Snow Amplification of Persistent Organic Pollutants ... - ACS Publications

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Snow Amplification of Persistent Organic Pollutants at Coastal Antarctica Paulo Casal,† Gemma Casas,† Maria Vila-Costa,† Ana Cabrerizo,† Mariana Pizarro,† Begoña Jimeń ez,‡ and Jordi Dachs*,† †

Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Barcelona, Catalonia 08034, Spain Department of Instrumental Analysis and Environmental Chemistry, Institute of Organic Chemistry (IQOG-CSIC), Madrid 28006, Spain

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‡

S Supporting Information *

ABSTRACT: Many legacy and emerging persistent organic pollutants (POPs) have been reported in polar regions, and act as sentinels of global pollution. Maritime Antarctica is recipient of abundant snow precipitation. Snow scavenges air pollutants, and after snow melting, it can induce an unquantified and poorly understood amplification of concentrations of POPs. Air, snow, the fugacity in soils and snow, seawater and plankton were sampled concurrently from late spring to late summer at Livingston Island (Antarctica). Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) concentrations in snow and air were close to equilibrium. POPs in soils showed concentrations close to soil−air equilibrium or net volatilization depending on chemical volatility. Seawater−air fugacity ratios were highly correlated with the product of the snow−air partition coefficient and the Henry’s law constant (KSA H’), a measure of snow amplification of fugacity. Therefore, coastal seawater mirrored the PCB congener profile and increased concentrations in snowmelt due to snowpack releasing POPs to seawater. The influence of snowpack and glacier inputs was further evidenced by the correlation between net volatilization fluxes of PCBs and seawater salinity. A meta-analysis of KSA, estimated as the ratio of POP concentrations in snow and air from previously reported simultaneous field measurements, showed that snow amplification is relevant for diverse families of POPs, independent of their volatility. We claim that the potential impact of atmospheric pollution on aquatic ecosystems has been under-predicted by only considering air−water partitioning, as snow amplification influences, and may even control, the POP occurrence in cold environments.

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INTRODUCTION Persistent organic pollutants (POPs), including polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs), such as hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB), are under international regulation by the Stockholm Convention of the United Nations Environment Programme.1 POPs have raised concern due to their toxicity, bioaccumulation, biomagnification potential, potential for long-range transport (LRT), and their high persistence.2 PCBs and OCPs have often been studied as model compounds to understand the atmospheric transport and deposition, accumulation in organisms, and biogeochemical cycling of POPs in polar regions.3−8 Semivolatile POPs reach remote regions due to their persistence and atmospheric LRT potential. Oceanic transport of POPs to Antarctica is hindered by the Antarctic circumpolar current, which minimizes the north−south exchange of POPs.3 Despite this, POPs have been detected in water masses south of the Antarctic circumpolar current due to atmospheric LRT and deposition.4−7 Local sources of POPs have also been reported from tourism and research stations,9,10 although represent a limited impact due to a low seasonal population. © XXXX American Chemical Society

Wet deposition by snow, due to the effectiveness of snow scavenging of air pollutants,11 can contribute as a unidirectional air to seawater/soil/snow flux of POPs.12 In addition there is an enhanced partitioning of POPs from the atmosphere to seawater, soils, and snow at low temperatures, a process known as “cold-trapping”.13 Remobilization from snow, soils, and seawater to the atmosphere may occur due to climate and seasonal fluctuations.14−16 Glacier retreat has been documented for the Western Antarctic Peninsula,17−19 and both snow melting and glaciers could also be a source of POPs to seawater.7,20,21 Legacy POPs, such as PCBs, HCHs, and HCB have already been reported in the Antarctic atmosphere, seawater, soils, and snow.5,6,14,15,22−25 In terms of depositional processes, previous assessments focused on individual air to surface processes such as air−soil or air−water exchange. Received: Revised: Accepted: Published: A

May 20, 2019 July 2, 2019 July 12, 2019 July 12, 2019 DOI: 10.1021/acs.est.9b03006 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Snow efficiently scavenges atmospheric pollutants due to its large specific surface area.12,20 There is a reduction of fugacity capacity during melting, inducing fugacity amplification.26 The melted snow−air fugacity amplification of POPs previously scavenged by snow from the gas phase is given by,27 fms fa

= KSAH′

(1)

Where f ms and fa are the POP fugacities in melted snow and air, respectively, KSA is the snow−air partition coefficient, and H’ is the temperature corrected dimensionless Henry’s law constant. Even though snow amplification has been identified as a process occurring in cold environments,11,12,27,28 to which degree the observed pattern of concentrations of POPs in seawater reflects the pattern in snowmelt remains unknown. The objectives of this work are (i) to report the largest multicompartment data set for HCHs, HCB, and PCBs at coastal Antarctica (ii) to evaluate the POP air−soil and air− seawater exchanges and their biogeochemical controls, and (iii) to assess the air−snow exchange and the role of snow amplification on the occurrence of POPs in seawater and its implications.

