5
Persistent Organic Pollutants

SOURCES AND GLOBAL REGULATORY STATUS

In the Stockholm Convention on Persistent Organic Pollutants (POPs), a chemical is considered persistent (from an atmospheric standpoint) if it has been measured at locations distant from sources of potential concern, if monitoring data show that long-range atmospheric transport may have occurred, or if modeling results show that the chemical has a potential for long-range atmospheric transport, with an atmospheric half-life exceeding two days (http://www.pops.int/documents/convtext/convtext_en.pdf). Under typical windspeeds, a chemical can travel 150-800 km in two days and result in contamination in remote locations (Scheringer, 2009).

Estimated atmospheric gas-phase reaction half-lives of POPs (Table 5.1) range from less than one day to greater than a year. In general, POPs with gas-phase reaction half-lives greater than two days and POPs absorbed onto fine particulate matter are capable of undergoing long-range atmospheric transport. The reaction half-lives of POPs in other environmental media (including water, soil, and sediment), are considerably longer than in the atmosphere. Because of their long reaction half-lives in water, soil, and sediment, and their potential for revolatilization and atmospheric transport, some POPs cycle in the global environment for many decades, similar to mercury.

In this chapter, we focus on the POPs identified in the United Nations Economic Commission of Europe’s Convention on Long-range Transboundary Air Pollution (UNECE LRTAP) (http://www.unece.org/env/lrtap/pops_h1.htm), which include aldrin, chlordanes, chlordecone, DDT,



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 113
5 Persistent Organic Pollutants SOURCES AND GLOBAL REGULATORY STATUS In the Stockholm Convention on Persistent Organic Pollutants (POPs), a chemical is considered persistent (from an atmospheric standpoint) if it has been measured at locations distant from sources of potential concern, if monitoring data show that long-range atmospheric transport may have occurred, or if modeling results show that the chemical has a potential for long-range atmospheric transport, with an atmospheric half-life exceed- ing two days (http://www.pops.int/documents/convtext/convtext_en.pdf). Under typical windspeeds, a chemical can travel 150-800 km in two days and result in contamination in remote locations (Scheringer, 2009). Estimated atmospheric gas-phase reaction half-lives of POPs (Table 5.1) range from less than one day to greater than a year. In general, POPs with gas-phase reaction half-lives greater than two days and POPs absorbed onto fine particulate matter are capable of undergoing long-range atmospheric transport. The reaction half-lives of POPs in other environmental media (including water, soil, and sediment), are considerably longer than in the atmosphere. Because of their long reaction half-lives in water, soil, and sediment, and their potential for revolatilization and atmospheric transport, some POPs cycle in the global environment for many decades, similar to mercury. In this chapter, we focus on the POPs identified in the United Nations Economic Commission of Europe’s Convention on Long-range Trans- boundary Air Pollution (UNECE LRTAP) (http://www.unece.org/env/ lrtap/pops_h1.htm), which include aldrin, chlordanes, chlordecone, DDT, 

