CHAPTER 9
ACID RAIN

WHAT IS ACID RAIN?

Sulfur dioxide and nitrogen oxides are gases that occur naturally in the Earth's atmosphere. These gases react with water, oxygen, and other chemicals in the atmosphere to form various acidic compounds, including mild sulfuric acid and nitric acid. In nature, the combination of rain and these oxides is part of a natural balance that nourishes plants and aquatic life. However, when human activity increases the amount of acid-forming chemicals in the air, the results can be harmful to humans and the environment.

Acid rain is the common name applied to any form of precipitation that contains a greater than normal amount of acid. It would be more accurate to call it "acid deposition." It occurs in two ways: wet and dry. Acidic fog, hail, rain, sleet, and dew are wet deposition. Dry deposition consists of acidic aerosols, particles, and gases. About half of the acid in the atmosphere falls to earth as dry deposition.

Dry deposition introduces acidic particles and gases into water in two ways: direct deposition onto the water body surface and indirect deposition. One example of indirect deposition is snowpack melt. In cold parts of the country, pollutants are concentrated in the upper layers of the snowpack because of wet deposition during snowfall, and dry deposition during periods of clear weather. During the spring snowmelt, runoff containing large amounts of acidic particles accumulated over the winter can flow into a lake or river, causing acid shock to aquatic inhabitants. (See Figure 9.1.)

Origin of the Term "Acid Rain"

The concept of acid rain originated in 1872 when the term was first used by Robert Angus Smith, an English chemist, to describe acidic precipitation in and around the city of Manchester, England. Subsequent scientific research on acid rain was sporadic and largely focused on local problems until the late 1960s, when Scandinavian scientists began more systematic studies. Acid precipitation was not identified in North America until 1972, when scientists found that precipitation was acidic in eastern North America, especially in the northeastern United States and eastern Canada. The 1975 First International Symposium on Acid Precipitation and the Forest Ecosystem (in Columbus, Ohio) helped scientists define the acid rain problem and initiated further research.

Formation of Acid Rain

One of the main components of acid rain is sulfur dioxide. When sulfur dioxide reaches the atmosphere, it oxidizes to first form a sulfate ion. It becomes sulfuric acid when it joins with hydrogen atoms in the air and falls back to earth. The most oxidation occurs in clouds, especially in heavily polluted air where other compounds such as ammonia and ozone help catalyze (accelerate) the reaction, converting more sulfur dioxide to sulfuric acid. Not all of the sulfur dioxide, however, is converted. In fact, a substantial amount of sulfur dioxide can float up into the atmosphere, be transported to another location, and return to earth unconverted.

Nitric oxide and nitric dioxide are the other major components of acid rain. Like sulfur dioxide, these nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. These reactions are also catalyzed in heavily polluted clouds where iron, manganese, ammonia, and hydrogen peroxide are present.

Figure 9.2 illustrates how sulfur and nitrogen oxides, as well as hydrocarbons, are carried into the air to become acid deposition. Gases and particulate matter are carried into the atmosphere where they mix with moisture and other pollutants to form dry and wet acid deposition. Wet deposition returns to earth as precipitation, which enters the water body directly, percolates through the soil, or becomes runoff to nearby water bodies. Dry deposition builds up over time on all dry surfaces to be transported to water bodies in runoff during periods of precipitation, or falls directly onto a water surface.

Measuring Acid Rain

The acidity of any solution is measured on a pH (potential hydrogen) scale, numbered from zero to fourteen, with a pH value of seven considered neutral. Values higher than seven are considered more alkaline or basic (the pH of baking soda, a mild alkali, is eight); values that are lower than seven are considered acidic (the pH of lemon juice, an acid, is two). Pure, distilled water has a pH level of seven. The pH scale is a logarithmic measure. This means that every pH drop of one is a tenfold increase in acid content. Therefore, a decrease from pH six to pH five is a tenfold increase in acidity; a drop from pH six to pH four is a hundredfold increase in acidity; and a drop from pH six to pH three is a thousandfold increase. (See Figure 9.3.)

Normal rainfall has a pH value of 5.65. It is not pure because it accumulates naturally occurring sulfur oxides and nitrogen oxides as it passes through the atmosphere. In comparison, acid rain has a pH of about four. "Normal" pH for freshwater streams and lakes in the United States is about six. The introduction over time of large volumes of acid deposition into unbuffered water bodies (mostly freshwater systems) can increase natural acidity as much as a hundredfold. Buffers are substances in the soils or water that offer resistance to changes in pH. When levels of alkaline chemicals that neutralize the acid rain are low in the soil or water, acid deposition directly affects the surface water pH.

FACTORS AFFECTING ACID DEPOSITION

The interplay of soil, water, climate, and winds can have a profound impact on the effects of acid deposition. The effects of acid rain can be greatly reduced by the presence of alkali (basic) substances. Sodium, potassium, magnesium, calcium, and bicarbonate are examples of chemicals with buffering (neutralizing) capacity. In areas where soils contain limestone (calcium carbonate) or other minerals with high buffering capacities, acidity is reduced as runoff travels over the soil, mixes with dust, and percolates through the soil. Brackish and salt water are more resistant to pH change from acid deposition than freshwater because they contain many substances with good buffering capacity.

Areas most sensitive to acid deposition have hard, crystalline bedrock and very thin surface soils. When no buffering particles are in the soil, acid rainfall and runoff directly affect surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity neutralizes acid rain better. Generally, lakes tend to be most susceptible to acid rain because of low alkaline content in lakebeds, the water, and the watershed soils, and the longer residence time of water in lakes.

Like lakes, freshwater streams flowing over stream-beds and draining watersheds with low buffering capacity can also be susceptible to acid deposition. For example, according to the Environmental Protection Agency (EPA) in "Effects of Acid Rain: Lakes & Streams" (http://www.epa.gov/acidrain/effects/surfacewater.html, November 12, 2003), 580 freshwater streams in the Mid-Atlantic Coastal Plain have been identified as acidic because of acid deposition.

In drier climates, such as the western United States, windblown alkaline dust blows freely and tends to help neutralize atmospheric acidity. On the other hand, the more acidic dust on the eastern seaboard contributes to atmospheric acidity.

Some acid deposition events are more severe than others. Episodic acidification refers to brief periods during which pH levels drop because of runoff from the influx of large amounts of water, such as heavy downpours and snowmelt. An example would be heavy rainfall following a long dry period. The runoff would be very acidic because of the combination of the acidic rain and the dry acid deposition washed from all surfaces.

Freshwater lakes and streams in many areas of the United States are susceptible to episodic acidification, that is, they become temporarily acidic during and immediately after storms and snowmelt. Effects may last for several hours or days, depending on the water flow, as opposed to waters that are acidic year-round. For example, during rainstorms and snowmelts, mountain streams in New York, North Carolina, Pennsylvania, Tennessee, and Arkansas have shown acidity from three to twenty times the level occurring the rest of the year. If episodic acidification occurs during periods when fish are spawning or seed is germinating, the results can be devastating. In severe cases, it has caused fish kills.

The prevailing winds in an area are determinants in the transport systems that distribute acid pollutants in definite patterns across the planet. The movement of air masses transports air pollutants many miles, during which time these pollutants are transformed into sulfuric and nitric acid by mixing with other pollutants, clouds, and water. This process is known as "transport and transformation." For example, a typical European transport pattern carries pollutants from the smokestacks of the United Kingdom over Sweden. In southwestern Germany, many trees of the famed Black Forest (mostly coniferous) are dying from the effects of acid rain transported from industrial sites to the region by wind.

Northeastern United States Hit Hardest

The areas of greatest acidity (lowest pH values) in the nation occur in the northeastern United States. This high acidity is caused by the large number of cities, dense populations, and the concentration of power and industrial plants in the Northeast. Because the area is characterized by generally acidic soils and copious freshwater lakes and streams with low buffering capacity, it is very vulnerable to the effects of acid deposition.

The prevailing wind direction in the Northeast also brings storms and air pollutants from the Ohio River Valley, an area rich in coal-fired utilities. A typical transport pattern brings pollutants from the Ohio River Valley to the northeastern United States on prevailing winds that tend to move from west to east and from south to north. As the Attorney General for the State of Maine, Steven Rowe, said in a November 2002 press release, the Clean Air Act benefits people in Maine most because the state is at the end of the country's "air pollution tailpipe"("AG Rowe to Sue Bush Administration for Gutting Clean Air Act," http://www.maine.gov/ag/press_release_pop_up.php?press_id=111, November 22, 2002). About one-third of the total sulfur compounds deposited over the eastern United States originate from sources in the Midwest, more than 300 miles away. In addition to the problems in the northeastern United States, eastern areas of Canada have also been affected by pollutants from the Ohio River Valley.

