Occurrence and Air−Sea Exchange of Phthalates in the Arctic

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Environ. Sci. Technol. 2007, 41, 4555-4560

Occurrence and Air-Sea Exchange of Phthalates in the Arctic Z H I Y O N G X I E , †,‡ R A L F E B I N G H A U S , † CHRISTIAN TEMME,† R A I N E R L O H M A N N , * ,§ ARMANDO CABA,† AND WOLFGANG RUCK‡ GKSS Research Centre Geesthacht, Institute for Coastal Research, Max-Planck Strasse 1, D-21502 Geesthacht, Germany, Institute of Ecology and Environmental Chemistry, University of Lueneburg, Scharmhorst Strasse 1, D-21335 Lueneburg, Germany, and Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882-1197

Air and seawater samples were taken simultaneously to investigate the distribution and air-sea gas exchange of phthalates in the Arctic onboard the German Research Ship FS Polarstern. Samples were collected on expeditions ARK XX1&2 from the North Sea to the high Arctic (60° N-85° N) in the summer of 2004. The concentration of Σ6 phthalates (dimethyl phthalate (DMP), diethyl phthalate (DEP), di-i-butyl phthalate (DiBP), di-n-butyl phthalate (DnBP), butylbenzyl phthalate (BBP), and diethylhexyl phthalate (DEHP)) ranged from 30 to 5030 pg L-1 in the aqueous dissolved phase and from 1110 to 3090 pg m-3 in the atmospheric gas phase. A decreasing latitudinal trend was present in the seawater and to a lesser degree in the atmosphere from the Norwegian coast to the high Arctic. Overall, deposition dominated the air-sea gas exchange for DEHP, while volatilization from seawater took place in the near-coast environment. The estimated net gas deposition of DEHP was 5, 30, and 190 t year-1 for the Norwegian Sea, the Greenland Sea, and the Arctic, respectively. This suggests that atmospheric transport and deposition of phthalates is a significant process for their occurrence in the remote Atlantic and Arctic Ocean.

Introduction Phthalates are manufactured worldwide on a large scale, mainly as plasticizers in resins and polymers, and especially as a softener in polyvinylchloride (PVC). Other industrial applications include the manufacturing of cosmetics, insect repellents, insecticide carriers, and propellants (1). In 2004, the world production of phthalates was estimated to be 6 million t year-1 (2). As plasticizers, phthalates are not physically bound to the polymer and can thus diffuse out of the plastic and leach into the environment. They can enter the environment via emissions from household and industrial products, as releases via wastewater from production and processing activities, and be released from the use and disposal of materials (3). Health and environmental hazards of phthalates have been the subject of scientific discussion and public concern (4-5). * Corresponding author phone: 401-874-6612; fax: 401-874-6811; e-mail: [email protected]. † Institute for Coastal Research. ‡ University of Lueneburg. § University of Rhode Island. 10.1021/es0630240 CCC: $37.00 Published on Web 05/25/2007

 2007 American Chemical Society

The release of phthalates directly to the atmosphere is believed to be the most important means of their entry into the environment (3). Since the 1970s, phthalates have been determined in various environmental media and biota (69). Studies of phthalates in the atmosphere over the North Atlantic, the Great Lakes, and the North Sea indicated that atmospheric deposition is a significant source for open waters (10-12). Phthalates have several degradation pathways, e.g., photodegradation in the atmosphere, biodegradation in water, and anaerobic degradation in sediments and soil (3). Laboratory studies suggest that phthalates are not expected to be highly persistent in most environments (13-16). However, longer half-lives are likely under anaerobic conditions, low concentrations, and in cold and nutrient-poor environments (17). Evaluative models indicate that phthalates have moderate long-range atmospheric transport potentials (16). Therefore, we wanted to take a closer look into their occurrence and distribution in the remote North Atlantic/ Arctic Ocean. This work was designed to improve our understanding of the distribution and transport mechanisms of phthalates to the Arctic, especially the role of the air-sea gas exchange in delivering these compounds. In this study, concentrations of six different phthalates (Figure 1) in surface water and in the atmosphere over the Arctic were determined by combining short-term atmospheric samples with the collection of representative water samples from the Norwegian Sea to the Arctic. Furthermore, air-sea gas exchange fluxes of phthalates were calculated based on paired air/water samples.