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MATERIALS AND METHODS Site Description and Sampling. The sampling campaign was carried out at Livingston Island, in the South Shetland Archipelago (Figure 1), Antarctica, from December first, 2014 to March first, 2015. During early December, which is late spring in this region, all land was covered with a snowpack of 50−150 cm. Snow melted during the austral summer at locations close to the coast line (snow samples S1−S6, Figure 1), however at inland locations (snow samples S7−S10, Figure 1), snow melting was not complete, with a remainder snow cover during the season. Surface snow samples (n = 10, Figure 1) were collected with a stainless steel shovel and left to melt into Teflon bottles for 24−48 h at 4−6 °C. The melted snow was filtered through precombusted GF/F glass fiber filters (142 mm diameter, Whatmann 0.7 μm mesh size) before passing through a precleaned XAD-2 (25 g, Supelco) packed stainless steel columns. This sampling was performed at a flow-rate of 100 mL/min. The filtration and extraction of melted snow was performed outdoors in order to maintain environmental temperatures and avoid contamination from indoor air. The XAD-2 columns were stored at 4 °C for refrigerated transport until further analysis. Twenty-six seawater and twenty-six plankton samples were collected at two sampling sites: Johnsons (62° 39,556′S, 60° 22,132′W, 14m depth) and Raquelias (62° 39,438′S, 60° 23,306′W, 30 m depth) (Figure 1). CTD (conductivity, temperature, density and other variables such as fluorescence, radiation and turbidity) depth profiles were taken before sample collection over the sampling campaign (Supporting Information (SI) Table S1). Surface seawater samples (100− 120 L, 1 m depth) were collected in 20 L aluminum cans and transported to the Juan Carlos I station (JC1) where they were further processed. Seawater samples followed the same filtration and extraction method as for snow samples, also carried out outdoors. Stainless steel adsorbent columns for seawater contained 50 g of XAD-2, and sampling was performed at a flow rate of 100−150 mL/min. Plankton samples were collected by vertical hauls with a conical

Figure 1. Sampling sites at Livingston Island (upper panel) and schematics of POP dynamics at coastal Antarctica (lower panel). Five fugacity samplers were deployed to collect gas phase and gas phase equilibrated with snow/soil at F1−F5. Seawater and plankton samples were taken at Raquelias site (62° 39,438’ S, 60° 23,306’ W) and Johnsons site (62° 39,556’ S, 60° 22,132’ W). Snow samples were collected at S1−S10. At the lower panel, fa, f w, fs, and fsm are the POP fugacity in air, seawater, snow, and melted snow, respectively. KSA and H’ are the snow−air partition coefficient and dimensionless Henry’s law constant, respectively, and KSAH’ provides a measure of the potential for snow amplification.

plankton net with a 50 μm mesh from 14 and 40 m depth to surface at Johnson and Raquelias sampling stations. Plankton samples were filtered with precombusted and preweighted GF/D glass fiber filters (Whatmann 2.7 μm mesh size, 47 mm diameter), subsequently wrapped in precombusted aluminum foil, and stored at −20 °C in airtight plastic bags. Plankton biomass was estimated by weight differences of the filters after being freeze-dried. Five soil/snow fugacity samplers28,29 were deployed during the sampling campaign (Figure 1) to collect gas phase samples from ambient air and ambient air equilibrated in situ with the soil/snow surface (51 pairs of samples). These two simultaneous samples were taken on a weekly basis (SI Table S2), and allowed to estimate the POP concentration in the gas phase and the POP fugacity in ambient air and in soil/ snow, and thus the direction of the air-snow/soil exchange. The five samplers were deployed at different altitudes (10, 70, 125, 300 m above sea level) and different distances from the research station (Figure 1). The average sampled air volume was 86 m3. Both the ambient air and the air equilibrated with the soil/snow were prefiltered through precombusted glass fiber filters (GF/F, 47 mm diameter, Whatmann) to remove dust particles, and subsequently through precleaned polyurethane foam plugs (PUFs) in which the gas phase compounds were retained. PUF cartridges were 10 cm long and 2 cm diameter. After sample collection, PUFs were stored at −20 °C in airtight precombusted glass vials. B

