Combining Long-Range Transport and Bioaccumulation

Apr 23, 2008 - The identification of potential Arctic contaminants requires an assessment of ... Organic Pollutants to the Arctic in a Changing Global...
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Environ. Sci. Technol. 2008, 42, 3704–3709

Combining Long-Range Transport and Bioaccumulation Considerations to Identify Potential Arctic Contaminants G E R T J E C Z U B , * ,† F R A N K W A N I A , ‡ A N D MICHAEL S. MCLACHLAN† Department of Applied Environmental Science, Stockholm University, S-106 91 Stockholm, Sweden, and Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada M1C 1A4

Received November 14, 2007. Revised manuscript received February 19, 2008. Accepted February 21, 2008.

The identification of potential Arctic contaminants requires an assessment of both the long-range transport and the bioaccumulation of the chemicals, most particularly in the indigenous inhabitants of the Arctic. For this purpose, a nonsteady state, zonally averaged global distribution model was linked to a nonsteady state bioaccumulation model describing Inuit exposure from a marine diet. The potential of hypothetical, perfectly persistent chemicals with varying combinations of partitioning properties to enrich in the Arctic environment following emission in the lower latitudes and, additionally, to bioaccumulate in the Arctic food chains was evaluated using the Arctic contamination and bioaccumulation potential (AC-BAP). The ACBAP is defined as the quotient of the human body burden of the chemical and the quantity of chemical cumulatively emitted to the global environment. The highest AC-BAP values (up to 3.7 × 10-11 person-1) were obtained for hypothetical multimedia chemicals with intermediate volatility and hydrophobicity. Perfectly persistent chemicals with 3.5 < log KOW < 8.5 and log KOA > 6 had AC-BAP values of at least 10% of the maximum value, indicating that a broad range of chemicals are potential Arctic contaminants if they are persistent. Moreover, the simulation results suggest that a chemical’s potential to bioaccumulate has a stronger impact on the overall potential to become an Arctic contaminant in humans than its potential for long-range transport. This modeling exercise demonstrates how linking nonsteady state models of chemical bioaccumulation and of global chemical fate can provide a valuable tool for assessing a chemical’s potential to be a contaminant in remote regions.

Introduction Since the first observation of high concentrations of polychlorinated biphenyls (PCBs) in breast milk samples from the Canadian Arctic in the late 1980s (1), high exposure of Arctic indigenous people to persistent organic pollutants (POPs) has been confirmed in a variety of studies (2, 3). The diet of indigenous human subpopulations of the Arctic, which includes local foods derived from marine carnivores, makes * Corresponding author e-mail: [email protected]. † Stockholm University. ‡ University of Toronto Scarborough. 3704

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them exceptionally vulnerable to environmental contamination with POPs. In fact, a recent exercise comparing the potential of different human food chains to accumulate organic contaminants came to the conclusion that an Inuit living on a traditional diet may accumulate organic contaminants more than a thousand times more effectively from the environment than a typical resident of temperate regions (4). The study concluded that “the consequence of the enhanced bioaccumulation potential in the Arctic is that critical levels of human exposure will result from much lower levels of input of persistent chemicals.” As a consequence, the identification of the properties of organic contaminants with the ability to reach the remote Arctic and to bioaccumulate through food chains up to humans is a high priority in chemical risk assessment and management. In general, these properties are (i) persistence in air or water to survive long-range transport, (ii) semivolatility allowing the chemical to partition into and be transported with the air phase, but on the other hand also be deposited again in the Arctic, (iii) susceptibility to bioaccumulation, and finally (iv) resistance to metabolism in the food chain. For the identification and assessment of the governing properties, two concepts have been introduced: the Arctic contamination potential (ACP) (5, 6), and the environmental bioaccumulation potential (EBAP) (7). The ACP quantifies a chemical’s enrichment in the Arctic physical environment. It is the ratio of the contaminant’s quantity in the Arctic surface compartments (soil, water, and sediment) and the quantity emitted cumulatively to the global environment (6). The EBAP is a measure for the potential of a completely persistent contaminant to bioaccumulate in humans on the basis of the environmental concentrations (7). It is the quotient of the human body burden (g person-1) and the quantity of chemical in the environment of the region that the human is living in, normalized to the surface area of that environment (g m-2). The EBAP thus has a unit of m2 per person, i.e., it describes the area containing the same amount of chemical as has been accumulated in one person. In the current study, we sought to develop a single metric that encompasses all of the properties that determine an organic contaminant’s ability to reach the remote Arctic and bioaccumulate up to humans. This metric should combine the information that the concepts of ACP and EBAP convey. Specifically, we combined the zonally averaged global transport model Globo-POP, which has been used in previous ACP calculations (5, 6), with the human food-chain bioaccumulation model ACC-HUMAN (8), an Arctic version of which has been used to calculate the EBAP of Inuit. This model combination was used to relate the emission of hypothetical persistent organic chemicals, which is assumed to occur over an extended period of time in the densely populated regions of the globe, to the body burden that these chemicals establish in a woman living in the Arctic. The rationale was to identify physical-chemical properties enabling a completely persistent chemical to accumulate both in the physical environment and the human food chain of the Arctic, thus providing an integrated measure of the potential for human exposure in the Arctic to a chemical used in lower latitudes.

