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Locating POPs Sources with Tree Bark Angela A. Peverly, Amina Salamova, and Ronald A. Hites* School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405, United States the atmospheric concentrations on a grid and time-scale designed to pinpoint the source, but this is often prohibitively expensive. To set up so-called active atmospheric sampling stations, which pump the air through a filter and an adsorbent at a known flow rate, requires thousands of dollars in equipment, continuous electricity and security, and someone to retrieve and replace the sampling media on a regular schedule. Furthermore, given the variability of the air movement and its temperature, repeated measurements at each active air sampling site are needed. This is not to say that active air sampling systems are not warranted; they are when the problem is of sufficient complexity and importance to warrant the expense. For example, the Integrated Atmospheric Deposition Network (IADN) operates several, permanent, active-sampling stations around the Great Lakes with the goal of understanding atmospheric transport of selected persistent Locating sources of persistent organic pollutants (POPs) to the organic pollutants (POPs) into the lakes.4 atmosphere can sometimes be difficult. We suggest that tree Rather than use active atmospheric sampling, another bark makes an excellent passive atmospheric sampler and that approach is to use passive sampling, in which pollutants are spatial analysis of tree bark POPs concentrations can often adsorbed by a sampling medium in contact with the pinpoint their sources. This is an effective strategy because tree surrounding air. Passive sampling systems do not actively bark is lipophilic and readily adsorbs and collects POPs from pump air through a filter or adsorbent. Instead, they use a the atmosphere. As such, tree bark is an ideal sampler to find variety of adsorbents deployed in a variety of formats− POPs sources globally, regionally, or locally. This article sometimes in combination with one another. Perhaps the most summarizes some work on this subject with an emphasis on common passive air sampler is a simple polyurethane foam kriged maps and a simple power-law model, both of which have (PUF) disk, which is usually deployed in a housing consisting been used to locate sources. Three of the four examples led of two stainless steel salad bowls (see Figure 1). This system directly to the pollutant’s manufacturing plant. can be deployed almost anywhere, hanging from the side of a building, from a utility pole, or even from a tree. Once FINDING SOURCES deployed, these samplers can be left in place for several weeks, One of the questions an environmental chemist frequently faces and after retrieval, the PUF disks can be analyzed for the is: Where did a pollutant come from? Sometimes the pollutant’s structure is unusual, and the location(s) of its uses are known; in this case, the link back to its source is not too difficult to make. For example, high concentrations of flame retardants observed in the air near an e-waste recycling facility in China are easily linked to that source.1 Sometimes a pollutant’s structure is common, and the locations of its sources are either numerous or not known. For example, high concentrations of SO2 in the air in the Adirondack Mountains cannot be linked with any certainty to a specific point source.2 Sometimes to determine the source of a pollutant, environmental samples can be taken on a grid of sufficiently small spacing to suggest one or more sources. For example, repeated sediment measurements of mirex in Lake Ontario pinpointed the Oswego River as one of its sources.3 Figure 1. Schematic of a polyurethane foam (PUF) disk passive Isolating the sources of atmospheric pollutants is particularly sampler. The yellow stripe represents the PUF disk, which is typically troublesome. Unlike rivers, air moves in many directions at 10−15 cm in diameter and about 1 cm thick. various speeds. Thus, a compound found in the air at a particular location may have come from near-by or far away. In Special Issue: Ron Hites Tribute addition to modeling the meteorology, one approach to locating these sources is to make frequent measurements of
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© XXXX American Chemical Society
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Figure 2. Concentrations of total DDTs measured in tree bark samples collected from 90 sites worldwide. All concentrations are in ng per gram of bark lipids.19
