Examining the Gas-Particle Partitioning of Organophosphate Esters

Article Cite This: Environ. Sci. Technol. 2018, 52, 13834−13844

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Examining the Gas-Particle Partitioning of Organophosphate Esters: How Reliable Are Air Measurements? Joseph O. Okeme,† Timothy F. M. Rodgers,‡ Liisa M. Jantunen,§,∥ and Miriam L. Diamond*,∥,†,‡

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†

Department of Physical and Environmental Science, University of Toronto Scarborough, 1265 Military Trail Toronto, ON M1C 1A4, Canada ‡ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON M5S 3E5, Canada § Air Quality Processes Research Section, Environment and Climate Change Canada, 6248 Eighth Line Egbert, ON L0L 1N0, Canada ∥ Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada S Supporting Information *

ABSTRACT: Organophosphate esters (OPEs) in air have been found to be captured entirely on filters of typical active air samplers and thus designated as being in the particle phase. However, this particle fraction is unexpected, especially for more volatile tris(2-chloroethyl) phosphate (TCEP) and tris(chloroisopropyl) phosphate (TCIPP). We evaluated gas-particle partitioning in indoor and outdoor air for OPEs and polybrominated diphenyl ethers (PBDEs) using single-parameter models (Junge−Pankow, Harner−Bidleman) and poly-parameter linear free energy relationship (pp-LFER) models. We also used the pp-LFER to estimate filter-air partitioning in active air samplers. We found that all gas-particle partitioning models predicted that TCEP and TCIPP should be in the gas phase, contrary to measurements. The pp-LFER better accounted for OPE measurements than the single-parameter models, except for TCEP and TCIPP. Gas-particle partitioning of PBDEs was reasonably explained by all models. The pp-LFER for filter-air partitioning showed that gas-phase sorption to glass and especially quartz fiber filters used for active air samplers could account for up to 100% of filter capture and explain the high particle fractions reported for TCIPP, tris(1,3-dichloro-2-propyl) phosphate TDCIPP, and triphenyl phosphate TPhP, but not TCEP. The misclassification of gasparticle partitioning can result in erroneous estimates of the fraction of chemical subject to gas-phase reactions and atmospheric scavenging and, hence, atmospheric long-range transport.

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disadvantage of denuders is that they have low sampling rates12 and thus have high detection limits and low temporal resolution when run for short and longer deployment times, respectively. The sampling train for AAS commonly consists of a filter, usually a glass or quartz fiber filter (GFF or QFF), followed by a sorbent, usually polyurethane foam (PUF) and/or styrene divinylbenzene copolymer (XAD) resin. The concentration of a compound retained on the filter relative to the sorbent is used to define the gas-to-particle phase ratio.1 Active sampling measurements may be inaccurate due to sampling artifacts that result when (1) particles are blown off the filter, (2) particle-sorbed compounds are stripped off the filter onto the sorbent, (3) breakthrough occurs in the sorbent, and (4) gas-phase compounds sorb to the filter.13,14 Sampling parameters that effect these artifacts are air volume, flow rate, temperature, relative humidity, and filter characteristics.1,14,15

INTRODUCTION Semivolatile organic compounds (SVOCs), with vapor pressures ranging between 10−9 and 10 Pa, can theoretically exist in both the gas and particle phases, but in terms of measurement, they are found in the gas or particle phase or a mixture of both phases.1 The phase into which SVOCs partition is an important determinant of chemical hazard. Gas-particle partitioning influences SVOC bioavailability and hence toxicological significance.2,3 It also controls environmental fate, specifically the ability of the compounds to undergo atmospheric long-range transport, wet and dry deposition, and air−water exchange. Examples of this importance come from studies based on measurements conducted using active air samplers (AAS).4−7 Modeling studies have confirmed the importance of faithfully describing gas-particle partitioning in order to capture chemical fate.8−10 Gas- and particle-phase SVOCs are most commonly measured using AAS where low and high volume AAS (LVASS and HV-AAS) are used indoors and outdoors, respectively. Denuders are also used as they are more reliable for measuring gas-particle partitioning,11 but AAS are preferred because they are cheaper and much easier to use than denuders. Another © 2018 American Chemical Society