DOI: 10.1021/acs.est.9b03006 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Analytical Procedures. Snow and seawater samples collected on columns containing 25 and 50 g of XAD-2, respectively, were eluted with 200 mL of methanol, 200 mL of dichloromethane and 100 mL of hexane at a flow rate of 2 mL min−1 using an axial piston pump. The methanol fraction was concentrated and then followed three consecutive liquid− liquid extractions with 50 mL of hexane as reported elsewhere.28 Extracts were filtered through anhydrous sodium sulfate and combined with the previous dichloromethane and hexane fractions. After reducing the volume, the extracts followed the fractionation method reported elsewhere.28 Concentrations in snow are referred to melted-snow (water) volumes. Plankton samples were Soxhlet extracted with dichlorometane:hexane (1:1, v:v) for 24 h, and followed a previously described fractionation method.28 Briefly, extracts were fractionated on a 5 g of silica gel (silica 60, 200 mesh, activated at 250 °C for 24 h) and 3 g of 3% deactivated neutral alumina (aluminum oxide 90, activated at 250 °C for 12 h) column, with 25 mL of hexane and 40 mL of dichlorometane:hexane (1:3, v:v). The hexane fraction, which contained the PCBs and organochlorine pesticides (OCPs), was concentrated in iso-octane and transferred to an injection vial with a final volume of 100 μL. PUFs were Soxhlet extracted with acetone:hexane (3:1, v:v) during 24 h. PUFs fractionation details can be found elsewhere.14,28 Hexachlorobenzene (HCB), hexachlorocyclohexane HCH isomers (α-HCH, β-HCH, γ-HCH, δ-HCH), and PCB congeners were analyzed by a gas chromatograph equipped with a μ-electron capture detector (GC-μ-ECD, Agilent Technologies, model 7890N) with a 60 m (0.25 mm i.d × 0.25 μm film thickness) DB-5 capillary column. The instrument was operated in splitless mode (closed for 1.5 min) and the oven temperature program started at 90−190 °C at 15 °C/min, then to 203 °C at 3 °C/min (held for 5 min), to 290 °C at 3 °C/min, and finally to 310 °C at 5 °C/min (held for 10 min). Injector and detector temperatures were 280 and 320 °C, respectively. Helium and nitrogen were used as carrier (1.5 mL/min) and makeup (60 mL/min) gases, respectively, and 2 μL of sample were injected. The following PCB congeners were analyzed: tri-PCB 18, 17, 31, 28, 33; tetra-PCB 44, 49, 52, 70, 74; penta-PCB 87, 95, 99/101, 105, 110, 118; hexa-PCB, 128, 132, 138, 149, 151, 153, 156, 158, 169; heptaPCB, 170, 171, 177, 180, 183, 187, 191; octa-PCB 194, 195, 201/199, 205; nona-PCB 206, 208; deca-PCB 209. The confirmation of the identification of major PCB congeners was done by high resolution mass spectrometry (GC-Q-ExactiveMS). Nutrients (phosphate, nitrate plus nitrite, and ammonia) and bacterial abundance were sampled and analyzed as described elsewhere,28 and used here when needed. Quality Assurance/Quality Control. All recipients, tubes and connections used from the sampling to the chemical analysis were made of stainless steel, glass or PTFE. These were precleaned with acetone prior use in order to avoid contamination. All filters were precombusted at 450 °C over 4 h. XAD-2 was Soxhlet extracted in methanol:dichlorometane (1:1) before packed in columns. Before use, the XAD columns were pre-eluted with methanol, dichloromethane and hexane and the extracts were concentrated and run by GC-μ-ECD to check for potential blank contaminations. XAD-2 columns were kept in methanol until their use in the field. PUFs were

precleaned with acetone/hexane (3:1, v:v) over 24 h, the solvent was eliminated under vacuum in a desiccator, and the PUF stored in airtight precombusted glass jars. Field blanks consisted of GF/D filters, PUFs and XAD-2 columns that followed the same process as the samples albeit without the pass of plankton, air, snow or seawater. Procedural and/or field blank were analyzed with each batch of 4−6 samples to monitor potential contamination during sampling and extraction. The limits of quantification (LOQs) were defined as the mean concentration of field or procedural blanks (the highest) plus three times the standard deviation of the blank response. For the analytes not detected in procedural blanks, LOQ were derived from the lowest standard in the calibration curve. Surrogate recoveries for the different types of samples are given in SI Table S3.

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RESULTS AND DISCUSSION Soil−Air Exchange of PCBs and HCB. Atmospheric concentrations of individual PCBs (SI Figure S1, Table S4) and ∑41PCBs (41 ± 24 pg m−3) were within the range of concentrations previously reported for the Antarctic atmosphere.5,10,14,30−36 There were no significant differences in the atmospheric PCB concentrations between sampling stations at different altitude, and at different distance to the research station (see Figure 1 for sampling locations). Gas phase HCB concentrations (11 ± 3.5 pg m−3) were also comparable to previous studies in the Antarctic region (SI Table S4).6,24,37,38 ∑4HCHs were detected in less than 10% of air samples, with concentrations ranging from < LOQ to 22 pg m−3 (SI Table S4). This low detection frequency is consistent with the long-term decline of HCHs concentrations in Antarctic air over the last decades6,23,24,30,37,39 following the reduction of HCHs primary sources.40 Due to the low detection frequency of HCHs in air samples, these compounds were not included in the assessment of air-soil/snow/seawater exchange, even though HCHs were quantified for snow, seawater and plankton samples (see below). PCB and OCP atmospheric concentrations were generally not correlated with air temperature (T). Only PCB-149 and PCB-187 showed a significant correlation with ambient temperature (SI Figure S2), though explained a low percentage of the variability (56% and 38%, respectively). A lack of temperature dependence of atmospheric concentrations has been described previously for the maritime atmosphere of the southern ocean5 and elsewhere.41,42 Gas phase concentrations not correlated with ambient temperatures are consistent with a lack of a significant local source. In order to establish the potential of soils as local secondary sources to ambient air, the fugacity in air (fa, Pa) and in surface (soil or snow, fs, Pa) were calculated for HCB and the individual PCB congeners as fa = 10−12CART /MW

