Seasonality in Exchange of Organochlorines between Arctic Air and

Environ. Sci. Technol. 1997, 31, 3258-3266

Seasonality in Exchange of Organochlorines between Arctic Air and Seawater B A R R Y T . H A R G R A V E , * ,† LEONARD A. BARRIE,‡ TERRY F. BIDLEMAN,‡ AND HAROLD E. WELCH§ Habitat Ecology Section, Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2, Canada, Atmospheric Environment Service, Environment Canada, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4, Canada, and Fisheries and Oceans Canada, Central and Arctic Region, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada

Decreases in atmospheric concentrations of organochlorine (OC) pesticides in the Northern Hemisphere during the last decade have resulted in Arctic Ocean surface layer water that is now supersaturated for some compounds. Seasonal measurements of several OCs in air and seawater in the Canadian arctic archipelago during 1993 were made to study changes in air-sea exchange over an annual cycle. Fugacity ratios and Henry’s law constants, with correction for seasonal changes in ice cover, were used to calculate net air-sea fluxes on a monthly basis. The lack of concentration gradients between 1 and 50 m indicated that advection of polar mixed layer water flowing southward from under the polar ice cap was an important factor controlling seawater concentrations. Out-gassing of HCHs, HCB, and dieldrin during the ice-free period could have lowered surface layer inventories by 4-20%. In contrast, net deposition of CHBs, chlordanes, and endosulfan-I during the open water period was equivalent to 50->100% of the surface layer inventory. Air-sea fluxes of OCs were influenced by the combined effects of changes in atmospheric vapor pressure and water mass advection. Potential removal of OCs on sedimenting particles was calculated to be less important in controlling surface layer concentrations.

Introduction Evidence to support the idea of global distillation of volatile organic compounds such as organochlorines (OCs) comes from observations of relative concentrations in seawater and overlying air, where estimates of air-sea gas exchange are calculated on the basis of the departure from air-water equilibrium expressed by Henry’s law constants (1-3). For example, the relatively volatile hexachlorocyclohexanes (HCHs) are distributed fairly uniformly throughout the northern troposphere (4). Because their partitioning into water is favored at colder temperatures, HCHs concentrations in ocean surface water increase with latitude. Concentrations of HCHs in the world’s oceans are highest in the Arctic Ocean and * Corresponding author tel/fax: +1-902-426-3188; e-mail: barry. [email protected]. † Bedford Institute of Oceanography. ‡ Environment Canada. § Freshwater Institute.

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subarctic seas and about an order of magnitude lower in tropical waters (5-7). Polychlorinated biphenyls (PCBs) and chlordanes, which are generally less volatile than HCHS, are near air-water equilibrium or undergo net volatilization in the temperate North Pacific and North Atlantic, but show net deposition in subarctic waters (4). While previous studies in the 1980s (5, 7, 8) concluded that there was a general tendency for air-to-water transfer of OC pesticides into the ocean, recent investigations show that for some compounds there may be a reversal in exchange and the ocean may now be a source to the atmosphere (9). Data collected since 1992 suggest that a long-term decline in atmospheric concentration of HCH has resulted in a reversal of the direction of net gas exchange in arctic and subarctic waters, from deposition in the 1980s to volatilization in the 1990s (5, 6, 10, 11). This is consistent with observations in the Great Lakes (12-14) and Lake Baikal (15), which show that, while lakes may be a sink for some OCs, volatilization is also a major transport pathway that moves contaminants from water to air. OC pesticides also undergo high and low cycles in the arctic atmosphere in response to changing meterological conditions (16-18). Thus, seasonal changes in both the rate and direction of air-to-sea exchange might be expected in arctic marine areas where fluctuations in air concentrations occur. Although a few of the studies cited above have used limited data collected in different seasons and locations to assess air-water gas exchange, no investigation has been carried out in the Arctic Ocean or any other location over a full annual period. There have also been no previous studies that allow air-sea exchange to be compared to other factors that affect water column inventories directly such as advection and biologically mediated particle exchange and sedimentation. Here we present the results of a sampling program in which monthly measurements of ‘dissolved’ OCs in the water column of the Canadian archipelago during JanuaryDecember 1993 were paired with weekly air samples collected in the same year at Alert, NT, as part of a continuous air monitoring program (16). Measurements of OCs in air and water, Henry’s law constants, wind speed, and fractional ice cover were used to calculate gas flux to estimate seasonal changes in the direction and magnitude of air-sea exchange for organohalogens in the Candian archipelago.

