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Characterizing PUF disk passive air samplers for alkyl-substituted PAHs: Measured and modelled PUF-AIR partition coefficients with COSMO-RS
Chemosphere 145 (2016) 360e364
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Characterizing PUF disk passive air samplers for alkyl-substituted PAHs: Measured and modelled PUF-AIR partition coefficients with COSMO-RS J. Mark Parnis a, *, Anita Eng b, Donald Mackay a, Tom Harner b a b
Chemical Properties Research Group, Department of Chemistry, Trent University, Peterborough, ON K9J 7B8, Canada Air Quality Processes Research Section, Environment Canada, 4905 Dufferin St., Toronto, ON M3H 5T4, Canada
h i g h l i g h t s Estimates of partitioning are improved by using isomer-specific predictions. COSMO-RS performs better than the GAPS template for alkyl-substituted PAHs. An oligomeric model for polyurethane foam is effective for PUF modelling. RMS error is significantly reduced by explicitly considering isomers.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2015 Received in revised form 9 November 2015 Accepted 13 November 2015 Available online xxx
Isomers of alkyl-substituted polycyclic aromatic hydrocarbons (PAHs) and dibenzothiophenes are modelled with COSMO-RS theory to determine the effectiveness and accuracy of this approach for estimation of isomer-specific partition coefficients between air and polyurethane foam (PUF), i.e., KPUFAIR. Isomer-specific equilibrium partitioning coefficients for a series of 23 unsubstituted and isomeric alkyl-substituted PAHs and dibenzothiophenes were measured at 22 C. This data was used to determine the accuracy of estimated values using COSMO-RS, which is isomer specific, and the Global Atmospheric Passive Sampling (GAPS) template approach, which treats all alkyl-substitutions as a single species of a given side-chain carbon number. A recently developed oligomer-based model for PUF was employed, which consisted of a 1:1 condensed pair of 2,4-toluene-diisocyanide and glycerol. The COSMO-RS approach resulted in a significant reduction in the RMS error associated with simple PAHs and dibenzothiophene compared with the GAPS template approach. When used with alkylated PAHs and dibenzothiophenes grouped into carbon-number categories, the GAPS template approach gave lower RMS error (0.72) compared to the COSMO-RS result (0.87) when the latter estimates were averaged within the carbon-number-based categories. When the isomer-specific experimental results were used, the COSMO-RS approach resulted in a 21% reduction in RMS error with respect to the GAPS template approach, with a 0.57 RMS error for all alkylated PAHs and dibenzothiophenes studied. The results demonstrate that COSMO-RS theory is effective in generating isomer-specific PUF-air partition coefficients, supporting the application of PUF-based passive samplers for monitoring and research studies of polycyclic aromatic compounds (PACs) in air. © 2015 Elsevier Ltd. All rights reserved.
Handling Editor: Shane Snyder Keywords: COSMO-RS PAH isomers Passive sampling Polyurethane foam disk PUF Partitioning Environmental monitoring
1. Introduction It is well known that isomers of many molecules show similar partitioning properties between common media such as air, water, and soils. It is often the case that isomers such as alkyl-substituted
* Corresponding author. E-mail address: [email protected] (J.M. Parnis). http://dx.doi.org/10.1016/j.chemosphere.2015.11.060 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
polycyclic aromatic hydrocarbons (PAHs) are treated as a single species group when evaluating and modelling their partitioning and environmental fate. An example would be the grouping of 1,2,3-trimethylnaphthalene, 1-ethyl-2-methylnaphthalene, 1propylnaphthalene and a host of other alkyl-substituted naphthalenes with three side-chain carbon atoms as a single species designated C3-naphthalene. This simplification is often driven by the fact that commonly used quantitative structureeactivity
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relationship (QSAR)-type estimators of molecular partitioning properties do not generally have the capacity to distinguish between such isomers, making the labor-intensive measurement of individual isomer properties of limited value under many circumstances. The consideration of isomeric forms of molecular species of interest with respect to environmental fate cannot be understated. Most chemical compounds show significantly different properties with respect to isomerism. For example, Pegoraro et al. (2015) recently showed that the (E) and (Z) isomers of 2-ethylhexyl-4methoxycinnamate (a common UV-blocking additive) have vapor pressures that differ by a factor of 5. QSAR-based programs such as those used in EPI Suite do not distinguish between these two isomers, and estimate the same vapor pressure value for both (EPI Suite 4.11 2015). Chen et al. (2015) have recently demonstrated that the isomers of perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and perfluorooctanesulfonamide (PFOSA), show higher partition coefficients on particulate phases for linear isomers compared with branched ones. As well, Zhang et al. (2015) have shown that isomers of alkylated PAHs experience different dry deposition rates from air which is due partly to differences in their extent of partitioning to aerosols. COSMO-RS is a solvation properties theory that is specific to the exact molecular structure of the partitioning species in question, since it is based on the ideally solvated equilibrium geometry and charge distribution of the molecule obtained from quantummechanical calculations. Such properties vary in a non-trivial manner from one isomer to another, resulting in changes to partition coefficients that are significant, as demonstrated in this work. In this way, COSMO-RS is ideally suited for treatment of isomeric species and the prediction of their physico-chemical properties. Despite this ability, little has been published on the application of COSMO-RS theory to the question of variation in physico-chemical properties with isomerism. A recent example is that of Simpson et al. (2015), who used the isomer-specific estimation capabilities of COSMO-RS to accurately estimate gas chromatography (GC) retention times of polybrominated diphenyl ether (PBDE) isomer metabolites by correlating the COSMO-RS estimated boiling point of specific reference PBDEs with their GC retention time. The ability to theoretically “resolve” such isomers greatly facilitated identification of otherwise unknown metabolites. Our particular interest is focused on passive-sampling techniques, and the behavior of isomers of target species with respect to the kinetics and thermodynamics of uptake from air by passivesampling media. It is common to use passive air sampling devices based on polyurethane foam (PUF) and other polymeric materials for environmental monitoring over large areas (Klanova and Harner, 2013). In support of the Global Atmospheric Passive Sampling (GAPS) network (Pozo et al., 2006), Harner (2014) has developed a template for characterizing uptake curves for PUF disks for a wide range of chemical classes, including polycyclic aromatic compounds (PACs) which includes the unsubstituted and alkylated PAHs and dibenzothiophenes. The template is openly accessible through www.ResearchGate.net and is widely accessed by users of PUF disk samplers. The GAPS template relies on an empirical relationship between the octanol-air partition coefficient, KOA and the PUF-air partition coefficient, KPUFAIR (Shoeib and Harner, 2002; Harner et al., 2013), and does not account for different isomers of alkylated-PAHs. Recently, we developed an oligomer-based model for such PUF-based passive sampling devices based on a 1:1 condensed pair of 2,4-toluene-diisocyanide and glycerol (Parnis et al., 2015). Used in conjunction with the COSMO-RS theoretical approach to solvation and partitioning property estimation (Eckert and Klamt, 2002, 2013), this model was demonstrated to have a root-mean-square (RMS) error of 0.47 in log KPUFAIR based on
