When pollution from burning fossil fuel enters the atmosphere, it can react to create acids, which then return to the earth in precipitation, having environmental consequences. Environmentalists and those living in affected communities have challenged industry and government alike over the issue of acid rain.
I. Acid Rain Dynamics
II. Acidity and Ecosystem Damage
III. Initial Government Action
IV. Canada and the U.S. Acid Rain Controversy
V. Acid Rain and Art, Architecture, and Antiquities
VI. How Much Acid Is There in Rain?
VII. Sources of Acid Rain
Acid Rain Dynamics
Moisture in the atmosphere forms small droplets of rain that gradually become larger and heavier and sink to the ground as rain. These droplets can form around dust, particulate matter, and each other. There are many sources for the pollution that forms the acid in rain. The consensus is that it is primarily industrial air emissions that contribute to acid rain. Large coal-fired power plants and factories add to this problem. The prevailing air currents carry these emissions all across the United States and the rest of the world. Airborne emissions from other industrialized and newly industrialized nations also travel long distances to other countries. Thus, the direction of the prevailing winds can determine the deposition of acid onto the earth’s surface. Once the acidic gases have been emitted into the atmosphere, they follow prevailing wind circulation patterns. Most industrialized areas of the world are located within the mid-latitude westerly belt, and their emissions are carried eastward before being deposited. Acid rain is possible anywhere precipitation occurs. Early scientific controversies were about the number of tree species affected and the long-term ecological impacts. Current controversies are about whether the problem has been solved. It has a greater environmental impact than predicted by early studies. Although sulfur emissions have decreased, other emissions have increased. Some contend that mercury deposition has increased, and others consider the scope of environmental regulation inadequate.
Acidity and Ecosystem Damage
Historical weather data come from precipitation records, ice cores, and tree borings. They show an increase in acid rain starting in the late 1930s and on through the 1940s and 1950s. This was also the approximate time of a large industrial expansion in the United States and before the implementation of clean air policy in the early 1970s. Many U.S. cities used coal and natural gas in their everyday activities, depending on the dominant industry.
Acidity causes metals, such as aluminum or lead, to become soluble in water. Once in the water, acid affects plants and fish and is considered toxic to both. Acid deposition is directly damaging to human health. Its ability to corrode metals—from lead and copper pipes, for example — can be toxic to humans. The sewer and water infrastructures of many older U.S. cities have lead pipes. Increased concentrations of sulfur dioxide and other oxides of nitrogen in the air, common industrial emissions, have been causally related to increased hospital admissions for respiratory illness. In areas that have a large concentration of these airborne industrial emissions, there is an increase in chest colds, asthma, allergies, and coughs in children and other vulnerable populations. Thus, there is a strong public health concern associated with industrial emissions apart from the issue of acid rain.
Acid deposition refers to the process of moving acids to the land within a given ecology. The acids then move through the top surface of the earth, the soil, vegetation, and surface waters. As metals such as mercury, aluminum, and lead are set free owing to the increased acidity from the rain, they can have adverse ecological effects. There is also concern that some of the metals may bioaccumulate and intensify as they move up the food chain. The sustainability of an ecosystem depends on how long it takes for the system to recover, in this case from acid in the rain. The ability of some ecosystems to neutralize acid has decreased because of the cumulative impacts of acid rain over time. This slows the recovery of other parts of the ecosystem. This is why environmentalists contend that recent decreases in some industrial emissions are not likely to bring about full ecosystem recovery. In spite of partial environmental regulation, the cumulative impacts of acid rain pollutants have damaged sensitive areas of the Northeast, such as the Adirondack State Park, by impairing their ability to recover from acid shock events.
Acid rain has a range of effects on plant life. Sensitive species go first. Part of the acid rain controversy is determining which species are affected and the overall scope of the problem. Acid rain falls in any place where it rains or snows. Crops used for food or other purposes can be negatively affected. Acid rain’s effects on plants depend on the type of soil, the ability of the plant to tolerate acidity, and the actual chemicals in the precipitation. Soils vary greatly from one location to another. Soils with lime are better able to buffer or neutralize acids than those that are sandier or that contain weathered acidic bedrock. In other soils, increasing acidity causes the leaching of plant nutrients. The heavy metal aluminum causes damage to roots. This can interfere with the plants’ ability to absorb nutrients such as calcium and potassium. The loss of these nutrients affects the plants’ ability to grow at normal, productive rates, and the intake of metals increases their potential toxicity. For example, acid deposition has increased the concentration of aluminum in soil and water. Aluminum has an adverse ecological effect because it can slow the uptake of water by tree roots. This can leave the tree more vulnerable to freezing and disease.