Materials and Methods Sampling Locations. The meteorological station at Alert, NT (82.5° N, 62.3° W) has been the site where studies have been conducted to examine seasonality in the arrival of southerly emissions from fossil fuel combustion, smelting, and industrial processes (16, 19) and to determine source regions of aerosols transported to the Canadian high arctic (20). Studies of atmospheric concentrations of sulfate aerosols and organic contaminants (chlordane and nonachlor isomers) at this site over the past 15 years have shown widespread spatial homogeneity of the Arctic Air Mass (21), which supports our assumption that atmospheric observations of aerosols and gaseous contaminants at Alert are representative of those throughout the Canadian archipelago. Air temperature increased to a summer maximum (5 °C) in early August and minima (-30 to -35 °C) during winter months in 1993 (Figure 1A). Seasonal changes in aerosol concentrations at Alert were inversely correlated with air temperature, with maximum values during winter (Figure 1B). An oceanographic sampling station (depth 108 m) was maintained about 4 km offshore of Cornwallis Island in Barrow Strait (74°42′ N, 94°50′ W) during periods of ice cover from January to June and from mid-October to December 1993.

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FIGURE 1. (A) Seasonal changes in air temperature and (B) aerosol surface area (L. Barrie, unpublished data) at study sites in Resolute Bay and Alert during 1993; (C) percentage of open water in the Arctic Ocean, Canadian archipelago, and Beaufort Sea [from data averaged for 1982-1987 (22) and for 1978-1987 (26)]; (D) depth of surface mixed layer with period of ice cover indicated by the shaded areas. Tidal currents reach 60 cm s-1 during spring tides with an eastward residual flow of 10-15 cm s-1 (10-15 km d-1). This combines water flowing south (1.7 ( 0.4 Sv) (1 Sv ) 106 m3 s-1) from the surface polar mixed layer (upper 125 m) of the Arctic Ocean with water from the west (Viscount Melville Sound) and from the south (Peel Sound) (22-24). Water temperatures varied from -1.8 °C in winter to -0.25 °C in August 1993. Salinity in the surface layer in winter (32-33‰) decreased during the open water period (31-27‰) between mid-June to early October due to runoff and ice melt. The depth of the pycnocline was minimum (5 m) during August, and the water column was isothermal and unstratified between December and March (Figure 1D). Land-fast ice covered Barrow Strait from mid-October to late May in 1993 with maximum thickness (1.9 m) in April. Interannual variability in ice thickness is correlated with changes in the average depth of snow covering in this area (25). Seasonal variations in average open water area (Figure 1C), used to calculate actual air-to-sea OC fluxes, were determined from passive microwave remote sensing data obtained by SMMR (scanning multichannel microwave radiometer) from NIMBUS 5 and 7 satellites (26). Average monthly values were summarized between 1978 and 1987 for the Canadian archipelago (area ) 0.714 × 106 km2). Organochlorine Measurements in Air and Seawater. Air samples were collected weekly from January to December

1993 at a site 5 km from a base camp at Alert using a highvolume (700-3000 m3) air sampler with a glass fiber filter in front of two polyurethane plugs (PUFs) in series (16). Breakthrough for most OCs using this method is 80%. Confirmatory analyses for toxaphene (CHBs) were performed on selected samples by GC-ECD and GC-NIMS using an HP5890 Series II gas chromatograph connected to a VG AutoSpecEQ double-focusing spectrophotometer (27). Mean concentrations of CHBs determined by GC-NIMS analysis were 2.7 times higher than values determined by GC-ECD. This factor was used to correct all CHB concentrations determined by GC-ECD. Minimum detection limits for different OCs in air varied from 0.05 to 0.2 pg m-3. Water samples (200-400 L) were collected at monthly intervals (January-December 1993) using two in situ Seastar pumps (28). Water pumped at 150 mL min-1 passed through a prebaked filter (Gelman AE GFF) and XAD-2 resin in a Teflon