361
partition data from Kamprad and Goss (2007) between 15 and 95 C for a large set of organic molecules. In the current work, we apply this same oligomeric model for PUF to the question of isomer-specificity in passive sampling contexts. We measure directly, using a generator column approach, the KPUFAIR for a range of individual alkyl-substituted PAHs to compare the performance of the currently used GAPS template approach (Harner, 2014) and estimates based on COSMO-RS theory. 2. Experimental details 2.1. Chamber setup and experimental design Fig. 1 shows a schematic of the experimental setup used to directly measure the KPUFAIR partition coefficients. The lower compartment consists of a double-walled glass chamber that contains PUF disks that have been spiked with target compounds, and the upper compartment is a glass ‘sorbent trap’ containing ~3000 mg of sorbent (Chromatographic Specialties Inc, ISOLUTE PAH, Mid Glamorgan, U.K.). The PUF disks (14 cm diameter; 1.35 cm thick; Newterra TE1014, Brockville, ON Canada) were cleaned with an accelerated solvent extractor (Dionex ASE 350) using acetone, petroleum ether and acetonitrile, and then dried under nitrogen. A more detailed method of PUF disk cleaning is described in Harner et al. (2013). Two cleaned PUF disks were cut to a smaller diameter of 7.5 cm in order for their edges to fit snuggly and completely flush against the inner wall of the chamber. Prior to inserting the PUFs into the chamber, they were each spiked with an equal amount of PAC standards (see Table S1 for full list of PACs). To ensure even distribution, the standard was fortified in ~10 mL of petroleum ether that was used to soak each PUF disk (5 mL for each disk). The PUF disks were then dried using a nitrogen stream to remove the petroleum ether. The two PUF disks had a combined mass and volume of 3.28 g and 149 cm3 respectively, based on its density of 0.022 g/cm3. For the experimental determinations of KPUFAIR, a continuous stream of ultrapure nitrogen at 22 C was used to generate an upward flow through the chamber and through the porous PUF disks. Chemicals that partitioned off the PUF disks were captured
Fig. 1. Schematic of apparatus used to measure experimental KPUFAIR values.
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on the sorbent. Air flow rates were monitored using a soap film bubble flow meter attached to the outlet of the sorbent trap and were typically in the range 10e18 L/hr. Sampling times varied from 22 to 216 h, with sampling volumes in the range 400e2200 L (n ¼ 7). The flow rates used in the determinations of KPUFAIR were well within the equilibrium range confirmed in earlier studies using this same chamber for measuring chemical partitioning between soil and air (Meijer et al., 2003). Equilibrium sampling was also confirmed by plotting CAIR versus flow rate as shown in Fig. S1. 2.2. Extraction and analysis Each sorbent trap was extracted twice with a 25 mL mixture containing dichloromethane (DCM) and hexane (50:50). The first 25 mL elution served as the sample, and the second 25 mL elution served as a sample blank. All elutions were spiked with a methodrecovery standard prior to concentrating to ~ 1 mL using nitrogen evaporation. The method recovery standard consisted of 250 ng of d12-benzo[b]fluoranthene, d12-indeno[1,2,3-cd]pyrene, d12-benzo [e]pyrene, d14-dibenz[a,h]anthracene, d10-anthracene, d10-acenaphthene, d12-chrysene, 62.5 ng of d12-2,6-dimethylnaphthalene, and 125 ng of d10-benzo[b]naphtha[2,1-d]-thiophene (CIL, Andover, MA). Internal standards comprising d10-fluorene and d12benz(a)anthracene (100 ng each, CIL, Andover, MA) were added to the sample extract prior to instrumental analysis. All sample and blank extracts were analyzed for PAHs, alkylated PAHs (alk-PAHs), and dibenzothiophenes (see Table S1 for full list of compounds) against a standard mixture obtained from CIL (Andover, MA) and Chiron AS (Trondheim, Norway). An Agilent 6890 series GC system with a 5975 mass selective (MS) detector operating in electron impact-selected ion monitoring (EI-SIM) mode was used to analyze the extracts. Details of the analysis method are provided elsewhere (Harner et al., 2013) including information on GCMS conditions, EI parameters, and information for target/qualifier ions of target compounds.
on the surrogate recoveries for each sample that were on average 87 ± 24%. The method detection limit (MDL) was estimated as the BM þ 3 STDEV. MDL values were used to determine the PACs with reliable reportable data. A total of 33 PACs out of the 70 PACs used in the standard solution to spike the PUF disk had values above their MDL, and were selected to be reported in Table 1 and Table 2. The full list of PACs in the standard solution, their instrumental detection limits (IDL) and MDLs are reported in Table S1. To ensure that the sorbent trap had sufficient capacity to capture all PACs in the air stream, a break-through test was performed by connecting a second trap in series. The results confirmed negligible breakthrough. 2.4. Deriving KPUFAIR values The PUF-air partition coefficient, KPUFAIR, is one of two key parameter used for deriving the volume of air sampled by a PUF disk passive sampler (Shoeib and Harner, 2002; Harner et al., 2013). KPUFAIR values represent the point where a chemical reaches equilibrium between the PUF disk and air (Pozo et al., 2006). The other important parameter is the air-side mass transfer coefficient which can also be expressed as a linear sampling rate R (typically ~4 m3/day for a conventional PUF disk). For low volatility chemicals that have high KPUFAIR values (e.g. log KPUFAIR > 8) sampling by the PUF disk over typical deployments periods of a few months remains in the linear phase and sample volumes depend entirely on R. As volatility increases, the KPUFAIR values decrease and the sorptive capacity of the PUF disk sampler is diminished, resulting in approach to equilibrium. It is for these higher volatility chemicals with log KPUFAIR < ~7, where it is important that KPUFAIR is estimated accurately as it factors heavily into the estimate of sample air volume, which is used to derive the chemical concentration in air. In this study, direct measurements of KPUFAIR values were determined based on the equationKPUFAIR ¼ CPUF =CAIR , where CPUF is the concentration of PAC in the PUF disk (ng/m3), and CAIR is the PAC concentration in air (ng/m3).