Many important life forms cannot survive in soils below a pH of about 6.0. The loss of these life forms slows normal rates of decomposition, essentially making the soil sterile. When the acid rain is nitrogen-based, it can have a strong impact on plants. High concentrations of nitric acid can increase the nitrogen load on the plant and displace other nutrients. This condition is called nitrogen saturation. Acid precipitation can cause direct damage to plants’ foliage in some instances. Precipitation in the form of fog or cloud vapor is more acidic than rainfall. Other factors such as soil composition, the chemicals in the precipitation, and the plant’s tolerance also affect survival. Sensitive ecosystems such as mountain ranges may experience acidic fog and clouds before acid rain forms.
Acid rain falls to the earth and gradually drains to the oceans via rivers, lakes, and streams. Acid deposition erodes the quality of the water in lakes and streams. It causes lakes to age prematurely, speeding up the natural process of eutrophication. It does this in part by reducing species diversity and aquatic life. Fish are considered an indicator species of ecological health. However, ecosystems are made up of food webs of which fish are only one part. Entire food webs are often negatively affected and weakened. Environmentalists and those interested in sustainability are very concerned about ecosystem effects and are much involved in acid rain discussions.
Initial Government Action
On September 13, 2007, in Montreal, Canada, 24 nations signed the Montreal Protocol. Canada is highly motivated to solve air pollution problems because it is highly vulnerable to the effects of acid rain. This landmark environmental treaty required the phasing out of ozone-depleting chemicals and compounds such as chlorofluorocarbons, carbon tetrachloride, and methyl chloroform. There is scientific consensus that these compounds erode the stratospheric ozone layer, which protects the earth from harmful ultraviolet radiation. This radiation can cause cancer, among other environmental impacts. To date, 191 countries have signed the protocol. The United States has implemented many parts of it more quickly and at less cost than expected. The thinning of the ozone layer mostly stopped in 1988 and 1989, almost immediately after treaty reductions began to take effect. The U.S. Environmental Protection Agency estimates that 6.3 million U.S. lives will be saved as a direct result of worldwide efforts to implement the Montreal Protocol’s requirements.
The early successes of the Montreal Protocol laid the groundwork for a national approach to acid rain in the United States. The 1990 National Acid Precipitation Assessment Program (NAPAP) concluded that acid deposition had not caused the decline of trees other than the red spruce, which grows at high elevations. This became a significant scientific controversy because of its policy implications for the Clean Air Act Amendments of 1990. Some contended that 20 years of the Clean Air Act had been enough and that air pollution was no longer a severe problem. Others strongly disagreed. Recent research shows that acid deposition has contributed to the decline of red spruce trees and other important trees throughout the eastern United States. Other species, such as black flies, increase under acid rain conditions. Indicator species and mammals at the high end of the food chain show high levels of some of the pollutants in acid rain, indicating the pervasiveness of chemical exposure within the environment. For example, sugar maple trees in central and western Pennsylvania are now also declining. The Clean Air Act Amendments of 1990 included specific provisions concerning acid rain.
Canada and the U.S. Acid Rain Controversy
One intrinsic political problem highlighted by acid rain, and involving all air pollution, is that air and water currents do not follow political boundaries. If one country’s air pollution goes directly to the neighboring country, little can be done. Canadian concern about the damage from acid rain predates U.S. concern. Canada examines all possible sources for the acid rain problem, including its own contribution. Acid rain resulting from air pollution is severely affecting lakes and damaging forests. Eastern Canada is particularly hard hit because of the prevailing winds. There is a continuing controversy about actual site-specific impacts. Some studies indicate no significant changes in the presence of some degrees of acid rain. Others find that impacts for a given species are significant. Disputes of this kind are typical and tend to be ongoing. U.S.-Canadian relations have overcome some of the initial strain regarding the issue of acid rain. The U.S. government did not make much progress until land was at risk. Those efforts, exemplified by the Clean Air Act Amendments of 1990, put into policy proven measures that could reduce the emissions of pollutants causing acid rain. Since 1990, there have been a number of cooperative environmental programs involving both the United States and Canada.
Acid Rain and Art, Architecture, and Antiquities
Acid rain occurs all over the world. It is most common in areas with a history of industrialization. It has impacts on both the natural and urban environment. The impact of acid rain on the treasures of antiquity all across the planet is difficult to know. Other parts of the world still burn brown coal—that is, coal containing many impurities such as sulfur. Large industrial processes fueled by coal churn large emissions into the atmosphere with little regulation or regard for the environment. The scrubbing of coal is currently expensive but offers a way to remove the sulfur within it.