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TABLE 1. Henry’s Law Constants (H), Annual Means and Ranges for Various Organochlorines Measured in Air (n ) 34) (Alert, NT) and Seawater (1 m Depth, n ) 34) (Barrow Strait/Resolute Bay, NT) during 1993a compound

H (Pa m3 mol-1)

air (pg m-3)

seawater (pg L-1)

R-HCHb γ-HCHb HCBd CHBsf cis-chlordane trans-chlordane dieldrin endosulfan-I

0.10c 0.061c 9.0e 0.11c,g 2.8e 1.2c,e 0.64c 0.67

58 (52-83) 10 (7-22) 57 (27-75) 11 (3-34) 1.0 (0.5-1.9) 0.4 (0.3-1.2) 1.2 (0.5-2.0) 3.5 (0.05-7.0)

3640 (2900-4400) 520 (360-670) 15 (14-18) 85 (40-125) 1.3 (0.8-1.8) 0.5 (0.4-0.6) 12 (9-20) 2.6 (2.0-5.7)

a Data were used to calculate fugacity ratios (Figure 2) at a water temperature of 0 °C and air temperatures shown in Figure 1A. hexachlorocyclohexane. c Ref 7. d Hexachlorocyclobenzene. e Ref 15. f Toxaphene. g Refs 4 and 8. Average value for toxaphene complex mixture comparable to ‘weathered’ toxaphene (31). b

column. Precleaning with methanol (250 mL), dichloromethane (500 mL), and methanol (250 mL) reduced column blanks to below detection limits for all OCs. Pumps were suspended (1 and 50 m) on a Kevlar line for 24-48 h. Columns were capped, sealed with Teflon tape, and frozen (-10 °C) prior to GC analysis. The standard deviation of OCs in triplicate samples at the same depth using the same procedures in an earlier study (28) was 90% (29), and thus no correction was made for retention or recovery efficiencies. Calculation of Fugacity Ratios and Air-to-Sea Gas Exchange Rates. The partial pressures (fugacities) of OCs in water (fw) relative to values in overlying air (fa) can be used to calculate the potential for volatilization or deposition (812, 15). The air-gas phase concentration was calculated as the sum of both the gaseous and particulate species. The particulate fraction (f) was calculated from the Junge-Pankow adsorption model (30) as

f ) cq/(p°L + cq)

(1)

where p°L (Pa) is the liquid-phase vapor pressure at ambient water temperature, c ) 17.2 Pa‚cm and q ) cm2 aerosols cm-3 air (the particle surface area available for adsorption) estimated from the concentration and size distribution of aerosols measured at Alert (Figure 1B). No corrections were made for partitioning of OCs with dissolved colloidal organic matter in seawater. Previous studies in coastal Arctic Ocean surface water found negligible amounts of OCs associated with filterable particulate matter (28), and correction for partitioning resulted in small changes to calculated fugacity ratios (8). XAD-2 columns in our study were assumed to collect ‘dissolved’ OCs because water first passed through a glass fiber filter (1 µm pore diameter). Water/ air fugacity ratios (fw/fa) were calculated as

CwH/Ca(1 - f)RTa

(2)

where Cw is the concentration (pg m-3) dissolved in surface (1 m) water, Ca is the sum of the gas and particle concentrations (pg m-3) described above, R is the gas constant (8.314 Pa m3 deg-1 mol-1), H is the Henry’s law constant (Pa m3 mol-1) for various OCs (Table 1), and Ta is air temperature (K) (Figure 1A). Environmental samples of CHB usually contain a different mix of congeners from reference standards. Since different congeners have different H values, the choice of an appropriate value is problematic. The average value we have used is similar to a temperature-corrected value (0.12) calculated for ‘weathered’ toxaphene (31). Net air-to-sea fluxes were calculated from a two-film model (32) using fugacity-based definitions and mass transfer coefficients for the air (ka ) 6 × 10-3 m s-1) and water (kw )