Experimental Section Sample Collection. In the summer of 2004, the German research ship FS Polarstern completed her expeditions in the Arctic, namely ARK XX/1 (Bremerhaven-GreenlandLongyearbyen, 6/17-7/16/2004) and ARK XX/2 (Longyearbyen-Arctic-Tromsø, 7/17-8/28/2004). Air samples (ARK XX1/2 A1-A6 (500-2000 m3)) were collected from the Norwegian Sea to the Arctic using a high-volume air sampler equipped with a PUF/XAD-2 column for gas-phase compounds and a glass fiber filter (GF/F-8, pore size: 0.45 µm, Schleicher & Schuell, Dassel, Germany) for atmospheric particles, respectively (18). Measurements were taken on the observation deck only under headwind conditions with relative wind speeds exceeding 3 m s-1. Six field blanks were taken by exposing the sampling medium to the routine handling and deployment without actually sampling. These field blanks were used to derive the detection limits for the air samples based on the contamination incurred during the shipment and handling of the sampling media. Parallel to the air sampling, representative water samples (500-1000 L) were collected directly from the stainless steel seawater inlet located at the keel of the ship at 11 m depth. A PAD-2 column (polymeric adsorbent: DVB styrene, Serva Electrophresis, Heidelberg, Germany) was used for collecting the dissolved analytes and a GF/F 52 (pore size: 0.75 µm, Schleicher & Schuell, Dassel, Germany) for collecting total suspended matter (TSM). Three field blanks of water sampling were prepared onboard by sampling 200 mL of seawater. PAD-2 and PUF/XAD-2 columns were spiked with deuterated phthalates (DMP d4, DEP d4, DnBP d4, DEHP d4) as surrogates prior to sampling. The details of the air and seawater samples are shown in the Supporting Information (Table SI 1); mean sampling positions are shown in Figures 2 and 3. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Chemical structures of the phthalates.

FIGURE 2. Spatial distribution of the dissolved Σ6 phthalates in the Arctic. The bars are placed on the average position for each water sample. Sample Preparation. The PAD-2 columns and GF/Fs were individually extracted using DCM, and PUF/XAD-2 columns were extracted using hexane and diethyl ether (8:2 v/v) for 24 h. Before the extraction, 20 ng of dibenzyl phthalate was spiked as an internal standard. The extracts were cleaned on a 2.5 g silica gel column (5% H2O deactivation), eluted with 15 mL of hexane (F1), 25 mL of hexane/diethyl ether (3:1 v/v) (F2), 25 mL of hexane/diethyl ether (1:1 v/v) (F3), and 20 mL of hexane/acetone (1:1 v/v) (F4). F2 and F3 were combined and concentrated down to 100 µL. The extract was adjusted to 200 µL with hexane and analyzed using gas chromatography and mass spectrometry (GC-MS) as described previously (18). Quality Control. The analytical quality of the data was guaranteed through the use of field blanks to derive method 4556

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detection limits (MDLs), breakthrough testing of air sampling, and the recoveries of surrogates. The recoveries of surrogates in more than 80% of water samples were within a range from 70% to 140%. Recoveries of deuterated phthalates ranged from 80% to 140% for PUF/XAD-2 columns and PAD-2 columns. Extraction recoveries for phthalates in atmospheric particles ranged from 86% to 118%. The relative standard deviations (RSD) of phthalates ranged from 2 to 16% for the extraction procedure and from 2 to 10% for the matrix spiking experiments, showing the good reproducibility of laboratory preparation. The breakthrough tests (ARK XX1/2 A1, A6) showed that more than 90% of phthalates were retained on the upper column and thus indicated the absence of significant breakthrough during sampling. Furthermore, the recoveries present on the upper column were very compa-

TABLE 1. Summary of Results of Dissolved Phthalates in the Arctica phthalate DMP DEP DiBP DnBP BBP DEHP ∑6 phthalates

detectable MDL frequency mean median minimum maximum (pg L-1) (%) (pg L-1) (pg L-1) (pg L-1) (pg L-1) 13 8 5 3 0.2 24

42 71 65 100 95 88

40 138 22 51 8 448 705

13 40 6 29 6 221 359

DMP. Photodegradation halflives (days) of phthalates were estimated as DEHP (0.38) < BBP (0.75) < DnBP (0.89) < DEP (2.39) < DMP (14.41) (17), suggesting that DEHP, BBP, and DnBP should be preferentially degraded during atmospheric transport. The partitioning of high molecular phthalates to particles might reduce their photolysis rates, and thus increase their persistence in the atmosphere, as atmospheric reactions often deplete compounds in the gas phase. Gas-phase phthalates can also be efficiently scavenged by rain (e.g., 25). Additionally, dry and wet depositions of particles may remove the high molecular phthalates from the atmosphere (30, 31). These studies suggest that photodegradation, wet and dry deposi4558

