Potential of Degradable Organic Chemicals for Absolute and Relative

absolute and relative Arctic enrichment behavior, as quantified by the Arctic Contamination Potential. For substances that degrade faster in water tha...
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Environ. Sci. Technol. 2006, 40, 569-577

Potential of Degradable Organic Chemicals for Absolute and Relative Enrichment in the Arctic FRANK WANIA* Department of Physical and Environmental Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario, Canada, M1C 1A4

Model simulations of the fate of numerous hypothetical substances in the global environment can provide considerable insight into how an organic chemical’s degradability and partitioning properties influence its absolute and relative Arctic enrichment behavior, as quantified by the Arctic Contamination Potential. For substances that degrade faster in water than in soil, but are quite persistent in the atmosphere, highest Arctic contamination is expected to occur if the substances have intermediate volatility and high hydrophobicity. Organic substances that are degradable in the atmosphere can still accumulate in the Arctic if they are soluble and highly persistent in water. These latter substances, which reach the Arctic in the ocean, also show the highest potential for relative enrichment in the Arctic, i.e., high amounts in northern high latitudes relative to the amounts in the total global environment. Beyond a threshold persistence in surface media of the order of several months to a year, chemical degradability leads to further relative enrichment. This is because only chemicals that are sufficiently long-lived get transferred to polar regions and once there can persist longer than at lower latitudes. The model simulations can inform the search for new potential Arctic contaminants, and can highlight combinations of properties which should be avoided in high production volume chemicals with the potential for environmental release. Three categories of organic substances are singled out for troublesome combinations of persistence, distribution, and potential bioaccumulation characteristics, only one of which contains “classical” Arctic POPs. Examples of potential Arctic contaminants within each of these categories are named.

Introduction The threat posed by the pervasive occurrence of persistent organic pollutants (POPs) emitted in lower latitudes to the indigenous people and marine mammals of the Arctic was one of the driving forces leading to the Stockholm Convention (1). This agreement subjected the twelve most notorious POPs to legally binding global bans and restrictions, and provided provisions to add other substances that have similar characteristics. It is thus of considerable interest to understand the biological and physical processes that lead to elevated chemical exposure in remote regions in general, and in the Arctic environment in particular (2). This issue of contaminant accumulation in polar regions has been approached conceptually (3-6) and also in quantitative terms (7, 8). * Author phone: (416)287-7225; e-mail: [email protected]. 10.1021/es051406k CCC: $33.50 Published on Web 12/08/2005

 2006 American Chemical Society

In an earlier study, a zonally averaged global distribution model was used to identify the partitioning properties that make an organic chemical susceptible to transport to, and accumulation in, the Arctic (8). Specifically, an indicator variable of contaminant enrichment, called the Arctic Contamination Potential (ACP), was defined which relates the amount of a substance in the Arctic to that in the total global environment. Given a generic emission scenario, such a variable can be calculated with a global model and compared for a variety of real and hypothetical chemicals. As the first step in the process to constrain the properties of chemicals that have a high potential for Arctic contamination, the ACP was calculated for a large number of hypothetical, perfectly persistent organic chemicals that differ in terms of their distribution characteristics as expressed by the equilibrium partition coefficients between air (A), water (W) and n-octanol (O), KAW, KOA, and KOW (8). By illustrating the results in maps displaying the ACP as a function of log KOA and log KAW, combinations of partitioning properties which enhance the likelihood of an organic chemical contaminating the Arctic could be identified (8) and related to various modes of environmental transport (9). Most real chemicals are not perfectly persistent, but can undergo reactions at rates depending on characteristics of the chemical and the environment they reside in. In fact, the variability of degradation rates with climate can contribute to the relative enrichment of contaminants in the Arctic as “a chemical that is being degraded in warm environments, yet preserved in cold regions, will eventually show higher concentrations in the cold regions” (8). When assessing the ACP of real chemicals, it is therefore imperative to account for degradation and its variability across the globe. In this contribution, I will first outline how degradability in atmosphere and surface media can affect the extent for absolute and relative accumulation of a large set of hypothetical chemicals in the Arctic. I will then show how it is possible to distinguish to what extent the relative enrichment of a substance in the Arctic is a result of global re-distribution processes driven by temperature gradients vs climatedependent persistence. I will further illustrate how the calculations for hypothetical chemicals can be employed to understand quantitatively the global transport and accumulation behavior of known Arctic contaminants, and to identify candidate substances that may have the potential to be, or to become, contaminants of concern in polar regions.

