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During the past twenty five years acid rain, formally referred to as acid deposition, has been the focus of much political debate and scholarly research. Acid deposition occurs when important precursor pollutants, such as sulfur dioxide (SO) and nitrogen oxides (NO), chemically mix with water vapor and oxidants in the atmosphere and fall back to earth in wet or dry form. Wet deposition comes in the form of dew, fog, snow or rain, while dry deposition occurs as either gasses or dry particulates. Research has shown that acid deposition adversely affects freshwater lakes and streams, coastal habitats, agricultural production, forests, soils, human health and building materials. Fossil-fuel power plants, refineries, and paper and pulp mills are the major sources of SO emissions, while automobiles and other vehicles are the primary emitters of NO.

I began a fifteen-year career as a federal science program manager in the middle of the 1980s, when I took a position in the U.S. Environmental Protection Agency’s (EPA) Office of Research and Development on the Acid Deposition Research Staff. EPA was emerging from an unprecedented trough in public perception and official performance. William Ruckelshaus had returned as Administrator, in part to repair the damage done by the previous Administrator, and among the many vexing environmental issues that needed to be addressed, the challenge of acid deposition was among the greatest. In this chapter, I reflect not so much on the science of acid deposition per se, but on my personal experiences as a participant in an important federal science and assessment program on a very visible public environmental issue, and what lessons can be drawn from them.

Acidic atmospheric deposition, popularly referred to as acid rain, is the transfer of strong acids and acid forming substances from the atmosphere to the Earth’s surface. Acidic deposition is comprised of sulfuric and nitric acids, and ammonium derived from atmospheric emissions of sulfur dioxide, nitrogen oxides, and ammonia respectively. These compounds are emitted by the burning of fossil fuels and by agricultural activities. Once such compounds enter an ecosystem, they can acidify soil and surface waters and bring about a series of ecological changes. The term acidic deposition encompasses all forms in which these compounds are deposited to the Earth, including gases, particles, rain, snow, clouds, and fog (see Box 3.1). Acidic deposition was first reported in the United Kingdom in the later half of the 19th Century (). Ecological effects were first documented in Scandinavia in the 1960s with the link between acidic deposition, surface water acidification and loss of fisheries (). Atmospheric deposition of sulfate, nitrate and ammonium are elevated in eastern North America, Europe and large portions of Asia ().

In Canada, nearly 45% of the land area is considered sensitive to acid deposition. Lakes and watersheds located on the Canadian Shield are considered the most vulnerable, due to the low buffering capacity of the typically shallow soils that overlay the Shield bedrock. A large portion of eastern Canada, including much of Ontario and Quebec as well as parts of the Atlantic Provinces are underlain by silicate bedrock; these areas also receive the highest levels of acid deposition in the country.

Acid rain research on terrestrial ecosystems has increasingly focused on the effects of inorganic nitrogen (N) deposition, both as nitric acid (HNO) and ammonium (NH, which can produce acidity in soils when oxidized). This is largely because acidification of sensitive catchments in the northeastern United States and elsewhere continued following the 1970 and 1990 amendments to the U.S. Clean Air Act (). These amendments capped sulfur oxide (SO), but not N oxide (NO) emissions from electric utilities and industrial sources. Nevertheless, NO control programs, focused on utility and industrial sources as well as on vehicle emissions, have stabilized if not decreased NO emissions and resultant nitrate deposition in the eastern United States (See , this volume). As a result, nitrate deposition has remained relatively stable in the northeastern United States and eastern Canada through the past decade, but has increased relative to sulfate deposition (). Ammonium deposition, due largely to ammonia (NH) emissions from fertilized agroeco-systems and from concentrated animal feeding operations (CAFOs, ), accounts for 30 to >50% of inorganic N deposition on the land surface in North American and other industrialized regions of the world (); ().

Since the first appreciation of the widespread occurrence of acid rain in North America (), most public attention has focused on the acid component rather than effects from the associated elements in atmospheric deposition. The emphasis has been on freshwater ecosystems and forests in sensitive regions with relatively low buffering capacity. Effects of acid deposition on coastal marine ecosystems have usually not been considered, which makes sense in the context of acidity. Marine ecosystems are very well buffered, since they contain large amounts of dissolved carbonate and bicarbonate, and consequently are quite insensitive to acid inputs. Similarly, marine waters contain huge quantities of sulfate (∼ 28 mM) and thus are not sensitive at all to inputs of sulfate associated with acid deposition. On the other hand, nitrogen (N) pollution can cause severe degradation in coastal marine ecosystems, and the role of atmospheric deposition as a contributor of nitrogen to coastal waters has received increasing scrutiny over the past 15 years since Fisher and Oppenheimer (1991) noted that the nitrate anion associated with nitric acid in acid rain may be a major source of nitrogen to Chesapeake Bay.

Acid rain has been a pivotal issue in the development of European environmental policies and programs. Appreciating the history of European responses to acid rain is also useful for understanding how the world has moved towards greater use of international environmental agreements to address transboundary pollution issues, such as stratospheric ozone depletion and global climate change. Indeed, the 1972 United Nations Conference on the Human Environment (UNCHE), the first truly global environmental meeting of the world’s heads of state and government, was proposed in response to Sweden’s concerns that acid rain originating in Great Britain and Germany (East and West) was responsible for the acidification and death of Scandinavian lakes. At the time of the UNCHE, there was still no scientific consensus or political acceptance of the idea that acid rain could fall as far as a thousand kilometers (600 miles) or more away from its pollution source. Nor was there much appreciation of the need for political action to address the transboundary and global nature of many pollution problems. Sweden used the UNCHE to bring international attention to the problem of transboundary acid rain and other increasingly pressing global environmental concerns.

Air pollution was among the most salient environmental problems in Central and Eastern Europe (CEE) in the aftermath of communism. Photographs of damaged monuments, forests defoliated from acid rain, and degraded landscapes became emblematic of the environmental burden associated with centralized planning. CEE countries contributed significant amounts of transboundary acidification in Europe due to prevailing atmospheric and geographic patterns. The area bordering the Czech Republic, East Germany, and Poland—a region of high concentration of industrial enterprises and acidification—became know as the “Black Triangle” and was just one example of a regional pollution “hot spot.” The period of democratic transition brought promise as well as international pressure to tackle domestic and transboundary air pollution in transition countries (); (); ().

Acid rain emerged in the late 1970s both as a domestic issue within the United States and Canada and as a contentious problem between the two neighbors. A decade of debate over the 1980s led eventually to controls on the emissions that cause acid rain, a step that required a new federal-provincial agreement in Canada and significant amendment of the U.S. Clean Air Act, and led also to an international agreement—the bilateral Air Quality Agreement of 1991. In the decade that followed, these efforts were praised as highly successful, especially in Ottawa and Washington. While other transboundary issues have achieved more prominence in recent years, particularly ground-level ozone, acid rain remains on the bilateral agenda.

Recently, the U.S. federal government has pursued a determined strategy toward increased energy production while paying little heed to the impact of this strategy on air quality and failing to take effective measures to reduce emissions of pollutants from the fossil-fueled power plants that dominate U.S. energy generation. While the evolution of the Clean Air Act and its important amendments—particularly the 1990 Acid Rain Program amendment—demonstrated a strong popular and political commitment to improved air quality in previous decades, the excessive focus on fossil-fuel based energy production and absence of a coherent national energy strategy that have characterized U.S. policy since 2000 represent a troubling reversal. Thus it is critical to take a closer look at how U.S. energy policy is impacting on air quality, and review the steps taken prior to 2000 that might provide useful lessons for the formulation of more effective energy and air quality policies in the future.