OCR for page 113
 GLOBAL SOURCES OF LOCAL POLLUTION TABLE 5.1 Some Persistent Organic Pollutants (based on reaction with hydroxyl radical) Year Atmospheric Gas-Phase Half-Lifea Banned POP Use/Source in U.S. (Days) Aldrin Insecticide 1987 0.17 Chlordane Insecticide 1988 2.1 Chlordecone Insecticide 1978 > 365 DDT Insecticide 1972 3.1 Dieldrin Insecticide 1987 1.2 Heptachlor Insecticide 1988 0.2 Hexabromobiphenyl Flame retardant 1973 38 HCB Fungicide 1984 > 365 Mirex Insecticide 1978 >7 PCBs Industrial 1977 3 to 120 Toxaphene Insecticide 1980 4.7 HCHs (technical mixture) Insecticide 1978 19 PAHs Combustion NA 0.5 to 2 PCDD/Fs Combustion NA 2.6 to 200 aEPA EpiSuite: http://www.epa.gov/oppt/exposure/pubs/episuitedl.htm. dieldrin, heptachlor, hexabromobiphenyl, hexachlorobenzene (HCB), mirex, polychlorinated biphenyls (PCBs), toxaphene, hexachlorocyclohexanes (HCHs), polycyclic aromatic hydrocarbons (PAHs), and polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) (Table 5.1) (http://www.unece. org/env/lrtap/pops_h1.htm). This list includes products of incomplete com- bustion (PAHs and PCDD/Fs), pesticides (aldrin, chlordanes, chlordecone, DDT, dieldrin, heptachlor, hexachlorobenzene, mirex, toxaphene, and HCHs), and industrial chemicals (PCBs and hexabromobiphenyl). It is noted that while the use of the synthetic organic POPs such as pesticides and industrial chemicals was banned in the United States several decades ago, they continue to volatilize from historically-contaminated soils and cycle in the environment. In addition, the emission of combustion derived POPs (PAHs and PCDD/Fs) may be reduced through combustion emission control but cannot be realistically eliminated. The objective of the UNECE POPs protocol is to eliminate any emis- sion, discharges, or losses of POPs. The protocol bans the use and pro- duction of some POPs (aldrin, chlordane, chlordecone, dieldrin, endrin, hexabromobiphenyl, mirex, and toxaphene), has severe restrictions on the use of some POPs (DDT, HCHs, and PCBs), and plans for a scheduled elimination of other POPs (DDT, heptachlor, HCB, PCBs) at a later date. In addition, the protocol requires parties to reduce emissions of HCB,

OCR for page 113
 PERSISTENT ORGANIC POLLUTANTS PCDD/Fs, and PAHs below 1990 emissions. The United States was a signa- tory party to the protocol in 1998 but has not ratified it (http://www.unece. org/env/lrtap/status/98pop_st.htm). Although persistent organic pesticides are banned or restricted in most developed countries throughout the world, they continue to cycle in the global environment due to revolatilization from historically contaminated soils, vegetation, and water bodies. As a result, some persistent organic pesticides continue to undergo long-range atmospheric transport, deposi- tion, and bioaccumulation in remote U.S. high-elevation and high-latitude ecosystems (Clarkson, 2002; Kucklick et al., 2002, 2006; Howe et al., 2004; Vander Pol et al., 2004; Kannan et al., 2005; Hageman et al., 2006; Muir et al., 2006; Su et al., 2006, 2008; Usenko et al., 2007; Ackerman et al., 2008; Landers, 2008). Over long periods of time (years to decades) these compounds eventually degrade or become sequestered in deep soils and sediments. As a result, atmospheric concentrations of persistent organic pesticides are generally decreasing in remote U.S. ecosystems (Sun et al., 2006b, 2007; Usenko et al., 2007; Landers, 2008). In contrast, the atmospheric concentrations of POPs emitted during incomplete combustion (including particulate-phase PAHs and PCDD/Fs) have generally remained the same or increased in remote locations, due to increased global combustion (Prevedouros et al., 2004; Becker et al., 2006; Sun et al., 2006c; Usenko et al., 2007; Venier et al., 2009). In addition, there has been a significant shift in the emission of PAHs (and possibly PCDD/F emission) from developed countries to developing coun- tries (Zhu et al., 2008; Zhang and Tao, 2009). As of 2004, Chinese and Indian emissions of 16 PAHs were significantly greater than U.S. emissions (114 Ggy–1, 90 Ggy–1, and 32 Ggy–1 for China, India, and the United States, respectively) (Zhang and Tao, 2009). This suggests that given the expected increased energy demands in developing countries, human and ecosystem exposure to PAHs (and possibly PCDD/Fs) may increase in coming years, especially in the rapidly developing regions of the world, while exposure to the other identified POPs may stay the same or decrease. TOXICOLOGICAL RELEVANCE POPs typically have low water solubility, high lipid solubility, and an intrinsic resistance to natural degradation processes. Because of these prop- erties, POPs are environmentally persistent and tend to bioaccumulate in adipose tissue, putting breast-feeding infants at higher risk of adverse health effects. The individual compounds have different toxicological properties that can result in adverse biological effects in fish, wildlife, and humans. A wide range of adverse health outcomes has been associated with exposure to individual POPs (see Agency for Toxic Substances and Disease Regis-

