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Atmospheric Environment
journal homepage: www.elsevier.com/locate/atmosenv
Short communication
Calibration and application of PUF disk passive air samplers for tracking polycyclic aromatic compounds (PACs)q
Tom Harner3, *, Ky Sua, Susie Genualdia, Jessica Karpowicza, Lutz Ahrensa, Cristian Mihelea, Jasmin Schuster a, Jean-Pierre Charlandb, Julie Narayana
aAir Quality Processes Research Section, Environment Canada, 4905 Dufferin Street, Toronto, ON M3H 5T4, Canada b Analysis and Air Quality Section, Environment Canada, Ottawa, ON K1A 0H3, Canada
CrossMark
HIGHLIGHTS
• PUF disk passive air samplers are characterized for polycyclic aromatic compounds (PACs).
• PUF disk sampling rates are on the order of 5 m3 day-1.
• There are no differences in sampling rates between particle-phase and gas-phase PACs.
• This is the first application of the samplers for alkylated PAHs and dibenzothiophenes.
ARTICLE INFO ABSTRACT
Results are reported from a field calibration of the polyurethane foam (PUF) disk passive air sampler for measuring polycyclic aromatic compounds (PACs) in the atmosphere of the Alberta oil sands region of Canada. Passive samplers were co-deployed alongside conventional high volume samplers at three sites. The results demonstrate the ability of the PUF disk sampler to capture PACs, including polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs and parent and alkylated dibenzothiophenes. Both gas- and particle-phase PACs were captured with an average sampling rate of approximately 5 m3 day-1, similar to what has been previously observed for other semivolatile compounds. This is the first application of the PUF disk sampler for alkylated PAHs and dibenzothiophenes in air. The derived sampling rates are combined with estimates of the equilibrium partitioning of the PACs in the PUF disk samplers to estimate effective sample air volumes for all targeted PACs. This information is then applied to the passive sampling results from two deployments across 17 sites in the region to generate spatial maps of PACs. The successful calibration of the sampler and development of the methodology for deriving air concentrations lends support to the application of this cost-effective and simple sampler in longer term studies of PACs in the oil sands region.
© 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
Article history: Received 30 January 2013 Received in revised form 27 March 2013 Accepted 5 April 2013
Keywords: Alberta oil sands Air sampling PACs PAHs
Alkylated PAH Dibenzothiophenes PUF disk
Passive air sampling
1. Introduction
Development of the oil sands industry in Alberta has led to concerns regarding health risk to humans, and other terrestrial and aquatic wildlife associated with exposure to toxic chemicals that are emitted as a by-product of the oil sands industry (Kelly et al., 2009, 2010). In response to these concerns the federal
q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.
* Corresponding author. E-mail address: tom.harner@ec.gc.ca (T. Harner).
government developed the Integrated Monitoring Plan for the Oil Sands in July 2011 (Canada, 2011). This was followed by a joint Canada/Alberta implementation plan for oil sands monitoring, released in early 2012 (Government of Canada and Government of Alberta, 2012). During the time period that these plans were being developed, smaller-scale reconnaissance studies were initiated to assess immediate concerns associated with air toxics deposition and related ecosystem effects. One of these studies included a feasibility assessment of the GAPS-type (Global Atmospheric Passive Sampling Network) (e.g. Pozo et al., 2009) polyurethane foam (PUF) disk sampler to measure the spatial distribution of polycyclic aromatic compounds (PACs) in air. PUF disk samplers have been used previously for polycyclic aromatic hydrocarbons (PAHs) (e.g. Jaward et al., 2004; Motelay-Massei et al., 2005; Bartkow et al.,
1352-2310/$ — see front matter © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2013.04.012
2006; Santiago and Cayetano, 2007; Klanova et al., 2008; He and Balasubramanian, 2010; Pribylova et al., 2012) but have so far not been tested for broader PACs including alkylated PAHs and parent and alkylated dibenzothiophenes. These compounds are present in bitumen and also emitted from pyrogenic sources (e.g. combustion processes such as forest fires, waste burning and vehicular traffic). Alkylated PAHs in air are primarily associated with petrogenic sources (e.g., bitumen and upgrading activities) (Hawthorne et al., 2006). These are PAH by-products that contain straight and/or branched carbon side chains. They are often more persistent and more toxic compared to the parent PAHs (Hodgson, 2007; Kelly et al., 2009). Dibenzothiophenes are sulfur containing aromatic chemicals (with and without straight/branched carbon side chains) that are associated with bitumen and an indicator of petrogenic sources (i.e. oil sands activity).
