Polychlorinated Biphenyls in Global Air and Surface Soil

Figure 1 presents concentrations of total PCBs (ΣPCBs) in air at global background, .... The concept of maximum reservoir capacity (MRC), the ratio o...
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Environ. Sci. Technol. 2010, 44, 2784–2790

Polychlorinated Biphenyls in Global Air and Surface Soil: Distributions, Air-Soil Exchange, and Fractionation Effect† Y I - F A N L I , * ,‡,§ T O M H A R N E R , ‡ L I Y A N L I U , § Z H I Z H A N G , §,| NAN-QI REN,§ HONGLIANG JIA,⊥ J I A N M I N M A , ‡ A N D E D S V E R K O ‡,# Science and Technology Branch, Toronto/Burlington, Environment Canada, International Joint Research Center for Persistent Toxic Substances (IJRC-PTS), State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, China, Heilongjiang University, Harbin, China, IJRC-PTS, Dalian Maritime University, Dalian, China, and IJRC-PTS, McMaster University, Hamilton, Canada

Received June 27, 2009. Revised manuscript received October 9, 2009. Accepted October 26, 2009.

Polychlorinated biphenyl (PCB) concentrations in air and soil, measured by various research groups from around the world, were compiled and analyzed. Data for air were available from most regions, particularly in Europe and Asia. The average air concentrations (pg/m3) for ΣPCB at background sites were 70 (5.1-170) for Europe, 79 (49-120) for North America, 66 (18-110) for South America, 270 (9-670) for Central America, 59 (17-150) for Asia, and 15 (13-17) for Australia. Data for soils exhibited better global coverage compared to air and were available from most regions. The average soil concentrations (pg/g dry weight) for ΣPCB at background sites were 7500 (47-97 000) for Europe, 4300 (110-25 000) for North America, 1400 (61-9 500) for South America, 580 (120-2 900) for Asia, 390 (94-620) for Africa, and 280 (140-540) for Australia. Based on available studies where coupled measurements of PCBs in air and soil were made, the equilibrium status of PCBs in the air-soil system was investigated for China, West Midlands of the UK, central and southern Europe, and along a latitudinal transect from the south of the UK to the north of Norway. Differences were observed in plots of the soil-air equilibrium status (expressed as the soil-air fugacity fraction, ff) for different PCB homologues. This was explained by varying contributions from primary and secondary emissionssspatially and temporally. The net effect after several decades of PCB emissions to air, preferential transport of lower molecular weight PCBs through primary and secondary emission, and reductions in emissions to air in recent decades is that the lower molecular weight PCBs have

† Part of the special section “Sources, Exposures, and Toxicities of PCBs in Humans and the Environment”. ‡ Environment Canada. § Harbin Institute of Technology. | Heilongjiang University. ⊥ Dalian Maritime University. # McMaster University. * Corresponding author tel: (416)739-4892; fax: (416)739-4288; e-mail: [email protected].

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achieved (and in some cases exceeded) soil-air equilibrium inmanypartsoftheworld.Theexceptionisremoteandbackground sites that are still dominated by primary sources.