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9 × 10-6 m s-1) films at a wind speed of 5 m s-1. Similar values were assumed for all OCs. Monthly averaged wind speeds measured at sea level in Resolute Bay in 1993 varied from 5.6 to 6.7 m s-1. We assumed that no volatilization occurred in the presence of ice cover and that monthly potential fluxes were multiplied by the fraction of open water in the Canadian archipelago (Figure 1C) to estimate actual air-sea exchange. Monthly integrated fluxes were compared with the OC inventories in the surface layer (Figure 1D) and to the 50 m depth (the maximum depth sampled) in the same month to determine the potential impact of net air-to-sea exchange. Statistical methods for Mann-Whitney nonparametric one-way analysis of variance and regression analyses were performed using SYSTAT (33).

Results and Discussion Seasonal Changes of Organochlorines in Air. R-HCH and HCB were the two most abundant OCs in air at Alert in 1993 with lower concentrations of γ-HCH and CHBs, followed by even lower levels of endosulfan-I, dieldrin, and chlordanes (Table 1). All OCs showed a bimodal seasonal distribution of concentrations with maxima in February-May and JulyAugust separated by minimum values in June (Figure 2). OC levels in air increased sharply during April and May and decreased during June coincident with the onset of the open water period (Figure 1C). Previous studies, with sampling usually restricted to summer months, also found that R-HCH and HCB were the dominant OC pesticides in the arctic atmosphere. Air concentrations measured at various arctic locations during the 1970s and 1980s (100-1000 pg m-3) (7, 10, 11, 16-18) were from 5 to 10 times higher than values we observed, confirming the downward trend in HCHs concentrations in the arctic atmosphere in the 1990s (8). Air concentrations of R-HCH measured at Resolute Bay in 1992 (10) and over the Bering and Chukchi Seas in 1993 (9) span the range observed at Alert in 1993 (Figure 2A). Concentrations of CHBs in air (Figure 2J) are also similar to the range of values reported from weekly observations between January and June 1992 at Alert (19) and Resolute Bay (10). Atmospheric levels of cisand trans-chlordane along a transect from Norway to Spitzbergen (18); Mould Bay, NT, in June-July 1984 (21); and Resolute Bay in August 1992 (10) are also similar to values that we observed (Figure 2M and 2P). The range of cis/transchlordane ratios in these studies (1.5-2.5) is comparable to the mean value for these isomers (2.2) in our study. Modeled backward trajectories of air parcels in the arctic from 1982 to 1987 indicated that locations in the former USSR and eastern Europe were probable emission sources for atmospheric contaminants (21). Calculations showed 5-day west-to-east trajectories from Alaska and the Beaufort Sea to Alert between August 17 and 21, 1993 (L. Barrie, unpublished data). Industrial emmissions from these regions account for a significant fraction of atmospheric sulfate released into the