SOURCES OF SULFATE AND NITRATE IN
THE ATMOSPHERE

Natural Causes

Natural causes of sulfate (sulfur oxides) in the atmosphere include ocean spray, volcanic emissions, and readily oxidized hydrogen sulfide released from the decomposition of organic matter found on land and in water. Natural sources of nitrogen or nitrates include nitrogen oxides produced by microorganisms in soils, by lightning during thunderstorms, and by forest fires. Scientists believe that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources.

The island of Hawaii provides a good example of the natural occurrence of acid rain. Sulfur dioxide gas and other pollutants emitted from the Kilauea volcano on the island of Hawaii combine and interact chemically in the atmosphere with water, oxygen, dust, and sunlight to produce "vog" (volcanic smog) and acid rain. Vog is a visible haze consisting of gas and aerosols (a suspended mix of very tiny liquid and solid particles) that can be a health hazard because it aggravates preexisting respiratory ailments. When rain falls in areas that have vog, the crops and local water supplies can be damaged by the resulting acid rain.

Many residents and visitors on the island of Hawaii report physical symptoms associated with vog. These include headache, breathing difficulties, greater susceptibility to respiratory ailments, general lack of energy, sore throat, watery eyes, and other flu-like symptoms. Although the amount of particulate material in the air does not routinely exceed the federal standards, sulfur dioxide concentrations do. Sulfur dioxide emission rates from Kilauea were first measured in 1975 and have been measured on a regular basis since 1979. Periodic reporting of these sulfuric dioxide emission rates is done by the U.S. Geological Survey.

The tiny sulfuric acid droplets in vog have the corrosive properties of diluted battery acid. When these droplets combine with moisture in the air to form acid rain, plant damage and acceleration of the rusting of metal objects such as vehicles and machinery occurs. Crop damage is another frequent occurrence, even in greenhouses, because the vog enters through vents and mixes with the moisture on plant leaves.

The combination of vog and acid rain created an unusual water supply problem on Hawaii. Many homes relied on rooftop rainwater catchment basins for drinking water. In 1988 the drinking water in more than 40% of the homes was found to contain elevated lead levels. Upon further study it was determined that the process of acid-induced leaching from lead roofing and plumbing materials was the cause of the elevated lead levels. Tests confirmed elevated lead levels in the blood of residents. This finding led to a major island-wide effort to remove lead materials from rainwater catchment systems.

Human Causes

Most human-made emissions of sulfur dioxide and nitrogen oxides are the result of burning fossil fuels (coal, oil, and gas) for energy. This includes fossil-fueled electric utilities and industrial plants, motor vehicles using gasoline or diesel fuel, and commercial or residential heating. Nonenergy sources of emission include metal smelters that emit sulfur compounds and nitrogen compounds from agricultural fertilizers that are carried by the wind to other areas.

Levels of pollutants are measured in two ways: emissions and concentrations. Emissions are those pollutants expelled into the air by a source, whereas concentrations are the total saturation of a contaminant over time. Table 9.1 shows sulfur dioxide emission sources from 1970 to 2002, while Table 9.2 shows nitrogen oxide emissions for that same period. Fuel combustion from fossil-fueled utilities accounts for most of the man-made sulfur dioxide emissions but less than half of the nitrogen oxides. The primary source of human-generated nitrogen oxides is transportation (car, truck, bus, and other vehicle emissions), which accounts for a little more than half of man-made nitrogen oxides emissions.

There has been great progress in reducing sulfur dioxide concentration. Between 1980 and 1999 the average annual mean concentration of sulfur dioxide dropped by half. According to EPA data, annual mean nitrogen oxides concentrations declined in the early 1980s, were relatively stable during the mid-to-late 1980s, and resumed their decline in the 1990s. Concentrations of nitrogen oxides declined 25% between 1980 and 1999.

EFFECTS OF ACID DEPOSITION ON
LIVING ORGANISMS

An ecosystem is a particular environment and the biological organisms that live there. Ecosystems can be global or tiny. The plants and animals living within an ecosystem are interdependent. For example, frogs eat water insects. If the insects disappear because of acid deposition effects, the frogs may not thrive because part of their food supply has disappeared. Because of the many and varied interconnections among the plants, animals, and microorganisms living in an ecosystem, changes in pH may change the ecosystem's biodiversity or overall health.

The duration of the effects of acid rain on living organisms can vary from a few hours to many years. For example, soils that are depleted of essential nutrients may take decades or even centuries to recover. Once acid rain is reduced to normal levels, the slow process of nutrient buildup in the soil is dependent on the gradual succession over time of plant life. Plants that are tolerant of depleted soils will restore nutrients over time as they grow, die, and decompose, putting essential nutrients back in the soils. In the natural order, these plants will be followed by other plants that require more nutrient-rich soil, and as they grow, die, and decompose they will return more nutrients to the soil. Animals attracted to each stage of plant succession will also add their wastes to the process, bringing in additional nutrients. This process will continue through time until a healthy, balanced population appropriate to the ecosystem is restored.

Aquatic Systems

The effects of acid rain on aquatic systems are varied and many. They include great harm or death to fish, diminished fish populations, loss of a species in a particular water body, and reduction in biodiversity. As acid rain moves through soils in a watershed, aluminum is released from soils into the lakes and streams. As the pH lowers in a water body, the aluminum level climbs. Both low pH and elevated aluminum levels are toxic to fish (aluminum burns the gills of fish and accumulates in organs, causing organ damage). They can also cause chronic stress, which does not immediately kill an individual fish, but impairs its ability to take in the oxygen, salts, and nutrients needed to stay alive.

Freshwater fish need to maintain their osmoregulation to stay alive. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules stimulate the formation of mucus in the gills, which interferes with their ability to absorb oxygen. If mucus buildup continues, the fish suffocate. In addition, a low pH disrupts the balance of salts in fish and other aquatic life, interfering with reproduction and maintenance of bones or exoskeleton.

Some aquatic plants and animals are able to tolerate more acidic waters. For example, frogs can tolerate lower pH than trout, crayfish, or clams. Acid-sensitive species, however, are lost as pH declines. It is usually the young of most species that are the most sensitive to environmental conditions. For example, at less than pH 5, trout and salmon eggs cannot hatch. At lower pH levels (pH 4 to 4.9), some adult fish die. Some extremely hardy fish such as the roach (a type of carp) can survive at a pH as low as 3.5 if the change is gradual and they have time to adjust.

Other effects include:

• Sudden, short-term shifts in pH levels, resulting in acid shock to freshwater ecosystems
• Gradual declines in fish populations and numbers of adult and juvenile fish over time as pH decreases
• Unsuccessful reproduction by many aquatic species, including poor egg production, abnormal eggs, and poor juvenile survival
• Physical impairment in juveniles of some species
• Loss of the ability in salmon to find home streams because of impaired sense of smell
• Loss of important components of the food web, leading to poor nutrition or starvation in species dependent on those components
• Changes in the plant and animal species within an ecosystem

Nitrogen has been shown to play an important role in both episodic and long-term acidification. It is also an important nutrient, but excess nitrogen can cause water quality degradation. Of the nitrogen released into the atmosphere through human activities, 10% to 45% is transported to U.S. oceans and estuaries through air deposition. In Chesapeake Bay, for example, 30% of the nitrogen contributed from human-made sources is atmospheric deposition.

Another area in which excess nitrogen from acid deposition, among other sources, is being watched very carefully is along the coastline. Studies of a phenomenon nicknamed dead zones have been underway for many years. The term dead zone actually refers to a state of hypoxia. A water body that is suffering from hypoxia is one in which excess nitrogen has caused the dissolved oxygen in the water to deplete to the point where the water can no longer support life. The three sources of nitrogen known to cause large dead zones to appear every summer in the Gulf of Mexico are agricultural run-off, industrial waste, and acid deposition.

Forest Systems

Acid deposition can have serious impacts on trees and soils, causing slower growth, injury, or death of forests. Acid deposition has been implicated in forest and soil degradation in the eastern United States, particularly in the high elevations of the Appalachian Mountains from Georgia to Maine, an area including the Shenandoah and Great Smoky Mountain National Parks.

When rain falls to the forest floor, the buffering capacity of the soil may neutralize some or all of its acidity. Differences in soil-buffering capacity is the reason that some areas that receive a lot of acid rain show little damage while other areas that receive the same amount show a lot of damage. The ability of forest soil to resist becoming acidified depends on the thickness of the soil and the type of bedrock below the soil.

When the soils cannot buffer the acid rain, vital nutrients present in the soil, such as calcium and magnesium, are stripped away by the acid-driven reactions. Aluminum, a toxic element present in all soils, is made more available to the trees and taken up by their roots. The combination of toxic aluminum and poor nutrition retard growth, make the trees more vulnerable to infection, and can eventually kill the trees.

