On the Mechanism of Mountain Cold-Trapping of Organic Chemicals

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Environ. Sci. Technol. 2008, 42, 9092–9098

On the Mechanism of Mountain Cold-Trapping of Organic Chemicals FRANK WANIA* AND JOHN N. WESTGATE Department of Physical and Environmental Sciences and Department of Chemistry, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada

Received May 13, 2008. Revised manuscript received October 22, 2008. Accepted October 23, 2008.

The preferential accumulation of selected organic pollutants at higher altitude has been observed in a number of mountain regions. It is proposed that this phenomenon is due to differences in the efficiency of precipitation scavenging at various elevations, which, in turn, is due to the temperature dependence of organic vapor partitioning into rain, snow, and aerosols. The occurrence and extent of enrichment with elevation depends on whether the scavenging efficiency of a chemical is sensitive to temperature within the range encountered along a mountain slope. A multicompartment fate and transport model parameterized for mountain systems suggests that substances with equilibrium partitioning coefficients at 25 °C between water and air from 103.5 to 105.5 and between atmospheric particles and air from 109 to 1011 are most likely to be subject to mountain cold-trapping. Such substances remain in the atmospheric vapor phase at higher valley temperatures, but are scavenged efficiently at the lower temperatures prevailing at higher altitudes. This implies that substances subject to mountain cold-trapping are approximately 2 orders of magnitude less volatile than substances that experience global cold-trapping. For example, while lighter PCBs get preferentially trapped at higher latitudes, the heavier PCBs are predicted to experience the strongest mountain coldtrapping. These model results agree with the results of field studies, with the exception of those studies that rely on sample media such as plant foliage for which precipitation is not the dominant deposition pathway. It appears that very fast deposition processes are required to trap contaminants along mountain slopes, whereas such processes reduce contaminant transport to remote polar regions.

Introduction Concentrations of contaminants in the environment typically decrease with increasing distance from the source because of dilution, dispersion, and degradation. Intriguingly, sometimes the opposite is true and concentrations in samples taken distant from sources are higher than in samples taken close to the point of release. The study of such counterintuitive phenomena is useful as it may reveal insight into important, exposure-relevant contaminant amplification processes (1, 2). Among the best known cases of such inverted gradients are concentrations of selected persistent organic pollutants (POPs) that increase with increasing Northern latitude. Such behavior has, for example, been observed for hexachlorobenzene (HCB) in foliage (3, 4) and for R-hexachlo* Corresponding author tel.: 1-416-287-7225; e-mail: frank.wania@ utoronto.ca. 9092