Received: Revised: Accepted: Published: 13834

August 15, 2018 October 15, 2018 October 26, 2018 October 26, 2018 DOI: 10.1021/acs.est.8b04588 Environ. Sci. Technol. 2018, 52, 13834−13844

Article

Environmental Science & Technology Gas-particle partitioning and sampling artifacts are reasonably well understood for long-studied nonpolar SVOCs such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and polychlorinated dibenzodioxins and furans (PCDD/Fs).1,14,15 However, gas-particle partitioning is uncertain for compounds of emerging concern such as perfluoroalkyl carboxylic acids for which measurements and modeled environmental behavior present conflicting expectations.16,17 Gas-particle partitioning is also not well characterized for organophosphate esters (OPEs), which is the focus of this paper. OPEs have commonly been characterized as particle-phase compounds because most studies have detected them primarily on the filters of LV-AAS18 and HV-AAS.7,19,20 The exception is the work of Wolschke et al.21 and Li et al.,22,23 who reported high sorbent capture for some of the OPEs. Strict particle-phase distribution is a questionable expectation for all OPEs since they span a wide range of volatilities.24,25 Specifically, tris(2chloroisopropyl) phosphate (TCEP), tris(2-chloropropyl) phosphate (TCIPP, also referred to as TCPP),26 and tris(1,3dichloro-2-propyl) phosphate (TDCIPP) have vapor pressures higher than or similar to those of some pentabrominated diphenyl ethers (penta-BDEs) that have been measured above 50% in the gas phase.27−29 Single-parameter models based on subcooled vapor pressure (P°L) and octanol-air partition coefficient (KOA) predict that these OPEs should be in the gas phase.24,25 A plausible explanation for this discrepancy between measured and modeled results for OPEs is that modeled estimates may be incorrect and/or that, during measurement, relative humidity (RH) may facilitate sorption of the gas phase onto polar filters such as GFF and QFF.16,30−32 For example, gas-phase sorption to filters has been shown to be a significant source of sampling artifacts when measuring polar compounds such as perfluorooctanoic acids.17 Thus, we questioned whether some OPEs may be gas-phase compounds that are mischaracterized as those in the particle phase because they exhibit “pseudo-particle-phase” behavior when measured using AAS. The term pseudo-particle phase is used here to mean false particle-phase compounds rather than a mixture of gas- and particle-phase product described elsewhere.33 Here, we assessed how pseudoparticle-phase behavior may affect gas-particle characterization of OPEs. To this end, we first compared measured and modeled gas-particle partitioning behavior of OPEs and polybrominated diphenyl ethers (PBDEs). Then, through modeling, we assessed the mechanism and importance of gas-phase sorption of OPEs and PBDEs to GFF and QFF. Target OPEs were: TCEP, TCIPP, TDCIPP, triphenyl phosphate (TPhP), isomers of tricresyl phosphate (TCPs, o, m, and p), 2-ethylhexyldiphenyl phosphate (EHDPP), tris(2-butoxyethyl) phosphate (TBOEP), tris(2-ethylhexyl) phosphate (TEHP), and PBDEs (BDE-28, -47, -100, -99, -154, and -153). Names and CAS numbers of target compounds are presented in the Table S1.