Material and Methods Global Distribution Model Globo-POP. Chemical enrichment in the Arctic environment as a result of global emission was calculated with Globo-POP, a dynamic, zonally averaged global distribution model (9). Globo-POP is subdivided into 10 interconnected multimedia box models, each representing 10.1021/es7028679 CCC: $40.75

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a well-mixed latitudinal band of homogeneous climatic characteristics. Each of these submodels consists of four air, two water, one sediment, and two soil compartments. Chemical transport between the interconnected submodels is described two-dimensionally in the atmosphere and onedimensionally in the surface ocean. A detailed model description is given in refs 9 and 10. Globo-POP has been evaluated for R-hexachlorcyclohexane (R-HCH) (11) and has been used in earlier studies for simulations of the global fate of polychlorinated biphenyls (PCBs) as well as for ACP calculations including a detailed sensitivity analysis (5, 6, 10, 12). In this study, time-variant fugacities of 223 hypothetical perfectly persistent chemicals in Arctic air, water, soil, and sediment were calculated over a 70 year time period. The emission of all hypothetical chemicals to the global environment was assumed to occur at a steady rate of 100 t per year into the atmosphere. As was assumed during previous ACP calculations (5, 6), the zonal distribution of the emissions matched that of the global population. The ACP was calculated as previously, namely as the fraction of the cumulative global emission present in Arctic soil, sediment, and water at a given time point. Bioaccumulation Model ACC-HUMAN. Chemical transfer from Arctic air and water through the marine food chain to humans was calculated with a modified version of ACCHUMAN, a mechanistically based, nonsteady state bioaccumulation model (8). ACC-HUMAN consists of a series of linked submodels, each representing a different organism or organism group along the food chains that are the key vectors of human exposure. A chemical mass balance is conducted for each organism, including humans of both sexes, taking into account chemical exposure from both the surrounding environment and food (i.e., from other organisms in the model) as well as chemical elimination via a variety of mechanisms including egestion, lactation, and metabolism. ACC-HUMAN has been evaluated for PCBs in southern Sweden (8) and has been used in earlier studies for EBAP calculations (4, 7). For this study, ACC-HUMAN was modified to reflect the key vectors of human exposure to persistent organic contaminants in the Arctic environment. Seal, and in particular seal blubber, is one of the main vectors of exposure to persistent, lipophilic organic pollutants for Arctic indigenous people living on a traditional diet (13). Therefore, a marine mammal module based on the model presented in ref 14 was added to ACC-HUMAN. The model was parametrized for the food web air/waterszooplankton and amphipods–polar cod (Boreogadus saida)sringed seal (Phoca hispida)sInuit. A detailed description of the modifications of ACC-HUMAN and the model parametrization is given in the Supporting Information. The calculated time variant fugacities of the hypothetical persistent chemicals in air and ocean water calculated with Globo-POP were used as input into the bioaccumulation model over the simulation period of 70 years. The model simulations included women born every 10 years before and during the 70 year emissions period. At birth, each child was assigned a chemical fugacity equal to that of the woman who had turned 20 at the time of the child’s birth, simulating the mother-to-child contaminant transfer. Children are highly vulnerable to the exposure to lipophilic organic contaminants and they receive the largest load of contamination from their mothers. Therefore, the end point of the simulation chosen for the assessment of chemical accumulation in Arctic residents was the body burden in a 30 year old woman as in earlier EBAP calculations. The Inuit mother was exposed to the hypothetical contaminants entirely from seal blubber as part of the traditional marine diet, and was nursing her third child 50 days after its birth. The environmental bioaccumulation potential (EBAP) was calculated as the

