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Acid Deposition: A Case Study of Scientific Uncertainly and International Decision Making COURTNEY RIORDAN U.S. Environmental Protection Agency Ed'~or's Note: This case study of the acid deposition issue was selected and designed to illustrate some of the generic and specifw difficulties of using scientific and technical information in environmental decision making especially when the interests of more than a single nation are at stake. This chapter was prepared bya US. scientist who wasinvolved in the negotiations with Canada on an air-quality treaty in 1981-83. D': Riordan was also responsible for management of an important part of the interagency program of research on acid deposition and its effects in the United States. International concern about acid deposition was raised significantly by Sweden's case study for the United Nations' Conference on the Human Environment in 1972 which was entitled "Air Pollution Across National Boundaries: The Impact on the Environment of Sulfur in Air and Precipi- tation" (Bolin et al., 1972~. However, awareness of the "acid rain" problem in North America began in the late 1960s when scientists in Canada and the United States first began to study the changing acidity of precipitation and its effects on the continent. In both Europe and North America, scientific data pointed strongly to two important conclusions: . Emissions of sulfur dioxide (SO2) and nitrogen oxides (NO=) were causing unnaturally high acidity in precipitation; and these increased loadings of acidic substances were leading to acidi- fication of certain lakes and streams, and perhaps adversely affecting crops, forests, and human health. By the late 1970s, acid deposition had become a major domestic political issue inside the United States and a major international political issue with Canada. Studies of the geographic distribution of sources of 342
ENVIRONMENTAL MANAGEMENT CASE STUDIES 343 emissions, acidic precipitation, and acidic lakes indicated that lakes were acidic in the northeastern United States and southeastern Canada, and that a major contributing cause of the problem was the heavy concentration of SO2 emissions in the midwestern United States. In 1978, the United States Congress passed a resolution calling for bilateral discussions with Canada to Preserve and protect mutual air re- sources." This resolution was stimulated in part by concern about construc- tion of a new coal-fired power plant in western Ontario near the Boundary Waters Canoe Area of northeastern Minnesota. In response to this res- olution, the governments of Canada and the United States issued a joint statement in July 1979, and on August 5, 1980, signed a Memorandum of Intent (MOI) to negotiate a treaty on transbounda~y air pollution. The MOI noted that: The Governments share a concern about damage resulting from transboundary air pollution . . . including the already serious problem of acid rain. Are resolved to . . . improve scientific understanding of the long-range transport of air pollutants [and] develop and implement policies to combat its impact. Are convinced that the best means to protect the environment . . . is through achievement of necessary reductions in pollutant loadings. Also in August 1979, President Jimmy Carter recommended a 10-year program of research on the causes and consequences of acid precipitation. In June 1980, the United States Congress passed the Acid Precipitation Act of 1980 (Public Law 96-294~. This law added legislative authority for a 10-year research program which was later named the National Acid Precip- itation Assessment Program (NAPAP). This program was to be carried out jointly by the major agencies of the U.S. government, chief among them being the Departments of Interior and Energy, the Forest Service, the National Oceanic and Atmospheric Administration, and the Environmental Protection Agency (Cowling, 1982~. The annual budget for NAPAP grew from $17.4 million in 1983 to $85.6 million in 1987. SCIENCE, POLITICS, AND RESEARCH Although there was a certain incongruity in these two nearly simulta- neous eventsestablishment of a major scientific research program while conducting international negotiations there were also some benefits. The MOI called for establishment of: technical and scientific work groups to assist in preparations for and the conduct of negotiations on a bilateral transboundazy air pollution agreement.... The Work Groups shall provide reports assembling and analyzing information and identifying measures which will provide the basis of proposals for inclusion in a transbounda~y air pollution agreement. These reports shall be provided by January 1982 and shall be based on available information.