In March 1999 the U.S. Geological Survey (USGS), in Soil Calcium Depletion Linked to Acid Rain and Forest Growth in the Eastern United States, reported that calcium levels in forest soils had declined at locations in ten states in the eastern United States. Calcium is necessary to neutralize acid rain and is an essential nutrient for tree growth. Sugar maple and red spruce trees, in particular, showed reduced resistance to stresses such as insect defoliation and low winter temperatures. Although the specific relationships among calcium availability, acid rain, and forest growth are uncertain, Dr. Gregory Lawrence, scientist and coauthor of the report, speculated:

Acid rain releases aluminum from the underlying mineral soil layer, which is followed by the upward transport of the aluminum into the forest floor (the nutrient-rich organic soil layer where root activity is greatest) by root uptake and water movement. The result is that aluminum replaces calcium, and the trees have a harder time trying to get the needed calcium from the soil layer.

Acid deposition can affect trees in other ways. Sulfur dioxide that has not been converted to sulfuric acid has been shown to clog up the leaf stomata (tiny openings in leaves where gases diffuse in and out), impairing plant respiration and photosynthesis. Nitric acid and nitrogen oxide have been shown to stimulate tree growth outside the growing season, leaving trees vulnerable to winterkill. Forests in high mountain regions often are surrounded by acidic clouds and fog that are more acidic than rainfall. Scientists believe that when the tree leaves and needles are frequently wetted in this acid fog, essential nutrients are stripped away. Loss of nutrients in the foliage makes the trees more vulnerable to other environmental threats, particularly winterkill. Winterkill resulting in damage or death is the result of naturally occurring stress caused by cold, wind, ice, and dehydration on trees and other woody plants that have been weakened by insect damage, nutrient deficiency, or drought.

Plants that are found in locations that are susceptible to high acid deposition experience the same fate as trees. The processes causing growth retardation and ultimately death are believed to be the same.

Human Health

Acid rain feels, tastes, and looks just like clean rain. Sulfur dioxide and nitric oxides, the pollutants that cause acid rain, however, can damage human health. These gases interact with particulate in the atmosphere to form aerosols (a mixture of very tiny liquid and solid particles) that can travel long distances transported by winds. When inhaled, aerosols penetrate deep into the lungs and are readily retained. Because of their very fine size, they can also penetrate indoors through ventilation systems.

Air pollution studies have indicated that elevated levels of acidic particles can cause asthma attacks, particularly in adolescents, and can also impair the ability of the upper respiratory tract to remove other potentially harmful particles. Some scientific studies have also established a relationship between elevated levels of fine particles and increased deaths from heart and lung disorders, such as bronchitis and asthma. Other scientists believe that these pollutants may increase the health risks to those over age sixty-five; those with asthma, chronic bronchitis, and emphysema; pregnant women; and those with histories of heart disease.

In "Effects of Acid Rain: Human Health" (Environmental Protection Agency, http://www.epa.gov/airmarkets/acidrain/effects/health.html, November 12, 2003), the EPA reported that sulfate aerosols make up about 25% of fine particles in the air in the eastern United States. Lowering emissions from power plants that contain fine sulfate and nitrate particles should eventually reduce the incidence and severity of the health problems believed related to these pollutants. The EPA estimated that when fully implemented in 2010, the public health benefits of the Acid Rain Program (created by Congress under Title IV of the 1990 Clean Air Act Amendments) would be about $50 billion annually in reduced health care costs because of decreases in emergency room visits, hospital admissions, and number of deaths.

Decreased nitric acid emissions are expected to lower the amount of ozone formed. Ozone is believed to increase the risk of illness or death from lung inflammation, including asthma and emphysema.

An indirect effect of acid deposition on human health is the increased reactivity in acid water of toxic metals and other chemicals. Increased reactivity means that the chemicals and toxic metals in the water are more likely to be taken up in fruits, vegetables, and animal tissue. Air deposition is believed to be the leading source of mercury bioaccumulation in fish. This sort of bioaccumulation has led to advisories against eating certain kinds of fish.

The principal pollutants generated by coal combustion that can cause health problems are particulate, sulfur and nitrogen oxides, trace elements (such as arsenic, fluorine, selenium, and the radionuclides, uranium, and thorium), and organic compounds as a result of incomplete coal combustion. Some of these trace elements have been shown to cause severe health effects in other countries, such as China, Romania, and Bulgaria.

The EPA conducted a detailed study of possible health effects that can come from the exposure to emissions of about twenty potentially toxic substances from coal-burning electric utilities. In this study, the EPA used USGS information on U.S. coal quality to assess the potential health impact of fourteen potentially toxic trace elements that may be mobilized by coal burning. The USGS fact sheet Health Impacts of Coal Combustion (July 2000) reported that, with the possible exception of mercury, there is no compelling evidence to indicate that emissions from U.S. coal-burning electric utilities cause human health problems. The absence of detectable health problems was credited in part to the use in the United States of coals that contain low to moderate amounts of sulfur and other potentially toxic trace elements. Another reason for the absence of detectable health problems was the common use of sophisticated pollution control systems by coal-burning utilities. These systems are specifically designed to reduce the emission of hazardous elements.

OTHER EFFECTS OF ACID RAIN

Reduction in Visibility

Sulfates and nitrates in the air contribute to reduced visibility, which means that people cannot see clearly or as far through the air. The air looks "hazy." Sulfate particles account for 50% to 70% of the visibility reduction in the eastern United States. Visibility in the East was expected to improve by 30% by 2010 because of acid rain program controls. The EPA has projected that this improvement will be worth more than $1 billion per year to the tourist industry in and around the eastern national parks.

In the western states, sulfates, nitrates, and carbon all play roles in reduced visibility. Sulfates have been shown to be important contributors to reduced visibility in many of the national parks found on the Colorado Plateau. These include Bryce Canyon, Grand Canyon, and Canyonlands. On the other hand, increased particulate in the air contributes to the spectacular sunrises and sunsets in the Red Rock country.

Human-Made Objects

Limestone and marble turn to gypsum, a crumbling substance, when exposed to acid. Many of the world's most beautiful buildings, monuments, and statuary are composed of these materials. Throughout the world, important art treasures and cultural and historic sites, such as the Taj Mahal in India; the Colosseum in Rome, Italy; and the Lincoln Memorial in Washington, D.C., are at risk. (See Figure 9.4.)

Investigations into the effects of acid rain on human-made objects in the United States, such as buildings, statues, metals, and paints, began only in the 1990s. A joint study conducted by the EPA, the Brookhaven National Laboratory, and the Army Corps of Engineers in 1993 found that acid rain caused $5 billion worth of damage annually in a seventeen-state region. Two-thirds of the damage was created by pollution whose source was less than thirty miles away.

New kinds of protective chemicals called consolidants, which adhere to limestone and marble, are being used to save some of the world's decomposing monuments from acid rain and other pollutants. Consolidants were developed in the 1960s in response to widespread water damage to stone buildings in Venice, Italy. Experts reported, however, that these chemicals have many limitations. They are toxic and difficult to apply. Their effects are only temporary, yet they permanently alter the nature of the stone. Most important, their long-term effects are uncertain. For those reasons, their use was banned on the Acropolis in Athens, Greece.

Automotive Coatings

During the 1990s, reports of damage to automotive textured roofs and paints increased. The damage generally occurs on flat, horizontal surfaces and appears as permanently etched, irregularly shaped areas. The damage is most easily observed on dark-hued vehicles, and can be detected with the aid of fluorescent lights on many vehicles that show no signs visible to the naked eye. Usually the damage is permanent.

The general consensus within the automobile industry is that the damage is caused by some form of environmental fallout. In the auto industry, the term "environmental fallout" refers to a wide variety of happenings, including bird droppings, decaying insects, acid rain, and tree sap. Chemical analysis of the damaged areas of some car finishes, however, has shown elevated levels of sulfate, implicating acid rain.

Quantifying the contribution of acid rain to paint finish damage relative to other forms of environmental fallout, deficient paint formulas, or improper paint application has been difficult. The best way to determine the exact cause of the damage is chemical testing, an expensive proposition.

The auto industry is actively pursuing the development of coatings that are more resistant to acid rain and other environmental fallout. Until acid rain is controlled or a universal protective technology is developed, the best protection for a vehicle is to keep it covered during precipitation events and wash it frequently, followed by hand drying.

POLITICS OF ACID RAIN

The early acid rain debate centered almost exclusively on the eastern portion of the United States and Canada. The controversy was often defined as a problem of property rights. The highly valued production of electricity in coal-fired utilities in the Ohio River Valley caused acid rain to fall on land in the Northeast and Canada. An important part of the acid rain controversy in the 1980s was the adversarial relationship between the U.S. and Canadian federal governments over emission controls of sulfur dioxide and nitrogen oxides. More of these pollutants crossed the border into Canada than the reverse. Canadian officials very quickly came to a consensus over the need for more stringent controls, while this consensus was lacking in the United States.

Throughout the 1980s, the major lawsuits involving acid rain all came from eastern states. States that passed their own acid rain legislation were also from the eastern part of the United States. There has been a clear difference in the intensity of interest between the eastern and western states regarding acid rain.