quotient of the woman’s body burden and the quantity of chemical in the Arctic submodel of Globo-POP at this time, normalized to the surface area of the Arctic region. Arctic Contamination and Bioaccumulation Potential AC-BAP. Here, the Arctic contamination and bioaccumulation potential (AC-BAP) is introduced, a metric which relates global chemical emission to the body burden and thus the internal exposure of a human living in the remote Arctic. Related to the product of the EBAP and the area normalized ACP, the AC-BAP is defined as the ratio of human body burden mH (g person-1) and the cumulative emissions of the hypothetical persistent chemical to the global environment eTG (g): AC-BAP )

mH eTG

(1)

AC-BAP has a unit of person-1 and represents the fraction of the cumulative global emissions stored in a single Arctic resident. A high value stands for a high potential of a hypothetical persistent chemical to reach the remote Arctic and to bioaccumulate in humans. Note that although it is comparable to the product of the separately calculated area-normalized ACP and EBAP, the AC-BAP gives slightly higher values. This is because ACP is determined from the chemical quantity in the Arctic environment excluding the atmosphere, whereas for EBAP, the human body burden is normalized to the chemical quantity in the Arctic including the atmosphere. However, as illustrated in Figure S3 (Supporting Information), this results in significant differences only for volatile chemicals, i.e., for compounds which only have a low potential to accumulate in the Arctic (both ACP and EBAP are small). As Gouin and Wania (15) demonstrated, the application of dynamic models implies that the duration of emission can affect the magnitude of model descriptors such as ACP. This may be amplified when considering EBAP and AC-BAP due to the slow approach of persistent chemicals to steady state in humans and the contribution of contaminant transfer from mother to child. Consequently, the simulations performed in this study considered a short-term scenario with 10 years of continuous chemical emission into air (indicated by the index 10 in ACP10, EBAP10 and AC-BAP10), and a longterm scenario with 70 years of exposure (indicated by the index 70 in ACP70, EBAP70 and AC-BAP70). In the first case, the 30 year old nursing mother was born 20 years before the start of emissions, whereas in the latter she represents the third generation of women exposed to the given chemical. Hypothetical Completely Persistent Chemicals. For the simulations, hypothetical completely persistent chemicals were defined. Assuming complete persistence reduces the number of variables when exploring the influence of chemical properties. It results in an estimate of the maximum potential of a chemical to accumulate in the nursing mother, which in the case of labile substances will be higher than the true accumulation. The hypothetical chemicals differed from each other only in their air-water and octanol-air partition coefficients (KAW and KOA, respectively). The octanol-water partition coefficient KOW was calculated as the quotient of KOA and KAW. The chemical partitioning space investigated in this study covered a range of -5 to 3 for log KAW and 4 to 12 for log KOA (using steps of half a log unit), which comprises the majority of the organic contaminants of environmental relevance. Other properties were kept constant in the simulations. For each hypothetical chemical a molar mass of 100 g mol-1 was assumed, and the heats of air-water, octanol-air, and octanol-water phase transfer were set to 60, -80, and -20 kJ mol-1, respectively, which is in accordance with earlier simulation studies (5, 7). For any two hypothetical persistent chemicals with the same combination of KOA and KAW, GloboVOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. ACP, EBAP, and AC-BAP in the Arctic as a function of KOA and KAW. The results are presented as the percentage of the maximum parameter value within the investigated chemical partitioning space (specified in the upper right corner of each panel). Panels A-C show ACP, EBAP, and AC-BAP, respectively, after 10 years, panels D-F after 70 years of continuous global emission into air. The white lines show the KOW-thresholds for an elevated AC-BAP. POP and ACC-HUMAN thus predict identical environmental fate and bioaccumulation behavior.