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rocyclohexane (R-HCH) in seawater (5). The shift in phase distribution equilibria of semivolatile chemicals from the atmospheric gas phase to the Earth’s surface with decreasing temperature can explain this distribution (6). This process has been termed “cold condensation”, but should more appropriately be called “polar cold-trapping” because the organic contaminants are not actually condensing at the colder temperatures of polar latitudes. The transport to higher latitudes can, at least in part, occur via repeated cycles of deposition and evaporation, driven by seasonal, frontal, and diurnal changes in temperature (“grass-hopping”). Globalcold-trapping therefore is most pronounced for so-called “multi-hoppers”, i.e., chemicals that readily exchange between the Earth’s surface and the gas phase (7). When comparing levels of POPs in foliage from around the world, Calamari et al. (3) noted not only higher HCB concentrations at higher latitudes, but also that samples from tropical latitudes had higher concentrations if they had been taken from high elevations. Since then, numerous studies have observed concentration gradients of POPs and other anthropogenic organic contaminants in various environmental media that increase with increasing elevation (8-14), a phenomenon thatsdue to its obvious similarities with the inverted latitudinal concentration gradientsshas been termed “orographic” or mountain cold-trapping (12). Because of those similarities, it appears to have been implicitly assumed that the same mechanism of repeated temperature-driven air-surface exchange is also responsible for mountain coldtrapping, i.e,. it was assumed that POPs are hopping up the mountains (3, 12). However, several observations are challenging this assumption: Differences in the Fractionation Pattern along Latitudinal and Elevational Gradients. One of the fascinating aspects of “polar cold-trapping” is that it leads to chemical fractionation of compound mixtures that differ in their susceptibility for air-surface exchange. In particular, the more volatile components of mixtures such as the polychlorinated biphenyls (PCBs) were shown to hop poleward much more efficiently than the less volatile ones, leading to a compositional shift to more volatile constituents with increasing Northern latitude (15-17). This “global fractionation” can be quantitatively reproduced by global transport and distribution models (18). Although the same families of pollutants, in particular the PCBs, have been shown to concentrate in both high latitude and high altitude locations, a detailed comparison of studies shows that different chemicals concentrate in high latitudes than at high elevations. For instance, burbot (Lota lota) from Canadian fresh waters show higher concentrations of trichlorinated biphenyls with increasing latitude, but not of the heavier congeners (15). However, measurements in fish in European mountains and in various other media show preferential concentration of the heavier hexachlorinated biphenyls PCB-138 and PCB153 (e.g., refs 12 and 19). And while lighter polybrominated diphenyl ethers (PBDEs) preferentially concentrate poleward in fish (20), no such upslope trend is evident in fish in mountain lakes (11, 21). In other words, even though we observe fractionation of compound mixtures along both latitudinal and elevational gradients, different chemicals become enriched at higher altitude and higher latitude. Initial attempts at explaining these differences remain vague and ultimately unconvincing (12, 22, 23). Differences in the Mountain Cold-Trapping Observed by Different Studies. While reviewing the field data on concentration changes with altitude published at the time, Daly and Wania (22) noted a wide variety of patterns. For 10.1021/es8013198 CCC: $40.75

 2008 American Chemical Society

Published on Web 11/17/2008

example, while HCB was shown to be elevated in foliage sampled at high elevations in tropical latitudes (3) it was not seen to concentrate at higher elevations in the soils of the Andes Mountains (24). In some cases, such apparent discrepancies may be due to differences between the media sampled, such as moss or snow, which are in direct contact with air and able to undergo rapid exchange, versus fish, sediment, or soil, which are not (25). More recently differences in composition of POPs contamination with altitude in soil of the Peruvian Andes compared to the Italian Alps were noted (23). It was suggested that differences in the pattern and amount of precipitation was at least partly responsible. It has also been proposed that the differences in the profiles of PBDEs between the Pyrenees and the Tatras mountains were due to the lack of time for secondary distribution to occur (11, 19). However, no cogent and comprehensive explanation for these differences has so far been forthcoming. Here we propose a mechanism for the mountain coldtrapping of organic contaminants, which is different from the process responsible for polar cold-trapping, even though it is also driven by the temperature dependence of the partitioning equilibria between the atmospheric gas phase and condensed phases. Specifically, building upon the work by Daly et al. (13), we hypothesize that it is the temperature dependence of the precipitation scavenging efficiency of organic chemicals which underlies mountain cold-trapping. Here we introduce this hypothesized mechanism in detail, and expand on calculations that indicate which organic chemicals are most strongly affected by it. This will help us explain why different chemicals are subject to polar and mountain cold trapping. We will also attempt to shed light on the somewhat bewildering differences in the cold-trapping behavior observed in different studies.