Method Global Distribution Model Globo-POP. All the calculations presented here were performed with the same zonally averaged global distribution model, called Globo-POP, which was used in the earlier study (8). The model consists of a series of 10 interconnected multimedia box models, each consisting of four air, two soil, two water, and one sediment compartments, and each representing a well-mixed latitudinal band of homogeneous climatic characteristics. Whereas the global environment is considered well-mixed zonally, chemical transport in the meridional direction is described two-dimensionally in the atmosphere and one-dimensionally in the surface ocean. In contrast to most other models used in the assessment of long-range transport potential, GloboPOP is a dynamic model. A global steady-state situation of a persistent chemical may take centuries to establish and thus provides no realistic representation of the distribution adopted by most synthetic organic chemicals. The model is described in detail elsewhere (10, 11). VOL. 40, NO. 2, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Arctic Contamination Potential. When first introduced, the ACP was defined as the fraction of the total global mass in the environment present in the surface media (all media except the atmosphere) of the northernmost latitudinal band of the model after a certain time period (8). Contamination in the Arctic was normalized by the total global contamination, the parameter mACP thus expressing the potential for relative enrichment in the Arctic:

mACP ) [mT1 - mA1]/mTG × 100%

(1)

where mT1 and mA1 are the mass of chemical in all compartments and in the four atmospheric compartments of zone 1 (N-Polar) of the Globo-POP model, respectively. mTG is the mass of chemicals in all model compartments. The deep sea and buried sediments are not compartments and chemical mass lost to these reservoirs is therefore not included in mT1 and mTG. A useful alternative normalization is the total amount cumulatively emitted to the global environment over time (9). This parameter, expressing the potential for absolute contamination of the Arctic environment, is referred to as the eACP:

eACP ) [mT1 - mA1]/eTG × 100%

(2)

where eTG is the mass of chemical emitted cumulatively to the global environment. mTG is always smaller than eTG because of chemical losses by degradation, transfer to the deep sea, and burial in deeper sediment layers. Although the differences between mACP and eACP tend to be minor for perfectly persistent chemicals, they can be very significant for degradable substances. To compare different real or hypothetical chemicals in terms of their ACP, it is useful to define a generic emission scenario. Adopting the scenario introduced in the earlier description of ACP (8), emissions are assumed to be continuous with a zonal distribution approximating that of the global population. Simulations were performed for 10 years of emission to the atmosphere. The eACP10air calculated using this scenario is a target-oriented indicator of longrange transport potential that has been compared with other model-based indices of long-range transport (12). The earlier study (8) also looked at shorter emission time periods (1 year) and different modes of emission (to cultivated soils and freshwater), but these will not be considered here, because they consistently led to lower calculated ACP values. Hypothetical Chemical Space. Although it is possible to calculate and interpret ACP values for specific organic substances (9), it is instructing to comprehensively investigate the Arctic contamination behavior of all those organic chemicals that are amenable to simulation with the GloboPOP model. We have previously argued that two parameters, namely two of the three partition coefficients KAW, KOA, and KOW, are sufficient to characterize the distribution behavior of nondissociating organic substances in the environment, if generic values for the energies of phase transfer are assumed to apply (8). Two-dimensional graphs were constructed in which ACP values for perfectly persistent chemicals were plotted as a function of log KAW and log KOA (8). This facilitated the visual identification of those combinations of partitioning properties that lead to an enhanced potential for a perfectly persistent organic substance to contaminate remote polar regions (8). Additional assumptions underlying this approach are outlined in ref 8. Additional dimensions are required for degradable organic chemicals in a comprehensive hypothetical dataset. Chemical degradability in the Globo-POP model is described with 12 user-defined parameters: pseudo-first-order degradation half-lives in five different media (uncultivated and cultivated soil, freshwater, seawater, and freshwater sediment), a rate 570

