Article pubs.acs.org/est
PAHs on a West-to-East Transect Across the Tropical Atlantic Ocean Rainer Lohmann,*,† Jana Klanova,‡ Petra Pribylova,‡ Hana Liskova,‡ Shifra Yonis,† and Kevyn Bollinger† †
Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, 02882 Rhode Island, United States Research Centre for Toxic Compounds in the Environment (RECETOX), Faculty of Science, Masaryk University, Kamenice 573/5, 625 00 Brno, Czech Republic
‡
S Supporting Information *
ABSTRACT: Surface water and atmospheric samples were collected across the tropical Atlantic Ocean on a transect of the R/V Endeavor in summer 2009 and analyzed for polycyclic aromatic hydrocarbons (PAHs). Across the entire tropical Atlantic Ocean, phenanthrene displayed on average highest dissolved concentrations (170 pg L−1), followed by pyrene (70 pg L−1) and fluoranthene (30 pg L−1). The Amazon plume was characterized by elevated dissolved concentrations of phenanthrene and benzo(g,h,i)fluoranthene. The warm eddy that we accidentally sampled at 66° W displayed highest concentrations of PAHs across the entire cruise, with phenanthrene, pyrene, and fluoranthrene all >1 ng L−1. After having crossed the warm core, concentrations decreased back to previous levels. Samples taken in the Gulf Stream were below detection limit for all parent PAHs, implying very efficient removal processes. Dissolved dimethylphenanthrenes were frequently detected in the samples from the southern hemisphere, the Amazon plume, and in samples characteristic of the Gulf Stream and the U.S. East Coast. Atmospheric concentrations were dominated by gas-phase fluoranthene, pyrene, phenanthrene, and retene. Air−water gradients indicated that PAHs are mostly undergoing net deposition across the tropical Atlantic Ocean, with conditions closer to equilibrium off the U.S. East Coast and in Rhode Island Sound.
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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are known carcinogens and mutagens, and their sources, transport, and effects have been studied extensively.1−4 Surprisingly little is known about the concentrations, trends, and fate of PAHs across the open oceans, whereas they have received much more attention on land and in the terrestrial atmosphere.5,6 Sampling across the oceans is made more difficult by the fact that research vessels, as other ships, produce vast quantities of PAHs in their exhaust plumes, resulting in the potential for contaminated sampling efforts.7,8 In spite of these concerns, several studies have reported atmospheric PAH concentrations from cruises covering the various oceans.9−12 Only few results are available for PAHs in the ocean water, and these have mostly covered the Atlantic/Arctic Ocean basins.11,13,14 Air−water exchange gradients of PAHs were only calculated by Nizzetto et al. (2008): Depending on the region considered, either net deposition was observed in regions with high atmospheric emissions or net volatilization in the oligotrophic southeast Atlantic Ocean.11 Basically, we lack an appreciation and understanding of the presence of PAHs for most of the oceans. A notable exception are PAHs in the Mediterranean Sea, for which several studies have reported concentrations in various media and key processes.15−18 In general, though, previous studies of oceanic processes had to assume that there were no © 2013 American Chemical Society
significant east-to-west gradients of organic contaminants in air or water.19,20 A research cruise of the R/V Endeavor in July−August 2009 from Namibia via Barbados back to her home port in Narragansett (RI), the U.S., offered a unique opportunity to determine PAHs in air, water, and their air−water exchange gradients (Figure 1). In contrast to previous cruise tracks, which were mostly along the eastern part of the Atlantic Ocean, this cruise covered both large east−west and north−south gradients and was mostly far away from shore. In a previous publication we reported on trends of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) for the same cruise across the tropical Atlantic. The continuous sampling of water and air facilitated the detection of the influence of major ocean currents and river plumes on concentrations of PCBs and OCPs in the water, while the atmospheric samples clearly showed the change from the (atmospheric) southern to northern hemisphere.21 In addition to the traditional PAHs sampling on ships using active high-volume air and water sampling, we also deployed passive samplers passive samplers as complementary apReceived: Revised: Accepted: Published: 2570
November 21, 2012 February 11, 2013 February 12, 2013 February 12, 2013 dx.doi.org/10.1021/es304764e | Environ. Sci. Technol. 2013, 47, 2570−2578