2.3. Quality assurance/quality control 3. Computational details All samples were blank-corrected by subtracting the blank mean (BM) from each sample. Samples were also recovery corrected based
COSMO-RS theory, as implemented in the COSMOtherm
Table 1 Measured and calculated average log KPUFAIR values for simple PAHs, dibenzothiophene, and isomers grouped by total side-chain carbon number at T ¼ 22 C. Peak interferences in the chromatograph are labeled with “Int”.
Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Dibenzothiophene TOTAL-C1-Naphthalenes TOTAL-C2-Napthalenes TOTAL-C3-Naphthalenes TOTAL-C4-Naphthalenes TOTAL-C1-Fluorenes TOTAL-C2-Fluorenes TOTAL-C3-Fluroenes TOTAL-C4-Fluorenes TOTAL-C1-Phenanthrenes/Anthracenes TOTAL-C2-Phenanthrenes/Anthracenes TOTAL-C3-Phenanthrenes/Anthracenes TOTAL-C4-Phenanthrenes/Anthracenes Retene a
COSMO-therm
GAPS templatea
Measured PUF (average)
5.00 5.92 5.82 6.52 7.01 7.04 7.88 7.68 7.11 5.40 5.99 6.06 6.50 6.98 7.37 7.53 8.03 7.37 7.86 8.14 8.21 8.56
4.26 5.40 5.08 5.14 5.69 5.69 6.57 6.57 5.79 4.66 4.97 5.25 5.58 5.53 5.64 5.80 6.87 5.98 6.35 6.67 6.99 6.77
4.99 5.79 5.38 5.90 6.51 6.97 7.50 7.49 6.69 4.92 5.14 5.51 Int 5.96 6.74 6.45 6.67 6.28 6.38 7.91 8.03 7.99
(from Harner, 2014); note: the average relative standard deviations for measurements of CAIR using the experimental system was 55%.
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Table 2 Measured and calculated KPUFAIR values at 22 C for isomers of alkyl-substituted PAHs and dibenzothiophene.
Naphthalene-1-methyl Naphthalene-2-methyl Naphthalene-1,5-dimethyl Naphthalene-2-ethyl-6-methyl Naphthalene-1,2,4-trimethyl Naphthalene-Isopropyl-1-methyl (eudalene) Naphthalene-1,2,5,6-tetramethyl Fluorene-9-methyl Fluorene-1,7-dimethyl Fluorene-9-n-butyl Phenanthrene-9-methyl Phenanthrene-1-methyl Anthracene-1-methyl Phenanthrene-3,6-dimethyl Phenanthrene-1,3-dimethyl Anthracene-2,3-dimethyl Phenanthrene-9-n-propyl Phenanthrene-1,2,6,9-tetramethyl Fluoranthene-2-methyl Dibenzothiophene-2-methyl Dibenzothiophene-2,8-dimethyl Dibenzothiophene-2,4,7-trimethyl Dibenzothiophene-4,6-diethyl
COSMO-therm
Measured PUF(average)
5.35 5.43 5.70 6.26 6.07 6.54 6.45 6.85 7.37 8.03 7.36 7.39 7.39 7.91 7.81 7.91 8.14 8.57 8.33 7.52 7.96 8.28 8.44
5.19 5.23 5.25 5.52 5.64 5.51 6.26 5.93 6.90 6.81 7.29 7.29 7.46 7.41 7.40 7.47 7.68 8.51 8.61 7.23 7.68 7.89 7.78
Note: the average relative standard deviations for measurements of CAIR using the experimental system was 49%.
program suite (Version C3.0 Release 13.01, Eckert and Klamt, 2013), was used to generate optimized geometries for a series of simple and isomeric alkyl-substituted PAHs in the COSMO ideal solvation environment, using the TURBOMOLE quantum-mechanical program (TURBOMOLE V6.5 2013). Structures were geometrically optimized at the TZVPD basis set level and COSMO charge density distributions computed using a “FINE” cavity construction. The PUF model used was a simple oligomeric 1:1 condensed pair of 2,4toluene-diisocyanide and glycerol with ethyl and acetate endcaps (CH3eCH2eOeC(¼O)eNHeC6H3(eCH3)eNHeC(]O)e OeCH2eCH(eOH)eCH2eOeC(]O)eCH3), optimized in the same manner as the PAHs above. Mole-fraction based air-PUF partition coefficients were computed at 22 C (the temperatures corresponding to the experimental data) and converted using the molar mass of the oligomer into estimated concentration-based log KPUFAIR values. COSMO-RS estimates of KPUFAIR for alkyl PAH isomers with the same side-chain carbon number were generated by averaging the results for individual PAH species from the isomers studied here. This therefore does not represent a weighted average based on typical alkyl PAH concentrations in any particular context.
5. Discussion The present study demonstrates that COSMO-RS solvation theory is an effective and powerful tool for estimation of partition coefficients between polyurethane foam passive sampling media and air. Our results show that for simple PAHs, the oligomeric PUF model significantly outperforms the GAPs template approach with improved correlation coefficient (0.95 vs 0.91), regression slope
4. Results KPUFAIR partition coefficients were measured experimentally for PAH and dibenzothiophene species for PUF disks with which 400e2200 L volume sampling was employed (See Table 1). Estimates of KPUFAIR were also made using COSMO-RS theory as implemented in the COSMOtherm program. Experimentally measured KPUFAIR values for isomer-specific alkyl-substituted PAHs and dibenzothiophenes were also determined and are presented in Table 2 with COSMO-RS estimates for each isomer. The correlation of both the GAPS and COSMO-RS estimates with respect to the experimental data for these experimental datasets was determined and is illustrated graphically in Fig. 2. The correlation coefficient and regression line slope and intercept are summarized for each of these plots in Table 3, with the RMS error for each dataset comparison.
Fig. 2. Plots of calculated log KPUFAIR values versus experimentally measured data with unsubstituted PAHs and alkyl-substituted PAHs, grouped by side-chain carbon number (alk-PAH), and further resolved by individual alkyl isomer (isomer-specific). Computed estimates are from the GAPS template approach (Harner, 2014) and the COSMO-RS approach.