OCR for page 113
 GLOBAL SOURCES OF LOCAL POLLUTION try reviews of chlordane, DDT, aldrin/dieldrin, HCH, heptachlor, mirex, PAHs, PPBs. PCBs, and toxaphene). Potential human health effects include impairment of the immune system, nervous system, hormonal system, and reproductive functions. Concerns over the risks of POPs have led to the establishment of worldwide monitoring programs to determine concen- trations of POPs in adipose tissue and associated adverse consequences (Jorgenson, 2001; Li et al., 2006b). A Joint WHO and Convention Task Force on the Health Aspects of Air Pollution assessed the health risks of priority POPs in relation to long- range transboundary pollution (WHO, 2003). The risks associated with the following groups of substances were reviewed: pentachlorophenol, DDT, hexachlorocyclohexanes, hexachlorobenzene, heptachlor, polychlorinated dibenzo-p-dioxins and dibenzofurans, polychlorinated biphenyls, and poly- cyclic aromatic hydrocarbons. The task force also performed a short hazard assessment for polychlorinated terphenyls, polybrominated diphenylethers, polybrominated dibenzo-p-dioxins and dibenzofurans, and short-chain chlorinated paraffins to identify the main gaps in information necessary for risk assessment. ATMOSPHERIC FATE AND INTERMEDIA TRANSPORT The more volatile POPs (such as HCB and HCHs) exist primarily as gases in the atmosphere, while less volatile POPs exist primarily in atmo- spheric particulate phases (including most PCDD/Fs). Still other POPs (such as PAHs) are distributed between both the atmospheric gas and aerosol phases. As a result, POPs are subject to both gas-phase and aerosol-phase removal mechanisms in the atmosphere, including wet and dry deposition, gas exchange, and direct and indirect photolysis. Precipitation, particularly snow, is an efficient scavenger of POPs from the atmosphere (Wania et al., 1999; Halsall, 2004; Lei and Wania, 2004). When this snow melts, it releases a pulse of POPs to the surrounding ecosystem (Daly and Wania, 2004; Lafreniere et al., 2006; Meyer et al., 2006). Gas-phase POPs react with photochemically generated OH radical in the atmosphere, and has been shown to be the most significant envi- ronmental transformation reaction for some POPs (Anderson and Hites, 1996; Mandalakis et al., 2003). In general, the photochemical degrada- tion rate decreases (and atmospheric half-life increases) for aerosol-phase POPs, increasing the potential for long-range atmospheric transport of POPs sorbed to fine particles. The reaction of PAHs with NOx and O3 are signifi- cant in that these reactions may result in the formation of more toxic nitro and oxy-PAHs (Pitts et al., 1985; Helmig et al., 1992; Sasaki et al., 1997). Because POPs are emitted primarily from anthropogenic combustion, industrial, and agricultural sources, their atmospheric concentrations tend to

OCR for page 113
 PERSISTENT ORGANIC POLLUTANTS decrease with distance from populated areas in the United States (Hafner et al., 2005). The concentrations of some POPs have been shown to increase in remote areas due to episodic transport events from source regions (Hageman et al., 2006; Usenko et al., 2007; Primbs et al., 2008a,b; Genualdi et al., 2009). The global fractionation of some POPs to high latitudes (Simonich and Hites, 1995) and the “cold trapping” of some POPs at high elevations has also been observed (Blais et al., 1998; Landers et al., 2008). EMISSION INVENTORIES The development of global emission inventories for POPs is challeng- ing, especially for synthetic organic POPs, because of the lack of informa- tion on how much of a chemical was manufactured and used throughout the world. Historically, many POPs have been deposited or applied to soils and continue be released as secondary emissions (similar to mercury). These secondary emissions are difficult to estimate, given that the magnitude and distribution of the original deposition and resulting emissions are largely unknown. Thus, global emission inventories for POPs have high degrees of uncertainty. Global emission estimates have been developed only for PCBs (Breivik et al., 2002a,b, 2007), HCHs (Li et al., 2003; Su et al., 2006), HCB (Barber et al., 2005) and PAHs (Zhang and Tao, 2009). European emission estimates exist for DDT, HCB, PAHs, and PCDD/Fs (Pacyna et al., 2003a). In many cases POP emissions from developing countries are largely unknown. U.S. INFLOW AND OUTFLOW OF POPS In recent years the inflow of POPs from Eurasia to the western United States via transpacific atmospheric transport has been identified and docu- mented (Killin et al., 2004; Primbs et al., 2008a,b; Genualdi et al., 2009). Although strong transpacific transport events are episodic, occurring pri- marily in the late winter and spring, the inflow of POPs from Eurasia to the western United States likely occurs at a low level throughout the year. In particular, elevated concentrations of aerosol-phase PAHs, HCB, and alpha-HCH have been measured in transpacific air masses relative to regional North American air masses at remote sites in the Pacific North- western United States (Killin et al., 2004; Primbs et al., 2008a,b; Zhang et al., 2008a; Genualdi et al., 2009). Figure 1.3 shows an example of the . transport of several POPs to Mt.Bachelor in Oregon, along with several other indicators of Asian anthropogenic sources (Primbs et al., 2008a,b). These same POPs have also been measured in outflow from Asia (Primbs et al., 2007). Other studies show outflow of PAHs from China (Guo et al., 2006; Lang et al., 2008). Studies have documented the emission of PAHs