The objectives of the study were i.) to conduct the first field calibration of GAPS-type PUF disk samplers for PACs by co-locating with conventional high volume air samplers at multiple sites; ii.) to derive sampling rates for both gas- and particle-phase PACs; iii.) to derive effective air sample volumes for individual PACs for PUF disk samplers, that accounts for sampling rate and approach to equilibrium (i.e. kinetic and equilibrium phases) so that air concentrations can be derived from the amount of chemical accumulated in the samplers, and iv.) to use the above methodology to generate the first spatial maps of PAC air concentrations across the oil sands region. Ultimately, this proof of concept of the PUF disk sampler for generating volumetric air concentrations for PACs will contribute to the longer term study of PACs and the assessment of their deposition across the oil sands region.
2. Methods
Specific PACs that are targeted in this study are listed in Table 1. These include PAHs, alkylated PAHs, and dibenzothiophene and its
Table 1
Target analytes.
PAHs Alk-PAHs Dibenzothiophenes
and alkylated dibenzothiophenes
Dibenzothiophene C1 Dibenzothiophenes C2 Dibenzothiophenes C3 Dibenzothiophenes C4 Dibenzothiophenes
a PAHs not included in US EPA 16 PAH list.
alkylated homologs. Standards for the parent PAHs were purchased from Cambridge Isotope Labs (Andover, MA) and Chiron (Trond-heim, Norway). For the alkyl homologs of the PAHs and dibenzo-thiophenes, one to three individual standards were obtained (Chiron, Trondheim, Norway) for each homolog group (i.e. C1, C2, C3, C4, representing the number of carbon atoms). The designation C1, C2, C3 etc. refers to the length of the carbon-chain attached to the parent compound (i.e. C1 is a methyl group, C2 is an ethyl side chain or dimethyl, and so on). Details regarding sample preparation, analysis and quality assurance/quality control (QA/QC) is given in the Supplementary Information (SI).
2.1. Sampler deployment
The PUF disks were deployed for two consecutive periods starting in November 2010, at various locations in the oil sands region near the Fort McMurray area in Alberta, Canada (see map, Fig. S1). Period 1 started in early November 2010 and ran for approximately 30 days and period 2 started in the first week of December and ran for approximately 60 days. Details regarding sampling sites and deployment are provided in Table S2. Fig. S2 is a schematic of PUF disk sampler and a photo showing deployment at a remote site. The seventeen monitoring sites that were used in this study were based on an existing network of air sampling sites operated by the Wood Buffalo Environmental Association (www. wbea.org). Nine "remote sites" had no electrical power and were accessible only by helicopter. The sites consisted of a sampling tower erected in a forest clearing. The passive samplers were mounted at approximately 15 m height using a cable and pulley system. The remaining eight "local sites" were accessible by road and were typically within a few kilometers of industrial/urban activity. At the local sites, samplers were mounted at about 5 m above ground level, on the rooftop railing of the air quality monitoring shelters. Additional information about the PUF disk sampler and deployment is provided in the SI. As part of the PUF disk calibration exercise, active high volume air samplers that collected total gas-and particle-phase compounds were co-deployed at three of the near-source monitoring sites (AMS5, AMS11, and AMS13) during period 2 (see Fig. S1). Samples were collected for 24 h, 1 day in 6. Additional details regarding high volume air sample collection are provided in the SI.
Passive samples were analyzed at the Hazardous Air Pollutants Laboratory in Toronto, Canada. This is the same laboratory that analyzes samples under the GAPS Network. High volume air samples were analyzed by the National Air Pollution Surveillance (NAPS) lab in Ottawa, Canada (http://www.ec.gc.ca/rnspa-naps/).
An intercalibration exercise was conducted between the two laboratories to harmonize datasets. As part of this exercise both laboratories analyzed urban dust standard reference material for PACs and both laboratories employed isotopically labeled recovery surrogates in their methods for ensuring data integrity. Details regarding the analytical method and quality assurance and quality control measures and results are presented further in the SI.
3. Theory
The ability to express PUF disk results as volumetric air concentrations (e.g. ng m~3) requires that the effective air volume for each analyte is estimated. These effective sample volumes depend on the following three key factors (Shoeib and Harner, 2002).