344 The topics covered in the Work Group Reports included: ECOLOGICAL RISES · the nature and extent of effects of acidic deposition (Bangay and Riordan, 1983~; · quantification of sources of emissions and costs for their reduction (Riegel and Rivers, 1982~; ~ · the relationship between decreases in emissions and decreases in acidic deposition (Ferguson and Machta, 1982~; and · alternative strategies by which to allocate and achieve emissions de- creases among geographical areas and types of pollution sources (Hawkins and Robinson, 1982~. Canadian scientists entered into the MOI work-group process with expectation that, around the beginning of 1982, an agreement would be signed by the two countries to decrease emissions of SO2 on a prescribed schedule in order to decrease existing damage and prevent future environ- mental damage from acidic deposition. However, the United States had a different perspective on the work-group process. As a result, the MOI process broke down during 1982. The Work Groups were actually able to reach agreement in a number of important areas, e.g., the amounts of emissions; the relative efficiencies and costs of technologies to decrease emissions from existing and new sources; and the relative lack of knowledge about the effects of acidic deposition on crops, forests, and materials. The Work Groups even reached agreement on the expected degree of accuracy of regional models to predict changes in annual average wet deposition in major receptor areas that would result from emissions reductions in major source regions. It was in the aquatics-effects area, however, that the work-group pro- cess and, ultimately, the entire MOI negotiation process broke down. The scientists on both sides all agreed that total sulfur loadings from atmospheric deposition were the likely cause of long-term acidification of a number of lakes in the northeastern United States and southeastern Canada. But the U.S. and Canadian scientists disagreed on the conclusions that could be reached with respect to two basic scientific questions which were of critical importance to policy makers in both countries (Bangay and Riordan, 1983~. The first question was "What is the extent of acidification of surface waters?" As is often the case in emerging environmental problems, much of the early research on acidic deposition had concentrated on known problem areas, e.g., the Adirondacks in the United States and southern Ontario in Canada. The number of lakes observed to have an average pH below 5.0 was less than 180 in the northeastern United States and less than 100 in Canada. Further, essentially no data were available on the hydrology' soils, and geology of these lakes or how these characteristics might compare to
ENVIRONMENTAL MANAGEMENT CASE STUDIES 345 those of the thousands of other lakes in the eastern United States and Canada. Ib estimate the extent of existing acidification, Canadian scientists wanted to extrapolate from the lakes that had already been studied to the entire population of lakes. The basis for this extrapolation was their conviction of a similarity in regional scale soil type and geology. However, the U.S. scientists would not support this extrapolation for two reasons. First, they were convinced that many of the already affected lakes were impacted by local smelter emissions and not just by regionally transported deposition. Second, existing knowledge indicated that subregional soil and geology variation was an important factor in determining the response of lakes to acidic deposition. The second major disagreement was related to the "target loading" issue. Canadian scientists wanted the Work Group on Impact Assess- ment to conclude that a reduction of the annual average wet deposition of sulfate to 20 kilograms per hectare per year would protect all but the most sensitive lakes from the adverse effects of acid deposition. The basis for this conclusion was empirical association. They had found acidic lakes in areas which were experiencing wet deposition greater than 25-35 kg~a/yr and had not observed acid lakes where deposition was less than 20 kg/ha/yr. The U.S. scientists would not agree to this view because they did not see the world so simply. Futher, in Norway and Sweden, there was evidence that some lakes could be acidic with wet deposition as low as 10 kg/ha/yr. In the opinion of the U.S. scientists, the difference between 10 and 20 kg/ha/yr could have been either the result of differences in the ratios of dry to wet deposition, or differences in the acid-neutralizing capacity of watersheds, or some combination of both factors. According to the U.S. scientists, a more defensible scientific position would be to estimate how many lakes would be acidic if annual average amounts of wet deposition were limited to a range of possible loadings, e.g., 10, 15, 20, 25, 30, 35, and 40 kg~alyr. In that way, policy makers would be aware that the situation was not black or white; rather, that there was a continuum on which lakes were more likely to be acidic as the amounts of annual average wet deposition increased. With reasonable scientific confidence, both the U.S. and Canadian sci- entists agreed that atmospheric deposition of sulfur had caused acidification of some lakes, on the order of a few hundred in the United States and about 100 in Canada. But the U.S. scientists were not willing to extrapolate from the limited data that were then available in order to provide an estimate of damage for the entire population of lakes in the northeastern United States and southeastern Canada. Three critical elements were lacking, in their opinion. First, there was a lack of an adequate dose/response func- tion relating acidic deposition to lake-water chemistry based on soil trans- port and transformation processes in watersheds; second, there was a lack