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Table 3 Linear regression and error analysis data for correlation of GAPS and COSMO-RS estimates of KPUFAIR for simple and alkyl-substituted PAHs and dibenzothiophene, based on experimental data correlations in Fig. 2. R2 correlation coefficient Data with isomers grouped by side-chain C-number: GAPS simple PAHs 0.914 COSMO-RS simple PAHs 0.949 GAPS alk-PAH 0.807 COSMO-RS alk-PAH 0.844 Isomer-specific data: COSMO-RS isomer-resolved alk-PAH 0.898
Regression slope
Regression intercept
Sample size N
RMS error
0.776 1.02 0.661 0.868
0.642 0.199 1.66 1.65
9 9 12 12
0.83 0.37 0.72 0.89
0.876
1.33
28
0.57
and intercept much closer to the ideal values of 1 and 0, and RMS error reduced to about half that of the GAPS template (0.37 vs 0.83). For the alkylated PAHs studied, the GAPS and COSMO-RS approaches are similarly effective when molecules were grouped by alkyl side-chain C-number, with the former yielding slightly better RMS error and the latter slightly better correlation. Both are significantly outperformed by the isomer-specific COSMO-RS modelling, in which the RMS error is reduced to 0.57 (cf. 0.72 GAPS and 0.89 COSMO-RS for the side-chain averaged approach), with minimal change in the correlation coefficient, slope and intercept. The superior performance of the COSMO-RS approach with respect to the GAPS template indicates that the naïve averaging approach taken here to generate COSMO-RS estimates of alkylPAHs based on carbon chain number is not realistic. Note that there is no reason why one would want to take such an average with COSMO-RS estimates, since the individual isomer data is more powerful and would be required for averaging in any case. The current format of the GAPS template (based on carbon chain number and not individual isomers) is driven by the reporting format for air monitoring data that is typically summarized according to carbon chain number and not by individual isomers. This approach is necessary due to analytical complexity of the alkylated PAHs and limited availability of standards for the very large number of alkyl-substituted PAHs present in air. Analysis of the RMS error for the isomer-specific alkyl-PAH group indicates no particular trend in the COSMO-RS estimation error, based on number of substituents. Specifically, the RMS error for isomer groups with one, two and three substituents is 0.55, 0.63, and 0.57 (sample size n ¼ 11, 10, and 3, respectively). This analysis, while of limited scope, does suggest that the COSMO-RS technique is robust from the point of view of isomer complexity, and would be expected to perform well irrespective of the nature of the isomerism in alkyl-substituted PAHs. The generator column approach developed here to measure KPUFAIR is also robust and simple but relies on the availability of environmental standards. This technique could be applied to measure KPUFAIR for other PUF types and other chemical classes. The most relevant chemicals will be the more volatile ones that approach equilibrium with PUF during typical deployment periods and for which KPUFAIR factors heavily into the derivation of air concentrations. Acknowledgments This work was partially funded by the Chemicals Management
Plan (CMP) and United Nations Environment Programme (UNEP). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.11.060. References Chen, X., Zhu, L., Pan, X., Fang, S., Zhang, Y., Yang, L., 2015. Isomeric specific partitioning behaviors of perfluoroalkyl substances in water dissolved phase, suspended particulate matters and sediments in Liao River Basin and Taihu Lake, China. Water Res. 80, 235e244. Eckert, F., Klamt, A., 2002. Fast solvent screening via quantum chemistry: COSMORS approach. AIChE J. 48, 369e385. Eckert, F., Klamt, A., 2013. COSMOtherm, Version C3.0, Release 13.01. COSMOlogic GmbH & Co. KG, Leverkusen, Germany. EPI Suite 4.11: US EPA, 2015. Estimation Programs Interface Suite™ for Microsoft® Windows, V. 4.11 or Insert Version Used. United States Environmental Protection Agency, Washington, DC, USA. Harner, T., 2014. 2014_GAPS Template for Calculating PUF and SIP Disk Sample Air Volumes. http://dx.doi.org/10.13140/RG.2.1.2819.2807. November 6 2014, Downloaded February 9, 2015. www.researchgate.net/profile/Tom_Harner? ev¼hdr_xprf. Harner, T., Su, K., Genualdi, S., Karpowicz, J., Ahrens, L., Mihele, C., Schuster, J., Charland, J.-P., Narayan, J., 2013. Calibration and application of PUF disk passive air samplers for tracking polycyclic aromatic compounds (PACs). Atmos. Environ. 75, 123e128. Kamprad, I., Goss, K.-U., 2007. Systematic investigation of the sorption properties of polyurethane foams for organic vapors. Anal. Chem. 79, 4222e4227. Klanova, J., Harner, T., 2013. The challenge of producing reliable results under highly variable conditions and the role of passive air samplers in the global monitoring Plan. Trends Anal. Chem. 45, 139e149. Meijer, S.N., Shoeib, M., Jones, K.C., Harner, T., 2003. Air-soil exchange of organochlorine pesticides in agricultural soils. 2. Laboratory measurements of the soilair partition coefficient. Environ. Sci. Technol. 37, 1300e1305. Parnis, J.M., Mackay, D., Harner, T., 2015. Temperature dependence of Henry's Law constants and KOA for simple and heteroatom-substituted PAHs by COSMO-RS. Atmos. Environ. 110, 27e35. Pegoraro, C.N., Chiappero, M.S., Montejano, H.A., 2015. Measurements of octanoleair partition coefficients, vapor pressures and vaporization enthalpies of the (E) and (Z) isomers of the 2-ethylhexyl 4-methoxycinnamate as parameters of environmental impact assessment. Chemosphere 138, 546e552. Pozo, K., Harner, T., Wania, F., Muir, D.C.G., Jones, K.C., Barrie, L.A., 2006. Toward a global network for persistent organic pollutants in air: results from the GAPS study. Environ. Sci. Technol. 40, 4867e4873. Shoeib, M., Harner, T., 2002. Characterization and comparison of three passive air samplers for persistent organic pollutants. Environ. Sci. Technol. 36, 4142e4151. Simpson, S., Gross, M.S., Olson, J.R., Zurek, E., Aga, D.S., 2015. Identification of polybrominated diphenyl ether metabolites based on calculated boiling points from COSMO-RS, experimental retention times, and mass spectral fragmentation patterns. Anal. Chem. 87, 2299e2305. Zhang, L., Cheng, I., Wu, Z., Harner, T., Schuster, J., Charland, J.-P., Muir, D., Parnis, J.M., 2015. Dry deposition of polycyclic aromatic compounds to various land covers in the athabasca oil sands region. J. Adv. Model. Earth Sys. http:// dx.doi.org/10.1002/2015MS000473.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Characterizing PUF disk passive air samplers for alkyl-substituted PAHs: Measured and modelled PUF-AIR partition coefficients with COSMO-RS J. Mark Parnis a, *, Anita Eng b, Donald Mackay a, Tom Harner b a b
Chemical Properties Research Group, Department of Chemistry, Trent University, Peterborough, ON K9J 7B8, Canada Air Quality Processes Research Section, Environment Canada, 4905 Dufferin St., Toronto, ON M3H 5T4, Canada