1.) Linear-phase sampling of the PUF disk sampler is driven by
the air-side mass transfer coefficient, kA. As discussed previously, kA is in the range of 100 m d_1 which is equivalent to a
sampling rate, R, of ~4 m3 d_1, based on the surface area of a
Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Retenea
Benz(a)anthracenea Chrysene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzo(a)pyrene
Perylenea
Indeno(1,2,3-c,d)pyrene Dibenz(a,h)anthracene
Benzo(g,h,i)perylene
C1 Naphthalenes C2 Naphthalenes C3 Naphthalenes C4 Naphthalenes C1 Fluorenes C2 Fluorenes C3 Fluorenes C4 Fluorenes C1 Phenanthrenes/ anthracenes C2 Phenanthrenes/ anthracenes C3 Phenanthrenes/ anthracenes C4 Phenanthrenes/ anthracenes
C1 Fluoranthenes/pyrene C2 Fluoranthenes/pyrene C3 Fluoranthenes/pyrene C4 Fluoranthenes/pyrene C1 Benz(a)anthracenes/ chrysene/triphenylenes C2 Benz(a)anthracenes/ chrysene/triphenylenes C3 Benz(a)anthracenes/ chrysene/triphenylenes C4 Benz(a)anthracenes/ chrysene/triphenylenes
PUF disk (i.e. R = kA x surface area) (Pozo et al., 2009; He and Balasubramanian, 2010).
2.) The PUF-air partition coefficient, KPUF-AIR determines the capacity of the PUF disk for each analyte.
3.) Lastly, for analytes that exist on ambient particles, the particle-phase sampling rate is also important. if collection of ambient particles is diminished by the sampling chamber, this will result in a reduced effective air sample volume for particle-associated chemicals that needs to be accounted for (Klanova et al., 2008).
g m-3), VPSM is the volume of the passive sampling medium (m3), kA is the air-side mass transfer coefficient (m d-1), Dfilm is the effective film thickness (m), and t is time (days). The kA value is simply the sampling rate R (m3 day-1) divided by the surface area of the PUF disk sampler (365 cm2). A template for estimating air volumes is available from the GAPS Network, www.ec.gc.ca/rs-mn/default. asp?lang=En&n=22D58893-1. The calculation of VAK for PACs is explained in the SI.
3.3. Particle-phase sampling
3.1. Linear sampling
Previous studies have shown that the linear phase sampling rate, R, is on the order of about 4 m3 d-1 for most non-polar hy-drophobic chemicals (Shoeib and Harner, 2002; Pozo et al., 2009; Chaemfa et al., 2008; He and Balasubramanian, 2010) including PAHs (Klanova et al., 2008; He and Balasubramanian, 2010). These linear sampling rates are typically derived by calibrating the passive sampler against an accurate determination of air concentration using high volume air samplers. The linear phase sampling rate for PUF disk samplers is thought to be largely driven by the air-side mass transfer coefficient (Tuduri et al., 2006). Diffusivity values for air are a weak function of temperature and molar volume (Shoeib and Harner, 2002). Higher wind speed has the effect of increasing the linear-phase sampling rate by diminishing the boundary layer surrounding the PUF disk (Tuduri et al., 2006; Klanova et al., 2008; May et al., 2011). The response of sampling rate to increased wind speed also suggests an air-side controlled uptake mechanism. The estimate of the linear-phase sampling rate can be further improved to account for site-specific variability (e.g. high winds) by performing calibrations against continuous high volume air samplers and/or by using depuration compounds (Pozo et al., 2009; Gouin et al., 2005; He and Balasubramanian, 2010). Depuration compounds are usually isotopically labeled chemicals that do not exist in air that are added to the PUF disk prior to deployment. Note: depuration compounds were not included in the preliminary phase of the study due to the required added complexity of the analytical method and the existing large number of target peaks that could result in potential interferences. The use of depuration compounds will be an area for further study.
3.2. Equilibrium correction
For most of the target compounds, the effective air sample volume (Vajr, m3) can be estimated fairly accurately and simply by using the air sampling rate, R (m3 d-1) multiplied by the number of days the sampler is deployed. However, for more volatile compounds, a correction must be made that accounts for the PUF disk reaching saturation (equilibrium with air) during the deployment period (Pozo et al., 2009). The equilibrium point has been shown to be proportional to the PUF-air partition coefficient, KPUF-AIR, which in turn is correlated to the octanol-air partition coefficient (KOA) of the chemical (Shoeib and Harner, 2002). In the current study where deployment times are on the order of 1 —2 months, this correction for approach to equilibrium is only needed for the more volatile 2-ring (and sometimes the 3-ring) PACs due to their higher vapor pressures and low KOA value.