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constant for the gas-phase reaction with the hydroxy (OH•) radical in air, and six activation energies describing the temperature dependence of the degradation reactions (10). The degradation half-lives in different environmental surface media are typically not independent of each other, because the hydrolytic, oxidative, or microbial degradation processes occurring in soils, sediments, and surface waters are often the same. Half-lives in water tend to be shorter than those in soils, which in turn tend to be much shorter than those in sediments. General rules of thumb for the relationship between degradation half-lives for organic chemicals in water, soil, and sediments have been developed by both the European Chemicals Bureau and the U.S. Office of Pollution Prevention and Toxics (12). By assuming that the degradation half-life of hypothetical organic chemicals in soils is twice that in water, and that the half-life in sediments is 10 times longer, and by further applying generic values for the 6 activation energies (30 kJ‚mol-1 for surface media, and 10 kJ‚mol-1 for the gas-phase reaction with OH•), we can reduce degradability to two dimensions: persistence in the atmosphere, represented by the reaction rate constant with OH• at 25 °C, kair in units of cm3‚molecules-1‚ s-1, and persistence in surface media, represented by the degradation half-life in water at 25 °C, HLref in units of time. Together with the two partitioning coefficients, these define a four-dimensional hypothetical chemical space, which can be graphed by having multiple two-dimensional panels representing the chemical partitioning space (defined by log KAW and log KOA) arranged in a two-dimensional grid representing different combinations of degradability in atmosphere and surface media. A similar approach was taken when comparing various models calculating indicators of persistence and long-range transport potential (12). Calculations of eACP and mACP for Hypothetical Organic Chemicals. Using the Globo-POP model, we calculated the mACP10air and eACP10air for 2000 hypothetical chemicals within the following property ranges: log KOA between 3 and 12, with steps of 1 log unit; log KAW between -4 and 3, with steps of 1 log unit; log(kair/cm3‚molecules-1‚ s-1) between -15 and -11, with steps of 1 log unit; and log(HLref/year) between -2 and +2, with steps of 1 log units. The log kair values of -11, -12, -13, -14, and -15 correspond to atmospheric degradation half-lives of approximately 1 day, 10 days, 100 days, 3 years, and 30 years, respectively, whereas log(HLref/year) of -2, -1, 0, 1, and 2 represent half-lives of 4 days, 37 days, and 1, 10, and 100 years, respectively. The size of the simulated chemical partitioning space is relatively small, when compared to the full range of reported KAW and KOA values and to that used in ref 12. Specifically, there are numerous volatile substances with log KOA values of less than 3, and many organic substances, including some of considerable environmental concern (e.g., currently used pesticides, pharmaceuticals, etc.), have log KAW values below -4. The subset of partitioning properties used here includes, however, the full range of true multimedia chemicals (see Figure 1A), and all known organic contaminants of Arctic concern. The atmospheric fate of substances with a log KAW of less than -4, which are very water-soluble and easily scavenged by rain, should not be described by models that ignore the intermittent character of precipitation (13). Since such substances typically have one or more polar functional groups, the empirical relationships on which the Globo-POP model relies to estimate environmental phase distribution may not be applicable either (14).

Results The following discussion is facilitated by understanding the distribution and transport behavior of organic chemicals of different partitioning properties. For this purpose, Figure 1A

FIGURE 1. Primary environmental compartments (A) and major modes of transport (B) of perfectly persistent, hypothetical chemicals defined by their partitioning properties log KAW and log KOA, when calculated with the Globo-POP model assuming 10 years of steady emissions into air. The white lines delineate partitioning properties of three categories of chemicals of potential concern discussed in the text. displays the simulated phase distribution of hypothetical perfectly persistent chemicals in the global environment after 10 years of continuous emission to the atmosphere. The chemicals in the upper left, shaded in red, partition predominantly (> 66%) into the global atmosphere; those in the lower left, shaded in blue, are found mostly in the world’s surface ocean and freshwater bodies; and the chemicals on the right, in yellow, are distributed primarily into global soils and freshwater sediments. The chemicals with partitioning properties corresponding to the green region are true multimedia chemicals with less than two-thirds of the total being found in any one compartment. The distribution in Figure 1A is slightly different from that displayed in Figure 3 of ref 8, i.e., the air to soil transition occurs at slightly higher log KOA values, because the current simulation no longer assumes simultaneous emission to air, soil, and water compartments. Figure 1B shows the four major modes of chemical transport behavior on a global scale, which have been assigned to specific partitioning property combinations based on the results of mACP10air calculations for perfectly persistent chemicals (8, 9). The chemicals in the upper left with a log KAW > 0 and a log KOA < 6.5, called “fliers”, are so volatile that they do not deposit to the Earth’s surface, even under the conditions of the Arctic environment (mACP10air < 1.5%). Those on the right with a log KOA > 10, called “single hoppers”, tend to be associated with particles in the atmosphere and are usually deposited irreversibly to the Earth’s surface, which limits their potential for Arctic contamination (mACP10air < 1.5%). Those in the red region forming an inverted L, referred to as “multiple hoppers”, are semi-volatile substances that readily undergo atmospheresurface exchange in response to temperature changes. Those in the horizontal leg of the L have intermediate log KAW values between -4 and 0 and exchange mostly with aqueous surfaces. Those in the vertical leg have intermediate log KOA values between 6.5 and 10 and exchange primarily with continental surfaces. Where these two legs intersect are chemicals that exchange with both aquatic and terrestrial surfaces. “Multiple hoppers” have relatively high ACP if perfectly persistent (mACP10air > 1.5%). Chemicals in the lower left of the diagram with a log KAW less than -2, designated by blue stripes and termed “swimmers”, undergo significant meridional transport in the oceans. Values of mACP10air for