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tion are important processes removing phthalates from the atmosphere. Air-Sea Gas Exchange Models. The direction of net airsea gas exchange may be determined by calculating dissolved/gas-phase fugacity quotients (Q) (32)

Q)

fw CwH′ ) (fw > fa) volatilization fa Ca

Q)-

fa Ca )fw CwH′

(fa > fw) deposition

(1)

(2)

where fw and fa are the fugacities in water and air (Pa), Cw and Ca are the dissolved- and gas-phase concentrations (ng m-3), and H′ is the dimensionless Henry’s law constant. H′ is obtained by dividing Henry’s law constant (H in Pa m3 mol-1) by the gas constant R (8.314 m3 Pa mol-1 K-1) and the average air temperature Ta (in K) during the water sampling. The larger fugacity was placed in the numerator so that the absolute value of Q was always >1. A positive fugacity quotient indicates net volatilization, while a negative value indicates net deposition. H values were taken from Cousins and Mackay (33) and corrected for average water temperature (Tw, K) and salinity (Cs, 0.5 mol L-1) (34) (for details see Table SI 4). Uncertainties in the calculation of the fugacity quotients were propagated from analytical errors of Ca and Cw, and the uncertainty in H values (32). The analytical error was estimated to be 0.15 for Ca and Cw. H values used in this study are different from H values estimated by Staples et al. (3) by a factor of 3. The uncertainties of H values render the analytical errors negligible; hence the propagated uncertainty for Q will be on the order of ∼ (1.5. Fugacity Quotients. From the Norwegian Sea to the Arctic Ocean, fugacity quotients of DMP, DEP, DnBP, DiBP, and BBP far exceeded the uncertainty, ranging from -100s to -1000s (for details see Table SI 5), indicating net air-towater deposition. Fugacity quotients of DEHP calculated for the Norwegian Sea ranged from 2.5 to -5. In the Greenland Sea and the Arctic Ocean, most fugacity quotients of DEHP were slightly beyond the uncertainty of 1.5, indicating air to water deposition. However, this was not certain with 95% confidence for all sample pairs (32). The positive flux values calculated for the paired samples W1/A1 and W13/A5 were just around the uncertainty limits of H. This reinforces the need for a more accurate determination of H values of phthalates and other organic compounds (32). With the exception of DEHP, fugacity gradients for phthalates were very steep, and implied strong air-to-water depositional fluxes. These could be caused by (i) unusually high concentrations of phthalates in the atmosphere (“dirty plume”), (ii) a very fast removal of phthalates in the water column, (iii) H values that are incorrect, wrongly implying fluxes when there are none; (iv) loss of compounds in the

dissolved phase, and/or contamination of atmospheric samples; and last a combination of these factors. As discussed above, most air masses originated from the Arctic/Greenland area, suggesting that the air should have been carrying low concentrations of phthalates to the sampling stations. In a literature review, Staples et al. (3) suggest that phthalates are fairly stable once deposited into surface waters. Estimates of H values for the different phthalates were within a factor of 2-3, suggesting a fairly good agreement on the “true” values. Based on the QA/QC samples, the fact that water sampling was performed in closed systems, and the observed strong gradient in dissolved concentrations, we have a high degree of confidence in phthalates in the aqueous phase. Atmospheric sampling onboard ship is notoriously difficult to perform (e.g., 35). Compared to urban samples, we observed a strong gradient for phthalates in the remote Atlantic/Arctic air. However, the gradient in atmospheric samples from near Europe to the high Arctic on this cruise was far less pronounced than that observed in the dissolved phase (see Figures 1 and 2). This might indicate that atmospheric samples were affected by diffusive emissions from the ship. Surprisingly, though, the atmospheric profiles of the different phthalates followed expected trends with respect to atmospheric degradation reactions (see above). Hence the overall picture is complicated, and might indicate contamination of atmospheric samples, but is not conclusive. In the following, we will focus on DEHP, which displayed fugacity gradients of less than (10. We take this as evidence that atmospheric concentrations were not strongly affected by any potential ship-side contamination. Air-Sea Gas Exchange Fluxes. Air-sea gas exchange fluxes were estimated using the modified version of the Whitman two-film resistance model (36, 37). The overall flux calculation is defined by