(2)

fs = 10−12CSART /MW

(3)

where CA is the measured ambient air concentration at 1.5 m height (pg m−3), R is the gas constant (8.314 Pa m3 mol−1 K−1), MW is the chemical’s molecular weight (g mol−1), and CSA (pg m−3) is the gas phase concentration that has been equilibrated with the soil (or snow) surface (SI Table S5) as measured using the fugacity sampler. C


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Environmental Science & Technology CSA PCB concentrations and profiles were similar to those of CA (SI Table S5). Gas phase concentrations equilibrated with the soil of PCB-149 and PCB-187 also showed significant correlations with air temperature (SI Figure S2). There was a significant least-squares linear regression of Log fs versus Log fa for PCBs (Figure 2), consistent with a close coupling of PCBs between the soil and the atmosphere. This close coupling has been described previously for polar and temperate regions.14,43,44 The net direction of air-soil exchange of PCBs and HCB was evaluated by comparison of fa and fs. Where fs/fa ratios higher than 1 indicate net volatilization, while net deposition occurs when fs/fa ratios are lower than 1. Due to the uncertainty of these measurements,12,27 values of Ln fs/fa between −1.2 and 0.53 indicate concentrations too close to equilibrium to discern a significant net volatilization or deposition. PCBs Ln fs/fa ratios and the octanol−air partition coefficient (Log KOA) were significantly correlated (Figure S3), even though this least-squares linear regression explained only 10% of the variability. For PCBs with Log KOA below 10.5, Ln fs/fa ratios ranged between −1.02 and 4.1 (SI Figure S3, Table S6), with more than 57% of ratios indicating net-volatilization. On the other hand, PCB congeners with higher hydrophobicity generally showed air−soil equilibrium to net deposition. No significant differences were observed between sampling sites with bare soil or soil with vegetation (SI Figure S4). For HCB, Ln fs/fa ratios were not correlated with Log KOA (SI Figure S5) probably due to the narrow temperature range; however, HCB showed significant differences between sampling sites. While Ln fs/fa ratios in bare soils indicated concentrations close to air−soil equilibrium, there was a netvolatilization from soil with a vegetation cover. Snow−Air Partitioning of PCBs and OCPs. Snow ∑41PCBs concentrations (190 ± 100 pg L−1, SI Figure S1, Table S7) and the predominance of low MW congeners, mostly tetra and penta- chlorinated congeners (39 ± 10% and 35 ± 7.1%, respectively, of ∑41PCBs), were similar to those reported in previous studies from the Antarctic Peninsula,15,45 and Northern Victoria Land.46 Average snow concentrations for HCB and ∑4HCHs were of 8.4 ± 3.5 pg L−1 and 18 ± 10 pg L−1, respectively (SI Table S7). These concentrations of OCPs in snow are also in agreement with recent studies,7,15,45,46 but 2−3 orders of magnitude lower than those reported in field assessments performed between 1960 and 1980.30 Changes in snow properties, particularly the increase in snow density during snow aging after the snow deposition event, have been described as a descriptor of PCBs and OCPs snowpack concentrations.47,48 Snow density is a useful surrogate for snow surface area which is directly related to the sorption capacity of snow, but which is much more difficult to measure. Even though snow samples S1−S10 were taken at different sampling sites and the snow deposition events occurred over the austral summer, ∑41PCBs and ∑5OCPs surface snow concentrations showed significant inverse correlations with snow density (Figure 3). These correlations were also observed for individual PCB congeners and OCPs (SI Table S8). These results are consistent with a previous study performed in the Antarctic Peninsula,15 where the sample with lowest snow density presented significantly higher concentrations for all POPs. Furthermore, ∑ 5 OCPs (∑4HCHs + HCB) and 14 of the target PCBs in that study also showed significant correlations between snow concen-

Figure 2. PCB fugacity in air (fa) versus fugacity in soil/snow (fs).

trations and snow density (SI Table S8), even though these correlations were not reported in the original publication.15 Snow scavenges gas and particle phase compounds during deposition,12 resulting in high concentrations in snow11,12,26 D