Legislative History

The U.S. Congress passed the first federal legislation aimed at reducing air pollution in 1967 (Air Quality Act of 1967). In 1970 the Environmental Protection Agency was founded and the Clean Air Act was passed. This law mandated the EPA to identify and set standards for pollutants identified as harmful to human health. The six pollutants identified and labeled "criteria" pollutants were:

• Sulfur dioxide
• Nitrogen dioxide
• Carbon monoxide
• Particulate matter less than or equal in size to ten micrometers
• Lead
• Ozone

Sulfur dioxide and nitrogen dioxide are the biggest contributors in the production of acid deposition.

In 1975 the First International Symposium on Acid Precipitation and the Forest Ecosystem convened in Columbus, Ohio, to define the acid rain problem. The scientists who took part in the symposium used the meeting to propose a precipitation monitoring network in the United States to cooperate with the European and Scandinavian networks and to set up protocols for collecting and testing precipitation.

In 1977 President Jimmy Carter's Council of Environmental Quality was asked to develop a national acid rain research program. Several scientists drafted a report that eventually became the basis for the National Acid Precipitation Assessment Program (NAPAP). Carter's initiative eventually translated into legislative action with the passage of the Energy Security Act (PL 96–264) in June 1980. Title VII of the Act (the Acid Precipitation Act of 1980) created the NAPAP and authorized federally financed support.

The Clean Air Act was amended in 1977. New legislation was added to address the problem of older fossil-fuel electric power producers that were not covered in the original law. The new program was called the New Source Review, under which these older plants would be required to undergo an EPA assessment if they chose to make changes to their operations. The EPA review would determine whether the planned changes would result in significantly higher emissions rates and, if so, these plants would be required to install pollution control technologies that brought them up to the new standards.

The first international treaty aimed at limiting air pollution was the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Trans-boundary Air Pollution, which went into effect in 1983. It was ratified by thirty-eight of the fifty-four UNECE member states, which included not only European countries but also Canada and the United States. The treaty targeted sulfur emissions, requiring that the parties reduce emissions 30% from 1980 levels, the so-called 30% club.

In 1990 the Clean Air Act was amended for a second time, and provisions designed specifically for reducing acid deposition were a significant part of the amendments passed.

Acid Rain Program—1990 Clean Air Act Amendments,
Title IV

Title IV of the 1990 Clean Air Act Amendments (PL 101–549) set as its objective achieving a ten-million-ton annual reduction in emissions from 1980 levels by the year 2010. Traditionally, environmental regulation has been achieved by the "command and control" approach, in which the regulator specifies how to reduce pollution, by what amounts, and what technology to use. Title IV, however, gave utilities flexibility in choosing how to achieve these reductions. For example, utilities may reduce emissions by switching to low-sulfur coal, installing pollution control devices called scrubbers, or shutting down plants.

SULFUR DIOXIDE EMISSIONS.

Title IV introduced a new regulatory approach to reduce acid rain—allowing electric utilities to trade allowances to emit sulfur dioxide. Utilities that reduce their emissions below the required levels can sell their extra allowances to other utilities to help them meet their requirements. Because in 1990 electric utilities were the source of 70% of sulfur dioxide emissions and 30% of nitrogen oxide emissions, the Act targeted emissions from electric utilities. Of the desired ten-million-ton reduction in sulfur dioxide, 8.5 million tons is to come from the nation's major source, electric utilities.

The emissions reduction was implemented in two phases. In Phase I, the 263 units at 110 utility plants in twenty-one states with the highest levels of emissions were mandated to reduce their annual emissions by 3.5 million tons beginning January 1995. An additional 182 units joined Phase I voluntarily, bringing the total of Phase I units to 445. Phase II, which began January 1, 2000, affected 2,000 more units in all forty-eight contiguous states and the District of Columbia. Figure 9.5 shows the location of the highest-emitting plants in Phase I and the approximately 2,000 cleaner and smaller units throughout the nation that became involved in Phase II.

ALLOWANCE TRADING FOR SULFUR DIOXIDE.

Title IV allows companies to buy, sell, trade, and bank sulfur dioxide pollution rights. Utility units are allocated allowances based on their historic fuel consumption and a specific emissions rate. Each allowance permits a unit to emit one ton of sulfur dioxide during or after a specific year. For each ton of sulfur dioxide discharged in a given year, one allowance is retired and can no longer be used. Companies that pollute less than the set standards will have allowances left over (banked allowances). They can then sell the difference to companies that pollute more than they are allowed, bringing them into compliance with overall standards. Companies that can clean up their pollution less expensively by changing fuel or persuading their customers to conserve energy would recover some of their costs by selling their pollution rights to other companies. The EPA holds an allowance auction each year. The sale offers allowances at a fixed price. This use of market-based incentives by Title IV is regarded by many as a major new method for controlling pollution.

Utilities also took advantage of their flexibility under Title IV to choose less costly ways to reduce emissions, such as switching from high- to low-sulfur coal, and are achieving sizable reductions in their sulfur dioxide emissions. More than half of Phase I plants opted to switch to low-sulfur coal, and 16% chose to install scrubbers. Air scrubbers are treatment devices placed on the exhaust or smoke stack and used to reduce the particulate matter and other contaminants in plant emissions. Only 3% of plants initially planned to purchase allowances. Not surprisingly, the market for low-sulfur coal is growing as a result of Title IV, and the market for high-sulfur coal is decreasing.

From 1995 to 1998, however, there was considerable buying and selling of allowances among utilities. Because the utilities that participated in Phase I reduced their sulfur emissions more than the minimum required, they did not use as many allowances as they were allocated for the first four years of the program. Those unused allowances could be used to offset sulfur dioxide emissions in future years. According to figures published by the EPA in the Acid Rain Program, 2003 Progress Report (September 2004), a total of 9.5 million allowances were granted nationally for the year. To this was added the large stockpile of banked allowances carried over from prior years, making the total allowance stockpile available for 2003 19.93 million tons.

In 2003 utility sources emitted 10.6 million tons of sulfur dioxide, about a million tons more than the allowances granted in 2003 but far fewer than were in the pool of allowances available since that pool includes all the stockpiled allowances from prior years. Over time the bank of allowances is expected to be depleted further as plants use stockpiled allowances to comply with the more stringent emissions requirement of Phase II.

NITROGEN OXIDE EMISSIONS.

Title IV of the Clean Air Act Amendments (PL 101–549) maintained the traditional environmental "command and control by regulation" approach for nitrogen oxides. Under this approach, the EPA specifies how the pollution will be reduced, by what amounts, and what technology to use. The nitrogen oxides program establishes standard emissions limitations (the amount that can be discharged) for the affected units.

To encourage industry to reduce nitrogen oxides emissions before the required January 2000 date, the EPA adopted regulations in April 1995 that provided an incentive. The EPA allowed Phase II, Group I nitrogen oxides affected units, which would not have been subject to nitrogen oxide emission limits until January 2000, to use an "early election" compliance option. Under this provision, these Phase II, Group I units can demonstrate compliance with the higher Phase I limits for their boiler type from 1997 through 2007 and postpone having to meet the more stringent Phase II limits until 2008. There is one catch, however. If the utility fails to meet the annual Phase I limit for the boiler in any year, the unit is subject to the more stringent Phase II limit for Group I boilers beginning in 2000, or the year following the exceedance, whichever is later.

Clear Skies Act

On February 27, 2003, new legislation was introduced to the U.S. House of Representatives (HR 999) and the U.S. Senate (S 485), entitled the Clear Skies Act of 2003. This legislation would amend the Clean Air Act and create a mandatory program that would reduce power plant emissions of sulfur dioxide, nitrogen oxides, and mercury by setting national caps on each pollutant. The stated pollution caps are as follows:

• Cut emissions of sulfur dioxide by 73%, reducing the emissions experienced in 2000 (eleven million tons) to a cap of three million tons in 2018.
• Reduce nitrogen oxide emissions by 67%, reducing the 2000 emissions of five million tons to a cap of 1.7 million tons in 2018.
• Cut mercury emissions by 69%, reducing the forty-eight tons emitted in 1999 to a cap of fifteen tons in 2018.

Environmental groups such as the Sierra Club, however, argued that the Clear Skies Act was essentially written by the energy industry, was riddled with loopholes for big polluters, and would actually weaken existing pollution rules. The Clear Skies Act of 2003 stalled in Congress. It was reintroduced in January 2005, and, as of that spring, appeared to have died in the Senate Environment and Public Works Committee.

ARE EFFORTS TO REDUCE
ACID RAIN WORKING?

The Good News

During the years since the enactment of the Acid Rain Program, much progress has been made in reducing power plant emissions across the nation, particularly in the Ohio Valley and the Northeast, where emissions have been historically highest. Figure 9.6 shows the average sulfur dioxide emissions from power plants by state for the year 1990 and for the two phases of the Program.