these chemicals tend to be highly sensitive to the values of the oceanic macro-diffusive mixing coefficients. Some chemicals are both “swimmers” and “multiple hoppers”, i.e., they undergo coupled transport in the atmosphere and the oceans (15), which explains why they have particularly high mACP10air values if perfectly persistent. MacLeod and Mackay (16) derived a similar categorization of chemical transport behavior on a regional scale, suggesting that Figure 1B may have validity at the regional as well as the global scale. Figures 2 and 3 display the eACP10air and mACP10air values for hypothetical chemicals as a function of KOA, KAW, kair, and HLref, whereas Figures 4 and 5 depict, respectively, the percentage change δe and δm of eACP10air and mACP10air relative to the respective ACP of the perfectly persistent analogue (PPA):

δe ) [eACP10air(PPA) - eACP10air]/eACP10air(PPA) × 100% (3) δm ) [mACP10air(PPA) - mACP10air]/mACP10air(PPA) × 100% (4) By quantifying the differences between the ACP10air of degradable chemicals and their perfectly persistent analogues, δe and δm emphasize the relationship between degradability and partitioning. Low eACP10air values below 1% are calculated for hypothetical substances with log kair of -11 (atmospheric degradation half-life of 1 day) or log(HLref/year) of -2 (degradation half-life in water of 4 days) indicating a very limited potential for reaching the Arctic. It may not be appropriate to calculate the global fate of such readily degradable chemicals in a coarsely resolved global box model, which assumes well-mixed conditions in very large geographical units. The results for these hypothetical substances are therefore not included in the figures, which are arranged in such a way that panels in the upper left (labeled A) designate chemicals that are fairly degradable in both air and surface media, whereas the panels in the lower right (P) have very long degradation half-lives in both air and surface media. It is thus not surprising that panel P in Figure 3 resembles closely the plot for the mACP10air of perfectly persistent chemicals (8). The lower left panels (M) show predictions for VOL. 40, NO. 2, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Absolute Arctic Contamination Potential eACP10air of hypothetical organic chemicals emitted continuously for 10 years to the atmosphere, displayed as a function of their partitioning properties (log KAW and log KOA), degradation rate constant in air (log kair), and degradation half-life in surface media (HLref). A number of confirmed Arctic contaminants are also shown. substances that are degradable in air, but persistent in surface media, whereas the upper right panels (D) show chemicals that are persistent in air, but degradable in surface media. In Figures 2 and 3 green colors indicate low ACP values, and areas of increasing ACP are shaded in yellow, orange, and red. In Figure 4, red and brown colors indicate large differences between the eACP of degradable chemicals and their perfectly persistent analogue, whereas green colors indicate small differences. In Figure 5, yellow and orange tones are used for positive δm values, indicating enhancement of Arctic enrichment through degradability. Negative δm values, denoting no such enhancement, are shown in green.

Discussion Influence of Degradability on the Absolute Arctic Contamination Potential eACP. As expected when moving from the panels on the lower right (P) to those on the upper left (A) of Figure 2, the displayed eACP10air values decrease, because of increasing degradability. Accordingly, values of δe (Figure 4) increase from panels P to A, as the absolute amount of a chemical predicted to reach the Arctic decreases with degradability compared to that of its perfectly persistent analogue. Chemicals with fairly high degradability in both air and surface media (e.g., panels A, B, E, and F in Figure 2) can never achieve eACP10air values above 1% irrespective of their partitioning properties, illustrated by the prevalence of green shadings in these plots. However, to what extent degradability in air and surface media decreases the potential for Arctic contamination does depend on the partitioning properties. 572