Northern Hemisphere (34). In addition, soil particles carried in the atmosphere from Asia are deposited in the arctic (35). The bimodal seasonal pattern of OC air concentrations with peaks in April-May and July at Alert in 1993 could be due to changes in aerosols and particulate matter that arise from fossil fuel combustion and industrial emissions. Generally, aerosols and particulate matter that provide surfaces for scavenging of OCs reach maximum concentrations between March and April (20, 34), as occurred in 1993 (Figure 1B). Atmospheric OCs at Alert increased to their spring maxima between April and May following this period. Gasparticle scavenging would decrease as air temperature increased to seasonal maxima above 0 °C between June and August (Figure 1A) (32, 36). The release of pesticides in gas and particle-bound phases during spring in southern latitudes may contribute to the increased air concentrations in some OCs that we observed during April and May (Figure 2). Supply through long-range atmospheric transport to the sampling site at Alert was evident from ratios of R/γ-HCH, which were lowest (8) during late summer and winter months. The decrease during May may be evidence for delivery of the active γ-isomer (lindane) released during spring plowing. Ratios of these isomers in technical HCH (4-15) are much higher than in lindane (0.2-1.0) (14). Other studies in arctic latitudes have also observed low R/γ-HCH ratios (between 2 and 4) in air (7-9, 18) and snow (17, 28) during spring and early summer that indicated a possible supply of fresh γ-HCH from agricultural use. Higher ratios (>10) may be due to photochemical transformation of γ- to R-HCH or differential air-sea transfer rates (11). The presence of endosulfan-I in arctic air (and seawater discussed below) shows that a compound considered to be of low persistence in southern latitudes, with a short half-life in seawater (7), is more persistent in northern latitudes. Ratios of trans/cis-chlordane in air were also minimum (0.3) between August and October with higher values (0.6) during other months, possibly indicative of air transport from southern latitudes. A similar late summer minimum trans/cis-chlordane ratio (∼0.2) with higher values earlier and later in the year was also observed at Alert in 1992 and in Svalbard, Norway, in 1993 (16, 18). Seasonal Changes of Organochlorines in Seawater. R-HCH was also the most abundant OC in seawater followed by γ-HCH and CHBs (Table 1). Concentrations of HCB and dieldrin, endosulfan-I, and chlordanes were 1-2 orders of magnitude lower. Although similar relative proportions have been reported in previous studies of surface Arctic Ocean water, levels in our study for all OCs, except dieldrin, are 10-50% lower than reported for samples collected prior to 1990 (8, 28) and reflect the general global decline in atmospheric concentrations (5, 6, 10, 11). The seasonal pattern of changes in concentrations of γ- and R-HCH, CHBs, chlordanes, and dieldrin with their winter/spring maximum and late summer minimum contrasted with the fall maximum for endosulfan-I and the relatively constant levels of HCB observed throughout the year. The R/γ-HCH ratios in seawater, while generally similar to those in air, varied over a narrower range (6-9). Ratios were most similar (6-7) (Mann-Whitney U test, n ) 20, p ) 0.009) at 1 and 50 m between January and March when the depth of the mixed layer was maximum (Figure 1D). The pulse of ‘fresh’ lindane observed in air in May was not observed in seawater. Volatilization fluxes relative to inventories of HCHs in surface layer seawater, discussed below, would have to be large for gas exchange to have an impact on the isomeric ratios. However, following ice melt, the summer-fall increase in the isomeric ratio in air also occurred in seawater. Ratio values were significantly lower at 1 m than at 50 m (n ) 14, p ) 0.033) in the summer and fall

months, and the maximum ratio in surface seawater during September (9) was similar to the value in air in August. Ratios for cis/trans-chlordane in air and water were similar (1.24.5, mean 2.7) and did not show any seasonality. For most OCs, there were no significant depth-dependent differences in concentrations for samples collected at 1 and 50 m (n ) 12, p > 0.15) throughout the year (Figure 2). The similarity implies that advection of polar mixed water southward through the archipelago is an important variable controlling seawater concentrations particularily during the period of ice cover. Temperature-salinity measurements between January and June showed that a single water mass type (-1.7 °C, 32-33‰) existed over the surface 50 m, which ressembled polar mixed layer water (37, 38). The absence of a large density gradient would allow mixing throughout the water column. With the onset of melt conditions in June, water at 1 m became progressively warmer (-0.25 to -0.5 °C) and less saline (1 (HCHs, HCB, cis-chlordane, and dieldrin) became even more oversaturated. Volatilization would have been reduced following the formation of ice cover. Since concentrations in air decreased after freeze-up, OCs that were undersaturated in surface seawater approached equilibrium. Compounds that were already oversaturated relative to levels in air continued to increase in concentration and potential for volatilization. Monthly integrals were calculated over three 4-month periods to compare fluxes during periods of ice cover with those during the time of open water (Table 2). With the exception of cis-chlordane and dieldrin, integrals for actual fluxes between June and September represented >60% of the annual total. The strongest seasonality occurred for CHBs and HCB. Integral fluxes between February and May were smaller than values between October and January for most OCs. Air-to-sea exchange could occur during freeze-up between October and December, which would not be possible with more continuous ice cover during the winter. Comparisons of cumulative monthly integrals of air-tosea fluxes with water column inventories in the upper 1 m surface layer and to 50 m depth (Figure 3, lower panels) showed that loss of R- and γ-HCH through volatilization during the ice-free period would have been small (