K p(M) =

Cpart /TSP

fpart(M) =

Cgas

(1)

Cpart Cgas + Cpart

(2)

where C part and C gas are the particle- and gas-phase concentrations (ng m−3) measured for the AAS filter and sorbent, respectively, and TSP (g m−3) is the concentration of total suspended particles. Estimated Gas-Particle Partitioning. To model the expected gas-particle partition (Kp) behavior of target compounds, we estimated their particle fractions (f part) using single-parameter relationships of Junge−Pankow34 and Harner−Bidleman35 and a poly-parameter linear free energy relationship (pp-LFER) of Arp et al.32 The Junge−Pankow (J−P) model expresses sorption of a compound to the active sites of particles as a function of subcooled liquid vapor pressure34 fpart(J − P) =

K p(J − P) =

cθ P°L + cθ

(3)

fpart(J − P) TSP(1 − fpart(J − P) )

(4)

−1

where c Pa cm is a constant with an assumed value of 17.2 and θ (cm2 aerosol cm−3 air) is the surface area absorbing aerosols per volume of air with values of 1.1 × 10−5, 1.0 × 10−6, and 1.0 × 10−7 cm2 aerosol cm−3 air used for outdoor urban, background, and remote air, respectively. These θ values correspond to total suspended particles (TSP) of 0.055, 0.014, and 0.0077 g m−3, respectively.15 P°L (Pa) is the subcooled liquid vapor pressure. The Harner−Bidleman (H−B) model35 correlates adsorption of a compound to particles, Kp(H−B) (m3 μg−1), to its octanol−air partition coefficient KOA log K p(H − B) = log K OA + log fOM − 11.91

(5)

where f OM is the fraction of organic matter of particles in air with default values of 0.40, 0.19, and 0.08 (gom gTSP−1) for urban, background, and remote air, respectively.15 The particle fraction, f part(H−B), was obtained by substituting Kp(H−B) into eq 6: fpart(H − B) =

K p(H − B) × TSP K p(H − B) × TSP + 1

(6)

32

The pp-LFER model relates the gas-particle partition coefficient, Kp(pp‑LFERg/p) (m3 g−1), to the Gibb’s free energies that describe specific and nonspecific equilibrium interactions of a compound between the gas and the water-insoluble organic matter (WIOM) component of particles log K p(pp‐LFER g/p) = 1.01S + 3.17A + 0.30B + 0.78L + 0.51V − 7.42

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METHODS FOR MEASURED AND THEORETICAL ESTIMATES OF GAS-PARTICLE AND FILTER-AIR PARTITIONING Measured Gas-Particle Partitioning. Measured gasparticle partitioning, Kp(M) (m3 g−1) and particle fractions (f part(M)) of OPEs and PBDEs were calculated as follows34

fpart(pp‐LFER g/p) =

(7)

K p(pp‐LFER g/p) × TSP K p(pp‐LFER g/p) × TSP + 1

(8)

where the numbers of eq 7 are the system constants of the multilinear regression of log Kp(pp‑LFERg/p) against Abraham solvation parameters denoted by the letters. S characterizes the dipolarity/polarizability of the compound, A is the electron 13835

DOI: 10.1021/acs.est.8b04588 Environ. Sci. Technol. 2018, 52, 13834−13844

Article

Environmental Science & Technology

Figure 1. Relationship between measured and predicted percent particle phase in urban air as a function of log P°L and log KOA. Measured OPEs were taken from the work of Salamova et al.,44 and minimum and maximum PBDE values were based on eight outdoor studies (Table S2). Modeled TCPs, TBOEP, and TEHP are hidden behind measured values. Log P°L values were taken from Brommer et al.24 and Tittlemier et al.,45 and log KOA values were taken from Okeme et al.46 and Harner and Shoeib47 for OPEs and PBDEs, respectively.