Results and Discussion Arctic Contamination Potential ACP. The ACP10 and ACP70 of the hypothetical persistent chemicals are shown in Figure 1A and D as a function of KOA and KAW. They are presented as the percentage of the maximum ACP within the investigated chemical partitioning space. As discussed elsewhere (5), it is chemicals subject to atmospheric long-range transport that show the highest ACP after 10 years, namely those with low KOA and intermediate KAW (so-called multiplehoppers that partition to water) and those with intermediate KOA (multiple-hoppers that partition to the terrestrial environment). The continuous emission caused an increase of the maximum ACP from 4.7% after 10 years of emission to 6.9% after 70 years. The partitioning properties of the chemicals achieving the maximum ACP changed slightly during the simulated time period from a log KOA of 5 to 4.5 and from a log KAW of -1.5 to -2, i.e., toward more hydrophilic and more volatile compounds. Also, the region of enhanced ACP on the partitioning map expanded downward, indicating an increase in the ACP of hydrophilic compounds with time which exceeded the simultaneous increase of the multiplehoppers’ ACP. These compounds with low KAW values are so-called swimmers that reach the Arctic by long-range oceanic transport (6). This oceanic transport is rather slow compared to long-range atmospheric transport and causes the delay in the Arctic enrichment of swimmers (15). As a 3706

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consequence, the swimmers’ concentrations increased up to 20-fold from 10 to 70 years after the start of emissions, whereas the multiple-hoppers’ concentrations increased by at most a factor of 8. A detailed description of the ACP as a function of the chemicals’ partitioning properties and the length of the emission period can be found in refs 5,6, and 15. Environmental Bioaccumulation Potential EBAP. The partitioning maps for the EBAP10 and EBAP70 of a 30 year old Inuit woman are presented in Figure 1B and E as a percentage of the maximum EBAP. Compared to the ACP partitioning map, EBAP shows steeper gradients, and a large fraction of the investigated partitioning space shows EBAP values less than 10% of the maximum. The EBAP also occupies a different part of the chemical partitioning space than the ACP; the highest EBAP is obtained for combinations of partitioning properties that give a comparatively low ACP. Presuming that a chemical could be classified as potentially bioaccumulative in humans if it possesses an EBAP of at least 10% of the maximum EBAP, the bioaccumulation thresholds for the Arctic are log KOA > 6 and 3.5 < log KOW < 9. These thresholds can be understood as follows: Compounds with a log KOW < 3.5 will not bioaccumulate sufficiently in fish due to low fish-water partition coefficients coupled with rapid gill elimination that prevents pronounced biomagnification; compounds with a log KOW > 9 will not bioaccumulate sufficiently in either fish or mammals due to less efficient dietary absorption; compounds with a log KOA < 6 will not bioaccumulate sufficiently in seals and humans because they