h i g h l i g h t s Estimates of partitioning are improved by using isomer-specific predictions. COSMO-RS performs better than the GAPS template for alkyl-substituted PAHs. An oligomeric model for polyurethane foam is effective for PUF modelling. RMS error is significantly reduced by explicitly considering isomers.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2015 Received in revised form 9 November 2015 Accepted 13 November 2015 Available online xxx
Isomers of alkyl-substituted polycyclic aromatic hydrocarbons (PAHs) and dibenzothiophenes are modelled with COSMO-RS theory to determine the effectiveness and accuracy of this approach for estimation of isomer-specific partition coefficients between air and polyurethane foam (PUF), i.e., KPUFAIR. Isomer-specific equilibrium partitioning coefficients for a series of 23 unsubstituted and isomeric alkyl-substituted PAHs and dibenzothiophenes were measured at 22 C. This data was used to determine the accuracy of estimated values using COSMO-RS, which is isomer specific, and the Global Atmospheric Passive Sampling (GAPS) template approach, which treats all alkyl-substitutions as a single species of a given side-chain carbon number. A recently developed oligomer-based model for PUF was employed, which consisted of a 1:1 condensed pair of 2,4-toluene-diisocyanide and glycerol. The COSMO-RS approach resulted in a significant reduction in the RMS error associated with simple PAHs and dibenzothiophene compared with the GAPS template approach. When used with alkylated PAHs and dibenzothiophenes grouped into carbon-number categories, the GAPS template approach gave lower RMS error (0.72) compared to the COSMO-RS result (0.87) when the latter estimates were averaged within the carbon-number-based categories. When the isomer-specific experimental results were used, the COSMO-RS approach resulted in a 21% reduction in RMS error with respect to the GAPS template approach, with a 0.57 RMS error for all alkylated PAHs and dibenzothiophenes studied. The results demonstrate that COSMO-RS theory is effective in generating isomer-specific PUF-air partition coefficients, supporting the application of PUF-based passive samplers for monitoring and research studies of polycyclic aromatic compounds (PACs) in air. © 2015 Elsevier Ltd. All rights reserved.
Handling Editor: Shane Snyder Keywords: COSMO-RS PAH isomers Passive sampling Polyurethane foam disk PUF Partitioning Environmental monitoring
1. Introduction It is well known that isomers of many molecules show similar partitioning properties between common media such as air, water, and soils. It is often the case that isomers such as alkyl-substituted
* Corresponding author. E-mail address: [email protected] (J.M. Parnis). http://dx.doi.org/10.1016/j.chemosphere.2015.11.060 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
polycyclic aromatic hydrocarbons (PAHs) are treated as a single species group when evaluating and modelling their partitioning and environmental fate. An example would be the grouping of 1,2,3-trimethylnaphthalene, 1-ethyl-2-methylnaphthalene, 1propylnaphthalene and a host of other alkyl-substituted naphthalenes with three side-chain carbon atoms as a single species designated C3-naphthalene. This simplification is often driven by the fact that commonly used quantitative structureeactivity
J.M. Parnis et al. / Chemosphere 145 (2016) 360e364
relationship (QSAR)-type estimators of molecular partitioning properties do not generally have the capacity to distinguish between such isomers, making the labor-intensive measurement of individual isomer properties of limited value under many circumstances. The consideration of isomeric forms of molecular species of interest with respect to environmental fate cannot be understated. Most chemical compounds show significantly different properties with respect to isomerism. For example, Pegoraro et al. (2015) recently showed that the (E) and (Z) isomers of 2-ethylhexyl-4methoxycinnamate (a common UV-blocking additive) have vapor pressures that differ by a factor of 5. QSAR-based programs such as those used in EPI Suite do not distinguish between these two isomers, and estimate the same vapor pressure value for both (EPI Suite 4.11 2015). Chen et al. (2015) have recently demonstrated that the isomers of perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and perfluorooctanesulfonamide (PFOSA), show higher partition coefficients on particulate phases for linear isomers compared with branched ones. As well, Zhang et al. (2015) have shown that isomers of alkylated PAHs experience different dry deposition rates from air which is due partly to differences in their extent of partitioning to aerosols. COSMO-RS is a solvation properties theory that is specific to the exact molecular structure of the partitioning species in question, since it is based on the ideally solvated equilibrium geometry and charge distribution of the molecule obtained from quantummechanical calculations. Such properties vary in a non-trivial manner from one isomer to another, resulting in changes to partition coefficients that are significant, as demonstrated in this work. In this way, COSMO-RS is ideally suited for treatment of isomeric species and the prediction of their physico-chemical properties. Despite this ability, little has been published on the application of COSMO-RS theory to the question of variation in physico-chemical properties with isomerism. A recent example is that of Simpson et al. (2015), who used the isomer-specific estimation capabilities of COSMO-RS to accurately estimate gas chromatography (GC) retention times of polybrominated diphenyl ether (PBDE) isomer metabolites by correlating the COSMO-RS estimated boiling point of specific reference PBDEs with their GC retention time. The ability to theoretically “resolve” such isomers greatly facilitated identification of otherwise unknown metabolites. Our particular interest is focused on passive-sampling techniques, and the behavior of isomers of target species with respect to the kinetics and thermodynamics of uptake from air by passivesampling media. It is common to use passive air sampling devices based on polyurethane foam (PUF) and other polymeric materials for environmental monitoring over large areas (Klanova and Harner, 2013). In support of the Global Atmospheric Passive Sampling (GAPS) network (Pozo et al., 2006), Harner (2014) has developed a template for characterizing uptake curves for PUF disks for a wide range of chemical classes, including polycyclic aromatic compounds (PACs) which includes the unsubstituted and alkylated PAHs and dibenzothiophenes. The template is openly accessible through www.ResearchGate.net and is widely accessed by users of PUF disk samplers. The GAPS template relies on an empirical relationship between the octanol-air partition coefficient, KOA and the PUF-air partition coefficient, KPUFAIR (Shoeib and Harner, 2002; Harner et al., 2013), and does not account for different isomers of alkylated-PAHs. Recently, we developed an oligomer-based model for such PUF-based passive sampling devices based on a 1:1 condensed pair of 2,4-toluene-diisocyanide and glycerol (Parnis et al., 2015). Used in conjunction with the COSMO-RS theoretical approach to solvation and partitioning property estimation (Eckert and Klamt, 2002, 2013), this model was demonstrated to have a root-mean-square (RMS) error of 0.47 in log KPUFAIR based on