Vair = (KpSM_A) x (Vpsm) x < 1 - exp
In Eq. (1), the dimensionless K'PSM-A is equal to KPSM-AIR multiplied by the density of the passive sampling medium (5PSM in
PUF disks have been shown to be capable of entrapping airborne particles (Chaemfa et al., 2009) although Klanova et al. (2008) reported reduced sampling rates for particle associated chemicals. The particle-phase sampling rate is expected to exhibit some variability among field sites depending on the total suspended particle concentration (TSP), the size distribution of particles, and various meteorological factors (e.g. wind speed, turbulence) (Tuduri et al., 2006; May et al., 2011). Variations in chamber design is another variable which may result in different sampling behavior, so it is important to assess the sampling rates for particle-phase PACs for the GAPS-type chamber design used in the current study.
4. Results and discussion
4.1. Field calibration of PUF disk samplers
Results from 21 high volumes samples collected at three of the sites during period 2 are presented in Table S11. These data were adjusted based on the results from the intercomparison exercise (see Table S11 for details) and used to estimate compound-specific sampling rates (R-values) for the PUF disk samplers. The results from the passive samplers are expressed as ng sampler-1 day-1 and presented in Tables S6—S9. At the cold winter temperatures experienced during period 2, the volatility of the PACs is suppressed (Odabasi et al., 2006) and most PACs will be accumulated in the linear (kinetic phase) of the PUF disk uptake profile over the two month deployment. In this case, the sampling rate (R, m3 day-1) can be determined simply using the ratio of the passive sampling data (expressed as ng sampler-1 day-1) and the high volume result (expressed as ng m-3). This comparison is illustrated in Fig. 1, with the slope of the plot reflecting the value of the sampling rate R. The mean value for R (based on calculations for individual analytes) is on average 5.0 ± 3.6 m3 day-1 for PAHs only, and 5.1 ± 3.7 m3 day-1 when considering all target analytes. This agrees well with sampling rates derived from previous studies using PUF disk samplers (Pozo et al., 2009; Gouin et al., 2005).
The results indicate that sampling rates do not differ substantially between sites and that sampling rates are similar for the three target groups, i.e. PAHs, alkylated PAHs, and (parent and alkylated) dibenzothiophenes. This is the first field calibration of PUF disk samplers for most of these analytes.
Another important result is that there is no discrimination of sampling for the particle-bound PACs which has been reported in some previous studies (e.g. Klanova et al., 2008). At the cold winter temperatures experienced during period 2, many of the 4-ring and high molecular weight PACs will be mainly on particles (Harner and Bidleman, 1998; Odabasi et al., 2006). Sampling rates derived for the higher molecular weight PACs are similar to values for the lower molecular weight PACs that are mainly in the gas-phase (see Figs. S5 and S6). This is a key finding because it demonstrates that the PUF disk samplers adequately capture the particle-associated burden of the atmosphere in this region. Many of the toxicologi-cally significant PACs (e.g. benzo[a]pyrene) exist mainly on particles. Future studies that will include a full year of data will explore
100.00
IA —. M c
n 0.10
□ AMS5 ♦ AMS11 AMS13
0.001 0.010 0.100 1.000 10.000 100.000 High Vol. (ng/m3)
100.00
_0J Q.
K O.io
XPAH □ alkPAH DBTh
0.001 0.010 0.100 1.000 10.000 100.000 High Vol. (ng/m3)
Fig. 1. Calibration of air sample results from PUF disk samplers against high volume samplers for polycyclic aromatic compounds. Top panel showing agreement between sampling sites and bottom panel showing agreement for different target classes. Note different units on axes.
the potential seasonality of the sampling rate, R, including its dependency on temperature and wind speed at the site. Additional measurements of particle-size distributions at the sites and within the sampling chambers may help to resolve the mechanism by which particle-phase PACs are captured and how this compares to sampling of gas-phase PACs.
4.2. Estimation of PUF disk effective air sample volumes (VAIR) for PACs
The estimation of Vair (m3) values for the full suite of PACs, including alkylated PAHs and parent and alkylated dibenzothio-phenes required an estimation of their KOA values based on correlation of KoA against molecular weight that was demonstrated for PAHs, with known KOA values (Odabasi et al., 2006). The method is explained in the SI and VAIR values for PACs for period 1 and 2 are reported in Table S12. Under the GAPS Network, a template is used for calculating effective air sample volumes (VAIR values, m3) for various target compounds, based on Eq. (1). The template has been updated to include PACs and is available through the corresponding author of this paper.