346 ECOLOGICAL RISKS of good empirical data on the relationships among geographical variation in surface-water quality, watershed soils, and hydrology; and finally, there was a lack of acceptable model estimates or measurements of dry deposition for lakes in the potentially affected regions (Bangay and Riordan, 1983~. Just as the MOI discussions on acidic deposition between the United States and Canada broke down during 1982, so did the ability of the United States Government to deal with its own domestic differences in perspectives about the acid deposition issue. Congressmen from the New England states, New York, Minnesota, and Wisconsin pushed for legislation to require decreases in SO2 emissions from electrical utility boilers. Some support for these proposals was offered by western states which could provide low-sulfur coal if major eastern utilities were willing to shift from high- to low-sulfur coals. However, congressmen from several midwestern states which had large deposits of high-sulfur coal, and in many cases also had large SO2 emissions, were adamantly opposed to major new decreases in SO2 emissions as was then-President Ronald Reagan Reagan. They argued that the new controls being called for would probably cost as much as $4 billion per year for as long as 20 years. They believed such costs were not justified for two reasons. First, they wanted to "wait and see" what effect the 27% decreases in SO2 emissions during the period from 1974 to 1980 would have on the problem. Second, they wanted to "do more research before making a decision;" they did not believe that existing scientific knowledge was sufficient to estimate the number of existing lakes that were acidic as a result of acid deposition, or to predict how many additional lakes might become acidic if amounts of emissions and deposition were to remain constant or to increase. They also saw gaps in knowledge of source/receptor relationships, modeling of atmospheric processes, and watershed acidification processes. Thus, in part because of scientific uncertainties, the United States faced a political stalemate. WHEN TO ACT: SCIENCE VERSUS POLICY In the United States, issues such as acid deposition are hotly debated by the scientific community, elected officials, environmental groups, industry leaders, the media, and the public at large. These debates often become confused because of the seemingly unavoidable mixing of inherently distinct functions the scientific function of discovering how nature works and how its is influenced by human activities, and the political function of deciding what values the society holds dear, and what, if anything, society ought to do about a given social, economic, environmental, or political issue.
ENVIRONMENTAL MANAGEMENT CASE STUDIES 347 The problem of handling this confusion of functions is particularly difficult for scientists in government and in the private sector. Too often, elected officials and the public look to scientists for answers to questions that are not scientific but political. The problem of acidic deposition is a good example. As early as 1981, many scientists inside and outside of government had concluded that acidic deposition had, with acceptable scientific certainty, contributed in a major way to the long-term acidification of several hundred lakes in the northeastern United States and southeastern Canada. Because the lakes that had been studied were not known to be representative of the total number of lakes, there was no mutually acceptable scientific way to estimate the total number of lakes that might become acidic in the future. Nevertheless, scientists were being asked and some were providing answers to questions on how society should decrease emissions of SO2 and thereby decrease acidic deposition. The problem with this process is that the question of whether and how much society should pay to avoid a particular pollution effect is fundamen- tally not a scientific question. Rather, it is a political one, involving complex trade-offs between differing values of individuals and groups with respect to environmental quality and other activities that also affect the quality of life, e.g., food supply, education, public health, etc. In fact, even if scientists did know how many lakes were now acidic and how many more would become acidic in the future, it would still require a careful assessment of many non-scientific values to be able to determine what trade-o~s, if any, society ought to make between the costs of emissions decreases and the costs of acidic lakes. Such questions are inherently political and require political judgment, not scientific judgment. Of course, in 1982 as well as today, scientists cannot answer some of the important scientific questions about the environmental effects of acidic deposition, to say nothing of the political questions about what society ought to do about it. In view of the scientific uncertainties, many politicians in the United States have been unwilling to adopt new legislation for the purpose of decreasing SO2 emissions below the amounts that are already being achieved in some regions under the Clean Air Act of 1970 and its amendments of 1977. In the absence of a political solution to the acidic deposition problem, the United States conducted a major research program under the Acid Precipitation Act of 1980. President Reagan and his supporters in the United States Congress believed that a rational political decision was to invest in research that would close critical gaps in scientific knowledge. Thus, a conscious decision was made based on the following assumptions:
348 ECOLOGICAL RISKS · There are critical gaps in policy-relevant scientific knowledge. · Applied research can close many of these gaps during the 10-year research program that was begun in 1980. · Widespread environmental damage at present rates of deposition is unlikely in 5-10 years. MAJOR FEATURES OF THE NAPAP RESEARCH PROGRAM The nature and scope of the NAPAP research program changed signif- icantly as a result of the political stalemate that developed domestically and internationally during the period from 1982 to 1983. The extensive political and scientific debates that occurred at this time focused attention on certain gaps in scientific knowledge that were of major concern to policy makers. Those opposed to additional SO2 controls were obliged to identify those uncertainties in the science that they believed prevented rational decisions. Those favoring additional SO2 controls insisted on three basic conditions for their willingness to support research as opposed to action: . First, those opposed to controls had to demonstrate why a gap in knowledge or uncertainty was critical to policy; · Second, NAPAP research plans and approaches had to provide reasonable assurance that major decreases in scientific uncertainties could be achieved over a period of 5-10 years; and · Third, the research had to be affordable and actually funded. During the period from 1983 to 1984, the researchers in the NAPAP program worked closely with policy makers to develop a major expansion in the applied research program to address significant cans in knowledge with projects that met the three conditions listed above. In some ways, the timing of these discussions could not have been better. Research had already provided insights about some of the causes and effects of acidic deposition. These early results provided the foundation for a larger scale applied-research effort. Thus, the bulk of the new resources in NAPAP were directed to provide better answers to policy-relevant scientific questions in the following categories: of o--r~ -~~~~ Aquatic Effects . To what extent have surface waters been acidified by acidic depo- sition? · How many more lakes and streams are likely to be acidified if deposition rates remain constant or increase?