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using a sampling train, equipped with a precombusted glass fiber filter (GFF) and three polyurethane foam (PUF) plugs in series. Initially (samples 1−33), we collected 600−1100 L of water, and after Barbados, 200−500 L were filtered (samples 34−57). While water sampling proceeded continuously, the unfavorable winds often restricted air sampling. A total of 47 air samples were collected using a high-volume air sampler equipped with a precombusted GFFs and two pre-extracted PUF plugs. The active air sampler was setup above the bridge, facing the wind. From Namibia to Barbados, volumes of 400− 570 m3 (∼ 12 h each) were collected, and after Barbados, volumes were ∼230−350 m3 (∼ 8 h each). Passive Samplers. Passive polyethylene (PE) samplers were continuously exposed for approximately 48 h each to marine boundary layer air and flowing seawater inside the laboratory. The air PE samplers were exposed in inverted stainless steel bowls (“UFO disk”) on the ship’s main (chimney) mast 17.5 m above sea level (i.e., several meters above the high-volume sampler). The water PE was fixed in a steel pipe connected to the flowing seawater inside the ship’s special purpose laboratory, at a nominal flow rate of 10 L min−1. A total of seven PE samplers were towed on a line via the A-frame, ca. 100 m behind the ship, for 40−70 h each.21 Air Concentrations Inside the Ship. Seven passive PE samplers were deployed inside the ship at several locations (bridge, laboratories, and botswain’s locker) for 12−14 days each to assess the potential for contamination from the ship’s indoor air and ventilation system.7 They were deployed twice in the main and wet lab, the bridge, and once in the botswain’s locker for 1−2 weeks each (Supporting Information Table S8). Sample Analysis. High Volume Samples (PUFs, GFFs). PUFs and GFFs were extracted using automated warm Soxhlet extraction (40 min warm Soxhlet followed by 20 min of solvent rinsing) with dichloromethane (DCM) in a B-811 extraction unit (Büchi, Switzerland). The concentrated extracts were split into two portions: 1/4 of extract was used for PAHs analysis, and the remaining 3/4 of extract were used for PBDEs, indicator PCBs, and OCPs analysis. The extracts of water PUF or water GFF samples were first dried using Na2SO4. Analysis. Standards for PAHs were purchased from LGC (UK), and for alkylated PAHs from various vendors (see Supporting Information). The first portion of extract was fractionated on a silica column (5 g of silica 0.063−0.200 mm, activated at 150 °C for 12 h). The first fraction (10 mL nhexane) containing aliphatic hydrocarbons was discarded. The second fraction (20 mL DCM) containing PAHs was collected and then reduced by stream of nitrogen in a TurboVap II (Caliper LifeSciences, USA) concentrator unit and transferred into an insert in a vial. Terphenyl was added as syringe standard; the final volume was 200 μL. GC-MS analysis was performed on 6890N GC (Agilent, USA) equipped with a 60 m × 0.25 mm × 0.25 μm DB5-MS column (Agilent, J&W, USA) coupled to 5973N MS (Agilent, USA). Injection was 1 μL splitless at 280 °C, with He as carrier gas at constant flow 1.5 mL min−1. The GC program was 80 °C (1 min hold), then 15 °C min−1 to 180 °C, followed 5 °C min−1 to 310 °C (20 min hold) for analysis of parent PAHs, 80 °C (1 min hold), then 20 °C min−1 to 180 °C, 3 °C min−1 to 220 and 6 °C min−1 to 320 °C (10 min hold) for alkylated PAHs. The MS was operated in EI+ mode with selected ion monitoring (SIM) PE Sheet Samplers. Blanks and exposed sheets of PE were rinsed with Millipore water, dried with a disposable tissue, and
Figure 1. Cruise track of EN 464 with major ocean currents encountered. Map shows monthly mean ocean surface currents (m s−1), centered on July 15, 2009.47
proaches to measure truly dissolved and gas-phase concentrations. The potential for using passive samplers to determine POPs in the remote ocean had been earlier demonstrated by Booij et al. using semipermeable membrane devices (SPMDs).22 We were hoping that two-day deployments of polyethylene (PE) passive samplers would result in improved spatial resolution of both concentrations and PE-based net air− water exchange gradients.23 In summary, we analyzed samples collected across the tropical Atlantic to deduce whether (i) there were significant east−west and north−south gradients of selected PAHs, (ii) the Atlantic Ocean was a net sink or secondary source of PAHs, (iii) ocean current and river plumes would affect the presence and air−water gradients of selected PAHs, and (iv) to compare active and passive sampling approaches for PAHs. Our previous paper reported on results for OCPs and PCBs; this contribution describes results for parent and alkylated PAHs. Details of the cruise track are detailed in Figure 1. Briefly, the cruise covered several major Atlantic currents (the Benguela, South Equatorial, North Brazil, North Equatorial Currents, and Gulf Stream) and sampled parts of the Amazon plume. By chance, the cruise also sampled a warm eddy in the Sargasso Sea (Figure 1) located in between 25−27° N and 65−67° W. It was characterized by a water temperature increase by ca. 0.5 °C. The only plausible way for a warm ring to form in the Sargasso Sea involves first the formation of an aneurysm on the warm side of the Gulf Stream, followed by the detachment of the aneurysm.24 The most likely place where the Gulf Stream’s warm edge might get heavily polluted before flowing across the South Atlantic Bight is off Havana, Cuba. Pollutants from Florida/the Mississippi River would contaminate the cold side of the Gulf Stream not the warm side. Atmospherically, the ship traversed the southeasterly trade winds, passed the intertropical convergence zone (ITCZ), and continued in the northeasterly trade winds; the last few samples were affected by continental air outflow from the U.S. East Coast.