regional, or local scales. Different parts of a tree have been used for these types of studies. Sampling the inside of a tree using a corer (also called an increment borer) gives an environmental chemist access to the water being pumped from the roots of the tree to the leaves. The concentrations of pollutants that are even slightly soluble in this water can be measured and mapped spatially as a function of tree location. This approach has been termed “phytoforensics, dendrochemistry, and phytoscreening”; see the ES&T Feature article by Burken et al. for an excellent review of this topic.7 This experimental tactic has been widely and successfully applied to the location of underground plumes of, for example, chlorinated ethenes.8−11 However, for very lipophilic compounds, such as chlorinated pesticides or polychlorinated biphenyls (PCBs), this approach will not work because most of these compounds tend to stay with the soil lipids and do not move to the inside of the tree.12,13 In our laboratory, we have extensively used tree bark as a passive
persistent organic pollutants of interest. One of the problems with passive samplers is that it is difficult to know the volume of air that has been sampled, unlike active samplers, which operate at a known flow rate for a known time period. There are ways around this problem using sophisticated meteorological modeling methods,5 but it remains an issue. For a review of passive air sampling, see the special issue of Environmental Pollution organized by Harner et al.6
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TREES AS PASSIVE SAMPLERS
Another approach to passive air sampling is to use vegetation, particularly trees. Trees do not need to be deployed, they are present almost everywhere, they do not require electricity, and they can be sampled nondestructively. Their widespread availability makes them ideal for initial studies aimed at identifying compounds in the atmosphere or in groundwater and for more detailed studies aimed at pinpointing sources. Trees are useful passive air samplers that can be used on global, B
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Figure 3. Relative concentrations of (A) toxaphene in tree bark from North America,16 (B) polybrominated diphenyl ethers in tree bark from North America,17 (C) Dechlorane Plus in tree bark from the northeastern United States,22 and (D) total PCBs in tree bark from northeastern Alabama, United States.23 The black dots indicate sampling sites. Red represents “hot spots,” and blue represents the opposite. The width of each panel represents a total distance of: (A) 7400 km, (B) 7400 km, (C) 2100 km, and (D) 45 km. The white stars in each panel represent likely sources of each substance.
tration in the air surrounding a tree at a given site. Unlike active and passive air sampling, the amounts of a pollutant in tree bark are not given as mass per unit volume of air (for example, as pg/m3) but rather as mass of pollutant per mass of bark or mass of lipids in the bark (for example, as ng/g lipid). The normalization to bark lipids was an early attempt to compensate between different tree species,16 but it is not necessary if similar tree species are sampled. Because of their wide availability, we generally use pine trees. The most extensive example of using tree bark to measure the geographical distribution of selected POPs is a 1995 paper in which the concentrations of 22 organochlorine pesticides were measured in more than 200 bark samples from 90 sites worldwide.19 Relatively high amounts of some of these compounds were found at a few sites; for example, the samples from Romania were very high in DDT-related compounds probably because Romania was the second largest producer of chlorinated organic compounds in the former Soviet-bloc countries.20 Other sites, especially those in western sub-Sahara Africa, showed relatively low levels of most of these compounds, probably because these countries were too poor to afford the use of any pesticides. Figure 2 (top) shows these data for total DDTs (the sum of the concentrations of p,p′DDT, p,p′-DDE, and p,p′-DDD) as dot plots−the red dots indicating “hot spots” of DDT concentrations. It is clear from even this crude data visualization that there is a swath of high DDT tree bark levels running from Europe down through India and extending to Western Australia. A better way to visualize these data is to use kriging. Kriging is an interpolation technique, which accounts for the overall spatial arrangement among the measured points. Prediction maps are formed with three dimensions: latitude, longitude, and (in our case) the pollutant’s concentration in
sampler since 1987, and the rest of this article will focus on this approach.