ENVIRONMENTAL Af4NAGEMENT CASE STUDIES 349 · What is the dose/response function that relates acidic deposition to surface water acidification? Forest Ejects · Is acidic deposition alone or in combination with other factors responsible for observed growth reductions and damage to selected forests in the eastern United States? What is the extent of damage in these forests that might be at- tributable to air pollution? What is the dose/response function that relates acidic deposition to growth declines and/or damage in these forests? Emissions What are the historical and present amounts of emissions of acidic deposition precursors? · What techniques are available for decreasing these emissions and at what cost? Emission/Deposition Relationships · What are the present patterns of dry deposition? · What changes in patterns of wet and dry deposition of sulfur and nitrogen compounds would result from a change in the pattern of emissions? The research approaches used in pursuing these several applied research questions are summarized briefly below: The National Surface Water Survey (NSWS) was initiated in 1984 to provide a statistically based estimate of the number of acidified lakes and streams in various parts of the United States. In Phase I, samples were taken in regions of the country that were known to contain a significant percentage of lakes and streams with alkalinity less than 400 microequivalents per liter. In Phase II, representative subsets of lakes with alkalinity less than 200 microequivalents per liter were sampled to determine spatial and temporal variations of acidity in each lake over the spring, summer, and fall seasons. This study was also designed to determine the presence or absence of various fish species in some subregions. Phase III is a long-term monitoring program for a set of lakes with alkalinity less than 200 microequivalents per liter in areas with different acid deposition loads. The Direct/Delayed Response Program (DDRP) was designed to sup- plement the results of the NSWS by providing detailed information on the dynamic responses of watersheds and lakewater chemistry to acid inputs. The DDRP funded the development and application of models that can
350 ECOLOGICAL RISKS use data on vegetation, soils, and hydrogeology of watersheds to predict future changes in lakewater chemistry which may occur under a variety of future acid deposition scenarios. Three different models were run using the detailed data on soil type and watershed characteristics gathered for 145 watersheds in the northeastern and the southeastern sections of the United States. The three models were the so-called ILWAS (Integrated Lake-Water Acidification Study), TRICKLE DOWN, and MAGIC (Model of Acidification of Groundwater In Catchments) models. The Watershed Response Program is a watershed manipulation program designed to test critical features of the three models listed above in the field at the plot and catchment level. Simulated acidic deposition is applied to watersheds and then the response of vegetation, soils, and surface waters is observed. The data generated by these watershed-manipulation studies should provide a definitive test of the power and utility of the watershed models for predicting lakewater responses to possible future changes in acidic deposition loadings. The Forest Response Program was designed to determine the possible effects of acid deposition and other airborne pollutant chemicals on forests. During the late 1970s, acid deposition was considered a major contributing cause of damage to forests in certain areas of Germany. During the early 1980s, two sources of data for tree injury and decline that were not explained by natural causes began to appear in the United States. Many scientists were concerned that these initial reports of changes in the condition of forest trees in the United States might become comparable in magnitude and extent to those observed in Europe. A four-part program was initiated to include: deposition. · field studies to identify and quantify changes in forest health; controlled exposure/response experiments to determine the impact of acidic deposition on tree seedling growth; · research on physiological processes to identify cause-and-effect mechanisms; and development of models to predict tree and forest response to acidic After much debate about alternative approaches to the study of emis- sions/deposition relationships, NAPAP decided to develop the Regional Acid Deposition Model (RADM). This model is a six-elevation Eulerian Model with~horizontal grids that are 80 kilometers on a side. RADM 1 contains first generation descriptions of transport, clean-air chemistry, wet scavenging, and deposition. A preliminary evaluation of RADM 1 was completed during 1986 using two limited data sets: the Oxidant and Scavenging Characterization of April Rains (OSCAR) and the Cross-Appalachian Tracer Experiment