The Mechanism of Mountain Cold-Trapping Introducing the Hypothesis. We hypothesize that the main mechanism of mountain cold-trapping is the temperaturedriven difference in the efficiency of precipitation scavenging between lowland and mountain. The chemicals that achieve higher concentrations at higher altitudes than at lower elevations are those that are not efficiently scavenged by precipitation falling at the temperature prevailing at lower elevations, but whose rate of wet deposition increases as an orographically lifted air mass cools. Temperature Dependence of the Rate of Wet Deposition. In the context of quantifying atmospheric wet deposition processes, the ratio of a chemical’s concentration in precipitation to its concentration in air is known as the scavenging ratio, Wtot. Simple theoretical considerations (26) show that a chemical with a Wtot below approximately 3000 is not efficiently scavenged by precipitation, and dry deposition is its primary mode of transfer to the surface. If, on the other hand, a chemical’s Wtot exceeds approximately 300,000, it is quickly scavenged from the atmosphere at the onset of a precipitation event, and wet deposition is the dominant mode of transport to the surface. Wet deposition is much faster than dry deposition, so a shift in importance from dry to wet deposition also implies an increase in the total deposition rate. Only in the range between these two thresholds (3.5 < log Wtot < 5.5) is the wet deposition rate sensitive to the value of Wtot. For reasons that will be explained in detail below, Wtot is a function of temperature, generally increasing with decreasing temperature and vice versa. However, a change in temperature will only lead to a change in the rate of wet deposition of a chemical if that temperature change causes a change in that chemical’s Wtot within the range 3000 to 300,000. For the sake of brevity, these values of Wtot shall hereafter be called the Wet Scavenging Ratio Relevance Range, or WRR.

In general, an increase in elevation corresponds to a decrease in temperature, i.e. a typical mountain is colder at the summit than at the base. Even if we assume that the precipitation rate does not change with elevation, the rate of deposition of a compound increases with increasing elevation along a mountain slope if its log Wtot falls within the WRR within the temperature gradient encountered along the slope. Relating Wtot with Chemical Partitioning Properties. To predict and understand which organic chemicals will be subject to mountain cold-trapping, we will need to relate Wtot to chemical partitioning properties. Precipitation can scavenge organic chemicals from the atmosphere as vapors or with particles. Wtot is the sum of the scavenging ratios for the vapor WG and for the particle-sorbed chemical WP, weighted by the fraction that is particle sorbed Φ Wtot ) (1 - Φ)WG + ΦWP

(1)

The diffusive movement of organic vapors is sufficiently rapid to allow for the assumption of equilibrium between atmospheric phases (27-29). Assuming equilibrium between atmospheric gas phase and rain droplets implies that the gas scavenging ratio WG equals the equilibrium water-air partition coefficient KWA or Henry’s law constant. Making further the assumption of equilibrium between atmospheric gas and particle phase, Φ can be calculated from the equilibrium particle-air partition coefficient KPA (26), using 1

Φ) 1+

(2)

1 KPA · νPA

where vPA is the volume fraction of particles in the air. The scavenging ratio for particles WP depends on the characteristics of both particles and precipitation, but is generally on the order of 105. This implies that any compound that becomes particle-sorbed is also becoming subject to efficient precipitation scavenging. Temperature Dependence of Wtot. The temperature dependence of Wtot stems from the temperature dependence of KWA and KPA, which is quantitatively expressed by the energies of phase change between water and air, ∆UWA, and particles and air, ∆UPA, respectively.

( (

) )

log KWA(T) ) log KWA(Tref) -

∆UWA 1 1 ln(10) · R T Tref

(3)

log KPA(T) ) log KPA(Tref) -

∆UPA 1 1 ln(10) · R T Tref

(4)

Lei and Wania (26) have estimated Wtot as a function of temperature for a range of different organic compounds. As stated above, a change in wet deposition rate with temperature occurs if a compound’s total scavenging ratio log Wtot changes within the range 3.5-5.5, the WRR. This can happen under two possible scenarios: the first is that the log KWA of that chemical shifts within the range 3.5-5.5 under the relevant temperature change. The other is that the chemical’s KPA changes such that it becomes particle bound and therefore subject to efficient particle scavenging.