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TREE BARK Tree bark, unlike the wood inside the tree, is relatively high in lipids. In fact, these lipids keep the water inside the tree and the rain out. Some tree bark has sufficiently high lipid levels that it can provide nutrition to desperate people; for example, many of the trees in North Korea have had their bark stripped to a height of about 2 m by people looking for something to eat.14 In our laboratory’s use of this media, tree bark is typically sampled with a clean chisel over a 10 × 10 cm area at about chest height from fully grown trees. Samples are taken to a depth of only ∼1 cm so that the tree is not permanently damaged, and it will heal within a few weeks. The resulting sample of bark is cut into small pieces with clean shearers and Soxhlet extracted with various solvents. After suitable cleanup, the pollutants of interest are usually analyzed by gas chromatographic mass spectrometry. Tree bark is a low cost, simple, fast, and environmentally friendly sampling tool. Incidentally, each of the atmospheric sampling methods described here have different time constants. Active sampling usually runs for 1−3 days, and thus, it gives a good measurement of the average atmospheric concentration during that short time period. Passive sampling usually takes 3−8 weeks, and thus, it gives an estimate of the average atmospheric concentration over that longer time period. Tree bark accumulates organic pollutants over a much longer time period, which is dependent on the lifetime of the bark on that tree. This time period is not known with any precision, but for most trees without exfoliating bark, this time is probably in the 3−5 year range.15−19 In practice, this means that tree bark gives a long-term, integrated measurement of a pollutant’s concenC
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tree bark. Thus, a kriged map is a smoothed representation of the concentrations as a function of location and an extrapolation of these concentrations to places for which there are no measurements. Kriging interpolates the concentrations on a uniform surface, and variables such as terrain and wind direction are not considered. One decision that has to be made in using kriging is the radius of extrapolation as applied to sparse data. For example, if one had only one data point, say in Hawaii, and if one selected a 60° radius, then that one data point would be extrapolated to most of the Pacific Ocean− probably not a useful result. Therefore, as with all interpolation methods, one must examine the projections and confirm the conclusions carefully. Kriging has been applied to tree bark data by Zhao et al.,21 who used kriging to indicate regions of high concentrations of polycyclic aromatic hydrocarbons (PAH), several chlorinated pesticides and biphenyls (PCBs), and several brominated flame retardants throughout China. These authors found high tree bark levels of PAH around Beijing and high levels of brominated flame retardants around Shanghai. We have applied kriging to our previously published data on pesticides in tree bark sampled on a worldwide basis; see Figure 2 (bottom). Keep in mind that these are the same concentrations that were shown in the dot plot in Figure 2 (top). In this case, however, we have interpolated the data over a smooth circle with a radius of 25° centered on each sampling site, and we have used a smoothing factor of 0.2. The swath of high levels (red-orange color) still extends from Europe to Western Australia, but we see more clearly the low levels in the Caribbean, and Alaska. It is interesting that modest levels (orange to yellow) are apparent in central China and central Australia. In this case, the kriging map was computed with ArcGIS 10.1 software using empirical Bayesian kriging because it is the most accurate method for a small data set and is somewhat automatic, thus reducing human manipulation of the variables. In Figure 2, a large interpolation area (25° radius) was necessary because the data was on a global scale, but for smaller spatial scales, smaller areas are necessary. Figure 3 illustrates this idea for tree bark data for four different compounds obtained over decreasing spatial scales: continental, regional, and local. Figure 3A shows the concentrations of toxaphene in tree bark samples taken from the United States and Canada.16 Kriging of these data using a 5° radius prediction area clearly shows that the highest toxaphene concentrations were centered at Memphis, Tennessee. Remembering that toxaphene was the insecticide of choice to kill boll weevils on cotton after DDT was banned in 1972 and noting that the area covered by the orange oval in Figure 3A was, and is, the major cotton growing region of the United States, it did not come as a surprise that toxaphene levels in tree bark were particularly high in this region. We can model these concentrations as a function of distance from a source using a simple approach. It can be shown16 that this relationship has the form: Ci = a0Di−a1
ς=
∑ (Ci − a0Di−a )2 1
i
(2)
This calculation proceeds by changing the latitudes and longitudes of a possible source while changing the values of a0 and a1 such that ζ is minimized. This calculation has been implemented in an Excel spreadsheet using the Solver tool. Once the location of the source is known, then it is convenient to display the data by taking the logarithms of both sides of eq 1, simplifying the results to a linear regression. If the mechanism of concentration decrease from the source is simply radial diffusion, then the expected value of a1 is 2.16 If the mechanism is based on a Gaussian plume, then the expected value of a1 is about 1.5.22 Notice that wind direction is not included in this model. To calculate the distances for the toxaphene data shown in Figure 3A, we assumed that the source was Memphis, a location that had been previously pinpointed using eq 2.16 Given the latitude and longitudes of each tree bark sample and of this source, the spherical distances of each sample from the source were calculated, and the toxaphene concentrations as a function of these distances were plotted on a log−log scale (Figure 4A).