The much lower VAIR values for naphthalene and other low molecular weight PACs (Table S12) reflects equilibrium or approach to equilibrium in the PUF disk during the deployment period.
It is important to elaborate on the sources of uncertainty and propagated errors associated with the passive sampler derived air concentrations. These include: laboratory analytical uncertainty (typically 10—30%) for each lab (which highlights the importance of an interlaboratory comparison exercise as performed in this study); and uncertainty in the estimate of average air concentrations from
high volume air samplers operating on a 1 day in 6 sampling schedule, or about 17% of the time that the passive samplers were deployed. Previous studies have estimated that passive sampler-derived air concentrations are accurate within a factor of about 2 or 3 of actual values (Harner et al., 2006). The uncertainty associated with the passive data will be reduced and better quantified as additional calibration data become available from this program.
Furthermore, for PACs which vary substantially in size, a trend of decreasing diffusivity in air is expected with an increase in molar volume (molecular weight). Based on theory presented in Shoeib and Harner (2002) and molar volumes reported in Mackay et al. (2006), we expect diffusivity in air to be reduced by a factor of about 1.5 for 6-ring PACs compared to 3-ring PACs. Investigation of the variability of sampling rate with PAC molar volumes and other parameters is beyond the scope of this dataset and will be explored further in future studies.
4.3. Spatial distribution of PACs
Based on the derived air sample volumes for the passive samples (Table S12), deployment times, and the amount of chemical accumulated by the passive samplers (Table S6—S9), air concentration at the 17 sites were derived. Results are presented in Fig. 2 for dibenzothiophenes and in Figs. S3 and S4 for PAHs and alkylated PAHs, respectively. A detailed analysis of the PAC air concentration and composition is beyond the scope of this paper which focuses on the technical aspect of the PUF disk sampler calibration. However, some general observations regarding the data are given below.
Fig. 2 shows the spatial map of dibenzothiophenes (parent and alkylated) air concentration across the oil sands region. Highest concentrations, on the order of about 5—40 ng m~3, are observed at local sites that are in close proximity to active mining and/or upgrading activities suggesting the utility of this compound class as a marker of oil sands activity. Air concentrations at background sites are lower, on average, by a factor of about 2. These are the first measurements of dibenzothiophenes in air using PUF disk type passive samplers.
Results for PAHs and alkylated PAHs are presented in Figs. S3 and S4. Parent and alkylated naphthalenes are not included in the sum as their contributions dominate the air profile. It is common for naphthalene to be excluded from reported sum PAHs values as not all programs report naphthalene. Air sampling of naphthalene is challenging due to its high volatility (Bidleman and You, 1984). The SPAH also exhibit a gradient in air concentrations moving from local to remote sites, however the gradient is less well defined compared to gradient observed for the dibenzothiophenes. Furthermore, SPAH are elevated near Fort McMurray which is consistent with urban areas acting as sources of PAHs (Odabasi et al., 2006; Halsall et al., 1994). Highest air concentrations are observed for the sum of alkylated PAHs (mean for all sites of ~20 ng m~3) which also exhibit the local-remote concentration gradient observed above. Alkylated PAHs have previously been associated with oil sands activity (e.g. Hawthorne et al., 2006). These data represent the first measurements of alkylated PAHs in air using passive PUF disk samplers.
A more detailed presentation and interpretation of the air concentrations, congener profiles and longer times series data will be undertaken in future publications.
4.4. Implications
This study presents a method for estimating volumetric air concentrations for PACs using the GAPS-type PUF disk sampler. This is the first application of the sampler to alkylated PAHs and parent and alkylated dibenzothiophenes, which are important oil sands
Fig. 2. Dibenzothiophene air concentrations (ng m 3), derived from deployments of PUF disk passive air samplers at 17 sites in November—December 2010 (left panel) and December—February, 2010/11 (right panel).
compounds. This proof of concept of the PUF disk sampler lends confidence to its future application as a cost-effective method for investigating PACs across the oil sands region. Future work will explore the effect of changing seasons on the performance of the PUF disks sampler, for collecting both gas-phase and particle-phase PACs, and will explore the application of the sampler to a broader list of relevant compounds including, inter alia, nitro-PAHs and hydroxyl-PAHs (Wang et al., 2011) that may also be associated with oil sands activities.
Acknowledgments
We thank WBEA for logistical support for air sample collection. We also acknowledge the support of Gary Poole, Environment Canada, in sample analysis and training.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2013.04.012.
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