361
partition data from Kamprad and Goss (2007) between 15 and 95 C for a large set of organic molecules. In the current work, we apply this same oligomeric model for PUF to the question of isomer-specificity in passive sampling contexts. We measure directly, using a generator column approach, the KPUFAIR for a range of individual alkyl-substituted PAHs to compare the performance of the currently used GAPS template approach (Harner, 2014) and estimates based on COSMO-RS theory. 2. Experimental details 2.1. Chamber setup and experimental design Fig. 1 shows a schematic of the experimental setup used to directly measure the KPUFAIR partition coefficients. The lower compartment consists of a double-walled glass chamber that contains PUF disks that have been spiked with target compounds, and the upper compartment is a glass ‘sorbent trap’ containing ~3000 mg of sorbent (Chromatographic Specialties Inc, ISOLUTE PAH, Mid Glamorgan, U.K.). The PUF disks (14 cm diameter; 1.35 cm thick; Newterra TE1014, Brockville, ON Canada) were cleaned with an accelerated solvent extractor (Dionex ASE 350) using acetone, petroleum ether and acetonitrile, and then dried under nitrogen. A more detailed method of PUF disk cleaning is described in Harner et al. (2013). Two cleaned PUF disks were cut to a smaller diameter of 7.5 cm in order for their edges to fit snuggly and completely flush against the inner wall of the chamber. Prior to inserting the PUFs into the chamber, they were each spiked with an equal amount of PAC standards (see Table S1 for full list of PACs). To ensure even distribution, the standard was fortified in ~10 mL of petroleum ether that was used to soak each PUF disk (5 mL for each disk). The PUF disks were then dried using a nitrogen stream to remove the petroleum ether. The two PUF disks had a combined mass and volume of 3.28 g and 149 cm3 respectively, based on its density of 0.022 g/cm3. For the experimental determinations of KPUFAIR, a continuous stream of ultrapure nitrogen at 22 C was used to generate an upward flow through the chamber and through the porous PUF disks. Chemicals that partitioned off the PUF disks were captured
Fig. 1. Schematic of apparatus used to measure experimental KPUFAIR values.
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on the sorbent. Air flow rates were monitored using a soap film bubble flow meter attached to the outlet of the sorbent trap and were typically in the range 10e18 L/hr. Sampling times varied from 22 to 216 h, with sampling volumes in the range 400e2200 L (n ¼ 7). The flow rates used in the determinations of KPUFAIR were well within the equilibrium range confirmed in earlier studies using this same chamber for measuring chemical partitioning between soil and air (Meijer et al., 2003). Equilibrium sampling was also confirmed by plotting CAIR versus flow rate as shown in Fig. S1. 2.2. Extraction and analysis Each sorbent trap was extracted twice with a 25 mL mixture containing dichloromethane (DCM) and hexane (50:50). The first 25 mL elution served as the sample, and the second 25 mL elution served as a sample blank. All elutions were spiked with a methodrecovery standard prior to concentrating to ~ 1 mL using nitrogen evaporation. The method recovery standard consisted of 250 ng of d12-benzo[b]fluoranthene, d12-indeno[1,2,3-cd]pyrene, d12-benzo [e]pyrene, d14-dibenz[a,h]anthracene, d10-anthracene, d10-acenaphthene, d12-chrysene, 62.5 ng of d12-2,6-dimethylnaphthalene, and 125 ng of d10-benzo[b]naphtha[2,1-d]-thiophene (CIL, Andover, MA). Internal standards comprising d10-fluorene and d12benz(a)anthracene (100 ng each, CIL, Andover, MA) were added to the sample extract prior to instrumental analysis. All sample and blank extracts were analyzed for PAHs, alkylated PAHs (alk-PAHs), and dibenzothiophenes (see Table S1 for full list of compounds) against a standard mixture obtained from CIL (Andover, MA) and Chiron AS (Trondheim, Norway). An Agilent 6890 series GC system with a 5975 mass selective (MS) detector operating in electron impact-selected ion monitoring (EI-SIM) mode was used to analyze the extracts. Details of the analysis method are provided elsewhere (Harner et al., 2013) including information on GCMS conditions, EI parameters, and information for target/qualifier ions of target compounds.
on the surrogate recoveries for each sample that were on average 87 ± 24%. The method detection limit (MDL) was estimated as the BM þ 3 STDEV. MDL values were used to determine the PACs with reliable reportable data. A total of 33 PACs out of the 70 PACs used in the standard solution to spike the PUF disk had values above their MDL, and were selected to be reported in Table 1 and Table 2. The full list of PACs in the standard solution, their instrumental detection limits (IDL) and MDLs are reported in Table S1. To ensure that the sorbent trap had sufficient capacity to capture all PACs in the air stream, a break-through test was performed by connecting a second trap in series. The results confirmed negligible breakthrough. 2.4. Deriving KPUFAIR values The PUF-air partition coefficient, KPUFAIR, is one of two key parameter used for deriving the volume of air sampled by a PUF disk passive sampler (Shoeib and Harner, 2002; Harner et al., 2013). KPUFAIR values represent the point where a chemical reaches equilibrium between the PUF disk and air (Pozo et al., 2006). The other important parameter is the air-side mass transfer coefficient which can also be expressed as a linear sampling rate R (typically ~4 m3/day for a conventional PUF disk). For low volatility chemicals that have high KPUFAIR values (e.g. log KPUFAIR > 8) sampling by the PUF disk over typical deployments periods of a few months remains in the linear phase and sample volumes depend entirely on R. As volatility increases, the KPUFAIR values decrease and the sorptive capacity of the PUF disk sampler is diminished, resulting in approach to equilibrium. It is for these higher volatility chemicals with log KPUFAIR < ~7, where it is important that KPUFAIR is estimated accurately as it factors heavily into the estimate of sample air volume, which is used to derive the chemical concentration in air. In this study, direct measurements of KPUFAIR values were determined based on the equationKPUFAIR ¼ CPUF =CAIR , where CPUF is the concentration of PAC in the PUF disk (ng/m3), and CAIR is the PAC concentration in air (ng/m3).