Figure 4. Concentrations of (A) toxaphene,16 (B) polybrominated diphenyl ethers,17 (C) Dechlorane Plus,22 and (D) polychlorinated biphenyls23 in tree bark collected at various distances from suspected sources. The red lines are regressions using eq 1; the P value indicates the probability of insignificance. For the first three substances, the bark samples were collected at each site in triplicate, and each was measured individually. The error bars are the standard errors of these replicate measurements. The concentrations for the first three substances are on a bark weight basis; the concentrations for the PCBs are on a lipid weight basis. There are no replicates for the PCB data.
The regression is highly significant, and the resulting value of a1 is 1.01, which is lower than we might have expected but still within a reasonable range. It is interesting to note that these concentrations are down to what we might consider to be background levels at about 1000 km from the source. Clearly, there is considerable long-range transport of toxaphene through the atmosphere. Another kriged map (Figure 3B) shows the relative concentrations of polybrominated diphenyl ethers (PBDEs) in tree bark in North America.17 In this case, a 5° radius prediction area was also used. PBDEs were widely used to flame
(1)
where Ci is the concentration of toxaphene (in this case) in tree bark taken at a spherical distance Di from the source, and a0 and a1 are fitted constants. The units of a0 and a1 depend on the units of C and D, which in this case are ng/g and km, respectively. We have previously located sources using nonlinear curve fitting and the following function: D
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log−log plot shown in Figure 4D exhibits high significance and an a1 value of 1.59. Note that the PCB concentrations drop by a factor of 100 within 10 km of the plant, clearly a PCB “hot spot.” One may be wondering if tree bark concentrations can be directly related to air concentrations, and could tree bark levels be used to predict air levels? This question was addressed in our laboratory for a range of flame retardants, PCBs, and pesticides measured in air at several sites and over several
retard plastics such as polyurethane foam. By the early 2000s, it had become apparent that these compounds had escaped from the products in which they had been used and had widely contaminated people and the environment.24 The manufacturers of these chemicals took them off the market starting in 2004 and ending in 2013. Nevertheless, PBDEs are present in tree bark from the United States and Canada, and the kriged map shows a spatial distribution similar to that of toxaphene except that it is shifted to the west. Unlike toxaphene, which was deliberately released into the environment, PBDEs were not. Thus, the explanation for the distribution shown in Figure 3B is likely related to the production of these chemicals at two plants on the southern border of Arkansas−one plant in El Dorado and one in Magnolia. Thus, we assumed that these two plants were the sources of atmospheric PBDEs to North America and used their average latitude and longitude to calculate the spherical distances for each tree bark sample. A log−log plot of these data is shown in Figure 4B. The regression is highly significant, and the a1 value is 1.13. We suggest that the linearity of this plot supports our assignment of the source location. The ability of this approach to pinpoint a suspected source is good−especially given that most of the samples were taken at distance of 100−1000 km from the source. Another case, but on a smaller spatial scale, is that of Dechlorane Plus (DP), which is also a flame retardant.22 DP is highly chlorinated and has been on the market for many years. It was first detected in the environment in air samples collected near the eastern shore of Lake Erie.25 DP continues to be manufactured by the OxyChem plant in Niagara Falls, New York. Figure 3C shows the kriged map for DP in tree bark; in this case, the radius for the interpolation area was set to 1° to give higher spatial resolution. Clearly, the highest tree bark concentrations of DP are centered in Niagara Falls, and thus, we assumed that the source location was the OxyChem plant itself. The spherical distances of each sample were calculated using this source, and the data are plotted in Figure 4C. Again the regression is excellent, and in this case, the value of a1 is 1.28. The linearity of this plot is highly suggestive that we have located the source correctly. Incidentally, Figure 3C indicates that meteorology does not have much of an effect on tree bark concentrations. In this case, the concentrations upwind of Niagara Falls (the prevailing winds come from the south and southwest) are not significantly different than those downwind of this site. The last example is at a more localized scale. Polychlorinated biphenyls (PCBs) were widely used all over the world. They