acceptor property, B is the electron donor property, L is the partition ratio between gas phase and hexadecane representing van de Waal’s property, and V is the McGowan’s volume.32 The solute descriptors were obtained from the UFZ-LSER database,36 except those of EHDPP, TBOEP, and TEHP, which were estimated using Absolv.37 Equation 8 was used to obtain the particle fraction, f part(pp‑LFERg/p). Equation 7 describes the absorption of nonpolar and polar SVOCs to the water-insoluble organic matter (WIOM) in TSP. Specific conditions for this pp-LFER were 15 °C and 50% RH and particles representative of Berlin in winter which are recommended as proxy for generic aerosols because these aerosols displayed sorption behavior typical of common terrestrial TSP.38 However, Arp et al.32 postulated and tested a dual-phase sorption mechanism of SVOC sorption to WIOM and sorption of polar and ionizable SVOCs to a mixed aqueousphase constituting TSP. As such, we used the dual-phase model of Arp et al.32 to account for the WIOM as well as the mixed aqueous phase fraction of TSP that contains salt and watersoluble organic matter (WSOM, eq S1). However, eq 7 at 50%

RH gave the same estimates as the dual-phase model (eq S1) at RH values of 28, 77, and 90%, which was expected for large and nonionizing compounds such as those tested here for which sorption is driven by interaction with WIOM, irrespective of RH.32 Thus, reference is made to only estimates produced using eq 7 onward to avoid duplication. We assessed the capacity of GFF and QFF to retain gas-phase compounds in the absence of particles using a pp-LFER model.16 The purpose here was to account for the component of the filter capture not explained by the tested gas-particle partitioning models. First, we used eq 9 taken from Arp et al.16 to estimate specific retention volumes, VG, (m3 g−1) of GFF and QFF at 15 °C and 50% RH for the test OPEs and PBDEs: log VG = 3.60A × EDsurf + 5.11B × EA surf vdW + 0.135 × L γsurf +C

(9)

Equation 9 is currently the only model available for estimating gas-phase sorption to GFF and QFF. The numbers are the constants of a multilinear regression obtained from plotting log 13836


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Figure 2. Relationship between sensitivity of pp-LFER estimates and discrepancy between pp-LFER and Harner−Bidleman estimates of particle fraction. Sensitivity was expressed as the percentage change in particle fraction when a solute descriptor was excluded in turn with replacement. Relative difference was calculated as absolute difference between f part(pp‑LFERg/p) and f part(J−P or H−B) divided by mean of absolute sum of estimates.

VG against the values of solute descriptors, A, B, and L. The terms EDsurf (0.56 ± 0.12 and 0.63 ± 0.05), EAsurf (0.56 ± 0.07