Introduction Polychlorinated biphenyls (PCBs) are one of twelve compounds or compound classes listed as persistent organic pollutants (POPs) by the Stockholm Convention on POPs that came into effect on May 22, 2001 (1). POPs are characterized by their stability in the environment, ability to undergo long-range atmospheric transport (LRAT), and their potential to bioaccumulate through the food chain where they impose a threat to human health and the environment. Under the first global monitoring plan of the Stockholm Convention on POPs, air and human tissues are the two core media recommended for investigating temporal trends of POPs for the purpose of assessing the effectiveness of international control measures (2). Although PCBs are now banned in most parts of the world, they continue to be released from old equipments and waste sites (3, 4). Of the approximated 1.3 × 106 t cumulative global production, about 440 to 92 000 t is estimated to have been emitted into the environment (4, 5). Although concentrated near emission sources, PCBs have become dispersed in the environment through LRAT. They are ubiquitous in air and present in soils from all over the world, where they tend to concentrate due to the large sorptive capacity of soil for PCBs (6). In recent years, numerous studies have investigated PCBs in air and soil to better understand their distribution and cycling between these media (see Table S1, Supporting Information (SI), and references within). Others have attempted to predict and/or investigate the affect that temperature and differences in physical chemicals properties between congeners and homologue groups may have on the transport and distribution of PCBs. For instance, the “global fractionation” of PCBs and other POPs hypothesized by Wania and Mackay (7, 8) has been supported by measurements from latitudinal transects in soil (9) and air (10). According to the fractionation hypothesis (7, 8) the lower molecular weight (LMW) PCBs, because they are more volatile and less associated with depositing particles, are able to travel further in air and become relatively enriched further away from sources. The more volatile PCBs are also more likely to undergo “grasshopping” or be re-emitted to air (secondary emission) once deposited because they are less strongly retained onto soils and other condensed phases compared to the high molecular weight (HMW) congeners. An analogous fractionation of PCBs has been reported on smaller spatial scales. For instance, Harner et al. (11), reported the “urban fractionation” of PCBs in air along a relatively short (∼75 km) urban-rural transect in Toronto, and Ren et al. (12) observed fractionation of PCBs in soils from an urban-rural transect in the city of Shanghai (∼200 km). The “urban fractionation” is less associated with “grasshopping” and/or soil-air exchange (cycling) and more the result of proximity to a source region (i.e., to a primary deposition). A so-called “longitudinal fractionation” was found in a Chinese rural region due to the particular use pattern of PCBs in China, with higher use in the eastern region compared to the western region (12). This review compiles the latest data for PCBs in air and soil to achieve the following: (i) better understand the spatial distribution of PCBs in air and soil at the regional and global scale; (ii) assess air-soil exchange of PCBs by investigating 10.1021/es901871e CCC: $40.75

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the equilibrium status of PCBs on regional and global scales using the compiled soil and air concentration data; and (iii) explain observed concentrations and distributions/patterns of PCBs in air and soil based on fractionation theories and building on these explanations using new information on soil-air equilibrium status (item ii above) at the global and regional scales.

The fugacity fraction (ff ) is used to assess equilibrium status of a chemical between two interacting phases, in this case soil and air

Method

By adjusting the units, pg/m3 for CA, pg/g dw (dry weight) for CS, and using the density of the dry soil FS (g/m3) to normalize to volume, we get

Data Compilation. In this study, concentration data of PCBs in air and soil from various international studies have been collected. Detailed information for air and soil sampling data is provided in Table S1, and site locations are shown in Figure S1. Tables S2 and S3 further summarize, on a country basis, the available data from background, rural, and urban sites for air and soil, respectively. Among all countries, China has the largest data sets for both air and soil. It is important to note that number of PCB congeners contributing to ΣPCB varies according to the study. This information is summarized in Table S1. Although this may lead to some comparability issues, it is often the case that the most dominant PCBs are determined, so we expect that data sets are comparable to within about a factor of 2. Theory. The fugacity approach (13) was used to investigate the air-soil equilibrium status of PCBs based on collected data. The fugacity capacity (Z value, mol/(m3 Pa)) describes the potential of some material (air, water, soil) to retain a chemical (a PCB congener, for example). The Z value for air is given by ZA ) 1/RT

(1)

where R is the gas constant and T is temperature in K. By assuming that most of the chemicals are associated with the organic matter content of the soil, the fugacity capacity for soil is given by ′ KOA /RT ZS ≈ φSOM

(2)

where φ′SOM is the mass fraction of SOM (soil organic matter, φ′SOM is often estimated as 1.8φ′SOC, where φ′SOC is the mass fraction of SOC (soil organic carbon)), and KOA is the octanol/air partition coefficient, which is strongly temperature-dependent and spans several orders of magnitude in the range of environmentally relevant temperatures (14, 15). The soil-air partition coefficient (KSA), which describes the equilibrium partitioning of a chemical between air and soil, is the ratio of the Z values between soil and air, ′ KOA KSA ) ZS/ZA ≈ φSOM

(3)