ENVIRONMENTAL MANAGEMENT CASE STUDIES 351 (CAPTEX). OSCAR measured wet deposition amounts and chemistry within certain specific weather events. CAPTEX measured plumes of inert tracer material across the northeastern United States and Canada also during selected meteorological events. The preliminary evaluation results are being used to revise RADM. NAPAP also decided that some measure of dry deposition and trends was essential Such measures were needed to improve estimates of total deposition of acidic materials in receptor areas and as a means of evaluating RADM. Unfortunately, dry deposition was a case where the need outpaced the feasibility of science and the availability of funding in the NAPAP research program. As a result, a decision was made to employ an indirect air concentration/deposition velocity approach at monitoring sites. This decision was made in the absence of demonstration that an appropriate deposition velocity algorithm could be developed for operational sites based on actual flus measurements developed at a limited number of core research sites. The rationale for this decision was that even if the technique failed, air quality information would still be available for model evaluation. NAPAP has not funded research on new combustion technologies or new post-combustion clean-up technologies. However, a great deal of research is being carried out by other programs in the U.S. government and by the private sector. NAPAP INTERIM ASSESSMENT In September 1987, NAPAP issued an Interim Assessment of its re- search program findings. The report was expected to be used by policy makers in the Executive and Legislative branches of the U.S. government in their reassessment of acid deposition policy. Although the scientific chap- ters of this Interim Assessment (NAPAP 1987b,c,d) provided a valuable summary of both NAPAP-sponsored and non-NAPAP-sponsored research findings, substantial controversy resulted from the disparities in substance and tone between the Executive Summary (NAPAP, 1987a) and the scientific chapters (LeFohn and Krupa, 1988~. From a policy perspective, the most important and most controversial conclusions related to the probability of future adverse environmental effects if current rates of acidic deposition were maintained in the future. The Executive Summary of the Interim Assessment emphasized that: · Available observations and current theory suggest that there will not be an abrupt change in aquatic systems, crops, or forests at present levels of air pollution. · Some lakes and streams in sensitive regions appear to have been acidified by atmospheric deposition at some point in the last 50 years.
352 ECOLOGICAL RISKS Available data suggest that most watersheds in the glaciated northeast are at or near steady state with respect to sulfur deposition, and that further significant surface water acidification is unlikely to occur rapidly at current deposition levels. Although no lakes and streams with a pH of less than 5.0 have been found in the Southern Blue Ridge Province, water bodies in this region are generally not at steady state with respect to sulfur deposition, and gradual increases in surface water sulfate and decreases in acid neutralizing capacity (ANC) may occur as the sulfur absorption capacity of the soil decreases. At current levels of acidic deposition, short-term direct foliar effects on crops or healthy forests are unlikely. Acidic deposition may have a cumulative effect on trees growing on certain low-nutrient soils, but this effect is expected to be gradual and has not been reported in the United States at current levels. It is unlikely that regional sulfur dioxide concentrations are causing damage to crops or forests. Bees and crops can exhibit severe damage and even mortality from high concentrations of ozone and sulfur dioxide. Such occurrences are rare today because of emission controls on most major point sources. At the more typical ambient chronic concentrations of ozone, some crop damage is observed. For many tree species in low-elevation forests, growth reduction may be occurring at ambient ozone concentrations. With the possible exception of above-cloud-base forests where high mortality has occurred from unknown causes, most U.S. forests are not expected to show an abrupt change in health at current ambient air pollutant concentration levels and deposition rates. Perhaps as important were the Interim Assessments' enumeration of scientific uncertainties which NAPAP hopes to reduce by 1990: · the sources, quantities, and reactivities of natural emissions of sulfur dioxide, nitrogen oxides, volatile organic compounds, methane, and alkaline substances (current emissions of these substances are uncertain by about a factor of about 3~; · the origin and distribution of hydrogen peroxide, a primary oxidiz- ing agent in clouds; · the influence of urban emissions on deposition locally (<30 km) and in the mesoscale (30 km to 200 km) downwind; · the source/receptor relationship resolved to a state level on a seasonal and annual basis; · the current spatial and seasonal distribution of dry deposition of sulfur dioxide and nitric acid; · identification of forest soils which are potentially sensitive to change by ambient acidic deposition and which might affect tree health;