some or all OPEs, with the discrepancy being highest for the more volatile OPEs with log P°L > −2, namely, TCEP and TCIPP. The models all agreed that high P°L compounds such as TCEP and TCIPP are expected to be mainly in the gas phase, which contradicts measurements that show them to be mostly, if not exclusively, in the particle phase. pp-LFER estimates were the most comparable to measurements for the remaining OPEs. For PBDEs, estimates of all three methods were within the range of measured literature values of 3−23% (BDE-28), 4−63% (BDE-47), 6−90% (BDE-100 and -99), and 10−98% (BDE-153 and -154) (Table S2). The comparison between measured and estimated f part outdoors was similar for indoors where measured f part is ∼99% for OPEs18,42,43 and background > remote air. Compounds with high P°L values remained in the gas phase, and all other compounds (except BDE-153 and -154) partitioned to the gas phase in remote air. f part(pp‑LFERg/p) showed no dependence on TSP for the low and high volatility OPEs and BDE-28, whereas they varied with TSP (urban > background > remote) for TDCIPP, TPhP, and the remaining PBDEs. To assess the mechanism driving the difference between the pp-LFER and the single-parameter model estimates, we regressed the sensitivity of f part(pp‑LFERg/p) to changes in the molecular interactions of eq 7 against the relative difference between estimates (Figure 2). The sensitivity was expressed as the percentage change in particle fraction when a solute descriptor was excluded in turn with replacement and the relative difference was calculated as f part(pp‑LFERg/p) and f part(J−P or H−B). Correlation was insignificant for all descriptors except B (determination coefficient, r2, = 0.73, p < 0.001), suggesting that the discrepancy between f part(pp‑LFERg/p) and f part(J−P or H−B) was positively correlated to electron donor− acceptor interactions. The correlation analysis is consistent with the conclusion that single linear parameter models based on P°L and KOA lack the mechanistic capability to account for all interactions occurring in partitioning systems, especially for polar compounds.32,48−50 Comparison of the gas-particle partitioning models and the sensitivity analysis suggested that the pp-LFER estimates were more reliable than the single linear parameter models, considering the mechanistic basis of the pp-LFER model. The particle fraction for OPEs and PBDEs was driven by molecular interactions between compounds and particles rather than P°L or KOA. Based on the pp-LFER results, the expectations are that TCPs, TBOEP, TEHP, and BDE-153 and -154 predominantly reside in the particle phase, and TCEP, TCIPP, and BDE-28 are in the gas phase, regardless of TSP, RH, and temperature values. TDCIPP, TPhP, and BDE-47, -99, and -100 have varying particle fractions depending on the TSP value. The pp-LFER model accounted reasonably well for the measured particle fraction of all the target compounds, except for TCEP and TCIPP for which the discrepancy was >98%. pp-LFER-Estimated VG and VB of Compounds on GFF. The difference between measurements and model estimates of particle sorption for TCEP and TCIPP is larger than can be justified by modeling uncertainties or variability in TSP concentrations. Therefore, we used the pp-LFER filter-air partitioning model of Arp et al. (eq 9)16 to assess the contribution to this difference that could result from gas-phase sorption of compounds to AAS filters. Specific retention volume (VG) and safe sampling volume (VB) values were used to describe the sorptive capacity of GFF and QFF. The modeled trend here was similar for VG and VB; as such, VB is used for simplicity and illustrative purposes. Within compound groups, log VB was inversely correlated to log P°L (Figure S2). This correlation was statistically significant for both PBDEs (r2 = 0.97, p < 0.01) and OPEs (r2 = 0.57, p = 0.02). However, greater scatter was observed for OPEs probably because the OPEs are not homologues, unlike the PBDEs. Compared to log P°L, a stronger correlation was found between

Figure 3. Relationships between predicted log specific retention volume VB on GFF and QFF and McGowan volume of selected OPEs and PBDEs at 15 °C and 50% RH. Log VB values were the same for TCPs isomers because the isomers had the same solvation parameters.

is a better predictor of filter sorption than vapor pressure. OPEs had VB values that were approximately 3 orders of magnitude higher than those of PBDEs of similar or lower P°L, indicating a much greater potential of the filters to retain gas-phase OPEs than PBDEs. Other studies also found polar compounds to sorb more than nonpolar compounds to polar surfaces.16,51 According to the pp-LFER for filter sorption, the intensity of van der Waals and electron-donor/acceptor interactions, and the resultant retention volume, increased with molecular volume within a compound group (Figure 3 and Table S12). VB values for GFF and QFF were higher for OPEs (e.g., TCEP and TCIPP) for which descriptor B values were approximately three times the values of PBDEs (e.g., BDEs-28 and 100) of comparable McGowan volume, suggesting that electron-donor capability seemed to drive between-group variability. The GFF and QFF are bipolar sorbents that consist mainly of silicon dioxide (SiO2). At ambient humidity, the Si−O and −O of the filters undergo electron donor/acceptor interactions with the OH and −O groups of water. These interactions result in a film of water covering the surface of the filter.13 A range of 1 molecular layer to 5−10 molecular layers has been estimated to cover the surface of quartz sand at 30% to 90% RH, respectively.13 Water molecules, in turn, interact with the compounds tested here by accepting electrons since the compounds can only donate electrons due to their monopolar nature. As discussed, modeling results for gas-particle partitioning showed that electron donor interaction is a key predictor for the higher sorption of OPEs compared to PBDEs. Thus, the two analyses of gas-particle and filter-air partitioning yield a consistent interpretation of the mechanism that could account for OPE retention on filters. Another factor contributing to between-group variability of VB may be that OPEs and PBDEs have different sorption mechanisms. Adsorption onto the surface of the water film is the common sorption mechanism for most organic compounds except for very polar compounds that can be absorbed into the water film.30 OPEs, as polar compounds, may be adsorbed and/ 13838