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Increasing degradability in the atmosphere (moving from right to left in Figures 2 and 4), decreases the eACP10air of chemicals, especially in the upper left of the partitioning space. Although the eACP10air of the “fliers” is very sensitive to the degradation rate constant in air (Figure 4), it is always low (Figure 2), because of the lack of significant deposition. The number of partitioning property combinations that is affected by degradability in the atmosphere increases with increasing degradation rate constant in air, which is reflected by an increase in the dark green area of panels P to M of Figure 2. For example, the eACP10air of chemicals with a log KOA below 7 and a log KAW above -1 is decreased by at least 30% (transition between green and yellow shadings in Figure 4) if the chemical has an atmospheric degradation half-life of 3 years (i.e., log kair ) -14) (Figure 4O). These thresholds are shifted to a log KOA of 9.5 and log KAW of -3 for chemicals with an atmospheric half-life of 100 days (i.e., log kair ) -13) (Figure 4N). For chemicals with half-lives in air of the order of 10 days (i.e., log kair ) -12) the eACP10air of almost all substances in the investigated partitioning space is reduced significantly (by at least 30%) (Figure 4M). In other words, with increasing degradation rate constants in air, the eACP10air of more and more of the “multi-hoppers” is reduced (in addition to that of the “fliers”), because even a small fraction of a chemical partitioning into the atmosphere results in reduced Arctic contamination, if it is sufficiently reactive with OH• radicals. It is particularly interesting to explore for which chemicals degradability in air has little effect on the calculated eACP10air. There are two such groups of substances. Chemicals with a high log KOA (i.e., values above 11) have a low eACP10air (Figure

FIGURE 3. Relative Arctic Contamination Potential mACP10air of hypothetical organic chemicals emitted continuously for 10 years to the atmosphere, displayed as a function of their partitioning properties (log KAW and log KOA), degradation rate constant in air (log kair), and degradation half-life in surface media (HLref). A number of confirmed Arctic contaminants are also shown. 2), which is hardly affected by introducing degradability in air (Figure 4). These “single hop” substances (Figure 1B) will tend to sorb to particles in the atmosphere, which not only hinders their efficient atmospheric transport (8), but also prevents them from reacting with OH• radicals, which is assumed to be a gas-phase reaction. A second group of compounds which is insensitive to kair are those with log KAW values below -2, which continue to have reasonably high eACP10air values despite having fairly high degradation rate constants in air (indicated by the occurrence of yellow shading in the lower left of panels M and N of Figure 2). These are the “swimmers”, which can undergo transport while dissolved in river and ocean water (Figure 1B). The model results suggest that these substances, if persistent in water, have the potential to accumulate in the Arctic (via ocean currents), even if they have short atmospheric degradation half-lives (of the order of 10 to 100 days) because they volatilize to a very limited extent. Introducing degradability in surface media (moving from bottom to top of Figures 2 and 4, especially in the columns to the right), tends to strongly reduce the eACP10air of most chemicals within the investigated partitioning space. For example, an HLref of 37 days reduces the eACP10air by more than 80% for almost all of the simulated combinations of partitioning properties (Figure 4D). Even an HLref of 1 year causes reductions in eACP10air of generally more than 30% for most chemicals (Figure 4H). Chemicals which partition strongly into air, i.e., the “fliers”, are less affected by changes in HLref (panels D and H in Figure 4), but these substances tend to have a low potential for Arctic accumulation anyway, because they lack the capability to deposit to Arctic surface