OCR for page 113
 GLOBAL SOURCES OF LOCAL POLLUTION and re-emission of synthetic organic POPs from soils and vegetation dur- ing the large 2003 Siberian fires and the subsequent transpacific transport of these emissions to two sites in the western United States (Genualdi et al., 2009). In a recent study conducted to identify POP source regions and emissions from recent use vs. re-emissions from historic use, one chemical form of HCH (racemic alpha-HCH) was measured in Asian, transpacific, and free tropospheric air masses, while a different chemical form (nonra- cemic alpha-HCH) was measured in regional U.S air masses (Genualdi, 2009). Nonracemic alpha-HCH is indicative of re-emissions from historic use (and “aged” signature) because of enantioselective biodegradation by soil organisms over time. Our current understanding of the magnitude of the inflow of POPs to the United States through transpacific transport of Eurasian emissions is limited but growing. Hageman et al. (2006) estimated the relative contribu- tion of regional (within 150 km radius) and long-range (> 150 km radius) atmospheric transport to the dieldrin, alpha-HCH, chlordane, and HCB concentrations in annual snow pack collected from remote, high-elevation sites in seven western U.S. national parks, by correlating their measured concentrations with the cropland intensity within 150 km of the park. They estimated that 100 percent of the POP concentrations measured in Alaskan parks were due to long-range transport, while 30 to 70 percent of the con- centrations of these POPs measured in the most westerly continental U.S. park (Mt. Rainier National Park) were due to long-range transport (includ- ing transpacific transport). At progressively more interior parks (Glacier and Rocky Mountain National Parks) the contribution from long-range transport decreased to 10 to 30 percent. In comparison, the inflow of POPs from Canada to the United States, Mexico to the United States, and West Africa to the southeastern United States is much less documented. Gamma-HCH (lindane) used in the Canadian prairie provinces (Saskatchewan, Alberta, and Manitoba) has been shown to be transported to the Great Lakes (Ma et al., 2003, 2004) and Glacier National Park (Hageman et al., 2006). In 2004 elevated POP concentrations were measured at a remote mountain site in the western United States and linked to emissions from forest fires in western Canada (Primbs et al., 2008a,b). Although high concentrations of POPs have been measured in air throughout Mexico (Wong et al., 2009), their direct atmo- spheric transport from Mexico to the United States has not been docu- mented. Saharan dust storms have the potential to transport POPs from Africa to the southeastern United States (Zhang et al., 2008b; Pozo et al., 2009), but this phenomenon has not been well characterized either. The outflow of POPs from the United States to the Great Lakes region, and the resulting bioaccumulation in this ecosystem, has long been recog- nized and modeled (Hafner and Hites, 2003, 2005; Ma et al., 2005c,d).