2.3. Quality assurance/quality control 3. Computational details All samples were blank-corrected by subtracting the blank mean (BM) from each sample. Samples were also recovery corrected based
COSMO-RS theory, as implemented in the COSMOtherm
Table 1 Measured and calculated average log KPUFAIR values for simple PAHs, dibenzothiophene, and isomers grouped by total side-chain carbon number at T ¼ 22 C. Peak interferences in the chromatograph are labeled with “Int”.
Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Dibenzothiophene TOTAL-C1-Naphthalenes TOTAL-C2-Napthalenes TOTAL-C3-Naphthalenes TOTAL-C4-Naphthalenes TOTAL-C1-Fluorenes TOTAL-C2-Fluorenes TOTAL-C3-Fluroenes TOTAL-C4-Fluorenes TOTAL-C1-Phenanthrenes/Anthracenes TOTAL-C2-Phenanthrenes/Anthracenes TOTAL-C3-Phenanthrenes/Anthracenes TOTAL-C4-Phenanthrenes/Anthracenes Retene a
COSMO-therm
GAPS templatea
Measured PUF (average)
5.00 5.92 5.82 6.52 7.01 7.04 7.88 7.68 7.11 5.40 5.99 6.06 6.50 6.98 7.37 7.53 8.03 7.37 7.86 8.14 8.21 8.56
4.26 5.40 5.08 5.14 5.69 5.69 6.57 6.57 5.79 4.66 4.97 5.25 5.58 5.53 5.64 5.80 6.87 5.98 6.35 6.67 6.99 6.77
4.99 5.79 5.38 5.90 6.51 6.97 7.50 7.49 6.69 4.92 5.14 5.51 Int 5.96 6.74 6.45 6.67 6.28 6.38 7.91 8.03 7.99
(from Harner, 2014); note: the average relative standard deviations for measurements of CAIR using the experimental system was 55%.
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Table 2 Measured and calculated KPUFAIR values at 22 C for isomers of alkyl-substituted PAHs and dibenzothiophene.
Naphthalene-1-methyl Naphthalene-2-methyl Naphthalene-1,5-dimethyl Naphthalene-2-ethyl-6-methyl Naphthalene-1,2,4-trimethyl Naphthalene-Isopropyl-1-methyl (eudalene) Naphthalene-1,2,5,6-tetramethyl Fluorene-9-methyl Fluorene-1,7-dimethyl Fluorene-9-n-butyl Phenanthrene-9-methyl Phenanthrene-1-methyl Anthracene-1-methyl Phenanthrene-3,6-dimethyl Phenanthrene-1,3-dimethyl Anthracene-2,3-dimethyl Phenanthrene-9-n-propyl Phenanthrene-1,2,6,9-tetramethyl Fluoranthene-2-methyl Dibenzothiophene-2-methyl Dibenzothiophene-2,8-dimethyl Dibenzothiophene-2,4,7-trimethyl Dibenzothiophene-4,6-diethyl
COSMO-therm
Measured PUF(average)
5.35 5.43 5.70 6.26 6.07 6.54 6.45 6.85 7.37 8.03 7.36 7.39 7.39 7.91 7.81 7.91 8.14 8.57 8.33 7.52 7.96 8.28 8.44
5.19 5.23 5.25 5.52 5.64 5.51 6.26 5.93 6.90 6.81 7.29 7.29 7.46 7.41 7.40 7.47 7.68 8.51 8.61 7.23 7.68 7.89 7.78
Note: the average relative standard deviations for measurements of CAIR using the experimental system was 49%.
program suite (Version C3.0 Release 13.01, Eckert and Klamt, 2013), was used to generate optimized geometries for a series of simple and isomeric alkyl-substituted PAHs in the COSMO ideal solvation environment, using the TURBOMOLE quantum-mechanical program (TURBOMOLE V6.5 2013). Structures were geometrically optimized at the TZVPD basis set level and COSMO charge density distributions computed using a “FINE” cavity construction. The PUF model used was a simple oligomeric 1:1 condensed pair of 2,4toluene-diisocyanide and glycerol with ethyl and acetate endcaps (CH3eCH2eOeC(¼O)eNHeC6H3(eCH3)eNHeC(]O)e OeCH2eCH(eOH)eCH2eOeC(]O)eCH3), optimized in the same manner as the PAHs above. Mole-fraction based air-PUF partition coefficients were computed at 22 C (the temperatures corresponding to the experimental data) and converted using the molar mass of the oligomer into estimated concentration-based log KPUFAIR values. COSMO-RS estimates of KPUFAIR for alkyl PAH isomers with the same side-chain carbon number were generated by averaging the results for individual PAH species from the isomers studied here. This therefore does not represent a weighted average based on typical alkyl PAH concentrations in any particular context.
5. Discussion The present study demonstrates that COSMO-RS solvation theory is an effective and powerful tool for estimation of partition coefficients between polyurethane foam passive sampling media and air. Our results show that for simple PAHs, the oligomeric PUF model significantly outperforms the GAPs template approach with improved correlation coefficient (0.95 vs 0.91), regression slope
4. Results KPUFAIR partition coefficients were measured experimentally for PAH and dibenzothiophene species for PUF disks with which 400e2200 L volume sampling was employed (See Table 1). Estimates of KPUFAIR were also made using COSMO-RS theory as implemented in the COSMOtherm program. Experimentally measured KPUFAIR values for isomer-specific alkyl-substituted PAHs and dibenzothiophenes were also determined and are presented in Table 2 with COSMO-RS estimates for each isomer. The correlation of both the GAPS and COSMO-RS estimates with respect to the experimental data for these experimental datasets was determined and is illustrated graphically in Fig. 2. The correlation coefficient and regression line slope and intercept are summarized for each of these plots in Table 3, with the RMS error for each dataset comparison.
Fig. 2. Plots of calculated log KPUFAIR values versus experimentally measured data with unsubstituted PAHs and alkyl-substituted PAHs, grouped by side-chain carbon number (alk-PAH), and further resolved by individual alkyl isomer (isomer-specific). Computed estimates are from the GAPS template approach (Harner, 2014) and the COSMO-RS approach.