The greater the value of KSA for a particular chemical, the more strongly it will be retained by soil. Furthermore, KSA is proportional to SOM content and increases at lower temperatures, in-line with KOA. The fugacity f, is a measure of a chemical’s escaping tendency from a particular medium. In other words, chemicals tend to move from media where they have high fugacity to media where they have low fugacity. Fugacity is also the partial pressure (Pa) of a chemical in a particular medium. Fugacity is proportional to concentration and is defined as C/Z (where C is concentration, mol/m3), and given by fa ) CART

(4)

′ KOA ) CSRT/KSA fs ) CSRT/φSOM

(5)

for chemical concentrations in air (CA) and soil (CS), respectively.

ff ) fS /(fS + fA) ) (CS /KSA)/(CS /KSA + CA) ) CS /(CS + KSACA)

(6)

ff ) CSFS /(CSFS + KSACA)

(7)

Note that alternate units may be used as long as they are consistent and result in a dimensionless ff. A slightly modified version of eq 7 was used, based on empirical studies (16, 17), and takes the form ff ) CSFS /(CSFS + 0.41KSACA)

(8)

Values of ff equal to 0.5 indicate soil-air equilibrium and no net gas exchange. Values >0.5 indicate net volatilization from soil and values 65°, there seem to be no clear definitions for the three site categories (background, rural, and urban), and these categories may have slightly different meanings in the studies from which the data are associated. For our purposes urban sites are within a city, rural sites are in the countryside and typically surrounded by farmland (but far away from industrial/urban areas), and background sites are usually in areas that are not often visited by people and far away from human activities. Data coverage is relatively good for Europe, East Asia, and the Great Lakes region of North America, but in many parts of the world data is lacking. For instance, there was much fewer data from Africa, Australasia, central and northern Asia, South America, and the Caribbean region. Global mean air concentrations (pg/m3) according to site type were 22 (5.1-59) at Arctic sites, 82 (5.1-670) at background sites, 170 (10-2800) at rural sites, and 1700 (10-150,000) at urban sites (Table S4). Large variability in air concentrations of PCBs of up to 3 and 5 orders of magnitude was observed at rural and urban sites, respectively, reflecting the influence of local/regional sources on these two site categories; whereas variability was lower at Arctic sites (about 1 order of magnitude) and at background sites (about 2 orders of magnitude). In comparing PCB air burdens in different regions of the world, it is beneficial to base the comparison on air concentrations at background sites as this removes some of the bias introduced by sites that are located near point sources. The average PCB air concentrations (pg/m3) at background sites in each region were: 70 (5.1-170) for Europe, 79 (49-120) for North America, 66 (18-110) for South America, 270 (9-670) for Central America, 59 (17-150) for Asia, and 15 (13-17) for Australia. The same analyses for rural sites produces 130 (25-690) for Europe, 110 (23-380) for North America, and 200 (10-2800) for Asia; and similarly for urban sites: 3400 (19-150,000) for Europe, 490 (18-1500) for North America, and 200 (10-870) for Asia (Table S4). These findings are consistent with the global emission history for PCBs (4, 5) with highest emissions in industrialized regions of Europe and North America. VOL. 44, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Concentrations (pg/m3) of ΣPCBs in air at global sites. Top: background sites; middle: Rural sites; bottom: Urban sites. Note the different scales.

FIGURE 2. Concentrations (pg/g dw) of ΣPCBs in surface soil at global background sites and Chinese rural/background sites. Soil. Figure 2 presents ΣPCBs in global surface soils at global background sites and rural/background sites in China. Similar to what was observed for air, ΣPCBs in soil (pg/g dw) 2786

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were highly variable (ranging from ∼40 to ∼100 000; mean, 4900) with highest concentrations in Europe and North America and much lower levels in Asia and Africa. Averages