OCR for page 113
 PERSISTENT ORGANIC POLLUTANTS The Integrated Atmospheric Deposition Network (IADN), a joint US and Canada monitoring program, has been in place since the 1990s (Glassmeyer et al., 1997, 2000; Hillery et al., 1998; Hites, 1999; Cortes et al., 2000; Buehler et al., 2002, 2004; James and Hites, 2002; Hafner and Hites, 2003, 2005; Carlson et al., 2004; Sun et al., 2006a,c, 2007; Venier et al., 2009). Atmospheric clearance rates of POPs (reported as half-lives) have been cal- culated from the long-term monitoring record for gas-phase PAHs, PCBs, HCHs, chlordanes, dieldrin, HCB and DDT, and range from 1 to 32 years (Sun et al., 2006b,c, 2007). These data suggest that the atmospheric con- centrations of these POPs are decreasing in the Great Lakes region. Aerosol- bound PAH and PCDD/F concentrations have not decreased significantly over the same period at these remote sites (Sun et al., 2006c; Venier et al., 2009), perhaps as result of increased combustion worldwide (Zhang and Tao, 2009). Numerous studies have documented the outflow of POPs from the United States to the Canadian Arctic, and since 1992, atmospheric mea- surements of POPs have been made at Alert, a monitoring station in the Canadian Northwest Territories (Hung et al., 2001, 2002a,b; Prevedouros et al., 2004; Becker et al., 2006; Su et al., 2006, 2008). In comparison, there is much more limited data on atmospheric POP concentrations and source regions to the U.S. Arctic (only Point Barrow, Alaska, from 2000 to 2003) (Su et al., 2006, 2008). Because of the low temporal resolution of the Point Barrow dataset (samples collected over the period of a week), a thorough investigation into the geographic location of POP source regions to the U.S. Arctic has not been conducted. Many studies have, however, documented the deposition of POPs in Alaska (Hageman et al., 2006), and their bio- accumulation in food webs (Kucklick et al., 2002, 2006; Howe et al., 2004; Vander Pol et al., 2004; Kannan et al., 2005; Ackerman et al., 2008). Although the outflow of POPs from the United States to Mexico and Europe likely occurs to some degree, this outflow has not been directly measured in discrete air masses and its magnitude is unknown. Atmo- spheric POP concentrations have been measured in remote Western Europe, including Mace Head, Ireland (Lee et al., 1999, 2004), but air masses with elevated POP concentrations were primarily tracked back to other parts of Europe. EXISTING POP MODELING CAPABILITIES Because of their global atmospheric transport potential, distribution between the atmospheric gas and aerosol phases, and potential to partition to and from various environmental media, global atmospheric transport models for POPs are not as refined as the global transport models for PM species or volatile compounds. Modeling the long-range transport of POPs

OCR for page 113
0 GLOBAL SOURCES OF LOCAL POLLUTION requires a global emission inventory, a transport model, and a detailed understanding of removal processes. Because each POP has unique chemical properties and sources, global modeling has been conducted for a limited number of POPs. In North America most POP modeling has focused on regional sources to the Great Lakes. For example, findings from Ma et al. (2005a,b) indicate episodic transport of toxaphene from the southeastern United States to the Great Lakes. Zhang et al. (2008b) showed episodic transpacific transport of lindane (γ-HCH) and transatantic transport to the Caribbean and south- eastern United States from Africa, although there is little data to validate these modeling results. A connection between the concentration anomalies of several POPs in the Great Lakes with sea-urface temperature anomalies in the tropical Pacific (an indicator of El Niño) was shown by Ma and Li (2006). While this long-distance correlation is somewhat surprising at first glance, it is plausible that there is a mechanism linking El Niño with trans- port patterns and re-emission of previously deposited toxic compounds, such as POPs or Hg. Gusev et al. (2007) describe global modeling activities at the Meteoro- logical Synthesizing Centre (MSC)-East for α-HCH and PCB-153. In their analysis European, North American, and Asian emissions of PCB-153 are 65, 14, and 8 percent of the global total, respectively. Using a 20 percent reduction scenario (similar to the HTAP O3 analyses discussed in Chapter 2), the authors show that intercontinental influence for PCB-153 is modest. A 20 percent reduction in PCB-153 emissions from Europe results in a 7, 3.5, and 2 percent decline in European, Arctic, and Asian deposition, respectively. The much smaller North American emissions of PCB-153 show minimal influence outside this region. For α-HCH, emissions are dominated by cer- tain countries, including Algeria, Tunisia, Spain, France, Romania, North Korea, Vietnam, Maylasia, and India. European, North American, and Asian emissions of α-HCH are 26, 0.3, and 35 percent of the global total, respec- tively. Intercontinental transport of α-HCH is more significant than PCB-153 because it is present in the atmospheric gas phase rather than the aerosol phase. For example, a 20 percent reduction in α-HCH emissions from East Asia results in a 3 percent reduction in deposition in North America. While these modeling exercises are an important step forward, to date there has been relatively little interaction between the modeling and obser- vational communities. As a result, these global model predictions are largely unverified. These results demonstrate that long-range transport of POPs is an important process to consider, but we are not yet able to make accurate quantitative predictions for most compounds. It is important to be able to model the episodic nature of long-range transport of POPs. CTMs with episodic transport prediction capability (such as GEOS-CHEM), have not yet been parameterized for POPs. Sat-