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Figure 4. Comparison of sampled volumes from indoor and outdoor studies with safe sampling volumes (VB) of GFF and QFF estimated here using pp-LFER at 15 °C and 50% RH. Error bars indicate standard errors and asterisks indicate negative log values for GFF. The standard errors were calculated from the standard errors of the scaling factors and constant c for eq 9.

or absorbed to the water film whereas PBDEs may only be adsorbed onto the surface of the water film. Within and between group variability also depends on the properties of the filter type. Estimated specific VG of QFF were two to three orders of magnitude higher than those of GFF, which is likely due to differences in molecular interactions for

outdoors. However, an assumption of 50% is conservative as gasphase sorption to surfaces is expected to be lower at higher RH.30 Using GFF Indoors. As a case study, Okeme and Yang et al.43 collected 140 m3 of air using an LV-AAS in an indoor calibration study. The GFF captured VB, leading to losses by blow off, or by “breakthrough” of the gas phase from the filter to the sorbent. At this point, AAS sampling is assumed to reflect the “actual” gasparticle distribution of the compound in air. To assess the gas-phase sorption artifact, we compared the VB values estimated here with air volumes collected in field studies (Figure 4). For indoors and outdoors, 1−200 m3 and 300−∼3000 m3 of air are commonly sampled using LV-AAS and HV-AAS, respectively. The comparison between V and VB assumed that the ambient temperature was 15 °C, RH was 50%, and the gas-particle equilibration in air was achieved for target compounds. Filter capacity indoors is likely to be overestimated here since indoor temperatures are 5−10 °C higher than the ppLFER temperature. RH could be higher than the modeled value of 50%. For example, Toronto’s long-term average RH is 70%

a

TCEP TCIPPa TDCIPPa TPhPa TBOEP BDE-28 BDE-47 BDE-99

f part(M)43 (%)

f part(pp‑LFERg/p)a (%)

f part artifact (%)

100 100 100 100 100 100 100% for the OPEs with lower P°L such as TCPs, EHDPP, TBOEP and TEHP for which VB > V. But as mentioned, this artifact is unlikely to affect these compounds because the pp-LFER estimates here and field measurements (e.g., Salamova et al.44) agreed that these compounds should be almost entirely in the particle phase, irrespective of TSP, RH, and temperature. Gas-phase sorption should be less of a concern when using GFF to sample PBDEs whether indoors or outdoors because estimated VB values were ≤2% of V values and PBDE gasparticle partitioning estimated using all models were comparable with measured values (Figure 1). For QFF, the VB for most OPEs and PBDEs exceeded V values, suggesting the possibility of substantial gas-phase sorption. Of course, the higher

Estimated average VB values for the measured PBDEs were ≤9% of 140 m3 of the volume of air sampled indoors43 (Figure 4), suggesting that gas-phase sorption to the GFF used in the study should be insignificant and that the measured f part values should be reliable. However, for OPEs, gas-phase sorption could account for up to 74−100% of GFF capture, considering the uncertainty margin of the pp-LFER model. As such, the contribution from gas-phase sorption seemed to reconcile the discrepancy between f part(pp‑LFERg/p) estimates and f part(M) for TCIPP, TDCIPP and TPhP but not for TCEP (Table 1) indoors.43 Measured f part for TBOEP is not in doubt given the good agreement observed between the measured and pp-LFER estimated values. The sorption artifact would be more substantial in situations18,52 where very low volumes of air (≤2 m3) were sampled. For such studies, distinguishing between gas- and particle-phase OPEs would be impossible because a sampling artifact could account for 100% of filter capture, regardless of modeling uncertainty. TCEP was expected to exceed 90% gasphase based on the gas-particle partitioning predictions of the three models, regardless of the RH. Therefore, both the gasparticle partitioning and sorption to filter models cannot reconcile the >90% measurement of TCEP for filters. Using GFF and QFF Outdoors. As mentioned, many field studies19,20,44,53,54 have reported unexpectedly high particle fractions of >80% for OPEs based on their concentrations captured on the filters of HV-AAS (Table S4). As such, some other studies55−57 have assumed filter-captured OPEs to represent total OPE concentrations. A few exceptions are studies that reported 80% f part, VB estimates were 13840