Figure 9.7 and Figure 9.8 present average power plant emissions of nitrogen oxide for the same time periods. Here one sees that the areas showing the most improvement are the Ohio Valley and the state of Texas.

According to the EPA's Acid Rain Program, 2003 Progress Report, great improvement has been made in reducing the pollutants that cause acid deposition. Sulfur dioxide emissions from power plants were reduced 38% between 1980 and 2003, though emissions were up slightly from 2002 to 2003. (See Figure 9.9.) Power plant emissions of nitrogen oxides over the period 1990–2003 were reduced 37%, from 6.7 million tons to 4.2 million tons. (See Figure 9.7.)

In 2003, the fourth year of the Acid Rain Program's Phase II implementation, all but one of the 3,497 Phase II affected utility units complied with the requirements to hold sufficient allowances to cover their sulfur dioxide emissions.

These reductions are even more encouraging in light of increased economic activity. The reduced sulfur and nitrogen oxide rates were achieved during a period of economic growth in which the amount of fuel burned to produceelectricityincreasedby28%.WithouttheAcid Rain Program's mandated reductions in emission rates, both sulfur dioxide and nitrogen oxide emissions from power plants would have been expected to rise in most parts of the nation as fossil fuel use rose. The fact that utility plant emissions of these pollutants have declined in the face of rising electrical production is very encouraging.

This news is offset somewhat by the contents of a report published by the U.S. Government Accountability Office, Air Pollution, Meeting the Future Electricity Demand Will Increase Emissions of Some Harmful Substances (October 2002). The report presents a forecast of energy requirements that was prepared by the Energy Information Administration. By the year 2020 electricity generation is expected to have risen by 42%. Emissions of pollutants by electric utilities are also expected to rise over this period. The good news in the report was that acid deposition producing pollutants were not expected to rise despite a significant increase in the production of electricity. Figure 9.10 presents pollution emissions by type that were projected to accompany the rise in electricity generation.

The Bad News

Important reductions in the emissions of acid rain producing pollutants have been documented. However, the ground-level data being collected on actual acid deposition has provided a somewhat less optimistic assessment. The Clean Air Status and Trends Network (CASTNET) is a scientific monitoring network established by the EPA under a mandate set forth in the 1990 Clean Air Act Amendments. The network is operated jointly by the EPA and the National Park Service.

In Clean Air Status and Trends Network 2002 Annual Report (November 2003), CASTNET presents data on the concentrations of sulfate and nitrogen particulates in the precipitation that has fallen on eastern states over the period 1990–2002. These data differ from the emissions data seen earlier. The power plant emissions of these two chemicals are down over the period in question. Their concentrations in precipitation are also down, but only slightly.

Figure 9.11, Figure 9.12, Figure 9.13, and Figure 9.14 present the sulfur particulate concentrations in precipitation collected by CASTNET in their eastern collection sites. Sulfate (SO₄²⁻) concentrations were down between 1990 and 2002, but by less than sulfate dioxide (SO₂) emissions were over the same period.

Nitrate (NO₃⁻) concentrations presented for the same geographical area and time frame showed a more discouraging result. Figure 9.15 and Figure 9.16 show that nitrate concentrations in the eastern states showed no real decline over the period 1990–2001 despite declining utility plant emissions of nitrogen oxides over the period. Here again, the fact that nitrogen oxide emissions are down from sources participating in the Acid Rain Program does not mean that all human-made emissions are down.

Rising Nitrogen Oxide Emissions

A detailed analysis of the nitrogen oxide emissions data presented for all sources of emissions in the EPA's report National Air Quality and Emissions Trends Report 1999 (March 2001) shows clearly that total nitrogen oxide emissions were higher in 1999 than in 1970, 1980, and 1990.

Nitrogen oxide emissions from transportation sources were responsible for the greatest portion of the increase between 1970 and 1999, rising from 9.3 million short tons to 14.1, an increase of nearly 52%. The transportation category is divided into two sections, on-road vehicles and non-road engines and vehicles. The on-road portion rose but at a much slower pace than the non-road portion. On-road vehicle emissions, which include all automobiles and trucks, rose from 7.4 million short tons in 1970 to 8.6 million short tons in 1999, an increase of 16%. The non-road engine and vehicle category includes recreational vehicles, construction machinery, lawn and garden equipment, airplanes, railroad machinery, and marine vessels. This category rose from 1.9 million short tons in 1970 to 5.5 million in 1999, an increase of 189%. Nitrogen oxide emissions from fuel combustion sources, the second largest category in 1999, remained statistically unchanged over the period.

The Acid Rain Program had greater success in stemming sulfur dioxide emissions than it had, as of 1999, in stemming nitrogen oxide emissions.

A 1999 Assessment

In April 1999 the National Acid Precipitation Assessment Program released findings from the study National Acid Precipitation Assessment Program Biennial Report to Congress: An Integrated Assessment. The study warned that, despite important strides in reducing air pollution, acid deposition remains a serious problem in sensitive areas and provided more evidence that acid deposition is more "complex and intractable than was believed ten years ago." Among the findings were:

• New York's Adirondack Mountain waterways suffer from serious levels of acid. Even though sulfur levels are declining, nitrogen levels are still climbing. The agency predicted that by 2040, about half the region's 2,800 lakes and ponds would be too acidic to sustain life.
• Chesapeake Bay is suffering from excess nitrogen, which is causing algae blooms that can suffocate other life forms.
• High elevation forests in Colorado, West Virginia, Tennessee, and Southern California are nearly saturated with nitrogen, a key ingredient in acid deposition. (Nitrogen saturation is a condition where the nitrogen levels in the soil exceed the plant needs with the result that excess nitrogen is flushed into streams where it can cause undesirable plant growth. As the nitrogen moves through the soil it strips away chemicals essential for forest fertility, increasing lake and stream acidity).
• High elevation lakes and streams in the Sierra Nevada, the Cascades, and the Rocky Mountains may be on the verge of "chronically high acidity."

In conclusion, the report recognized the important strides made in reducing sulfur dioxide emissions since passage of the 1990 Clean Air Act Amendments. However, the slow recovery that was being recorded in lakes, streams, and forests indicated that further reductions in sulfur and nitrogen were necessary.

Anticipated Benefits of Further Abatement

There are many anticipated benefits of recovering from the effects of acid rain. Table 9.3 summarizes some of them. The benefits to human health are believed to be reduced illness and death from lung disorders and heart disease, resulting in decreased need for medical services and medical treatment. Aquatic systems and forests are expected to experience less stress, thereby preserving vital habitat and resources important to the nation's economy. In some locations, the return of badly damaged ecosystems to healthy, thriving ecosystems is another anticipated benefit.

Reduced destruction of human-made objects is also a potential benefit of further abatement of acid deposition. The reduced deterioration of vehicles, buildings, monuments, and other structures should reduce the costs that society has to pay to repair and correct destruction of this kind, and a reduction in damage and loss of cultural objects preserves our heritage for future generations.

Taken together, the overall benefits of further reducing acid deposition should be a healthier, more environmentally sound habitat for people and all living organisms.

CHAPTER 5
ACID RAIN

WHAT IS ACID RAIN?

Acid rain is the common name for acidic deposits that fall to Earth from the atmosphere. The term was coined in 1872 by the Scottish chemist Robert Angus Smith (1817–1884) to describe the acidic precipitation in Manchester, England. In the twenty-first century scientists study both wet and dry acidic deposits. Even though there are natural sources of acid in the atmosphere, acid rain is primarily caused by emissions of sulfur dioxide (SO₂) and nitrous oxide (N₂ O) from electric utilities burning fossil fuels, especially coal. These chemicals are converted to sulfuric acid and nitric acid in the atmosphere and can be carried by the winds for many miles from where the original emissions took place. (See Figure 5.1.) Other chemicals contributing to acid rain include volatile organic compounds (VOCs). These are carbon-containing chemicals that easily become vapors or gases. VOC sources include paint thinners, degreasers, and other solvents and burning fuels such as coal, natural gas, gasoline, and wood.

Wet deposition occurs when the acid falls in rain, snow, or ice. Dry deposition is caused by tiny particles (or particulates) in combustion emissions. They may stay dry as they fall or pollute cloud water and precipitation. Moist deposition occurs when the acid is trapped in cloud or fog droplets. This is most common at high altitudes and in coastal areas. Whatever its form, acid rain can create dangerously high levels of acidic impurities in water, soil, and plants.

Measuring Acid Rain

The acidity of any solution is measured on a potential hydrogen (pH) scale numbered from zero to fourteen, with a pH value of seven considered neutral. (See Figure 5.2.) Values higher than seven are considered more alkaline or basic (the pH of baking soda is eight); values lower than seven are considered acidic (the pH of lemon juice is two). The pH scale is a logarithmic measure. This means that every pH change of one is a tenfold change in acid content. Therefore, a decrease from pH seven to pH six is a tenfold increase in acidity; a drop from pH seven to pH five is a one hundredfold increase in acidity; and a drop from pH seven to pH four is a one thousandfold increase.