The negative effects of acid rain on historic buildings and works of art differ depending on the materials of which they are made. The effects are far-ranging, especially over time. Washington, D.C.; Philadelphia; Milan, Italy; Bern, Switzerland; and many other cities feel the impact of acid rain. In terms of traditional Western classical works of art, Italy may face the highest risk of damage from acid rain. In Italy, many works of art are made of calcium carbonate, in the form of marble. As in the case of most choices of building stone, marble was selected because it was locally available. Calcium carbonate reacts upon contact with acid rain, thus gradually tending to dissolve. Italy has a severe acid rain problem owing to its geography, prevailing winds, and the dependence on the burning of coal as a source of energy. Many classic ancient marble structures and statutes are at risk of being corroded by acid rain in Italy’s cities. Northern Italy has the worst air quality in Western Europe. Some of the smaller sculptures have been encased in transparent cases, which are then filled with a preserving atmosphere. Others have continued to corrode.
In the United States, limestone is the second most commonly used building stone. It was widely used before Portland cement became available. Limestone was preferred because of its uniform color and texture and because it could be easily carved. Limestone from local sources was commonly used before 1900. Nationwide, marble is used much less often than other stone types. Granite is primarily composed of silicate minerals, which are resistant to acid rain. Sandstone is also composed of silica and is resistant to most types of acid rain. Limestone and marble are primarily composed of calcium carbonate, which dissolves in weak acid. Depending on the building materials, many older U.S. cities are suffering damage due to acid rain.
How Much Acid Is There in Rain?
The term acid deposition is used to encompass both the dry and wet deposition of acidic compounds in acid precipitation. The most recent term used in place of acid rain is atmospheric deposition, which includes acidic compounds as well as other airborne pollutants. The term reflects the recognition that air pollution involves the complex interaction of many compounds in chemical stew within the atmosphere.
Unpolluted rain is normally slightly acidic, with a pH of 5.6. Carbon dioxide from the atmosphere dissolves to form carbonic acid, which is why normal rain is slightly acidic. When acidic pollutants combine with the rain, the acidity increases greatly. The acidity of rainfall over parts of the United States, Canada, and Europe has increased over the past 40 years. This is primarily due to the increased emission of sulfur and nitrogen oxides that accompanies increased industrialization.
The sulfur and nitrogen oxides are the common pollutants from coal-burning activities such as power generation. Many if not most of these emissions are legal in that they are within the terms of their permits from the Environmental Protection Agency (EPA) or state environmental agency. Legal or not, these pollutants are oxidized in the atmosphere and converted into sulfuric and nitric acids. These acids are then absorbed by clouds laden with raindrops. As they become heavier, they fall to the earth. This process is called acid deposition. Acidic fog, snow, hail, and dust particles also occur. The acidity of these different forms of precipitation can vary greatly.
Sources of Acid Rain
Part of the controversy about the sources of acid rain has revolved around the question of whether environmental policy could really affect the acidity of rain. Scientific debate about natural, human, and industrial causes engulfed much of the political battleground. Although the policy question was answered in the affirmative—that, yes, environmental policy can make the air cleaner—the debate about sources continues.
All forms of precipitation are naturally acidic because of naturally occurring carbon dioxide; human activities tend to add to the acidity. Non-polluted rain is assumed to have a pH of 5.6. This is the pH of distilled water. Natural sources of these environmentally regulated chemicals may be significant. Emissions due to human activity tend to be concentrated near historic industrial sites and older population centers. The presence of other naturally occurring substances can produce pH values ranging from 4.9 to 6.5. This scientific dynamic has kept other debates alive regarding whether government has an effective role in environmental policy if the sources are natural. pH levels are among the many factors that are monitored.
The relationship between pollutant emission sources and the acidity of precipitation at affected areas has not yet been determined. More research on tracing the release of pollutants and measuring their deposition rates to evaluate the effects on the environment is under way. This is an area of much scientific and legal controversy. If it were possible to show that a given emission definitely came from a given plant, then government would be able to assign liability to the polluter. Governments would also be able to locate sources of acid rain that comes from other countries.
Acid rain as a controversy in the United States has been subsumed by controversies around global warming and climate change. Many of the debates are the same, especially in terms of science and legal issues. Scientific disputes mark the continuing evolution of ways to measure the actual environmental impacts. The controversy around acid rain was an early one, historically documented as a symptom of a larger problem. It is an important historic controversy because it promoted significant, successful policies. It is also a modern controversy because it continues to provide evidence of humans’ impact on the environment.
Robert William Collin
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