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Net diffusive snow−air exchange fluxes ranged from a net deposition of 9.0 pg m−2 d−1 to a net volatilization of 37 pg m−2 d−1 for ∑41PCBs (SI Figure S6, Table S9), and from a net deposition of 16 pg m−2 d−1 to a net volatilization of 4 pg m−2 d−1 for HCB (SI Table S9). The variability in the fluxes is consistent with the variability of fs and fa (Figure 2). These results highlight the difficulties in establishing a clear volatilization/deposition pattern between the snow and the atmosphere, as POPs concentrations are rapidly changing with changing snow properties during aging, in addition to varying air concentrations. These diffusive snow−air fluxes cannot induce a fast variation (within few days) of the POP concentration in bulk snow or in the overlying air. Therefore, most PCBs and OCPs are not released from the snowpack due to volatilization, but are instead mobilized during snowmelting. Snowmelt mobilizes POPs to soils and seawater, but because soil organic matter is low in Antarctic soils, most POPs are transferred to coastal seawaters. At inland areas, not all snow may melt, and snow becomes glacial ice. Livingston Island’s South Bay is also the recipient of important inputs of glacial ice, including Johnson’s glacier which is near one of the seawater sampling sites (Figure 1). PCBs, HCHs, and HCB in Coastal Seawater during the Austral Summer. Average ∑41PCBs concentrations in seawater and plankton were 59 ± 26 pg L−1 and 26 ± 29 ng gdw−1, respectively (SI Tables S10−11). Seawater and plankton PCBs concentrations reported here are comparable with previous studies in the Southern Ocean and Antarctic coastal sites.5,25,52,53 OCPs were detected in all seawater and plankton samples (SI Tables S10−11). Average ∑4HCHs concentrations in seawater (1.3 ± 1.6 pg L−1) were comparable to those reported in recent studies,6,24,34,54,55 but lower than those reported in the 80s.30 Plankton phase ∑4HCHs concentrations (1.5 ± 0.7 ng gdw−1) were also within the range of previous reports in Antarctic plankton.6,56 PCBs and OCPs concentrations in the dissolved and plankton phases at the Raquelias sampling site were not significantly different than at the Johnsons sampling site. Furthermore, there were no significant correlations of PCBs or OCPs concentrations with CTD ancillary data (temperature, salinity, photosynthetic active radiation, fluorescence, turbidity). This can be either due to the narrow range in temperature or salinity, and the covariability of these with other biogeochemical and physical controls. Inputs of POPs to coastal seawater due to glaciers or snow melting can be dissipated by volatilization, dilution, settling of particulate organic carbon (biological pump), microbial degradation, or photodegradation. Biotic and abiotic degradation are potential sinks of chemicals such as HCHs in the water column, as suggested previously for the Arctic and open Southern Ocean.6,57 Concentrations of α-HCH in plankton were inversely correlated with plankton biomass (r2 = 0.64, p < 0.001). Such inverse correlation has been observed in other oceans and attributed to biodegradation of HCHs in the water column.6,58 A multiparametric regression analysis of the concentrations of HCHs versus environmental variables, showed that the bacterial abundance of the previous time period interacting with seawater temperature explained part of the variability of the concentrations of α-HCH in seawater (r2 = 0.22, p < 0.05) and plankton (r2 = 0.25, p < 0.05). Bacterial abundance was correlated with ammonia concentrations (r2 = 0.47, p < 0.01),

Figure 3. Least squares linear regressions of ∑41PCBs (upper panel) and ∑5OCPs (lower panel) snow concentrations (pg L−1) versus snow density (kg L−1).

The dependence of snow concentrations (which are given in melted-snow volumes) with snow density is consistent with snow amplifying the concentrations due to the high specific surface area of fresh snow (that typically possesses the lowest snow density).47,48 Snow aging, and snow-melting, drive a leaching of POPs to water and soils, as well as snow-air gaseous exchange. Fugacity of PCBs in snow was significantly correlated to their fugacity in air (Figure 2), consistent with previous reports,12,26 and with a close coupling of the snow and air compartments. The Ln fs/fa values for PCBs and HCB varied for the different sampling events and did not show a significant correlation with Log KOA (SI Figure S3, Table S6), nor with vapor pressure (results not shown). Overall, PCBs were close to air-snow equilibrium, with a variability consistent with the correlations between PCB and OCP snow concentrations and snow density (Figure 3), but presumably also dependent on other unidentified variables and processes. The fugacity sampler integrated 7 days for each sample, which included highly variable snow deposition events over the course of each sampling event. The snow-air partition coefficient (KSA) was estimated from the concentrations in snow and air equilibrated with snow ( CS ). The mean for Log KSA was 3.5, and Log KSA ranged CSA

between 1.6 and 5.0. These values are not significantly different, but in the lower range of those reported for snow in the North America’s Great Lakes region,49 and recently in the Antarctic Peninsula.15 The snow-air diffusive exchange fluxes (Fsnow−air) were calculated by

ij C yz Fsnow − air = vjjj S − CA zzz j KSA z (4) k { −1 where v is the exchange velocity (m d ), calculated following the Whitman two-layer resistance method (see SI Text S1).50,51 E


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Figure 4. Net diffusive seawater−air fluxes (ng m−2 d−1) of polychlorinated biphenyls (PCBs), and correlation between fluxes and salinity.

temperature or salinity. Air−water fugacity ratios for HCB were close to one (equilibrium) for most sampling periods. These results are consistent with previous reports from the Southern Ocean showing close to air−water equilibrium of HCB.6,22 PCBs gross absorption (FAWabs) and volatilization fluxes (FAWvol) ranged from 0.14 to 6.5 ng m−2 d−1, and from 0.45 to 24 ng m−2 d−1, respectively (SI Table S12−13). The resulting net air−water diffusive flux ranged from a small net deposition of 0.55 ng m−2 d−1 for one sampling event, to a net volatilization of 21 ng m−2 d−1 for a sampling event occurring in late summer (Figure 4, SI Table S14). The observed net volatilization for most sampling events is consistent with runoff of PCBs from land supporting seawater concentrations. A leastsquares linear regression of PCBs FAW versus salinity (Figure 4) showed a significant inverse correlation, consistent with snow-melting and glacial discharge of PCB to seawater driving volatilization fluxes. The net volatilization of PCBs at coastal Antarctica contrasts with the deposition fluxes observed in the open Southern Ocean.5 However, the same study noted that such net deposition did not occur at coastal sites.5 Coastal seawaters, with snow amplified PCB concentrations, are diluted when mixed with open ocean waters. Concentrations in the open ocean are further depleted due to settling fluxes of particle bound PCBs (biological pump).5 An important issue is whether these important melted snow and glacial inputs influence the PCB congener profile in coastal seawater and amplify their seawater−air fugacity gradient following a congener specific pattern. Equation 1 predicts that the fugacity ratio between melted snow and air is proportional to KSAH’, a relationship that seawater receiving melted snow may follow (Figure 1, lower panel). We plotted the average seawater−air fugacity ratios (f w/fa) versus KSAH’ using the average field derived KSA, and temperature corrected H’ values (Figure 5 and SI Figure S7). The seawater−air fugacity ratio was strongly correlated with KSAH’ (r2 = 0.95, p < 0.05) (SI Figure S7), with a strong influence of two congeners (PCB 99 and PCB 138). Such influence on the regression coefficient