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high to hinder equilibration of compounds with both the front or back filters. Another attempt at eliminating gas-phase sorption was the deactivation of filters by siliconization.17 Siliconization reduced the sorption of gas-phase perfluorooctanoic acid to GFF but not sufficiently to prevent mischaracterization of the particle fraction.17 Denuders may be used as alternatives to conventional samplers due to their observed and anticipated lower artifact,1,11 but it is debatable whether denuders are significantly more efficient than conventional AAS for measuring gas-particle partitioning.65 Apart from measurement artifacts, field data and pp-LFER estimates can differ due to model uncertainties. For example, Arp et al.16 found that pp-LFER for filter partitioning estimated VG values that were 0.08−0.73 log units lower than the values measured by Mader and Pankow31 for the sorption of PAHs to QFF. Reasons for the discrepancy included variability of temperature and RH, which are challenging to capture in a model. Procedures for treating filters and conducting air sampling can also introduce error. Arp et al.16 showed, using a chromatography experiment, that baking at ∼600 °C changed the surface chemistry of a filter, thereby increasing gas-phase sorption. Whether filters would maintain the conferred sorption is uncertain during field studies. Prediction errors can also result from limitations of the pp-LFER training set.66,67 The filter model of Arp et al.16 has a calibration set that consisted of compounds such as alcohols, alkylbenzenes, halogenated benzenes, fluorotelomer alcohols, and fluorotelomer olefins that were more volatile than the OPEs and PBDEs estimated here. Ideally, calibration compounds should be analogous to target compounds to minimize prediction errors. Future work should test the sorption of OPEs to filters in a chamber at variable values of RH, temperature, and TSP and should assess denuders as alternatives to filter/sorbent active samplers. Based on the analysis presented here on the likelihood of there being a gas-phase sorption artifact, we suggest that studies of OPEs in air have likely mischaracterized the gas-particle partitioning of the more volatile OPEs, e.g., TCEP and TCIPP. As a precautionary note, measured particle fractions of OPEs examined here that fall substantially outside the error margin of the pp-LFER model of Arp et al.30 are most likely influenced, in part, by equilibrium sampling artifacts such as gasphase sorption to filters. Ways to minimize the error in reported gas-particle partitioning include the use of GFF instead of QFF, reporting OPEs as bulk air concentrations, and assuming more volatile OPEs (e.g., TCEP and TCIPP) to be gas-phase compounds when modeling their environmental behavior.