are efficiently eliminated via breathing. These thresholds are in agreement with the corresponding bioaccumulation thresholds identified for inhabitants of southern Sweden if they were on a largely marine diet (7), with the exception of the upper bound for KOW, which was 11 in that case. This difference is due to the fact that there is an additional filter in the Arctic food chain model, the ringed seal, which discriminates against the bioaccumulation of compounds with high KOW due to a decline of the gut absorption efficiency in mammals for chemicals with a log KOW > 8 (14, 16). The thresholds were not markedly influenced by the fact that the EBAP in southern Sweden was calculated assuming equilibrium partitioning of the hypothetical chemicals in the physical environment, whereas the calculations here were based on a nonsteady state simulation of the chemicals’ fate. It is interesting to consider if these thresholds would change if the dietary exposure was largely through a terrestrial food web. Although terrestrial food accounts for a very small portion of the exposure to organic contaminants in some Inuit communities (e.g., 0.04% in coastal areas of western Greenland (13)), there are other communities that rely much more heavily on terrestrial food sources. As shown in a previous study in southern Sweden (7), the lower KOA threshold for EBAP would be expected to be independent of the food chain structure, as it is a consequence of the human’s effective elimination of more volatile chemicals via breathing. However, the KOW thresholds may be different in a terrestrial food chain; in southern Sweden the lower KOW threshold was found to be considerably lower for terrestrial than for marine food chain exposure (7). However, it should be noted that contaminant levels are generally much lower in terrestrial food sources in the Arctic (17), and hence, marine food chain exposure will usually have priority from a global risk management perspective. A much more significant difference between the EBAP values for the Arctic and southern Sweden is their magnitude. The maximum values for the Arctic (2.4 × 104 and 5.4 × 104 m2 person-1 after 10 and 70 years of emission, respectively) exceeded the maximum value for southern Sweden (120 m2 person-1) by more than 2 orders of magnitude. This is in agreement with the results of ref 4 and further illustrates the much stronger capacity of the Arctic marine food web to bioaccumulate organic contaminants from the environment, which can, to a large extent, be attributed to the additional mammal in the food web. The EBAP, like the ACP, changed with the duration of emissions (compare Figures 1B and 1E). With 5.4 × 104 m2 person-1, the maximum EBAP70 (log KOA ∼ 10.5; log KOW ∼ 5.5) was more than twice as high as the maximum EBAP10. This is primarily due to the longer period of exposure (10 vs 30 years) of the 30 year old woman who was the end point of the simulation. An additional factor was that the environmental contamination (and thereby the dietary exposure) increased from zero during the 10 years of the woman’s exposure in the EBAP10 simulation, while it changed comparably little during the 30 years of the woman’s exposure in the EBAP70 simulation. These temporal changes of the environmental concentrations are not accounted for in the EBAP calculation, as the chemical inventory in the environment at the time the woman turns 30 was used. These influences were compensated to some extent by the enhanced sequestration of the chemical from water to soil and sediment, i.e., the (bioavailable) fraction of the chemical in the Arctic that was present in water decreased (Figure S4, Supporting Information). In addition, after 70 years of emissions, the region of maximum EBAP was shifted to lower KOW values and a second maximum developed at intermediate KOA and KAW values. This can be largely attributed to the chemical specific differences in the effects of the longer exposure period of the