Air Mass Transport and Mountain Cold-Trapping An interesting aspect of air mass transport in mountains is the occurrence of periodic reversals in atmospheric transport direction along mountain slopes. On a local scale, there are diurnal valley wind systems that blow upward during the day and downward during the night. During clear days, diurnal air temperature variations that are stronger within alpine valleys than in adjacent plains can create a diurnal thermal wind system that can propagate well into a mountain VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Total wet scavenging ratios (Wtot) of PCBs and HCB (A), PAHs (B), and pesticides (C) within the environmentally relevant temperature range. Wtot for flake (specific surface area 0.158 m2 g-1, upper line at subzero temperatures) and bullet snow (0.036 m2 g-1, lower line) were calculated using sorption coefficients estimated according to ref 45. The blue areas indicate the lapse range of a typical tropical mountain (A), the total lapse range of a temperate mountain (annually averaged range indicated with navy hatching) (B), and the winter and summer lapse ranges of the same temperate mountain (C). Dark blue indicates rain, and light blue represents snow. The orange area corresponds to the Wet Scavenging Ratio Relevance Range (WRR). The deposition of chemicals with Wtot lines in the green areas where WRR and lapse range overlap should be sensitive to temperature in the lapse range of a mountain and therefore lead to their mountain cold-trapping. The middle tick mark in the seasonal lapse range indicates the temperature at the tree line (see Figure 2A). range’s foreland (approximately 100 km) (30). Such regional scale wind systems lead to the phenomenon of “alpine pumping” or “mountain venting”, which can quickly transport boundary layer air from the foreland into higher layers of the atmosphere above the mountain range. On an even larger scale, there are also seasonal reversals of air mass transport in and out of mountain ranges, the most obvious being the Indian monsoon in which air from the subcontinent blows into the Himalayas from June to September and in the other direction during the rest of the year. All of these flow patterns have in common that they favor the net upward transfer of semivolatile contaminants (22). Air mass transport into the mountains occurs during warmer periods of the cycle and higher temperatures enhance contaminant mobility of persistent chemicals (31). Upward transport of air masses also increases the likelihood of precipitation at higher altitude and therefore the possibility of efficient contaminant deposition.

Predicting which Contaminants Experience Mountain Cold-Trapping To predict which organic chemicals are preferentially deposited at the higher elevations of a mountain range we need to identify which chemicals experience a change in log Wtot between 3.5 and 5.5 within the lapse range of the mountain. As mountains differ in terms of the lapse range, i.e., in terms of the temperature gradient encountered between valley bottom and summit, we should expect different chemicals to become enriched in different mountains. High mountains have a larger lapse range than low mountains. Whereas the lapse range of tropical mountains is generally constant throughout the year, those of temperate and polar mountains undergo large seasonal shifts. Furthermore, the lapse ranges of tropical mountains are in a higher temperature bracket than the lapse ranges of high latitude mountains, which therefore often straddle the freezing point. The latter implies that changes in Wtot with altitude become influenced by the relative scavenging efficiency of rain and snow. Cold-Trapping in Tropical Mountains. It transpires that cold-trapping in tropical mountains is conceptually the easiest, because it avoids the complications of seasonality and snow. Figure 1A graphically represents how to identify chemicals that become enriched on a hypothetical tropical mountain, the lapse range (9-26 °C) of which is shown in 9094

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blue. The Wtot as a function of temperature for each of the listed chemicals was calculated using the method by Lei and Wania (26). Tables with the physical-chemical properties are provided in the Supporting Information. The Wtot of the heavier (more chlorinated) PCBs pass through the WRR within the lapse range of the mountain, whereas the Wtot of the lighter PCBs and HCB do not. The wet deposition rate at the base of the mountain is thus lower than at the summit for the heavier PCBs but not for the lighter PCBs or HCB. This suggests that in environmental media that receive chemical input mostly by wet deposition (e.g., soils), we would expect enrichment of heavier PCBs at higher altitude in tropical mountains, but not for lighter PCBs and HCB. Cold-Trapping in Temperate Mountains. As we move to higher latitudes, the lapse ranges of mountains shift to lower temperatures (Figure 2A). This implies that the chemicals becoming enriched at higher elevations of tropical mountains are likely to be slightly less volatile than those becoming enriched in colder mountains. A complication arises in mid and high latitude mountains, and even in some very high tropical mountains in that the lapse range often includes the freezing point, i.e., whereas it rains at lower altitudes, precipitation at higher altitudes falls in the form of snow (Figure 1B). The enrichment of organic chemicals at higher altitudes then depends on the relative rate of deposition by rain and snow. When investigating this issue, Lei and Wania (26) concluded that the main reason that snow scavenging ratios of organic chemicals are often higher than rain scavenging ratios is the strong temperature dependence of gas phase to condensed phase partitioning. Small, polar chemicals, which readily dissolve in water, are expected to be more efficiently scavenged by rain than by snow. With increasing size and decreasing polarity the snow surface becomes as, or even more, efficient in accommodating organic vapors as liquid rain droplets (26). Overall, most semivolatile organic contaminants of interest in the context of mountain cold-trapping would be expected to be more efficiently scavenged by snow than by rain, and thus may become enriched at higher altitude, if the Wtot changes within the WRR. A seasonally shifting lapse range, as is common in temperate mountains, means that different substances may be experiencing changes in Wtot within the WRR during different times of the year (Figure 1C). In other words, summer rains may lead to the enrichment of slightly different chemicals at higher altitude than winter snow.