on a continental basis were 7 500 (47-97 000) for Europe, 4 300 (110-25,000) for North America, 1400 (61-9 500) for South America, 580 (120-2 900) for Asia, 390 (94-620) for Africa, and 280 (140-540) for Australia (Table S4). Air-Soil Partitioning Coefficient. As discussed previously, the sorptive capacity of a chemical in soil (ZS, eq 2) plays a key role in determining its overall environmental fate. More specifically, soil retention capacity governs how much chemical is able to accumulate in soil during decades of primary emission and deposition and how much is released back to the atmosphere (secondary emission when this occurs) for deposition somewhere else. The strong increase in soil retention capacity with decrease in temperature also plays an important role in the latitudinal distribution of chemicals and the “global fractionation effect”. In the case of PCBs, the LMW congeners that are volatile and mobile in air in higher temperate climates are much less volatile in cold climates and become effectively “trapped” in soil and other condensed phases, and consequently, they are less prone to secondary emission and become enriched in cold climate condensed phases. It is useful to explore the equilibrium status of the different PCB congeners or homologues in a given region. This not only provides some historical context on the accumulation history that has given rise to the current soil burdens but it also allows us to project into the future and explore the potential role of soil as a source of PCBs to the atmosphere through secondary emissions, and how this may vary among congeners or homologue groups. This is an increasingly important issue as a large portion of the global burden of emitted PCBs resides in soils (18). It has been hypothesized that slow migration of these persistent legacy PCBs, may ever result in a “secondary pulse” in remote arctic regions that follows a decade or two of declining concentrations of mainly “primary source” PCBs (19). The sorptive capacities of the soil-air system for different PCB homologues can be explored by first calculating the KSA value according to eq 3. KSA varies by orders of magnitude for realistic ranges of soil organic matter content and ambient temperature as demonstrated in Figure S2 for PCB-28, also indicating higher sensitivity of KSA to temperature (3 orders of magnitude from -30 to 30 °C) than to SOM contents (2 orders of magnitude from 0 to 1). A global gridded map of logKSA values was produced (Figure 3) for PCB-28, -101, and -180 using 2003 annual mean temperatures (required for KOA calculation (14)) (http://www.cdc.noaa.gov/Composites/) (Figure S3), and SOM (SOC) content (http://islscp2.sesda.com/ISLSCP2_1) (Figure S4). Figure 3 illustrates several things very nicely: (i) the higher KSA (higher Koa) values for HMW PCBs; (ii) the increase in KSA with latitude, which is attributed to lower temperatures as one moves away from the equator toward the poles; (iii) the vast majority of PCB sorbing capacity lies in the northern hemisphere; and (iv) the occurrence of regions or “pockets” within given latitudinal bands where KSA values deviate. For instance, higher KSA values exist in the Boreal region of North America and parts of equatorial Africa whereas lower KSA values are observed in desert regions where SOM is low (e.g., Saharan Africa). Regions with moderate or low logKSA values (e.g., less than about 6 or 7) will be temporary or transient sinks for PCBs and actively involved in soil-air exchange and secondary emission (“grasshopping”). Whereas regions where log KSA values are elevated (e.g., more than about 8 or 9) will act as longer-term “traps” and accumulate PCBs. The concept of maximum reservoir capacity (MRC), the ratio of the capacities of the surface soil and of the atmospheric mixed layer to hold chemical under equilibrium conditions, has been applied to selected POPs in the surface