media (8) (Figure 1B). The decrease in eACP10air upon introducing surface degradability is highest for chemicals that distribute into water compartments (i.e., those in the lower left of the partitioning space) (panels D, H, and L in Figure 4). This is simply the result of the assumption that degradation half-lives in water are shorter than those in soil and sediments. Obviously, the eACP10air of substances, for which the relative rates of degradation in different surface media deviate strongly from those assumed for the hypothetical chemicals, will display a different relationship between partitioning and surface degradability. The assumed longer degradation half-lives in soil when compared to water is also the reason, that for organic chemicals which are degradable in surface media, the maximum eACP10air is calculated for substances with intermediate log KOA between 6 and 9 and a log KAW above -2 (the vertical leg of the inverted “L” in panels C, D, G, and H in Figure 2). Such chemicals, combining intermediate volatility and high hydrophobicity, have been identified as likely to undergo repeated air-surface exchange with terrestrial surfaces, but do not partition readily into the aqueous phases (8). Influence of Degradability on the Relative Arctic Contamination Potential mACP. As previously stated (8), two mechanisms can contribute to the relative enrichment of an organic substance in the Arctic. One is the temperaturedriven redistribution process on a global scale, referred to as “global distillation” or “cold condensation”, which affects semi-volatile chemicals that can occur both in the atmospheric gas phase and in the Earth’s marine and terrestrial surface media (3, 4). However, relative enrichment in polar regions can also be the result of degradation in lower latitudes VOL. 40, NO. 2, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Relative difference of the absolute Arctic Contamination Potential eACP10air of hypothetical organic chemicals emitted continuously for 10 years to the atmosphere from that of its perfectly persistent analogue, displayed as a function of their partitioning properties (log KAW and log KOA), degradation rate constant in air (log kair), and degradation half-life in surface media (HLref). Green colors denote small changes from the persistent analogue and red colors denote large changes. occurring faster than at high latitudes (5, 7). By discussing only perfectly persistent chemicals, which do not undergo degradation, ref 8 focused on the impact of a chemical’s partitioning properties on its potential for Arctic contamination. The impact of a chemical’s persistence on its relative enrichment in Arctic latitudes can be assessed by comparing mACP10air calculated for a degradable chemical with that of a perfectly persistent chemical with the same partitioning properties mACP10air(PPA) (9) (Figure 5). For a chemical for which mACP10air is larger than mACP10air(PPA), the terms 1 - mACP10air(PPA)/mACP10air and mACP10air(PPA)/mACP10air may be interpreted as a quantitative indication of the importance of “climate-dependent persistence” and “global distillation” for this chemical’s relative enrichment in polar regions, respectively (9). It should be cautioned, however, that in the model temperature is the only environmental parameter influencing degradation rates, and the activation energies therefore determine how strongly persistence varies with latitude. The calculated influence of climate-dependent persistence on relative Arctic enrichment thus depends on the numerical value of these activation energies. Larger activation energies, corresponding to larger differences in degradation half-lives between latitudinal zones, will increase the importance of climate-dependent persistence. Figure 3 indicates that spatially variable persistence can indeed increase the relative enrichment of an organic chemical in the Arctic. It should be re-emphasized that the mACP expresses the relative enrichment of a chemical, i.e., its amount in the Arctic relative to the amount in other locations around the globe. A nonpersistent chemical will 574

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always have a lower eACP than its persistent analogue. Panel P in Figure 3 differs from 4D in ref 8 because the scale has been expanded: the maximum mACP10air for perfectly persistent chemicals was less than 5% (8), but mACP10air for degradable chemicals can adopt values above 10%. Overall, the pattern of maximum enrichment for multi-hop substances with intermediate log KAW and log KOA values (the inverted L region in the panels of Figure 3) that had been observed for perfectly persistent chemicals (8) is preserved, suggesting that irrespective of degradability, it is the chemicals capable of repeated air-surface exchange that have the highest potential for relative Arctic enrichment. How degradability affects the extent of relative enrichment is made more obvious in Figure 5, where green shading indicates a decrease, and yellow to orange shading indicates an enhancement of the relative enrichment in the Arctic. The patterns are quite complex, but in general it appears that increased degradability in the atmosphere increases the mACP10air, especially for relatively volatile chemicals (log KOA below 7.5) that are persistent in surface media (panels I, J, M, and N in Figure 5), whereas increased degradability in surface media decreases the mACP10air of the same partitioning property combinations (panels C, D, G, and H in Figure 5). A key observation is that a decrease in mACP10air is only observed for relatively volatile compounds that are fairly degradable in surface media, i.e., green shading only appears in the upper panels (A to D, G, H, and L) of Figure 5. The mechanistic interpretation is that only chemicals which are sufficiently long-lived to get transferred to polar regions in the first place can persist relatively longer in the Arctic than

FIGURE 5. Relative difference of the relative Arctic Contamination Potential mACP10air of hypothetical organic chemicals emitted continuously for 10 years to the atmosphere from that of its perfectly persistent analogue, as a function of their partitioning properties (log KAW and log KOA), degradation rate constant in air (log kair), and degradation half-life in surface media (HLref). in lower latitudes. The model calculations suggest that there may be a threshold persistence in surface media of the order of several months to a year, beyond which significant Arctic contamination is possible and degradability leads to further relative enrichment. Incidentally, degradability can still increase the mACP10air of an organic chemical that is degrading faster in surface media than this threshold, if it is fairly involatile, i.e., has a log KOA above 9 (panels A to D of Figure 5). The fact that this mACP10air enhancement occurs largely independently of the reaction rate in air, i.e., is similar in panels A through D of Figure 5, indicates that these “single hoppers” can reach the Arctic sorbed on atmospheric particles, while they are protected from degradation with OH•. However, whereas the enhancement of relative polar enrichment by latitudinally variable degradability may be large for these chemicals, their ACP10air values are generally very small (Figures 2 and 3). The largest enhancements in mACP10air by a factor of 2 and more (orange shadings in Figure 5) tend to be observed for fairly degradable chemicals, for which only very low eACP10air values were calculated (dark green shading in Figure 2). This suggests that the relative enrichment of such substances in the Arctic may be of little consequence, because of the small absolute amounts involved. An exception are the chemicals in the lower left of panels I, J, M, and N, identified as “swimmers” above, transported to the Arctic in the ocean. Such waterborne chemicals, when degradable in air, experience a particularly large relative enhancement of mACP10air (Figure 5), and indeed show the largest absolute mACP10air values (Figure 3). Warm temperatures at low latitudes not only favor their volatilization from the sea, butsalong with higher OH•