were especially favored as a nonflammable dielectric fluid for use in transformers and capacitors, but they were also used as heat transfer agents, dye carriers in carbonless copy paper, and even as solvents for pesticides. By the mid-1960s, it became apparent that PCBs were ubiquitous pollutants, and their production and use were banned−at least in North America, Japan, and Europe. While they were in production, about half of the United States’ supply was produced in Anniston, Alabama, and the associated waste was dumped near-by. Hermanson and Johnson investigated PCB concentrations in tree bark from this relatively small town,23 and their data are shown as a kriged map in Figure 3D. The radius of the prediction area was set to 0.04° because of the small spatial scale of these data. The location of the former Monsanto plant that produced PCBs at this location is shown, and it is clear that the tree bark PCB concentrations maximize around this production facility. The
Figure 5. Concentrations of several flame retardants, PCBs, and pesticides in air (measured with active sampling) and tree bark samples from the same sites around the North American Great Lakes showing the strong relationship between the two parameters. The red line is the log−log regression, which is highly significant (P < 0.001), and the blue lines are the 95% confidence limits of the regression. Replotted from Salamova and Hites.26
years.26 These data are shown in Figure 5 and gave the following regression line: log(Cair) = (0.980 ± 0.107)log(C bark) + (1.205 ± 0.083) (3)
where Cair is the total atmospheric concentration of a pollutant in the vapor and particle phases added together, both in units of pg/m3, and Cbark is the concentration of that pollutant in tree bark, in units of ng/g bark (not lipid). In this case, the prediction error is about an order of magnitude, which may seem like a lot. On the other hand, this relationship covers 4−5 orders of magnitude in both concentrations. If an error of this magnitude is acceptable, tree bark may be useful, keeping in mind how inexpensive and simple it is to collect. It is also worth saying the obvious: Three of our examples have pinpointed the source as the manufacturing facility of the chemical in question. In the case of Dechlorane Plus, this facility is still in operation; in the case of PCBs, this facility has long been closed. The PBDE plant is still in operation although its product mix has changed in recent years. So called “fugitive emissions” from these manufacturing facilities are clearly important, and in some cases, they can be tracked through the atmosphere for hundreds of kilometers. One should not neglect controlling these sources. Tree bark analyses can not only pinpoint sources, but such analyses can also be used for long-term monitoring of remediation. For example, a recent study has demonstrated that the Velsicol Superfund Site located E
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of measured concentrations in tree bark. Environ. Sci. Technol. 2003, 37, 475−484. (17) Zhu, L. Y.; Hites, R. A. Brominated flame retardants in tree bark from North America. Environ. Sci. Technol. 2006, 40, 3711−3716. (18) Peverly, A. A.; Salamova, A.; Hites, R. A. Air is still contaminated 40 years after the Michigan chemical plant disaster in St. Louis, Michigan. Environ. Sci. Technol. 2014, 48, 11157−11160. (19) Simonich, S. L.; Hites, R. A. Global distribution of persistent organochlorine compounds. Science 1995, 269, 1851−1854. (20) Romania: A Country Study; Bachman, R. D., Ed.; GPO for the Library of Congress: Washington, DC, 1989. (21) Zhao, Y.; Yang, L.; Wang, Q. Modeling persistent organic pollutant (POP) partitioning between tree bark and air and its application to spatial monitoring of atmospheric POPs in mainland China. Environ. Sci. Technol. 2008, 42, 6046−6051. (22) Qiu, X. H.; Hites, R. A. Dechlorane Plus and other flame retardants in tree bark from the Northeastern United States. Environ. Sci. Technol. 2008, 42, 31−36. (23) Hermanson, M. H.; Johnson, G. W. Polychlorinated biphenyls in tree bark near a former manufacturing plant in Anniston, Alabama. Chemosphere 2007, 68, 191−198. (24) Hites, R. A. Polybrominated diphenyl ethers in the environment and in people: A meta-analysis of concentrations. Environ. Sci. Technol. 2004, 38, 945−956. (25) Hoh, E.; Zhu, L. Y.; Hites, R. A. Dechlorane Plus, a chlorinated flame retardant, in the Great Lakes. Environ. Sci. Technol. 2006, 40, 1184−1189. (26) Salamova, A.; Hites, R. A. Evaluation of tree bark as a passive atmospheric sampler for flame retardants, PCBs, and organochlorine pesticides. Environ. Sci. Technol. 2010, 44, 6196−6201.
in St. Louis, Michigan, still is an atmospheric source of polybrominated biphenyls, DDT, and other chemicals made at the plant even after 40 years of remediation.18
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Mark Hermanson and Glenn Johnson for sharing the PCB data shown in Figure 3D.