FIGURE 2. 10% isolines of AC-BAP10 (blue line) and AC-BAP70 (red line) in comparison with the location of selected organic chemicals within the chemical partitioning space (physical chemical properties from refs 22-27. 30 year old woman and the relative changes in environmental contamination during the exposure period discussed above, as well as to the enhanced sequestration of the involatile and hydrophilic compounds (low KAW and high KOA values) into soil and sediment (Figure S4, Supporting Information). Arctic Contamination and Bioaccumulation Potential AC-BAP. The AC-BAP identifies those perfectly persistent compounds which have an elevated potential to reach the Arctic region and to bioaccumulate through the food web up to humans (Figure 1C and F). There is a strong similarity between the chemical partitioning space giving a high ACBAP and that giving a high EBAP. This suggests that bioaccumulation potential has a stronger influence on whether a chemical will be a contaminant in Arctic indigenous people than the long-range transport potential. In combination with earlier findings that the degree of a chemical’s persistence in the environment and the food web has a stronger impact on the overall human exposure than its partitioning properties (4), these findings suggest that of these three criteria for the identification of persistent organic pollutants according to the Stockholm Convention (18), persistence has the greatest significance, followed by the bioaccumulation potential and the susceptibility for longrange transport. Using again the 10% isoline as a criterion, the chemical partitioning space giving an elevated AC-BAP can be approximated by 3.5 < log KOW < 8.5 (white lines in Figure 1) and log KOA > 6. These thresholds are similar to those for EBAP, and the same processes limit the AC-BAP, namely low bioconcentration factors in fish (the low KOW threshold), reduced dietary absorption efficiency (the high KOW threshold), and elimination from seals and mammals to air (the low KOA threshold, see above). The most significant difference in the partitioning space occupied by AC-BAP and EBAP is in the lower right-hand portion of the plot, where chemicals with high KOA values show a lower AC-BAP. This threshold is determined by the long-range transport potential of the hypothetical persistent chemicals. The model predicts a lower long-range transport of these chemicals to the Arctic because they are largely associated with particles and or water in the atmosphere and are rapidly washed out. However, there are some doubts about this result since Globo-POP assumes continuous rainfall, which has been shown to considerably shorten the VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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long-range atmospheric transport of chemicals from this portion of the partitioning space compared to models using more realistic intermittent rain simulations (19). If this constraint were to be removed, the AC-BAP would likely be controlled by bioaccumulation in this part of the partitioning space as well. The maximum AC-BAP was obtained for multimedia chemicals with intermediate volatility and hydrophobicity (log KOA of 7.5; log KOW of 5.5-6). The difference in the maximum AC-BAP after 70 years of emissions compared to 10 years was a factor of 2.7, which was somewhat more than the increase in maximum EBAP (2.2), but less than the increase in EBAP of the chemicals with comparable partitioning properties (5.3). The maximum value after 70 years was 3.7 × 10-11 person-1, which means that at this time on average 37 ppt of any emission of the perfectly persistent chemical would have accumulated in an Inuit woman by the time she was 30. Note that both the AC-BAP and the EBAP are likely underestimated since only human exposure from seal blubber consumption was considered. Other foods, in particular those of marine origin, can contribute more than half of the dietary exposure of the Inuit to some Arctic contaminants (13). For comparison purposes, the AC-BAP was estimated from global emission estimates for PCBs from 1930 to 2000 (20) and from PCB body burdens reported for women of childbearing age from Greenland’s west coast (a region characterized by high seal consumption) at the end of the 1990s (21). The estimated AC-BAP values for the persistent PCB congeners 153 and 180 were 3 × 10-12 person-1 and 4 × 10-12 person-1, respectively. The predicted AC-BAP70 values for hypothetical chemicals with comparable KOA and KAW values (9.5/-2 and 10/-2.5, respectively) were 3 × 10-12 person-1 and 2 × 10-12 person-1. The good agreement between model predictions and observed values lends confidence to the model’s ability to predict chemical transfer to the Arctic and bioaccumulation in humans. Figure 2 shows the location of selected organic substances within the investigated chemical partitioning space. It reveals that the 10% isolines of AC-BAP10 and AC-BAP70 indeed encompass the chemicals of concern in the Arctic environment, e.g., PCBs, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), chlordanes, toxaphene, hexachlorocyclohexanes (HCHs), and polybrominated diphenyl ethers (PBDEs). This lends confidence to the utility of AC-BAP as a screening tool to identify compounds of potential concern in the Arctic. There is, on the other hand, a portion of the partitioning space with a high AC-BAP in which no known Arctic contaminants fall, namely 6 < log KOA < 9 and -1.5 < log KAW < 0.5. This could be an indication that there are many Arctic contaminants which have so far been overlooked. Partitioning property combinations in the range of a log KAW of 0 and a log KOA of 8 are rather exceptional; they indicate that a chemical has a strong aversion to water but a relatively strong affinity to both air and lipids. Some compounds with long perfluorinated alkyl chains likely have partitioning properties in this range (28). They warrant further investigation. The AC-BAP is a screening tool that incorporates knowledge of a chemical’s susceptibility to long-range transport and bioaccumulation in assessing its potential to become an Arctic contaminant in humans. In the future, this knowledge can be combined with information on the chemicals’ persistence, a third factor influencing the magnitude of exposure to Arctic contaminants (6). The AC-BAP could form a basis for more sophisticated screening tools that include persistence information, both in the physical environment and food chains (i.e., metabolism). 3708

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Acknowledgments Financial support from the European Union (FAMIZ grant EVK3-CT-2000-00024; Marie-Curie Intra-European Fellowship MEIF-CT-2005-010918) and from the CEFIC Long-Range Initiative is gratefully acknowledged. We thank Peter Ko¨mp, Marit Reigstad, and Paul Wassmann for helpful discussions, for providing deeper insights into the biology of the Arctic, and for making unpublished data available.

Supporting Information Available A description of the modifications made to the bioaccumulation model ACC-HUMAN for the application to Arctic conditions and Figures S3 (showing the fraction of the chemical in the Arctic present in the atmosphere after 10 and 70 years of continuous emissions) and S4 (showing the change of the fraction of the chemical sequestered into Arctic air, water, and soil over the same time period). This material is available free of charge via the Internet at http:// pubs.acs.org.

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