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Table 3 Linear regression and error analysis data for correlation of GAPS and COSMO-RS estimates of KPUFAIR for simple and alkyl-substituted PAHs and dibenzothiophene, based on experimental data correlations in Fig. 2. R2 correlation coefficient Data with isomers grouped by side-chain C-number: GAPS simple PAHs 0.914 COSMO-RS simple PAHs 0.949 GAPS alk-PAH 0.807 COSMO-RS alk-PAH 0.844 Isomer-specific data: COSMO-RS isomer-resolved alk-PAH 0.898
Regression slope
Regression intercept
Sample size N
RMS error
0.776 1.02 0.661 0.868
0.642 0.199 1.66 1.65
9 9 12 12
0.83 0.37 0.72 0.89
0.876
1.33
28
0.57
and intercept much closer to the ideal values of 1 and 0, and RMS error reduced to about half that of the GAPS template (0.37 vs 0.83). For the alkylated PAHs studied, the GAPS and COSMO-RS approaches are similarly effective when molecules were grouped by alkyl side-chain C-number, with the former yielding slightly better RMS error and the latter slightly better correlation. Both are significantly outperformed by the isomer-specific COSMO-RS modelling, in which the RMS error is reduced to 0.57 (cf. 0.72 GAPS and 0.89 COSMO-RS for the side-chain averaged approach), with minimal change in the correlation coefficient, slope and intercept. The superior performance of the COSMO-RS approach with respect to the GAPS template indicates that the naïve averaging approach taken here to generate COSMO-RS estimates of alkylPAHs based on carbon chain number is not realistic. Note that there is no reason why one would want to take such an average with COSMO-RS estimates, since the individual isomer data is more powerful and would be required for averaging in any case. The current format of the GAPS template (based on carbon chain number and not individual isomers) is driven by the reporting format for air monitoring data that is typically summarized according to carbon chain number and not by individual isomers. This approach is necessary due to analytical complexity of the alkylated PAHs and limited availability of standards for the very large number of alkyl-substituted PAHs present in air. Analysis of the RMS error for the isomer-specific alkyl-PAH group indicates no particular trend in the COSMO-RS estimation error, based on number of substituents. Specifically, the RMS error for isomer groups with one, two and three substituents is 0.55, 0.63, and 0.57 (sample size n ¼ 11, 10, and 3, respectively). This analysis, while of limited scope, does suggest that the COSMO-RS technique is robust from the point of view of isomer complexity, and would be expected to perform well irrespective of the nature of the isomerism in alkyl-substituted PAHs. The generator column approach developed here to measure KPUFAIR is also robust and simple but relies on the availability of environmental standards. This technique could be applied to measure KPUFAIR for other PUF types and other chemical classes. The most relevant chemicals will be the more volatile ones that approach equilibrium with PUF during typical deployment periods and for which KPUFAIR factors heavily into the derivation of air concentrations. Acknowledgments This work was partially funded by the Chemicals Management
Plan (CMP) and United Nations Environment Programme (UNEP). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.11.060. References Chen, X., Zhu, L., Pan, X., Fang, S., Zhang, Y., Yang, L., 2015. Isomeric specific partitioning behaviors of perfluoroalkyl substances in water dissolved phase, suspended particulate matters and sediments in Liao River Basin and Taihu Lake, China. Water Res. 80, 235e244. Eckert, F., Klamt, A., 2002. Fast solvent screening via quantum chemistry: COSMORS approach. AIChE J. 48, 369e385. Eckert, F., Klamt, A., 2013. COSMOtherm, Version C3.0, Release 13.01. COSMOlogic GmbH & Co. KG, Leverkusen, Germany. EPI Suite 4.11: US EPA, 2015. Estimation Programs Interface Suite™ for Microsoft® Windows, V. 4.11 or Insert Version Used. United States Environmental Protection Agency, Washington, DC, USA. Harner, T., 2014. 2014_GAPS Template for Calculating PUF and SIP Disk Sample Air Volumes. http://dx.doi.org/10.13140/RG.2.1.2819.2807. November 6 2014, Downloaded February 9, 2015. www.researchgate.net/profile/Tom_Harner? ev¼hdr_xprf. Harner, T., Su, K., Genualdi, S., Karpowicz, J., Ahrens, L., Mihele, C., Schuster, J., Charland, J.-P., Narayan, J., 2013. Calibration and application of PUF disk passive air samplers for tracking polycyclic aromatic compounds (PACs). Atmos. Environ. 75, 123e128. Kamprad, I., Goss, K.-U., 2007. Systematic investigation of the sorption properties of polyurethane foams for organic vapors. Anal. Chem. 79, 4222e4227. Klanova, J., Harner, T., 2013. The challenge of producing reliable results under highly variable conditions and the role of passive air samplers in the global monitoring Plan. Trends Anal. Chem. 45, 139e149. Meijer, S.N., Shoeib, M., Jones, K.C., Harner, T., 2003. Air-soil exchange of organochlorine pesticides in agricultural soils. 2. Laboratory measurements of the soilair partition coefficient. Environ. Sci. Technol. 37, 1300e1305. Parnis, J.M., Mackay, D., Harner, T., 2015. Temperature dependence of Henry's Law constants and KOA for simple and heteroatom-substituted PAHs by COSMO-RS. Atmos. Environ. 110, 27e35. Pegoraro, C.N., Chiappero, M.S., Montejano, H.A., 2015. Measurements of octanoleair partition coefficients, vapor pressures and vaporization enthalpies of the (E) and (Z) isomers of the 2-ethylhexyl 4-methoxycinnamate as parameters of environmental impact assessment. Chemosphere 138, 546e552. Pozo, K., Harner, T., Wania, F., Muir, D.C.G., Jones, K.C., Barrie, L.A., 2006. Toward a global network for persistent organic pollutants in air: results from the GAPS study. Environ. Sci. Technol. 40, 4867e4873. Shoeib, M., Harner, T., 2002. Characterization and comparison of three passive air samplers for persistent organic pollutants. Environ. Sci. Technol. 36, 4142e4151. Simpson, S., Gross, M.S., Olson, J.R., Zurek, E., Aga, D.S., 2015. Identification of polybrominated diphenyl ether metabolites based on calculated boiling points from COSMO-RS, experimental retention times, and mass spectral fragmentation patterns. Anal. Chem. 87, 2299e2305. Zhang, L., Cheng, I., Wu, Z., Harner, T., Schuster, J., Charland, J.-P., Muir, D., Parnis, J.M., 2015. Dry deposition of polycyclic aromatic compounds to various land covers in the athabasca oil sands region. J. Adv. Model. Earth Sys. http:// dx.doi.org/10.1002/2015MS000473.