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MATERIALS AND METHODS Sampling. High Volume Air and Water Sampling. Active air and water sampling procedures were described in our companion paper.21 In brief, a total of 57 water samples were collected in the ship’s laboratory from the ship’s seawater pipe 2571
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Figure 2. Dissolved concentrations of selected PAHs (pg L−1), water temperature (°C), and salinity (psu) as a function of longitude. Major ocean currents are highlighted.
soaked for 24 h in 200 mL of n-hexane followed by 24 h in 200 mL of DCM. The two solvents were then pooled and concentrated using stream of nitrogen in a TurboVap II concentrator, and the extract was split into two portions and processed using the same procedure as the high volume samples. Quality Assurance and Quality Control. The results for PAHs were not recovery corrected. Method performance was tested prior to sample preparation with an in-house background soil. Recovery of native analytes measured for the reference soil (n = 15) varied from 72 to 102% for PAHs. For PAHs, GFF (respectively PUF) blanks were similar for water and air (one PUF blank was excluded due to contamination). All GFF (respectively PUF) blanks were combined for blank correction and MDL determination. The method detection limit (MDL) was calculated as 3× standard deviations of blank concentrations (for more details, see Supporting Information). Physicochemical Properties. Relevant internally consistent physicochemical constants (octanol−water (Kow) and air− water partitioning (Kaw) values and aqueous solubility at saturation of the subcooled liquid (Ciwsat (L)) were taken from Ma et al. (2010).25 For all PAHs, an average value of 25 kJ mol−1 was taken as the enthalpy of PE−water exchange, while enthalpies of PE−air exchange increased with increasing molecular weight.26,27 The Setschnow constant was taken as 0.30, as reported elsewhere.28 Kaw was not corrected for the influence of dissolved organic carbon (DOC), as DOC concentrations are in general too low
in the open Ocean to sorb more than a few % of the most hydrophobic PAHs measured here. On average, an average enthalpy of 49 kJ mol−1 from Beyer et al. (2002) was used to temperature-correct Kaw.29 Truly dissolved concentrations, Cdiss (pg L−1 H2O) (and gasphase, Cgas in pg m−3) were derived from PE-normalized concentrations, CPE (pg L−1 PE) as detailed previously.21 Meteorological and Sea Surface Auxiliary Measurements. For each sample, we used averaged values of latitude, longitude, surface water temperature (Twater), salinity, and fluorescence of the flow-through seawater; air temperature (Tair), relative humidity (RH), relative and absolute wind speed and direction were as reported previously.21 Back-trajectories were back-calculated for 5 days with 6 h steps at 300 m above sea level using HYSPLIT.30
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RESULTS AND DISCUSSION Water Concentrations. Active Sampling Results. Of the parent PAHs, only pyrene and fluoranthene were routinely detected in the dissolved phase, while phenanthrene, benzo(g,h,i)fluoranthene, and triphenylene were detected in most samples (Figure 2). Few PAHs were detected in the particlebound phase. Across the entire Atlantic Ocean, phenanthrene displayed on average highest dissolved concentrations (170 pg L−1), followed by pyrene (70 pg L−1), fluoranthene (30 pg L−1), and triphenylene (25 pg L−1). General background concentrations ranged from 30 to 80 pg L−1 for pyrene and 15−50 pg L−1 for fluoranthene; few other PAHs were detected regularly. 2572
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Table 1. Comparison of Surface Water Dissolved PAH Concentrations (pg L−1) across the Atlantic Ocean phenanthrene
fluoranthene
pyrene
mean
range
mean
range
this study, 2009 East Atlantic, 200511 N Atlantic, 2004 13 Irminger Sea, 1993,14a
199 420 44 44
32−1400 200−700 8−180
69 130 1.2 38
25−240 84−220 0.4−5
this study, 2009 East Atlantic, 200511
39 135
mean
range
NH 29 110 36 49
13−190 nd−140 10−130
SH
a
22−94