Pure, distilled water has a neutral pH of seven. Normal rainfall has a pH value of about 5.6. It is slightly acidic because it accumulates naturally occurring sulfur oxides (SO_x) and nitrogen oxides (NO_x) as it passes through the atmosphere. Acid rain has a pH of less than 5.6.

Figure 5.3 shows the average rainfall pH measured during 2005 at various locations around the country by the National Atmospheric Deposition Program (NADP), a cooperative project between many state and federal government agencies and private entities. Rainfall was most acidic in the mid-Atlantic region and upper Southeast, particularly Ohio, Pennsylvania, West Virginia, Maryland, Delaware, Virginia, eastern Tennessee, and Kentucky. The areas with the lowest rainfall pH contain some of the country's most sensitive natural resources, such as the Appalachian Mountains, the Adirondack Mountains, Chesapeake Bay, and Great Smoky Mountains National Park. Overall, precipitation is much more acidic in the eastern United States than in the western United States because of a variety of natural and anthropogenic (human-caused) factors that are discussed below.

SOURCES OF SULFATE AND NITRATE IN THE ATMOSPHERE

Natural Sources

Natural sources of sulfate in the atmosphere include ocean spray, volcanic emissions, and readily oxidized hydrogen sulfide, which is released from the decomposition of organic matter found in the Earth. Natural sources of nitrogen or nitrates include NO_x produced by micro-organisms in soils, by lightning during thunderstorms,

FIGURE 5.1

and by forest fires. Scientists generally speculate that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources. (This is a rough estimate as there is no way to measure natural emissions as opposed to those that are manmade.)

Sources Caused by Human Activity

According to the U.S. Environmental Protection Agency (EPA), in "What Is Acid Rain?" (June 8, 2007, http://www.epa.gov/acidrain/what/index.html), the primary anthropogenic contributors to acid rain are SO₂ and NO_x, resulting from the burning of fossil fuels, such as coal, oil, and natural gas.

The EPA notes in "Clearinghouse for Inventories and Emissions Factors" (http://www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls) that approximately 70% of SO₂ emissions produced in 2006 were because of fuel combustion by fossil-fueled electric utilities. Fuel combustion at industrial facilities contributed another 13%. Lesser sources included transportation vehicles and industrial processes. Highway vehicles were the primary source of NO_x emissions, accounting for 36% of the total in 2006. Off-highway vehicles (such as bulldozers) contributed 22%. Fuel combustion in power plants was another major source,

FIGURE 5.2

accounting for 20% of the total. Lesser sources included industrial processes and waste disposal and recycling

NATURAL FACTORS THAT AFFECT ACID RAIN DEPOSITION

Major natural factors contributing to the impact of acid rain on an area include air movement, climate, and topography and geology. Transport systems—primarily the movement of air—distribute acid emissions in definite patterns around the planet. The movement of air masses transports emitted pollutants many miles, during which the pollutants are transformed into sulfuric and nitric acid by mixing with clouds of water vapor.

FIGURE 5.3

In drier climates, such as those of the western United States, windblown alkaline dust moves more freely through the air and tends to neutralize atmospheric acidity. The effects of acid rain can be greatly reduced by the presence of basic (also called alkali) substances. Sodium, potassium, and calcium are examples of basic chemicals. When a basic and an acid chemical come into contact, they react chemically and neutralize each other. By contrast, in more humid climates where there is less dust, such as along the eastern seaboard, precipitation is more acidic.

Areas most sensitive to acid rain contain hard, crystalline bedrock and thin surface soils. When no alkaline-buffering particles are in the soil, runoff from rainfall directly affects surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity, such as flat land, neutralizes acid rain better. Lakes tend to be most susceptible to acid rain because of low alkaline content in lake beds. A lake's depth, its watershed (the area draining into the lake), and the amount of time the water has been in the lake are also factors.

EFFECTS OF ACID RAIN ON THE ENVIRONMENT

In nature the combination of rain and oxides is part of a natural balance that nourishes plants and aquatic life. However, when the balance is upset by acid rain, the results to the environment can be harmful and destructive. (See Table 5.1.)

Aquatic Systems

Even though pH levels vary considerably from one body of water to another, a typical pH range for the lakes and rivers in the United States is six to eight. Low pH levels kill fish, their eggs, and fish food organisms. The

TABLE 5.1

degree of damage depends on several factors, one of which is the buffering capacity of the watershed soil—the higher the alkalinity, the more slowly the lakes and streams acidify. The exposure of fish to acidified freshwater lakes and streams has been intensely studied since the 1970s. Scientists distinguish between sudden shocks and chronic (long-term) exposure to low pH levels.

Sudden, short-term shifts in pH levels result from snowmelts, which release acidic materials accumulated during the winter, or sudden rainstorms that can wash residual acid into streams and lakes. The resulting acid shock can be devastating to fish and their ecosystems. At pH levels below 4.9, fish eggs are damaged. At acid levels below 4.5, some species of fish die. Below pH 3.5, most fish die within hours. (See Table 5.2.)

TABLE 5.2

Because many species of fish hatch in the spring, even mild increases in acidity can harm or kill the new life. Temporary increases in acidity also affect insects and other invertebrates, such as snails and crayfish, on which the fish feed.

Gradual decreases of pH levels over time affect fish reproduction and spawning. Moderate levels of acidity in water can confuse a salmon's sense of smell, which it uses to find the stream from which it came. Atlantic salmon are unable to find their home streams and rivers because of acid rain. In addition, excessive acid levels in female fish cause low amounts of calcium, thereby preventing the production of eggs. Even if eggs are produced, their development is often abnormal.

Increased acidity can also cause the release of aluminum and manganese particles that are stored in a lake or river bottom. High concentrations of these metals are toxic to fish.

Soil and Vegetation

Acid rain is believed to harm vegetation by changing soil chemistry. Soils exposed to acid rain can gradually lose valuable nutrients, such as calcium, magnesium, and potassium and become too concentrated with dissolved inorganic aluminum, which is toxic to vegetation. Long-term changes in soil chemistry may have already affected sensitive soils, particularly in forests. Forest soils saturated in nitrogen cannot retain other nutrients required for healthy vegetation. Subsequently, these nutrients are washed away. Nutrient-poor trees are more vulnerable to climatic extremes, pest invasion, and the effects of other air pollutants, such as ozone.

FIGURE 5.4

Some researchers believe that acid rain disrupts soil regeneration, which is the recycling of chemical and mineral nutrients through plants and animals back to the Earth. They also believe acids suppress decay of organic matter, a natural process needed to enrich the soils. Valuable nutrients such as calcium and magnesium are normally bound to soil particles and are, therefore, protected from being rapidly washed into groundwater. Acid rain, however, may accelerate the process of breaking these bonds to rob the soil of these nutrients. This, in turn, decreases plant uptake of vital nutrients. (See Figure 5.4.)

Acid deposition can cause leafy plants such as lettuce to hold increased amounts of potentially toxic substances such as the mineral cadmium. Research also finds a decrease in carbohydrate production in the photosynthesis process of some plants exposed to acid conditions. Research is under way to determine whether acid rain could ultimately lead to a permanent reduction in tree growth, food crop production, and soil quality. Effects on soils, forests, and crops are difficult to measure because of the many species of plants and animals, the slow rate at which ecological changes occur, and the complex interrelationships between plants and their environment.

trees

trees. The effect of acid rain on trees is influenced by many factors. Some trees adapt to environmental stress better than others; the type of tree, its height, and its leaf structure (deciduous or evergreen) influence how well it will adapt to acid rain. Scientists believe that acid rain directly harms trees by leaching calcium from their foliage and indirectly harms them by lowering their tolerance to other stresses.

According to the EPA, acid rain has also been implicated in impairing the winter hardening process of some trees, making them more susceptible to cold-weather damage. In some trees the roots are prone to damage because the movement of acidic rain through the soil releases aluminum ions, which are toxic to plants.

One area in which acid rain has been linked to direct effects on trees is from moist deposition via acidic fogs and clouds. The concentrations of acid and SO_x in fog droplets are much greater than in rainfall. In areas of frequent fog, such as London, significant damage has occurred to trees and other vegetation because the fog condenses directly on the leaves.

Birds

Increased freshwater acidity harms some species of migratory birds. Experts believe the dramatic decline of the North American black duck population since the 1950s is because of decreased food supplies in acidified wetlands. Acid rain leaches calcium out of the soil and robs snails of the calcium they need to form shells. Because titmice and other species of songbirds get most of their calcium from the shells of snails, the birds are also perishing. The eggs they lay are defective—thin and fragile. The chicks either do not hatch or have bone malformations and die.