OCR for page 113
 PERSISTENT ORGANIC POLLUTANTS ellite observations of POPs are not currently possible because of their extremely low concentrations in the atmosphere (pg/m3 to ng/m3). Existing satellite images for particulate matter and gas-phase combustion products may not be appropriate surrogates for pesticides because of differences in source regions, atmospheric chemistry, and deposition. NEW POPS Some currently used chemicals, including pesticides (such as endosulfan) and consumer product chemicals (such as fluorinated organic chemicals [FOCs] and polybrominated diphenyl ethers [PBDEs]) are now found in remote locations throughout the world and have the potential to be consid- ered POPs. Human exposure to the chemicals used in consumer products includes both direct exposure (skin and inhalation) and exposure via the food web; concentrations of some of these potentially new POPs have increased in the food web, human blood serum, and mother’s milk in recent years in the United States (Schecter et al., 2006, 2007; Schecter, 2008; Tao et al., 2008). . Some FOCs, such as fluorotelomer alcohols (FTOHs), have been shown to undergo microbial degradation (Dinglasan et al., 2004) and photochemical transformation (Wallington et al., 2006) to perfluorinated carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA). These compounds can undergo atmospheric long-range transport (Shoeib et al., 2006; Piekarz et al., 2007) and oceanic transport (Armitage et al., 2006; Wania, 2007). Their precursors can also be found throughout the globe, including in remote locations such as Alaska and the Arctic and high- elevation ecosystems (Smithwick et al., 2006; Stock et al., 2007; Loewen et al., 2008; Schenker et al., 2008). PBDEs are also distributed globally, including remote U.S. high- elevation and high-latitude ecosystems (Kannan et al., 2005; Muir et al., 2006; Usenko et al., 2007; Ackerman et al., 2008). Although there have been regulations and voluntary efforts to move from the persistent, bio- accumulative, and toxic tetra- and penta-brominated PBDEs to safer alter- natives, these alternatives have been shown to undergo photodegradation (Hua et al., 2003; Bezares-Cruz et al., 2004; Zeng et al., 2008) and micro- bial degradation in the environment (He et al., 2006; Robrock et al., 2008), resulting in the same tetra- and pent-brominated PBDEs. CLIMATE CHANGE AND POPS Because the global environmental fate of POPs (including their degra- dation and intermedia transport) is highly temperature dependent, global climate change has the potential to significantly change the current global