FIGURE 2. Summer (lower line of each pair) and winter (upper line of each pair) lapse ranges along the American Cordillera. The high temperatures are 24 h averages from the warmest and coldest months of the year from local weather stations. The lows were calculated based on a lapse rate of 5 °C km-1 from the elevation of the weather stations to the summits. The middle tick mark indicates the approximate temperature at the tree line (from elevation data in ref 34) calculated in the same manner. Only one line appears for Costa Rica which shows little temperature change over the year, and where the tree line is usually a function of volcanism rather than temperature and precipitation (A). Annual precipitation at different elevations for five latitudinal zones (from data in ref 33) (B). Figure 1B and C illustrate this for the polycyclic aromatic hydrocarbons (PAHs) and some pesticide chemicals, respectively. All PAHs have higher Wtot for lower temperatures overall, but the relative wet scavenging efficiencies of snow and rain can be compared within a few degrees of 0 °C. The most volatile PAHs (naphthalene, acenaphthene, fluorene) are more efficiently scavenged by rain than by snow at the freezing point, but their Wtots do not cross the WRR for either snow or rain, so wet deposition is not an important fate process for these PAHs. The least volatile PAHs (e.g., benzo[a]pyrene) are so strongly particle sorbed that their Wtots do not change appreciably over the lapse range of the mountain. Only the Wtot of the intermediate PAHs with three (anthracene, phenanthrene) and four rings (pyrene, chrysene) pass through the WRR of snow and rain, respectively, and thus would be expected to be preferentially enriched by winter and summer precipitation, respectively. Figure 1C suggests that the hexachlorocyclohexanes should be subject to strong cold trapping in mountains with a lapse range that includes the freezing point, because Wtot for snow is much larger than that for rain. Chlorothalonil and chlorpyrifos, on the other hand, would only be expected to show increasing deposition rates with elevation in warm mountains (13).

The Role of a Spatially and Temporally Variable Precipitation Rate Role of the Precipitation Rate Variability with Elevation. So far, we have only considered the change of temperature with altitude and season, yet precipitation also changes often

with time and space. If variations in the rate of wet deposition are at the core of the mechanism underlying mountain coldtrapping, changes in the precipitation rate with altitude play an important role. Specifically, precipitation rates that increase with elevation will enhance mountain cold-trapping. This has already been recognized by other workers (8, 23). Precipitation rates that increase with elevation can explain the mountain cold-trapping of compounds whose Wtot does not change with temperature and elevation. This includes substances that are particle-bound at all temperatures within the lapse range of a mountain slope. Zechmeister (32), for example, explained the enrichment of heavy metals in higher elevation moss samples from the Austrian Alps with precipitation rates that increase with altitude. Not in all mountains, however, does the precipitation rate peak at the highest elevations. The variation of precipitation with altitude varies widely on a global scale. A global survey of vertical precipitation profiles using data from 1300 long-term meteorological stations (33) reveals that it is mostly midlatitude mountains that experience a strong increase of precipitation with elevation. Tropical areas between 10° and 20° latitude (N and S) show a maximum in precipitation at 1-1.5 km, whereas equatorial areas, defined as those within 10° of the equator, even exhibit a general decrease in precipitation with elevation. A transitional type in the subtropics shows little height dependence, whereas polar areas tend to have higher precipitation amounts at low elevations. The variability in precipitation rate with elevation may thus reinforce, counteract, or confound the effect of the temperature-dependent scavenging efficiencies on mountain cold-trapping (Figure 2B). Role of Seasonally Changing Precipitation. Seasonal changes in the rate of precipitation could be of importance in the mountain cold-trapping of contaminants, when they interact with seasonally variable temperature lapse ranges. In particular, we may hypothesize that the lapse range during a “wet season” is more important in controlling the relative rate of wet deposition along the mountain slope than the temperature range during a “dry season” (Figure 1C). The range given by the annually averaged temperatures along the slope may not be particularly relevant, unless the “wet season” occurs during spring and/or fall, i.e., when those temperature ranges actually occur.