In "Adverse Effects of Acid Rain on the Distribution of the Wood Thrush Hylocichla mustelina in North America" (Proceedings of the National Academy of Sciences, August 12, 2002), Ralph S. Hames et al. discuss the results of their large-scale study, which shows a clear link between acid rain and widespread population declines in the wood thrush, a type of songbird. Hames and his colleagues believe that calcium depletion has had a negative impact on this bird's food source, mainly snails, earthworms, and centipedes. The bird may also be ingesting high levels of metals that are more likely to leach out of overly acidic soils. Declining wood thrush populations are most pronounced in the higher elevations of the Adirondack, Great Smoky, and Appalachian mountains. Hames and his cohorts warn that acid rain may also be contributing to population declines in other songbird species.

Materials

Acid rain can also be harmful to materials, such as building stones, marble statues, metals, and paints. Elaine McGee of the U.S. Geological Service reports in Acid Rain and Our Nation's Capital (1997, http://pubs.usgs.gov/gip/acidrain/contents.html) that limestone and marble are particularly vulnerable to acid rain. Historical monuments and buildings composed of these materials in the eastern United States have been hit hard by acid rain.

Human Health

Acid rain has several direct and indirect effects on humans. Particulates are extremely small pollutant particles that can threaten human health. Particulates related to acid rain include fine particles of SO_x and nitrates. These particles can travel long distances and, when inhaled, penetrate deep into the lungs. Acid rain and the pollutants that cause it can lead to the development of bronchitis and asthma in children. Acid rain is also believed to be responsible for increasing health risks for those over the age of sixty-five; those with asthma, chronic bronchitis, and emphysema; pregnant women; and those with histories of heart disease.

THE POLITICS OF ACID RAIN

Scientific research on acid rain was sporadic and largely focused on local problems until the late 1960s, when Scandinavian scientists began more systematic studies. Acid precipitation in North America was not identified until 1972, when scientists found that precipitation was acidic in eastern North America, especially in northeastern and eastern Canada. In 1975 the First International Symposium on Acid Precipitation and the Forest Ecosystem convened in Columbus, Ohio, to define the acid rain problem. Scientists used the meeting to propose a precipitation-monitoring network in the United States that would cooperate with the European and Scandinavian networks and to set up protocols for collecting and testing precipitation.

In 1977 the Council on Environmental Quality was asked to develop a national acid rain research program. Several scientists drafted a report that eventually became the basis for the National Acid Precipitation Assessment Program (NAPAP). This initiative eventually translated into legislative action with the Energy Security Act of 1980. Title VII (Acid Precipitation Act of 1980) of the act produced a formal proposal that created NAPAP and authorized federally financed support.

The first international treaty aimed at limiting air pollution was the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Trans-boundary Air Pollution, which went into effect in 1983. It was ratified by thirty-eight of the fifty-four UNECE members, which included not only European countries but also Canada and the United States. The treaty targeted sulfur emissions, requiring that countries reduce emissions 30% from 1980 levels—the so-called Thirty Percent Club.

The early acid rain debate centered almost exclusively on the eastern United States and Canada. The controversy was often defined as a problem of property rights. The highly valued production of electricity in coal-fired utilities in the Ohio River Valley caused acid rain to fall on land in the Northeast and Canada. An important part of the acid rain controversy in the 1980s was the adversarial relationship between U.S. and Canadian government officials over emission controls of SO₂ and NO₂. More of these pollutants crossed the border into Canada than the reverse. Canadian officials very quickly came to a consensus over the need for more stringent controls, whereas this consensus was lacking in the United States.

Throughout the 1980s the major lawsuits involving acid rain all came from eastern states, and the states that passed their own acid rain legislation were those in the eastern part of the United States.

Legislative attempts to restrict emissions of pollutants were often defeated after strong lobbying by the coal industry and utility companies. These industries advocated further research for pollution-control technology rather than placing restrictions on utility company emissions.

The NAPAP Controversy

In 1980 Congress established NAPAP to study the causes and effects of acid deposition and recommend policy approaches for controlling acid rain effects. About two thousand scientists worked on this unique inter-agency program, which ultimately cost more than $500 million. Even though its first report was due in 1985, the program was plagued by problems that resulted in numerous delays. In 1985 the first executive director, Christopher Bernabo, resigned and was replaced by Lawrence Kulp. In 1987 the study group released to Congress Interim Assessment: The Causes and Effects of Acidic Deposition, a massive four-volume preliminary report that caused a storm of controversy. The report contained detailed scientific information in its technical chapters about acid rain. The executive summary, written by Kulp, was released to the public and widely criticized for mis-representing the scientific findings of the report and downplaying the negative effects of acid rain. Philip Shabecoff notes in "Acid Rain Report Unleashes a Torrent of Controversy" (New York Times, March 20, 1990) that critics claimed Kulp had slanted the summary to match the political agenda of the administration of President Ronald Reagan (1911–2004), which advocated minimum regulation of business and industry.

Some of the scientific findings in the 1987 report included:

• Acid rain had adversely affected aquatic life in about 10% of eastern lakes and streams.
• Acid rain had contributed to the decline of red spruce at high elevations by reducing this species' cold tolerance.
• Acid rain had contributed to erosion and corrosion of buildings and materials.
• Acid rain and related pollutants had reduced visibility throughout the Northeast and in parts of the West.

The report concluded, however, that the incidence of serious acidification was more limited than originally feared. At that time the Adirondacks area of New York was the only region showing widespread, significant damage from acid. Furthermore, results indicated that electricity-generating power plants were responsible for two-thirds of SO₂ emissions and one-third of NO_x emissions.

Controversy over Kulp's role led to him being replaced by James Mulhoney. The new director ordered reassessments and revisions of the interim report. This was completed in 1991. However, by that time President George H. W. Bush (1924–) was in power, and he had made acid rain legislation a component of his election campaign. As a result, political forces, rather than NAPAP, largely drove the nation's emerging policy toward acid rain.

THE ACID RAIN PROGRAM—CLEAN AIR ACT AMENDMENTS, TITLE IV

Congress created the Acid Rain Program under Title IV (Acid Deposition Control) of the 1990 Clean Air Act Amendments. The goal of the program is to reduce annual emissions of SO₂ and NO_x from electric power plants nationwide. The program set a permanent cap on the total amount of SO₂ that could be emitted by these power plants. According to the EPA, in Acid Rain Program: 2005 Progress Report (October 2006, http://www.epa.gov/airmarkets/progress/docs/2005report.pdf), this cap was set at 8.9 million tons (approximately half the number of tons of SO₂ emitted by these plants during 1980). The program also established NO_x emissions limitations for certain coal-fired electric utility plants. The objective of these limitations was to achieve and maintain a two-million-ton reduction in NO_x emission levels by 2000 compared with the emissions that would have occurred in 2000 if the limitations had not been implemented.

In the 1999 Compliance Report: Acid Rain Program (July 2000, http://www.epa.gov/airmarkets/progress/docs/1999compreport.pdf), the EPA indicates that the reduction was implemented in two phases. Phase I began in 1995 and covered 263 units at 110 utility plants in 21 states with the highest levels of emissions. Most of these units were at coal-burning plants located in eastern and midwestern states. They were mandated to reduce their annual SO₂ emissions by 3.5 million tons. An additional 182 units joined Phase I voluntarily, bringing the total of Phase I units to 445.

Phase II began in 2000. It tightened annual emission limits on the Phase I group and set new limits for more than two thousand cleaner and smaller units in all forty-eight contiguous states and the District of Columbia.

A New Flexibility in Meeting Regulations

Traditionally, environmental regulation has been achieved by the "command and control" approach, in which the regulator specifies how to reduce pollution, by what amount, and what technology to use. Title IV, however, gave utilities flexibility in choosing how to achieve these reductions. For example, utilities could reduce emissions by switching to low-sulfur coal, installing pollution-control devices called scrubbers, or shutting down plants.

Utilities took advantage of their flexibility under Title IV to choose less costly ways to reduce emissions—many switching from high- to low-sulfur coal—and as a result, they have been achieving sizable reductions in their SO₂ emissions.

Allowance Trading

Title IV also allows electric utilities to trade allowances to emit SO₂. Utilities that reduce their emissions below the required levels can sell their extra allowances to other utilities to help them meet their requirements.

Title IV allows companies to buy, sell, trade, and bank pollution rights. Utility units are allocated allowances based on their historic fuel consumption and a specific emissions rate. Each allowance permits a unit to emit one ton of SO₂ during or after a specific year. For each ton of SO₂ discharged in a given year, one allowance is retired and can no longer be used. Companies that pollute less than the set standards will have allowances left over. They can then sell the difference to companies that pollute more than they are allowed, bringing them into compliance with overall standards. Companies that clean up their pollution would recover some of their costs by selling their pollution rights to other companies.

The EPA holds an allowance auction each year. The sale offers allowances at a fixed price. This use of market-based incentives under Title IV is regarded by many as a major new method for controlling pollution.