concentrationssalso their degradation in the atmosphere. Low temperatures, sea ice cover, and low OH• concentration make this sink inefficient in polar seas. Understanding the Enrichment of Known Arctic Contaminants. By placing known Arctic contaminants in the panels of Figures 2-5 based on their partitioning and persistence attributes, it is possible to assess their global transport and accumulation behavior relative to that of the hypothetical chemicals. Five polychlorinated biphenyls (PCBs) of different degrees of chlorination, the organochlorine pesticides DDT, chlordane, R-hexachlorocyclohexane (RHCH), and hexachlorobenzene (HCB) were located in the figures using substance properties compiled in refs 11 and 12. The values of eACP10air and mACP10air for these substances and the change relative to their respective perfectly persistent analogue can be deduced from the color coding of the place they occupy in the maps. This could be done for any chemical for which two of the three partition coefficients KOA, KAW, and KOW are known and for which an assignment to the degradability classes in air and surface media is possible. No actual model simulations are required. The values derived from the color coding agree well with those calculated explicitly for PCB congeners and R-HCH (9), although perfect agreement cannot be expected because the maps rely on categories of persistence rather than compound-specific degradation halflives. Also, the degradation half-lives of real chemicals in water, soil, sediment do not necessarily adhere to the fixed conversion factors (1:2:10) that were assumed to apply for the hypothetical chemicals making up the chemical space. This needs to be kept in mind when comparing ACP values of real and hypothetical substances. VOL. 40, NO. 2, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Figure 2 reveals that the PCBs with an intermediate degree of chlorination have higher eACP10air values than either light or heavy congeners. From Figure 4 we can deduce that the Arctic contamination potential of the smaller PCB congeners is much lower than that of their perfectly persistent analogues, whereas that of the heavier congeners is only marginally smaller. This means that degradation limits the potential of the lighter PCBs for Arctic accumulation, whereas that of the heavier ones is limited by their partitioning properties. Of all the displayed substances, HCB has by far the highest eACP10air value, which is due both to its high persistence in air and surface media and its partitioning characteristics. The other organochlorine pesticides have lower eACP10air values because of their lower persistence in both air and surface media. Figures 3 and 5 suggest that the relative enrichment of the PCBs in the Arctic is mostly the result of temperaturedependent redistribution processes, with the contribution of latitudinally variable degradability generally less than 40% (9), whereas that contribution is typically higher than 40% for the organochlorine pesticides. Searching for New Arctic Contaminants. Beyond their usefulness in gaining mechanistic understanding of global transport and accumulation patterns of known Arctic contaminants, it is hoped that the current simulations may facilitate the search for existing, but unknown Arctic contaminants, and prevent the production and large scale release of potential future pollutants with a propensity for accumulation in the Arctic and other cold, remote environments. The detection of novel environmental contaminants is usually not the result of a rational, directed search, but often has an element of serendipity. In searching for unknown Arctic contaminants analytical chemists should focus on substances which combine partitioning and degradability characteristics that yield high eACP10air values (Figure 2). That same combination of properties should be avoided in high production volume chemicals with the potential for environmental release (e.g., pesticides, consumer products, etc.). Three segments of the partitioning space, identified in Figure 1, are of particular concern and merit explicit discussion. Segment 1 (Figure 1) comprises “multiple hoppers” that exchange reversibly with marine and continental surfaces. The classical POPs with known occurrence in the Arctic, such as HCB, the PCBs, chlordane, and DDT, fall into this region. The simulations suggest that their Arctic accumulation is limited mostly by degradation in both air and surface media, and for the larger representatives of that group (such as PCB194) by efficient deposition, i.e., these latter compounds are crossing the boundary to the “single hoppers”. Log KOW values between 5 and 8 suggest the potential for efficient bioaccumulation and biomagnification (17). Because substances with partitioning properties corresponding to this segment fit the “classical” profile, the established methods and criteria for POPs should be able to identify and correctly classify substances that fall into this category. Examples are the polychlorinated naphthalenes and the smaller brominated diphenyl ethers. The second category of potentially troublesome substances is the “swimmers” (segment 2 in Figure 1). The simulations reveal their potential to reach the Arctic and achieve a high relative enrichment, if they are not rapidly degraded in surface media, but even if they are degradable in air. Although these substances have for the most part log KOW values below 5, and are therefore not believed to efficiently bioaccumulate, the chemicals at the right of the “swimmer” segment (with log KOA values above 7) may potentially be bioaccumulative (17). The question arises whether there are highly persistent substances within this segment of the partitioning space. Aliphatic alcohols, if sufficiently long, have such partitioning properties, but are not persistent. Potential candidates of concern for persistent 576