Why Do Measured Concentration Gradients with Elevation Vary? Mountains differ in terms of lapse range, change in precipitation rate with altitude, and how both temperature and precipitation change seasonally. It is thus not at all surprising that we should see a bewildering array of different concentration gradients with altitude (22). There are yet more confounding factors that contribute to this diversity of observations: Role of Contaminant Retention and Degradation in Determining Concentration Gradients. The relative rate of contaminant deposition at different altitudes is only part of the story, as concentration gradients can additionally be influenced by the rate of loss of contaminant from an environmental medium. The evaporation of contaminants from, and their degradation in, surface media are likely to vary with elevation. Some snow-scavenged contaminants will already evaporate during snow metamorphosis (35, 36), and further volatilization losses will occur during snow-free periods. Soils at different elevations may differ greatly in the extent to which they can retain the deposited contaminants. In temperate mountains, in particular, soils above the tree line, with little vegetation cover and low organic matter content, may have limited capacity for contaminant retention (14). If the rate of degradation processes decreases with VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. A conceptual representation of the Mountain-POP model. elevation, e.g., because of lower temperatures and decreased biological activity, this can enhance the enrichment of contaminants at higher altitudes. Concentration Gradients in Environmental Media for Which Precipitation Is Not the Dominant Deposition Mechanism. Wet deposition is very likely the chief route by which organic contaminants enter into mountain soils and watersand thus also fish. However, uptake of these chemicals by higher plants is rarely dominated by wet deposition. Snow, in particular, may not be particularly important in delivering contaminants to tree foliage because the foliage either is present only in summer or the snow falls from the foliage before it melts. If dry gaseous deposition is the dominant deposition process, higher concentrations in foliage sampled at higher elevation can be caused by an increase in the foliage/air partitioning coefficient at lower temperatures. If the same air concentration equilibrates with foliage at different temperatures, the coldest foliage will display the highest concentrations. Of course, this would only be expected to occur for substances that can achieve such equilibrium during the lifetime of the foliage, namely relatively volatile substances with a log KOA of less than approximately 9.5 (37, 38). We suggest that this can explain why lighter PCB congeners were observed to experience stronger cold-trapping than heavier PCB congeners in conifer needles sampled in the mountains of Western Canada (10). The concentration of light PCBs is controlled by the needles’ temperature dependent uptake capacity, whereas that of the heavy PCBs is kinetically controlled. It also explains why HCB concentrations in foliage are often shown to increase with elevation (3) even though the low KWA of HCB means that it is not subject to wet deposition. Finally, it can explain differences in the concentration gradients measured in needles, humus, and soils sampled along the same altitudinal transect (e.g., refs 39 and 40). Further complicating the interpretation of measurements in biota are differences in growth rate, whereby a rapidly growing organism may exhibit a dilution effect when contrasted to a slow growing one. In one study of trout in the Canadian Rocky Mountains, for instance, growth rate of the fish explained as much of the variation in organochlorine contaminant concentrations as elevation did (41). Finally, if a chemical is emitted along the mountain slope, distance from that source will often be the dominant determinant of contaminant concentrations (42). Which Mountains Are Most Efficient in Cold-Trapping Contaminants? Based on an understanding of the mechanism of contaminant cold-trapping in mountains, we may speculate which mountains are expected to be most susceptible to it. First of all, we would expect a higher extent of contaminant deposition in humid mountains compared to those in arid regions. The largest differences in deposition between high and low altitude we would expect to observe in mountains in which 9096