“skin” (1 mm) of soils (6). Although the MRC is also based on KOA and SOM, it is KSA that governs air-soil exchange and “fractionation” patterns for PCBs discussed in the next section. Air-Soil Exchange and “Fractionation” Patterns for PCBs. With knowledge of KSA values for all regions of the world, the next step is to investigate soil-air equilibrium status using coupled measurements of PCBs in air and soil, where such data is available. This analysis will reveal regions that are sources (secondary sources), “sinks” (soil undersaturated relative to air), or near equilibrium for different PCB congeners. It will also show patterns in the PCB distribution or “fractionation”. This may yield insight to the role of primary emissions and the potential of these sites to act as secondary sources for certain congeners. The soil-air equilibrium status was assessed on congener basis and averaged based on each homologue for several, coupled air and soil data sets and expressed as the soil-air fugacity fraction (ff ) (eq 8). Results presented in Figure 4 are on a homologue basis and, when available, for different site categories (background, rural, and urban). It should be noted that comparability issues exist for the data reported between these various studies. These sources of uncertainty are the result of factors such as the use of different laboratories, analytical methods, sampling methods, and quality assurance protocols, to name a few. However, resolving these issues is beyond the scope of this study. A margin of error is therefore applied when evaluating soil-air equilibrium status based on these data (20). China. The Chinese POPs Soil and Air Monitoring Program (SAMP) started in 2005 by the International Joint Research Center for Persistent Toxic Substances (IJRC-PTS) (12, 21). Under this program, passive air samples were collected on a seasonal (3 months) basis for 2 years from 2005 to 2007 at approximately 100 sites across China using polyurethane foam (PUF) disk-type samplers, and surface soils were also collected at the same sites in 2005. The objective of this national-scale program is to study and relate both temporal and spatial trends of POPs in air and surface soil with a focus on agricultural regions, although several urban and background sites were included for comparison purposes. In this study, air data were collected from mid-July to mid-October, 2005 at 97 sites (21) and data for surface soils (top 20 cm) were collected in a subset of 51 sites in the same year (12). The resulting soil-air fugacity fractions (ff values, Figure 4a) show a fractionation pattern enriched in the LMW congeners that are now near equilibrium and/or supersaturated with respect to air (ff g 0.5). This is the result of their greater mobility and lower KSA values compared to HMW PCBs. Soils become supersaturated relative to air because air concentrations are able to decline relatively quickly with time (as emissions to air decline) whereas concentrations in soils take longer to respond. Chinese soils may be important contributors (secondary sources) of the very volatile 2-Cl and in some cases 3-Cl PCBs that are observed in air. It is important to note that large values of ff identify net flux direction (i.e., identifying the role of soil as a “source” to the air) and the larger the ff, the greater the flux potential. However, the absolute quantity of PCBs associated with this flux is proportional to the concentrations of PCBs in soil and also depends on meteorological factors. The 4-Cl to 6-Cl PCBs have also approached equilibrium at urban and even rural sites. This is the result of many years of emission and deposition on a local or subregional scale. However, at background sites, ff values remain quite low reflecting the reduced mobility of higher molecular PCBs that preferentially deposit near sources. Background sites in China will likely continue to be “sinks” for the HMW PCBs for a long time into the future. VOL. 44, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Soil and air partition coefficient KSA for PCB-28, -101, and -180 on a 1° latitude by 1° longitude resolution. West Midlands of the United Kingdom. In this study conducted in 2003-04, concentrations and chiral signatures of PCBs were measured in outdoor air using PUF-disk passive samplers and in surface soils (top 5 cm) on a monthly basis for a 1-year period at ten sites on a rural-urban-rural transect (22). The pattern seen here (Figure 4b) is similar to that observed in Chinese urban and rural soils with the 3-Cl and 4-Cl PCBs near equilibrium and higher molecular weight PCB approaching equilibrium. In general however, the ff values for the same homologue group are lower for the UK urban study. This may be due to a slower decline in PCB air concentrations in this region compared to the China study. PCB elevated in indoor air is believed to be an important emission source in this region (22). Sweetman and Jones (23) have shown that PCBs at a number of UK locations were 2788

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decreasing with a half-life for individual PCBs of 2-6 years, while similar data is not available for China. Central and Southern Europe. Figure 4c summarizes results from coupled measurements at 47 sites between July and December, 2005 in central and southern Europe (24). Air samples were collected using PUF-based passive air samplers, and the soil samples (top 10 cm) were collected at each site at the beginning of the sampling campaign. The results are surprising in that they show equilibrium and/or near equilibrium for the full range of PCB homologues, even for the very involatile 7-Cl congeners. The apparent mobility of the 7-Cl PCBs may be due to the warmer climate across the sampling region. In other words, a 7-Cl PCB experiencing temperatures typical of southern Europe may have volatility similar to a 6-Cl PCB in a more northerly climate. This effect