(

F ) KOL Cw -

)

Ca H′

(3)

where F is the flux of a given phthalate (ng m-2 day-1), (Cw - Ca/H′) describes the concentration gradient (ng m-3), and KOL (m day-1) (1/KOL ) 1/Kw + 1/KaH′) is the mass transfer coefficient comprising resistances to mass transfer in both water (Kw) and air (Ka). The Schmidt number for CO2 at mean temperature of 5 °C (ScCO2 ) 1395) was applied for the estimation of Kw (34). As wind speed has a nonlinear effect on the water-side mass transfer coefficient, fluxes were normalized to the mean wind speed (5 m s-1 during ARK XX1/2) to better compare the air-sea exchange of phthalates for different days and areas in the Northern Oceans (38, 39). The estimated fluxes of DEHP in the Arctic are shown in Figure 4 and in the Supporting Information (Table SI 6). DEHP illustrated changing directions of the air-water exchange fluxes driven by varying concentration gradients. Relatively high dissolved DEHP concentration in seawater off the Norwegian coast caused a different air-water gradient, possibly indicating volatilization, with a flux up to +212 ng m-2 day-1. In contrast, depositional fluxes of DEHP were calculated for the Arctic Ocean. It should be noted (see above) that for some fluxes the direction of the air-sea gas exchange could not be determined with certainty. The net air/sea exchange flux for DEHP changed from volatilization in the German Bight to deposition in Northern Oceans. In comparison to the bulk and dry deposition fluxes estimated by Teil et al. (25), the air-sea gas exchange fluxes estimated in this work were lower by 2 orders of magnitude for DEHP. This suggests that dry deposition of DEHP might account for the majority of the bulk deposition fluxes. Net Deposition Fluxes of Phthalates in the Arctic. Given the surface area of the Arctic, the Norwegian, and the

FIGURE 4. Air-sea gas exchange fluxes of DEHP in the Arctic (W14, W15, and W16 are excluded, as they were collected under sea ice).

TABLE 3. Annual Gas Deposition of DEHP to the Norwegian Sea, the Greenland Sea, and the Arctic Based on the Mean Gas and Dissolved Concentrations Determined in This Study region surface area (km2) average flux (ng m-2 day-1) annual gas deposition (t year-1)

Norwegian Sea

Greenland Sea

Arctic

1.380 × 106 1.205 × 106 14.056 × 106 -9

-67

-37

5

30

190

Greenland Seas, net annual gas deposition fluxes of DEHP were estimated using the mean air-sea gas exchange fluxes shown in Table 3. The major input of phthalates to the Norwegian Sea and the Greenland Sea is from western and northern European countries. Our results suggest that the North Atlantic received ca. 0.02% of the annual market consumption of DEHP (∼20 000 t year-1). However, the input to the Arctic might be highly influenced by the atmospheric transport from North America, Russia, and the Pacific Ocean. The deposition estimated for the Arctic would account for ca. 0.01% of the annual world production (∼2 million t year-1). Gas exchange is only one pathway for atmospheric deposition; precipitation and dry particle deposition are also significant pathways in delivering phthalates to the Arctic (40, 41). As the higher part of the Arctic is covered with ice and snow throughout the year, atmospheric phthalates will presumably deposit onto the snow and ice, which could act as a cold trap. Therefore, further studies should focus on the air-snow/ice exchange and the chemical processing of phthalates in the Arctic summer and winter.

Acknowledgments We would like to express our great thanks to the Alfred Wegener Institute for Polar and Marine Research, as this study was part of the FS Polarstern Arctic cruise 2004. NOAA Air Resources Laboratory is also gratefully acknowledged for the provision of the HYSPLIT transport and dispersion model and the READY website (http://www.arl.noaa.gov/ready. html) used in this study.

Supporting Information Available Details on air and water sampling and individual phthalate concentrations, their properties, fugacity quotients, H′ values, and flux calculations; Figure SI 1 compares dissolved concentrations, and Figure SI 2 compares atmospheric concentrations of phthalates with other studies. This material is available free of charge via the Internet at http:// pubs.acs.org. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Received for review December 20, 2006. Revised manuscript received April 6, 2007. Accepted April 13, 2007. ES0630240