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the precipitation rate is inversely correlated with temperature (i.e., those in which the precipitation rate increases with altitude) (Figure 2B), and in mountains with the largest lapse ranges (i.e., those with the highest temperature difference between valley and summit) (Figure 2A). This suggests that high mountain ranges in the humid temperate zone should be most susceptible to the enrichment of contaminants at higher elevation. Examples are the European Alps, and the Coastal Ranges of temperate Western North America.

Simulating Contaminant Cold-Trapping in Mountains It should have become clear by now that the seasonal and spatial variability of temperature, precipitation rate and type, and surface characteristics makes it quite complex to understand and predict which chemicals are subject to mountain cold-trapping, and how different mountains differ in this regard. We suggest that simulation models, such as Mountain-POP (Figure 3), can play a useful role in unraveling that complexity. The dynamic multicompartment level IV fugacity-based fate and transport box model Mountain-POP was designed explicitly to simulate the transport and distribution of organic contaminants along elevational gradients (13). It is comprised of 10 compartments: one compartment for soil and one for the atmosphere for each of five elevation zones. To illustrate its usefulness for investigating the mechanisms of mountain cold-trapping the model was parameterized for a hypothetical mountain system, 120 km long from lowest to highest point. Whereas the precipitation rate was assumed uniform with altitude and constant in time at 1 m a-1, temporally uniform temperature was assumed to decrease by 5 K increments from 25 °C in the lowest altitudinal zone to 5 °C at the summit. To compare chemicals in terms of their susceptibility for mountain cold-trapping, we can define a “mountain contamination potential” (MCP) as the fraction of the total chemical amount in the mountain environment, which is present in the soils of the two highest altitudinal zones: 5

MCP )

∑M

i,soil

i)4

5

∑ i)1

(5)

5

Mi,soil +

∑

Mi,air

i)1

where Mi,soil and Mi,air are the mass of chemical in the soil and air compartment of zone i, respectively. Zone 1 is the lowest zone and zone 5 is the highest. To identify the organic chemicals that would become enriched at the higher elevations of this hypothetical mountain system, we calculated the MCP at steady state (after 10 years of simulation) for perfectly persistent, hypo-

will not. This is in agreement with the observations in the literature (3, 12, 19) as discussed above.

On the Difference Between Polar and Mountain Cold-Trapping

FIGURE 4. Mountain Contamination Potential (MCP, normalized to the maximum) in a warm mountain (5-25 °C) as a function of the equilibrium partitioning properties of a theoretical, perfectly persistent contaminant at 25 °C, as calculated by Mountain-POP. The partitioning properties of selected PCB congeners (white squares), PAHs (blue diamonds), and pesticides (colored circles; CTh, chlorothalonil; CPy, chlorpyrifos; Fnt, fenthion; TAI, triallate; Phn, phenol; alp, r-HCH; bet, β-HCH; gam, γ-HCH) are placed on the chemical space. thetical chemicals emitted steadily into the atmosphere of the lowest zone. The chemical’s equilibrium partitioning coefficients between water and air (0 < log KWA < 6) and octanol and air (6 < log KOA < 12) were varied over 6 orders of magnitude, respectively. In the model, KWA is used to describe vapor scavenging, whereas KOA is used to describe partitioning between gas phase and organic matter in both atmosphere and soil compartments. The temperature dependence of KWA and KOA was assumed to be the same for all hypothetical chemicals (∆UWA ) 60 kJ/mol, ∆UOA ) 60 kJ/mol). MCP is then plotted as a function of these partitioning values in what is known as a chemical space plot (Figure 4). Highest MCP values are predicted for chemicals with a log KOA between 8.5 and 11.5 and for those with a log KWA between 3.5 and 6 (Figure 4). The former substances with log KOA around 10 change from being gas-phase compounds at 25 °C to particle-bound substances at 5 °C, when the volume fraction of particles in the air vPA is 10-12, and the organic matter content of those particles is 5%. In other words, these are substances whose vapors are not efficiently rain scavenged at temperatures prevailing in lower zones, but who are efficiently scavenged with the particles to which they sorb at the lower temperatures at higher altitude. Similarly, the vapors of chemicals with a log KWA around 4.5 are not rain scavenged at 25 °C in the lowland, but are deposited with those rain droplets at 5 °C. Chemicals in the upper left of Figure 4 (log KWA < 2, log KOA < 7.5) are too volatile to be scavenged as either vapors or particle-bound species, even at the low temperatures of the higher altitudinal zones. Chemicals with partitioning properties corresponding to the right (log KOA > 12) and lower (log KWA > 7) edge of the chemical space in Figure 4, on the other hand, are so efficiently scavenged by rain, even at high valley temperature, that they do not become enriched at higher altitudes, unless the precipitation rate increases with elevation. One can use chemical space plots as a guide to the likelihood that a given chemical will or will not concentrate preferentially at high elevations in mountain systems. The partitioning properties of HCB and selected PCB congeners are included in Figure 4. The simulation predicts that the heavier PCBs 180 and 153 will preferentially concentrate upslope on a mountain, and that the lighter PCBs and HCB