and nutrient variability was consistent with an increase of productivity and glacial inputs.28 Such influence of bacterial abundances and plankton biomass on seawater concentrations were not found for the more persistent HCB. This is evidence of microbial degradation of HCHs at coastal Antarctica even though some of the correlations listed above explained a small fraction of the variability. Microbial degradation has been identified as an important sink for other chemicals, as low MW PAHs.28,59 Despite this degradation, the concentrations of these chemicals did not decrease during the austral summer, due to the snowmelt input to seawater. Snow Amplification of Concentrations and Fugacities. Diffusive fluxes between the gas and the dissolved phases have a dominant role in the transfer of PCBs and OCPs between the atmosphere and oceans.5,6,8,58,60 In the open Southern Ocean, net diffusive fluxes of PCBs and OCPs from air to water are orders of magnitude higher than dry deposition fluxes.5,6 As diffusive air−water fluxes tend to dissipate the fugacity gradient, the potential risk for atmospheric organic pollutants to reach aquatic ecosystems is often predicted assuming air−water equilibrium, thus from H’ values. In fact, chemicals with high H’ values have often been considered “flyers”2 with little potential to be deposited to ecosystems, even though this and most previous risk assessments neglected snow deposition. Diffusive air−water fluxes were calculated following a twofilm resistance model: ÄÅ ÉÑ ÅÅ CA Ñ Å FAW = FAWabs − FAWvol = kAWÅÅ − C W ÑÑÑÑ ÅÅÇ H′ ÑÑÖ (5) Where CW is the dissolved phase PCBs and HCB concentrations (ng m−3), and kAW is the air−water mass transfer velocity (m d−1) estimated as explained elsewhere.5,60 HCB gross absorption (FAWabs) and volatilization fluxes (FAWvol) ranged from 0.091 to 0.94 ng m−2 d−1 and from 0.075 to 1.4 ng m−2 d−1, respectively (SI Table S12−13). HCB mean net air−water diffusive flux was 0.0084 ± 0.34 ng m−2 d−1 (SI Table S14). These fluxes showed no correlation with seawater F


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This could also be relevant for other families of organic pollutants. KSAH’ as Indicator of Potential Concentration and Fugacity Amplification by Snow. There is a dearth of field measurements of KSA, and a meta-analysis of previous works reporting KSA for PCBs, PAHs, polybromodiphenyl ethers (PBDEs) and organophosphate esters (OPEs) showed that the average values of these ranged between 10 and 106.6 (SI Figure S8, SI Table S15, Text S2).15,28,49,62,63 KSA values for volatile POPs such as neutral polyfluoroalkyl substances (PFASs) derived from reported concurrent measures of PFASs in snow and air,64,65 show that these ranged between 103.3 and 105.5 (SI Figure S8). Therefore, the sorption capacity of snow is surprisingly unrelated to the chemical volatility, as neutral PFASs and some OPEs have significantly higher H’ than PCBs or PAHs, but similar empirical KSA values. Even for PCBs and PAHs, there is not a consistent correlation between KSA and volatility. The concentrations of POPs in snow determined experimentally may be biased low due to volatilization losses during melting and extraction, especially for the more volatile compounds. Further field measurements of KSA are needed to confirm the lack of dependence of KSA on volatility for these and other POP classes. Figure 6 shows the KSAH’ values (fugacity amplification potential by snow) for PCBs, PAHs, PBDEs, OPEs, and neutral PFASs from the field derived KSA reported here and in previous works.15,28,49,62−65 Despite an important variability in the magnitude, there is a fugacity amplification for PCBs, PAHs, PFASs, and OPEs due to snow melting, which is maximum for the more volatile neutral PFASs. From the only study that reported KSA for PBDEs, it seems that these do not amplify due to snow melting. The data set of KSA generated from this work shows that the more hydrophobic (higher chlorination) PCBs that could be quantified did not amplify in melted snow, consistent with their lower f w/fa ratios in seawater (Figures 5 and 6), and also lower fs/fa ratios at land (SI Figure S3). Unfortunately, there are no field or laboratory measurements of KSA for many legacy and emerging POPs previously reported in Antarctica,7,66−70 other cold environments, and the virtual totality of chemicals currently in commercial use. Snow scavenging of aerosol-bound POPs will increase the snow amplification potential discussed here, as KSAH’ indicates only the contaminant amplification potential due to scavenging of gas-phase organic pollutants. Scavenging of aerosol phase organic pollutants is the mechanism explaining the occurrence in snow of chemicals associated with carbonaceous or marine aerosols, such as high MW PAH28 and perfluoroalkyl carboxylic acids,71 respectively. Further research is needed on the influence of gas-particle partitioning on concentrations of POPs in snow. In Antarctica, there is a regional variability of the amount of snow precipitation. For example, snow accumulation is larger for the maritime Antarctica72 than for continental Antarctica, which is a dry environment. In addition, it has been reported that snow events have increased during the Anthropocene,73 probably related to climate change, which may be impacting the redistribution and occurrence of POPs, an issue that will require future research. The ultimate potential for a given POP to impact cold environments can be predicted from their KSAH’ values, but such approach is not considered in current risk assessments of chemicals. This has been a consequence of the use of models, which have been shown to under-predict KSA values,27 and the