predisposition of QFF than of GFF to gas-phase sorption does not change the true partitioning of the target compounds. It, however, increases the confusion in identifying particle fractions when using QFF. Long-Range Transport of OPEs. As mentioned, gasparticle partitioning (Kp) is a key parameter driving the longrange transport potential (LRTP) of SVOCs.8,59,60 LRTP is commonly assessed using multimedia modeling tools such as the Organization for Economic Co-operation and Development (OECD) LRTP Screening Tool.61 The OECD tool is a mass balance model designed to rapidly screen multiple compounds for the potential to undergo LRTP as judged against persistent organic pollutants (POPs) such as PBDEs, using commonly available physical−chemical properties. OPEs are enigmatic because several of them have been consistently found in air from remote regions,56,62 although the OECD model suggests that they do not have the physical−chemical properties consistent with POP-like LRTP. For example, Sühring et al.7 reported that TCEP, TCIPP, TDCIPP, and TPhP were measured with greater than 75% detection frequency in air samples across the Canadian Arctic from 2007 to 2013. We modeled LRTP of the target OPEs using the OECD LRTP Screening Tool61 based on Kp values calculated from literature measurements using eq 1 and estimated using Junge− Pankow (eq 4), Harner−Bidleman (eq 5), and pp-FERg/p (eq 7) models. A brief description of the model is provided in the SI. The tool calculates the characteristic travel distance (CTD, km) for air and water and displays the greater of air or water CTD values. Figure 5 shows the relationship between LRTP, expressed as CTD, and overall persistence (POV) of the target compounds in the environment with compounds falling in the top right quadrant exhibiting POP-like behavior using octaBDE63 as the reference. We varied Kp estimates according to the models presented here and assumed ∼100% in the particle fraction based on previous measurements, as discussed above.44 Under the Junge−Pankow, Harner−Bidleman, and ppLFERg/p models, none of the target compounds were in the upper-right POP-like quadrant, while the halogenated OPEs TCIPP, TCEP, and TDCIPP exhibited POP-like behavior under the measured Kp. Water transport determined the CTD for the chlorinated OPEs (TCEP, TCIPP, TDCIPP, and TCP) based on Junge−Pankow and Harner−Bidleman models, and TCEP and TCIPP under the pp-LFERg/p model. In contrast, air transport determined the CTD for all OPEs under the measured Kp scenario. Sühring et al.7 found that levels of chlorinated OPEs, dominated by TCEP and TCIPP, were highest around river mouths, from which they hypothesized that LRTP through rivers was the dominant transport pathway to the Arctic for these compounds and not atmospheric transport. Water-borne LRTP is consistent with the OECD tool results obtained here using the modeled and not measured Kp. The difference in LRTP between measured and estimated Kp scenarios illustrate one impact, in this case estimating LRTP, of the uncertainties in measuring and modeling OPE gas-particle partitioning, especially for the environmentally abundant compounds TCEP, TCIPP, and TDCIPP. Recommendations. Several studies have explored methods to reduce sampling artifacts. For example, studies have used a double filter to partially correct gas-phase sorption of PAHs and PCDD/Fs to QFF.12,64 Based on the pp-LFER results presented here for gas-phase sorption, back filter corrections may be unnecessary since the estimated sorption artifact was either negligible for some OPEs and most PBDEs or it was sufficiently

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b04588. Full names and CAS numbers of target compounds, values of Abraham solvation parameters, literature values of vapor pressure, octanol−air partition coefficients, and particle fractions. Modeled particle fractions and further details on the dual-phase model of gas-particle partitioning (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 13841


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Environmental Science & Technology ORCID

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Joseph O. Okeme: 0000-0001-7604-0736 Timothy F. M. Rodgers: 0000-0003-1850-404X Liisa M. Jantunen: 0000-0003-0261-9539 Miriam L. Diamond: 0000-0001-6296-6431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research funding was provided by Environment and Climate Change Canada (Contribution Agreement GCXE17S017), Health Canada (Agreement No. 4500308341), and the Natural Sciences and Engineering Research Council of Canada (NSERC, No. RGPAS 429679-12). The paper benefited from discussions with Prof. Kai-Uwe Goss and Dr. Hans Peter Arp, comments from Dr. Tom Harner, and data from Dr. Lisa Melymuk. We thank Prof. J.Mark Parnis for his thoughts and efforts at modeling gas-phase sorption using COSMOtherm.

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Environmental Science & Technology Organic Compounds by GC-Based Measurement and COSMO-RS Estimation: Rapid Measurements and Accurate Modelling. Chemosphere 2016, 156, 204−211.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on November 15, 2018, with minor errors in Figures 2 and 3 and equation 2. The corrected version was reposted on December 4, 2018.

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Examining the Gas-Particle Partitioning of Organophosphate Esters

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