Figure S1 in the Supporting Information compares the chemical space plots indicating the combination of chemical partitioning properties that make a persistent organic contaminant susceptible to enrichment in polar regions (7) and high mountain areas, respectively. Intriguing similarities and discrepancies become apparent. In both cases the highest potential for relative enrichment is predicted for chemicals with intermediate KWA and KOA, i.e., both plots have a vertical and a horizontal band indicative of a limited range of partitioning properties causing elevated cold-trapping. A closer look reveals that there are differences in the range of partitioning properties that lead to elevated mountain and polar cold-trapping (Figure S1). Specifically, the bands of elevated cold-trapping in mountains and polar regions are centered around a log KOA of 8 and 10 and a log KWA of 2 and 4.5, respectively. In other words, the chemicals becoming enriched in mountains tend to be less volatile than those preferentially accumulating in polar regions by about 2 orders of magnitude. When overlaying the properties of the PCBs onto the chemical space maps of Figure S1, it becomes clear that the model predicts enrichment of the lighter PCBs in the polar regions, but enrichment of the heavier PCBs at higher elevation. As discussed above, there is observational evidence to support those predictions. We conclude that in both polar and mountain coldtrapping, temperature gradients and their impact on gas phase/condensed phase partitioning play a crucial role. Nevertheless, these processes are controlled by different mechanisms and affect different chemicals. In the case of polar cold-trapping the temperature dependence of partitioning between the Earth’s surface and atmosphere is at the basis of the grass-hopper effect. In the case of mountain cold trapping the temperature dependence of partitioning between the various atmospheric components (gas phase vs particles, rain droplets and snow flakes) is important. We suggest that the key issue underlying these differences is the different scale of the transport distances along latitudinal and elevational gradients. Very fast deposition processes, such as precipitation scavenging, are required to compete with atmospheric advection and trap contaminants along mountain slopes, whereas such processes reduce contaminant transport to remote polar regions.

Implications of Mountain Cold Trapping Many of the chemicals that have been found to become enriched at higher elevations are bioaccumulative. Indeed, the partitioning properties favoring mountain cold-trapping (Figure 4) and the partitioning properties favoring bioaccumulation in both fish and bovine milk (43) overlap substantially (Figure S2). Mountain cold trapping could thus result in contaminant exposure of human populations that eat fish from alpine lakes or dairy products from livestock grazing a high altitude. Wildlife may also be affected, even if a substance does not biomagnify up the food chain. For example, pesticides have been implicated in the decline of the amphibian population in the Californian Sierra Nevada (44). It is thus clearly imperative to gain a full mechanistic and quantitative understanding of the process of mountain cold-trapping.

Acknowledgments We thank the Natural Sciences and Engineering Research Council of Canada for funding. VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available Additional figures and information on physical chemical properties used in the drawing of Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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