From 1995 to 1998 there was considerable buying and selling of allowances among utilities. Because the utilities that participated in Phase I reduced their sulfur emissions more than the minimum required, they did not use as many allowances as they were allocated for the first four years of the program. Those unused allowances could be used to offset SO₂ emissions in future years. In Acid Rain: Emissions Trends and Effects in the Eastern United States (March 2000, http://www.gao.gov/archive/2000/rc00047.pdf), the U.S. General Accounting Office (now the U.S. Government Accountability Office) notes that from 1995 to 1998 a total of 30.2 million allowances were allocated to utilities nationwide; almost 8.7 million,

FIGURE 5.5

or 29%, of the allowances were not used but were carried over (banked) for subsequent years.

Figure 5.5 shows the status of the allowance bank from 1995 through 2005. Banked allowances increased dramatically in 2000 due to the addition of the Phase II sources to the Acid Rain Program. Over the next five years the allowance bank steadily decreased in size. The EPA reports in Acid Rain Program: 2005 Progress Report that in 2005 a total of 9.5 million allowances were allocated. Another 6.9 million banked allowances were carried over from previous years. The EPA expects that the allowance bank will eventually be depleted as SO₂ emissions are further restricted by the implementation of the Clean Air Interstate Rule.

PERFORMANCE RESULTS OF THE ACID RAIN PROGRAM

There are three quantitative measures that environmental regulators use to gauge the performance of the Acid Rain Program: emissions, atmospheric concentrations, and deposition amounts.

U.S. Progress Report

The following information comes from the EPA's Acid Rain Program: 2005 Progress Report.

sources and emissions

sources and emissions. The report notes that in 2005 there were 3,456 electric generating units subject to the SO₂ provisions of the Acid Rain Program. Most emissions were associated with approximately 1,100 coal-fired units making up the total. In all, program sources emitted 10.2 million tons of SO₂ into the air. (See Figure 5.6.) The EPA expects that the 8.9-million-ton annual cap on emissions will be achieved by 2010. SO₂ emissions from sources covered by the program decreased by 41% between 1980 and 2005.

In 2005 the NO_x portion of the Acid Rain Program applied to a subset of the 3,456 units mentioned earlier, specifically 982 operating coal-fired units generating at least 25 megawatts. Between 1990 and 2005 NO_x emissions from power plants subject to the Acid Rain Program decreased from 5.5 million tons per year to 3.3 million tons per year. (See Figure 5.7.)

According to the report, in 2000 the program first achieved its goal of reducing emissions by at least 2 million tons; 8.1 million tons were originally predicted in 1990 to be emitted in 2000 without the program in place.

The report indicates that the SO₂ and NO_x emission reductions were achieved even though the amount of fuel used to produce electricity in the United States increased by more than 30% between 1990 and 2005. Coal was the

FIGURE 5.6

FIGURE 5.7

single-largest fuel source for U.S. electric generating plants in 2005, accounting for 50% of the total.

atmospheric concentrations and deposition amounts

atmospheric concentrations and deposition amounts. The EPA's Acid Rain Program uses two complementary monitoring networks to track trends in regional air quality and acid deposition: the Clean Air Status and Trends Network and the NADP's National Trends Network. Additional monitoring data are provided by national, state, and local ambient monitoring systems.

As shown in Figure 2.14 and Figure 2.6 in Chapter 2, atmospheric levels of SO₂ and NO₂ averaged nationwide since 1990 have been well below the national standards for these pollutants.

Table 5.3 shows trends in atmospheric concentrations and deposition for four key regions in the Acid Rain Program: mid-Atlantic, Midwest, Northeast, and Southeast. Overall, concentrations of ambient SO₂ and wet sulfates averaged over the period 2003–05 declined in all these regions, compared with the period 1989–91. The most dramatic differences are evident in the Northeast, where ambient SO₂ concentrations decreased by more than 50%. The results for nitrogen and nitrate compound concentrations are mixed, with decreases in some areas and increases in others. The same is true for wet inorganic nitrogen deposition, which decreased in the mid-Atlantic, Midwest, and Northeast, but increased slightly in the Southeast.

Canadian Progress Report

In November 2006 Environment Canada released a report on progress made by Canada and the United States on cross-border air pollution. The study, Canada–United States Air Quality Agreement: 2006 Progress Report (http://www.ec.gc.ca/cleanair-airpur/caol/canus/report/2006canus/toc_e.cfm), is the eighth biennial report related to the 1991 agreement between the two countries. The report states that Canada has been successful at reducing SO₂ emissions below its national cap. Canada's total SO₂ emissions were 2.3 million tonnes (metric tons) in 2004, which is 28% below the national cap of 3.2 million tonnes. However, Environment Canada notes that the reductions have not been sufficient to reduce acid deposition below the levels needed to ensure the recovery of ecosystems damaged by excess acidity in its eastern provinces.

ARE ECOSYSTEMS RECOVERING?

Monitoring data clearly indicate decreased emissions and atmospheric concentrations of SO₂ and NO_x and some reductions in deposition amounts. These improvements have not necessarily resulted in recovery of sensitive aquatic and terrestrial ecosystems. This is due, in part, to the long recovery times required to reverse damage done by acidification. The EPA reports that ecosystems

TABLE 5.3

harmed by acid rain deposition can take a long time to fully recover even after harmful emissions cease. The most chronic aquatic problems can take years to be resolved. Forest health is even slower to improve following decreases in emissions, taking decades to recover. Finally, soil nutrient reserves (such as calcium) can take centuries to replenish.

The most recent comprehensive analysis of acidified ecosystems was presented by NAPAP in the National Acid Precipitation Assessment Program Report to Congress: An Integrated Assessment (2003, http://www.cleartheair.org/documents/NAPAP_FINAL_print.pdf). The report presents a literature review summarizing findings from various government and academic studies. Overall, NAPAP finds that some ecosystems affected by acid deposition are showing limited signs of recovery. For example, one study shows that more than 25% of affected lakes and streams studied in the Adirondacks and northern Appalachians are no longer acidic. However, little to no improvement has been seen in examined water bodies in other regions, including New England and portions of Virginia. The report notes that even though chemical recovery has begun in some waterways, complete recovery for aquatic life forms, such as fish, is expected to take "significantly longer."

In regards to terrestrial ecosystems, NAPAP reports that forests are under many stresses besides acid rain, such as global warming, land use changes, and air pollution from urban, agricultural, and industrial sources. The combined effect of these stressors has greatly limited forest recovery from acidification. According to NAPAP, "There are as yet no forests in the U.S. where research indicates recovery from acid deposition is occurring." However, it is expected that reduced emissions under the Acid Rain Program will benefit forests in the long term.

The report acknowledges the future benefits of continued implementation of the Acid Rain Program, but it concludes that "the emission reductions achieved by Title IV are not sufficient to allow recovery of acid-sensitive ecosystems." Recent studies support the idea that additional emission cuts 40% to 80% beyond those of the existing program will be needed to protect acid-sensitive ecosystems. NAPAP modeling indicates that even virtual elimination of SO₂ emissions from power plants will be insufficient to provide this protection. It is believed that emission reductions from other sources (such as the industrial and transport sectors) will be necessary.

The Next Step: The Clean Air Interstate Rule

In 2005 the EPA issued the Clean Air Interstate Rule (CAIR; April 5, 2007, http://www.epa.gov/cair/) to address the transport of air pollutants across state lines in the eastern United States. CAIR puts permanent caps on emissions of SO₂ and NO_x in twenty-eight eastern states and the District of Columbia. It is expected to reduce SO₂ emissions by more than 70% and reduce NO_x emissions by more than 60% compared with 2003 levels. These measures should reduce the formation of acid rain and other pollutants, such as fine particulate matter and ground-level ozone.

The CAIR program will use a cap-and-trade system similar to that used in the SO₂ portion of the acid rain program. The EPA projects that complete implementation of CAIR in 2015 will result in up to $100 billion in annual health benefits and a substantial reduction in premature deaths because of air pollution in the eastern United States. It should also improve visibility in southeastern national parks that have been plagued by smog in recent years.

PUBLIC OPINION ABOUT ACID RAIN

Every year the Gallup Organization polls Americans about their attitudes regarding environmental issues. The most recent poll to assess acid rain was conducted in March 2007. Participants were asked to express their level of personal concern about various environmental issues, including acid rain, water pollution, soil contamination, air pollution, plant and animal extinctions, loss of tropical rain forests, damage to the ozone layer, and global warming. The results showed that acid rain ranked last among these environmental problems.

Analysis of historical Gallup poll results shows a dramatic decline in concern about acid rain since the late 1980s. (See Table 5.4.) In 1989 Gallup found that 41% of respondents felt a great deal of concern about acid rain and 11% felt none at all. By 2007 only 25% of people polled were concerned a great deal about acid rain and 20% expressed no concern about the acid rain issue.

TABLE 5.4