FIGURE 4. Fugacity fraction between air and soil in (a) China, (b) West Midlands, UK, (c) Central and South Europe, and (d) UK and Norway. The whiskers on the bars represent the range of the ff values; the dashed lines at ff ) 0.3 and 0.7 represent uncertainty in the equilibrium condition based on errors propagated in the calculation of ff (20). is supported by Figure 3 where KSA values in central and southern Europe are lower compared to the West Midlands of the UK for the same PCB. To re-emphasize, higher KSA values correspond to higher soil capacities (due to lower temperatures and/or higher SOM) and corresponding longer times to reach equilibrium. In fact it may be advantageous in the future to explore patterns in different studies according to KSA, rather than PCB homologue group, as this would help to normailize for the effect of temperature on PCB volatility. However, the analysis above does not explain why background soils in China (that have similar KSA values as soils in southern Europe, see Figure 3) seem to lag in their approach to equilibrium for the higher molecular weight PCBs (Figure 4a). It is possible that this could be due to the later introduction of PCBs in China and differences in the use profile. Indeed, PCB use in China started in 1965, approximately 35 years later than in Europe (3, 25). United Kingdom and Norway. Air samples were collected using SPMDs (semipermeable membrane devices) at background sites along a latitudinal transect from the south of the UK to the north of Norway during 1998-2000 (10); surface soils (top 5 cm) were sampled at the same sites in 1998 (9, 17). The original air concentration data (10) was reported as ng/ SPMD. For purposes of comparison we have converted these

to concentration units (pg/m3) taking into account ambient average temperatures at each sampling location and the uptake profile for SPMDs, as described by Shoeib and Harner (15). This accounts for lower air sample volumes for the LMW PCBs that approached equilibrium in the SPMD. Figure 4d shows a typical pattern for background sites, with approach to equilibrium for the LMW-PCBs (low KSA value) and large departure from equilibrium for the HMWPCBs (4-Cl to 8-Cl). As discussed previously, in the example of central and southern Europe, both temperature and SOM contents play a key role on the KSA values. The 3-Cl PCBs, because of the low temperatures and high SOM contents along this transect (especially in Norway) have KSA values similar to those for HMW-PCBs (4-Cl to 5-Cl PCB) in warmer parts of Europe (see Figure 3). This is another example of why it may be beneficial to investigate soil-air equilibrium status after correcting for KSA. It is worthwhile to point out that although PCB concentrations in Chinese soils are lower compared to those in UK and Norwegian soils (Figure 2), the Chinese soils are more “saturated” relative to air, particularly true for the LMW PCBs (Figure 4a and d). This is due to higher ambient temperatures and lower SOM contents in China that result in lower KSA values. The resulting soil-air fugacity fractions (ff ) along the UK-Norway transect (Figure 4d) show a typical fractionation pattern for background sites. When ff is far below 0.5, which was also the case for the HWM-PCBs in the Chinese and West Midlands, UK studies (Figures 4a, b), at the background sites in particular, this represents a situation where the primary sources dominate. This is because previously deposited PCBs are effectively trapped in the soil and are not available for re-emission to air. We refer to this as a “primary fractionation” pattern. It is characterized by absolute amounts of the individual PCB congeners decreasing with distance from the source area, with an increasing proportion of the more volatile compounds with distance from the source area. In contrast, when ff is above 0.5, which is the case for 3-Cl PCBs in UK and Norway background areas (Figure 4d), secondary sources begin to play a more important role and PCBs are more free to exchange between soil and air and become subject to “grasshopping”. We refer to this as a “secondary fractionation” pattern. Under this scenario, the transport of chemicals is mainly driven by temperature, SOM content, and ultimately the surface-air partition coefficients (e.g., KSA) for the various environmental compartments or climate zones. The phenomenon of “primary” and “secondary” fractionation with respect to soil-air systems is dynamic, changing with time and accordingly with the temporal emission profile of the chemical in question. In most cases, it is likely that both influences are involved to varying extents. Secondary fractionation, however, drives the longer-term accumulation of persistent chemicals in cold climates. This is a relatively slow process which, for PCBs, occurs over timescale of decades. It is plausible, according the secondary pulse hypothesis (19), that maximum concentrations in cold polar regions are yet to be realized for some classes of persistent chemicals such as the LMW PCBs.

Acknowledgments We are grateful to H. Dryfhout-Clark and A. Houfrom of Environment Canada for their help with IADN data, H. Hung and Y. Su of Environment Canada for their help with the Arctic data, and R. Kallenborn and K. Breivik of NILU, Norway for their help with EMEP data.

Supporting Information Available Many useful data sets, figures, and tables. This material is available free of charge via the Internet at http://pubs.acs.org. VOL. 44, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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