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chemicals in that segment are perfluorinated substances with a polar functional group, especially if they have a chain length that renders them bioaccumulative. Exact placement of substances such as PFOS, the fluorotelomer alcohols, or perfluorinated carboxylic acids within the plots of Figures 1-5 is not yet possible because of a general lack of reliable partitioning and persistence data. However, the pervasive presence of such substances in the circumpolar environment (e.g., 18) indicates the capability of at least some perfluorinated substances to contaminate the Arctic, even if the mode of transport has not yet been confirmed. A third category of concern is the hydrophobic “multihoppers” that exchange reversibly with continental surfaces only (segment 3 in Figure 1). They are potentially troublesome, because the eACP10air values of hypothetical chemicals in this segment often exceed those in segments 1 and 2 if they are persistent in the atmosphere, and their partitioning properties could render them potentially bioaccumulative (17). Again the question arises whether there are highly persistent substances with this combination of partitioning properties. Long chain aliphatic hydrocarbons have such properties, but are not persistent. Other substances with such partitioning properties are perfluoroalkanes and some of the cyclic and linear organosiloxanes. On the basis of data reported for the vapor pressure (19), Henry’s law constant (20, 21) and KOW (22) of the organosiloxanes, the larger representatives of this substance class (tetradecamethylhexasiloxane, hexadecamethylheptasiloxane, octadecamethyloctasiloxane, tetradecamethylcycloheptasiloxane, hexadecamethylcyclooctasiloxane, and octadecamethylcyclononasiloxane) fall into segment 3, as do the perfluoroalkanes with more than 14 carbon atoms. Whereas the organosiloxanes appear to be degradable in both air (23) and soils (24), the perfluoroalkanes are expected to be highly persistent, and would thus be prime candidates for Arctic accumulation if they were released in significant quantities into the environment. The smaller volatile organosiloxanes (e.g., dedecmethylpentasiloxane), which are in widespread commercial use, are “fliers” and will fail to deposit in Arctic latitudes, if they are sufficiently persistent in the atmosphere to get there. There are likely to be other persistent substances with properties corresponding to the segments of concern identified in Figure 1. Efforts should be undertaken to screen high production volume chemicals, with potential for environmental release, for such properties. If they do possess such properties, monitoring programs should actively seek to confirm their presence or absence in the Arctic environment. One may ask to what extent the insights gained from the ACP calculations with Globo-POP may depend on the chosen model, approach (e.g., definition of the ACP), simulation conditions (e.g., emission scenario), and assumptions, (e.g., fixed ratios between degradation half-lives, values of activation energies and energies of phase transfer). The Great Lakes Transfer Efficiency is another target-oriented approach to long range transport assessment which is based on a completely different model (BETR North America in steadystate mode), a different region (North America with the Great Lakes as a target region), and a different definition of the transport indicator (16). Despite such differences, a model comparison exercise revealed very similar classification of both hypothetical (12) and real (25) chemicals in terms of target-oriented long range transport behavior by Globo-POP and BETR North America. If two approaches that differ in so many regards yield comparable results, many of the assumptions that went into the simulations presented here, although yielding different numerical model results, are not expected to have an impact significantly large enough to put the major conclusions into question. Model Availability. A version of the Globo-POP model that includes the capability for eACP10air and mACP10air

calculations is available for download at www.utsc.utoronto.ca/∼wania.

Acknowledgments I am grateful to Catherine Oyiliagu for help with model calculations and graphs, and to the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support.

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Received for review July 19, 2005. Revised manuscript received November 1, 2005. Accepted November 4, 2005. ES051406K

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