrations below 1 pg m-3 show higher discrepancies ((60-80%), which may be the result of higher analytical errors close to the detection limits. For further independent evaluation at a different location, we compared the air concentrations of γ-HCH, endosulfan, and DDT-related compounds measured at Long Point by PAS and HiVol sampler (Table 3). An average sampling rate (R) of 1 m3 day-1 sampler-1 was assumed to apply. Even though no active air sampling was conducted during the latter part of the PAS exposure period (October-December 2000), the agreement is very good, especially for γ-HCH and the DDT-related compounds. R-Endosulfan is an exception, and the higher mean air concentration derived from the active sampling may be due to bias caused by higher concentration

Supporting Information Available A table with the method detection limits. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

FIGURE 7. Comparison of volumetric air concentrations of various POPs in units of pg m-3 determined from replicate passive samplers deployed for 1 yr at Point Petre, Burnt Island, and Alert with concentrations extrapolated from data reported for these sites by the Integrated Atmospheric Deposition Network (IADN) and the Northern Contaminants Program (NCP). The agreement is better for the contaminants with higher air concentrations.

TABLE 3. Mean and Standard Deviation of Air Concentrations (pg m-3) Derived from Two Passive Air Samplers (May-December 2000) and Nine Classical HiVol Samples (May-September 2000) at Long Point, ON γ-HCH p,p′-DDT p,p′-DDE p,p′-DDD o,p′-DDT R-endosulfan β-endosulfan

high volume

PAS

33 ( 35 4.2 ( 2.2 38 ( 31 0.9 ( 0.7 5.2 ( 2.7 215 ( 158 8.0 ( 8.5

34 ( 3 5.0 ( 1.3 36 ( 8 0.6 ( 0.3 1.6 ( 0.5 66 ( 12 10.3 ( 2.2

during the summer. The two comparisons suggest that PAS can at least be used to semiquantitatively measure atmospheric concentrations of POPs. In summary, the simple XAD2-based passive sampling system developed and tested in this study is suitable to derive time-weighted, semiquantitative information on concentrations of vapor-phase POPs in the atmosphere over time scales of months to years. The results described clearly demonstrate the usefulness of the passive samplers as a tool for monitoring POPs over a wide range of environmental conditions.

Acknowledgments We are grateful to Drs. Michael McLachlan and Gunther Umlauf for sharing the original design of the PAS and for helpful discussions; Dr. Feng Lin for the computer simulations; Darrell Smith, Floyd Orford, and Lori Leeder of the Meteorological Service of Canada (MSC) for their help in setting up the stations and retrieving the samplers; and Dr. Hayley Hung and Celine Audette of the MSC for providing IADN concentration and meteorological data. We further acknowledge funding from the Toxic Substances Research Initiative (TSRI 27) by Environment and Health Canada.

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Received for review September 18, 2002. Revised manuscript received January 8, 2003. Accepted January 13, 2003. ES026166C

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