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REFERENCES
(1) Tian, M.; Chen, S. J.; Wang, J.; Zheng, X. B.; Luo, X. J.; Mai, B. X. Brominated flame retardants in the atmosphere of e-waste and rural sites in southern China: Seasonal variation, temperature dependence, and gas-particle partitioning. Environ. Sci. Technol. 2011, 45, 8819− 8825. (2) Driscoll, C. T.; Driscoll, K. M.; Roy, K. M.; Mitchell, M. J. Chemical response of lakes in the Adirondack region of New York to declines in acidic deposition. Environ. Sci. Technol. 2003, 37, 2036− 2042. (3) Van Hove Holdrinet, M.; Frank, R.; Thomas, R. L.; Hetling, L. J. Mirex in the sediments of Lake Ontario. J. Great Lakes Res. 1978, 4, 69−74. (4) Salamova, A.; Ma, Y.; Venier, M.; Hites, R. A. High levels of organophosphate flame retardants in the Great Lakes atmosphere. Environ. Sci. Technol. Lett. 2014, 1, 8−14. (5) Petrich, N. T.; Spak, S. N.; Carmichael, G. R.; Hu, D.; Martinez, A.; Hornbuckle, K. C. Simulating and explaining passive air sampling rates for semivolatile compounds on polyurethane foam passive samplers. Environ. Sci. Technol. 2013, 47, 8591−8598. (6) Harner, T.; Bartkow, M.; Holoubek, I.; Klanova, J.; Wania, F.; Gioia, R.; Moeckel, C.; Sweetman, A. J.; Jones, K. C. Passive air sampling for persistent organic pollutants: Introductory remarks to the special issue. Environ. Pollut. 2006, 144, 361−364. (7) Burken, J. G.; Vroblesky, D. A.; Balouet, J. C. Phytoforensics, dendrochemistry, and phytoscreening: New green tools for delineating contaminants from past and present. Environ. Sci. Technol. 2011, 45, 6218−6226. (8) Larsen, M.; Burken, J.; Machackova, J.; Karlson, U. G.; Trapp, S. Using tree core samples to monitor natural attenuation and plume distribution after a PCE spill. Environ. Sci. Technol. 2008, 42, 1711− 1717. (9) Limmer, M. A.; Holmes, A. J.; Burken, J. G. Phytomonitoring of chlorinated ethenes in trees: A four-year study of seasonal chemodynamics in planta. Environ. Sci. Technol. 2014, 48, 10634−10640. (10) Balouet, J. C.; Burken, J. G.; Karg, F.; Vrobelsky, D.; Smith, K. T.; Grudd, H.; Rindby, A.; Beaujard, F.; Chalot, M. Dendrochemistry of multiple releases of chlorinated solvents at a former industrial site. Environ. Sci. Technol. 2012, 46, 9541−9547. (11) Sheehan, E. M.; Limmer, M. A.; Mayer, P.; Karlson, U. G.; Burken, J. G. Time-weighted average SPME analysis for in planta determination of [chlorinated] VOCs. Environ. Sci. Technol. 2012, 46, 3319−3325. (12) Meredith, M. L.; Hites, R. A. Polychlorinated biphenyl accumulation in tree bark and wood growth rings. Environ. Sci. Technol. 1987, 21, 709−712. (13) Trapp, S.; Miglioranza, K. S. B.; Mosbaek, H. Sorption of lipophilic organic compounds to wood and implications for their environmental fate. Environ. Sci. Technol. 2001, 35, 1561−1566. (14) Demick, B. Nothing to Envy; Ordinary Lives in North Korea; Spiegel & Grau, 2010, 336 pp. (15) Simonich, S. L.; Hites, R. A. Organic pollutant in vegetation. Environ. Sci. Technol. 1995, 29, 2905−2914. (16) McDonald, J. G.; Hites, R. A. Radial dilution model for the distribution of toxaphene in the United States and Canada on the basis F
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