ENVIRONMENTAL A{4NAGEMENT CASE STUDIES 353 · the relative contribution of acidic deposition, ozone, hydrogen peroxide, nitrate, and natural stresses to the decline of above-cloud-base forests in the Appalachians; · methods to extrapolate results of dose/response experiments of pollutants on seedlings and saplings to mature trees; methods to estimate change in the regional distribution of surface water chemistry (lakes and streams) over the next half~entury at present or changed rates of acidic deposition; and · the effect of episodic acidic events on the health and reproduction of fish in streams and lakes. The NAPAP Final Assessment is due to be completed in 1990 (Mahoney et al., 1989~. SUMMERY . .t In recent years, progress has been slow in resolving many scientific and public-policy questions about the causes, consequences, and management of acid deposition in North America. Part of the reason for this slow progress has been uncertainties about the science involved. But equally important has been the absence of a public consensus between Canada and the United States as well as among the several states within the United States about what, if anything, should be done about acidic deposition. It appears that the degree of scientific certainty that is required to reach a decision about such a complex issue of science and public policy is an inverse function of the degree of public consensus about the same issue. Acid deposition is certainly an example of this generalization. REFERENCES Bolin, B., et al. 1972. Air pollution across national boundaries: Lee impact of sulfur in air and precipitation. Case study prepared by Sweden for the United Nations Conference on the Human Environment. Stockholm, Sweden: Norstedt and Sons, p. 97. Bangay, G.E., and C Riordan. 1983. United States-Canada Memorandum of Intent on ansboundary Air Pollution. Final Report prepared by the Impact Assessment Work Group I. U.S. Department of State and the Embassy of Canada, Washington, DC. Cowling, E.B. 1982. Acid precipitation in historical perspective. Environmental Science and Technology 16:110A-123^ Hawkins, D.G., and R Robinson. 1982. United States-Canada Memorandum of Intent on lLansbounda~y Air Pollution. Anal Report prepared by the Strategies Development and Implementation Work Group 3^ U.S. Department of State and the Embassy of Canada, Washington, DC. Ferguson, H., and Lo Machta. 1982. United States-Canada Memorandum of Intent on lLansboundary Air Pollution. Anal Report prepared by the Atmospheric Modeling Work Group ~ U.S. Department of State and the Embassy of Canada, Washington, DC LeFohn, A., and S. Krupa. 1988. A technical amplification of NAPAP's Interim Assessment. Air Pollution Control Association, Washington DC.
354 ECOLOGICAL RISKS Mahoney, J.R., P.M. Irving, and J.L. Malanchuk. 1989. Plan and schedule for NAPAP's 1989 and 1990 assessment reports. Journal of the Air Pollution Association 38:1489-1496. NAPAP. 1987a. Executive summary. The Causes and Effects of Acidic Deposition: Interim Assessment, Volume I National Acid Precipitation Assessment Program. Washington, DC. NAPAP. 1987b. Emissions and control. The Causes and Effects of Acidic Deposition: Interim Assessment, Volume II. National Acid Precipitation Assessment Program. Washington, DC NAPAP. 1987c. Atmospheric processes. The Causes and Effects of Acidic Deposition: Interim Assessment, Volume III. National Add Precipitation Assessment Program. Washington, DC. NAPAP. 1987d. Effects of acidic deposition. The Causes and Effects of Acidic Deposition: Interim Assessment, Volume IV. National Acid Precipitation Assessment Program. Washington, DC. Riegel, Key, and M.E. Rivem. 1982. United States-Canada Memorandum of Intent on lLansboundary Air Pollution. Final Report prepared By the Emissions, Costs, and Engineering Assessment Work Group 3B. U.S. Department of State and the Embassy of Canada, Washington, DC. U.S. Department of State and the Embassy of Canada. 1980. Memorandum of Intent between the Government of the United States of America and the Government of Canada concerning transbounda~y air pollution. Washington, DC. Hi