OCR for page 113
 GLOBAL SOURCES OF LOCAL POLLUTION distribution of POPs. For example, increased surface temperatures could result in the volatilization of POPs from current temperate and tropical source regions and their deposition in colder regions, such as high-elevation and high-latitude ecosystems (Simonich and Hites, 1995; Wania and Mackay, 1995; Blais et al., 1998). On the other hand, if temperature increases more at higher latitudes compared with lower latitudes, fewer POPs will be stored at high latitudes. The melting of glaciers may result in the release of POPs stored decades ago into global circulation (Donald et al., 1999; Geisz et al., 2008) and POPs currently stored in soil and vegetation, may be re-released during fire events (Primbs et al., 2008a,b; Genualdi et al., 2009). Decreases in sea ice cover and increases in ocean temperature also have the potential to result in the redistribution of the more volatile POPs stored in ocean water (Macdonald et al., 2003, 2005; Macdonald, 2005). Although increased sur- face temperatures would theoretically increase the degradation rate of POPs in the environment, this benefit may be offset if POPs are redistributed to colder environments with more limited sunlight. Because rain and snow are efficient scavengers of airborne POPs, changes in precipitation patterns can affect where and how efficiently POPs are removed from the atmosphere. Because of their affinity for terrestrial surfaces, the global distribution of POPs will change along with vegetation patterns. Changes in the North Atlantic Oscillation, the El Niño-Southern Oscillation, the Arctic Oscillation, and the Pacific North American pattern could all affect the re-release and global redisribution of POPs currently stored in ocean water (Macdonald et al., 2003, 2005; Macdonald, 2005; Ma and Li, 2006). The tremendous uncertainty in predicting how POPs will be distributed globally in the future adds to the motivation for maintaining long-term atmospheric monitoring programs. KEY FINDINGS AND RECOMMENDATIONS Question: What do we know about the current import and export of POPs? Finding. There is substantial observational evidence that POPs can be transported over intracontinental scales, but only a few trans- port pathways have been documented. For instance, transpacific atmospheric transport of POPs to the contiguous United States is relatively well characterized, whereas inflow to Alaska is not. There is evidence of inflow from Canada to the United States, while inflow from Mexico is not well characterized. Outflow of POPs from the United States to Canada and the Arctic is fairly well characterized, whereas outflow to Europe is not. At present it is not possible to

OCR for page 113
 PERSISTENT ORGANIC POLLUTANTS quantify these transport fluxes due to the limited emission invento- ries and lack of validated quantitative models for POPs. Finding. There is evidence that atmospheric concentrations of banned or restricted-use pesticide-related POPs are declining due to global regulations, while concentrations of combustion-related POPs (PAHs and possibly PCDD/Fs) are increasing due to growing emissions from developing countries. Some chemicals currently in use that have the potential to be considered POPs due to their per- sistence, bioaccumulation potential, and toxicity (such as PBDEs and FOCs) are known to undergo long-range transport and have exhibited increasing concentrations in the food web and humans in recent years. Question: What are the potential implications of long-range transport of POPs on humans and ecosystems and environmental management goals? Finding. U.S. efforts to reduce exposures to POPs are clearly impacted by long-range transport. It is difficult to characterize the significance of this influence, both because of the scientific uncer- tainties described above and because there are currently no clear national goals for POPs deposition. Question: How might the factors influencing these issues change in the future? Finding. There is potential for the U.S. population (as well as some remote high-elevation and high-latitude U.S. ecosystems) to be exposed to increasing concentrations of certain POPs that have increasing emissions outside the United States (for instance, inha- lation exposure to carcinogenic PAHs and food web exposure to bioaccumulated PCDD/Fs). This potentially increasing exposure may be more pronounced in the western United States because of the patterns of transpacific transport from Asian countries. Finding. There is potential for the enhanced re-release of legacy POPs from melting glaciers, forest fires, and warming soils and oceans due to climate change influences. There is likewise poten- tial for remote high-elevation and high-latitude U.S. ecosystems to be exposed to increasing POP concentrations through re-release and redeposition of these compounds. The impacts of these future changes cannot currently be predicted quantitatively.

OCR for page 113
 GLOBAL SOURCES OF LOCAL POLLUTION Recommendation. Improve our ability to quantify the impacts of long-range transport of POPs on human and ecosystem health and to predict how this might change in the future. This requires efforts to • develop hemispheric emission inventories and projections for POPs (especially PAHs and PCDD/Fs). • parameterize and validate hemispheric transport models for POPs. Future research on global transport of POPs should focus on bringing together modeling and observation specialists dur- ing the design and execution of field campaigns to identify the most important parameters to measure and therefore reduce model uncertainties. • better quantify the current U.S. inflow and outflow of POPs through measurements and modeling. This includes the continu- ation of long-term atmospheric monitoring programs, which can aid our ability to track how POPs are redistributed due to climatic and global emission changes. • estimate future U.S. inflow and outflow based on projected changes in source regions and global climate change, using hemi- spheric transport models. • expand our understanding of the photochemical processes that affect POPs during transport. • evaluate long-range transport potential in the initial assessment and regulatory approval of new chemicals.