Figure 5. Snow amplification of the fugacity gradient between seawater and air. Regression of seawater−air fugacity ratio ( f w/fa) versus KSAH’ for PCBs. KSAH’ is a measure of snow amplification of concentrations and fugacity. SI Figure S7 shows the results in nonlog−log scale.

was eliminated by correlating log f w/fa versus log KSAH’ (Figure 5), and confirmed that this trend was maintained for a wide range of congener specific fugacity ratios (r2 = 0.58, p < 0.05). Therefore, seawater−air fugacity ratios of PCBs follow the pattern predicted for melted snow. This suggests a similar pattern of PCB concentrations in seawater than in melted snow. This further confirms that seawater PCBs quantitatively mirrored melted snow. The congener pattern of PCBs in seawater was correlated with that observed in plankton (r2 = 0.60, p < 0.05), such qualitatively similar patterns suggests that the influence of snow inputs was further transferred to the base of the food web. A comparison among PAHs and PCBs allows identifying differences between non-persistent and persistent chemicals. The water−air fugacity ratios for PAHs for the same campaign28 were also correlated with their field derived KSAH’ values (r2 = 0.38, p < 0.05) (SI Figure S6). The lower percentage of variability of the fugacity ratio explained by KSAH’ in the case of PAHs, when compared to PCBs, is consistent with the fact that PAHs undergo microbial and photodegradation in the water column,28,59 thus dissipating the snowmelt fingerprint and amplification. The ratio of methyland dimethyl phenanthrenes to parent phenanthrene was of 4.5 in seawater, and of 1.4 in snow. Microbial degradation is faster for parent PAHs than alkylated PAHs,61 therefore, a higher ratio of alkylated PAHs to parent PAH in seawater than in snow is consistent with microbial degradation being a key sink of PAHs in seawater. Alkylated PAHs showed higher f w/fa values, closer to the values predicted for melted snow, than parent PAHs (SI Figure S7). As far as we know, this is the first study that demonstrates that the pattern and amplification of fugacity in seawater, and for a large water body such as Livingston Island’s South Bay, is strongly related to snow amplification. This has important implications, because it shows that in cold environments with important snow precipitation, the concentrations in water are not governed by the magnitude of H’ (diffusive air−water exchange), but by the magnitude of KSAH’ (snow deposition). G


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Figure 6. Predicted snow amplification of the fugacity ratio between melted snow and air as given by the snow−air partition coefficient and the dimension-less Henry’s Law constant (f ms/fa = log KSAH’) for various families of organic pollutants. The results shown are the mean and standard deviation of log KSAH’ (see SI Text S2, Table S15, and Figure S8 for details).

scarcity of field measures of KSA. With the exception of the pioneering work by Franz and Eisenreich in 1997,49 concurrent measures of POP concentrations in snow and air have been reported only in recent years (SI Figure S8). The potential dissolved concentrations of semivolatile organic pollutants due to atmospheric deposition are often estimated from H’ and gas phase concentrations, but this leads to an underestimation of the seawater concentrations by several orders of magnitude depending on the chemical volatility (Figure 6). We claim that

an important effort should be undertaken in determining field derived KSA values and better H’ estimates for a wide range of chemicals and different environmental conditions, for chemicals with diverse dependence on primary and secondary sources, as well as detailed study of the impact of snow amplification controlling the occurrence of POPs in cold environments, and how this affects risk assessment and international regulation for the use of synthetic chemicals. H


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(10) Larsson, P.; Järnmark, C.; Södergren, A. PCBs and chlorinated pesticides in the atmosphere and aquatic organisms of Ross Island, Antarctica. Mar. Pollut. Bull. 1992, 25, 281−287. (11) Grannas, A. M.; Bogdal, C.; Hageman, K. J.; Halsall, C.; Harner, T.; Hung, H.; Kallenborn, R.; Klan, P.; Klanova, J.; Macdonald, R. W.; Meyer, T.; Wania, F. The role of the global cryosphere in the fate of organic contaminants. Atmos. Chem. Phys. 2013, 13, 3271−3305. (12) Franz, T. P., Gregor, D. J.; Eisenreich, S. J. Snow deposition of atmospheric semivolatile organic chemicals. In Atmospheric Deposition of Contaminants to the Great Lakes and Coastal Waters; Baker, J.E., Ed.; SETAC Technical Publications Series, 1997; pp 73−107. (13) Wania, F.; Mackay, D. Tracking the distribution of persistent organic pollutants. Environ. Sci. Technol. 1996, 30, 390A−397A. (14) Cabrerizo, A.; Dachs, J.; Barceló, D.; Jones, K. C. Climatic and biogeochemical controls on the remobilization and reservoirs of persistent organic pollutants in Antarctica. Environ. Sci. Technol. 2013, 47, 4299−4306. (15) Khairy, M. A.; Luek, J. L.; Dickhut, R.; Lohmann, R. Levels, sources and chemical fate of persistent organic pollutants in the atmosphere and snow along the western Antarctic Peninsula. Environ. Pollut. 2016, 216, 304−313. (16) Ma, J.; Hung, H.; Tian, C.; Kallenborn, R. Revolatilization of persistent organic pollutants in the Arctic induced by climate change. Nat. Clim. Change 2011, 1, 255−260. (17) Chapman, W. L.; Walsh, J. E. A synthesis of Antarctic temperatures. J. Clim. 2007, 20, 4096−4117. (18) Cook, A. J.; Fox, A. J.; Vauhan, D. G.; Ferrigno, J. G. Retreating glacier fronts on the Antarctic Peninsula over the past half century. Science 2005, 308, 541−544. (19) Turner, J.; Colwell, S. R.; Marshall, G. J.; Lachlan-Cope, T. A.; Carleton, A. M.; Jones, P. D.; Lagun, V.; Reid, P. A.; Iagovkina, S. Antarctic climate change during the last 50 years. Int. J. Climatol. 2005, 25, 279−294. (20) Burniston, D. A.; Strachan, W. J. M.; Hoff, J. T.; Wania, F. Changes in surface area and concentrations of semivolatile organic contaminants in aging snow. Environ. Sci. Technol. 2007, 41, 4932− 4937. (21) Geisz, H. N.; Dickhut, R. M.; Cochran, M. A.; Fraser, W. R.; Ducklow, H. W. Melting glaciers: a probable source of DDT to the antarctic marine ecosystem. Environ. Sci. Technol. 2008, 42, 3958− 3962. (22) Cabrerizo, A.; Dachs, J.; Barceló, D.; Jones, K. C. Influence of organic matter content and human activities on the occurrence of organic pollutants in Antarctic soils, lichens, grass, and mosses. Environ. Sci. Technol. 2012, 46, 1396−1405. (23) Bidleman, T. F.; Walla, M. D.; Roura, R.; Carr, E.; Schimidt, S. Organochlorine pesticides in the atmosphere of the Southern Ocean and Antarctica, January-March 1990. Mar. Pollut. Bull. 1993, 26, 258− 262. (24) Cincinelli, A.; Martellini, T.; Del Bubba, M.; Lepri, L.; Corsolini, S.; Borghesi, N.; King, M. D.; Dickhut, R. M. Organochlorine pesticide air-water exchange and bioconcentration in krill in the Ross sea. Environ. Pollut. 2009, 157, 2153−2158. (25) Fuoco, R.; Giannarelli, S.; Wei, Y.; Ceccarini, A.; Abete, C.; Francesconi, S.; Termine, M. Persistent organic pollutants (POPs) at Ross sea (Antarctica). Microchem. J. 2009, 92, 44−48. (26) Macdonald, R.; Mackay, D.; Hickie, B. Contaminant Amplification in the Environment. Environ. Sci. Technol. 2002, 36, 456A−462A. (27) Sanchís, J.; Cabrerizo, A.; Galbán-Malagón, C.; Barcelo, D.; Farré, M.; Dachs, J. Response to comments to “Unexpected occurrence of volatile dimethylsiloxanes in Antarctic soils, vegetation, phytoplankton and krill. Environ. Sci. Technol. 2015, 49, 7510−7512. (28) Casal, P.; Cabrerizo, A.; Vila-Costa, M.; Pizarro, M.; Jiménez, B.; Dachs, J. Pivotal role of snow deposition and melting driving fluxes of polycyclic aromatic hydrocarbons at coastal Livingston Island (Antarctica). Environ. Sci. Technol. 2018, 52, 12327−12337.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b03006. Tables S1−S15 and Figures S1−S8; (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maria Vila-Costa: 0000-0003-1730-8418 Jordi Dachs: 0000-0002-4237-169X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the staff of the Marine Technology UNIT (UTMCSIC) for their logistical support during the sampling campaign at Livingston Island. This work was supported by Spanish Ministry of science to P.C. through a predoctoral fellowship, European Commission to A.C. through a Marie Curie international outgoing fellowship, and by the Spanish MINECO through projects REMARCA (CTM2012-34673), SENTINEL (CTM2015-70535-P), and ISOMICS (CTM2015-65691-R). This research is part of POLARCSIC activities. The research group of Global Change and Genomic Biogeochemistry receives support from the Catalan Government (2017SGR800).

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