Sources Of Acidity

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Atmospheric acids mainly are produced in the air as a result of complex chemical reactions of the acid precursor gases. Direct emissions of acids such as sulfuric acid, hydrogen chloride, and hydrogen fluoride have been estimated but are not thought to play a significant role in the acidic deposition processes (NAPAP, 1991). Sources of the harmful chemical precursors affecting the acidity of deposited materials can be natural or human caused. The three pollutants of most concern in the acidic deposition process are sulfur dioxide (S02), nitrogen oxides (NO,) and volatile organic compounds (VOCs). Fossil-fuel-based power plants and motor vehicles are the major sources of all of these acid precursor pollutants in industrialized areas of the world (Graedel et al., 1993; Middleton, 1995).

Sulfur gases are primarily emitted from point sources, involving the combustion of coal in particular. Natural sources of sulfur gases include sea spray, volcanoes, and biologic activity. These sources, however, are at least a factor of 10 less than the human-caused emission for major industrial areas such as the United States. Nitrogen gases also result primarily from human activities involving fossil-fuel-derived energy use related to transportation and utilities. Major natural sources include soils and lightning and are thought to make up a significant portion of the overall emission totals, more than has been estimated for the sulfur gases. Estimates of these levels, however, are uncertain (NAPAP, 1991). The VOCs that produce the organic acids and influence the chemistry producing sulfuric and nitric acids also come mainly from automobile use. However, for the VOCs, natural production from vegetation can be quite significant. In highly vegetated, low industrialized regions natural sources become the dominant producers of VOCs.

Estimates of alkaline particulate and ammonia emissions indicate that there is a high potential for acid neutralization in some parts of the United States. The estimates, however, are subject to a high degree of uncertainty (NAPAP, 1991). On a global scale, emissions of these important acid neutralizes are among the least well-known chemical emissions (Graedel et al, 1993).


The effects of acid deposition are the subject of continuing controversy. The northeastern United States has experienced the worst reported impacts in the United States (NAPAP, 1991). Severe damages attributed to acid rain also have been documented for parts of western Europe (Graedel and Crutzen, 1989).

The most sensitive systems to acid deposition are poorly buffered lakes and streams. Buffering capacity refers to the availability of alkaline minerals from soil or rocks to neutralize the acids. When minerals are dissolved in a lake, buffering is able to diminish acid effects. However, this buffering ability or alkalinity can be used up with additions of acidic pollutants. Low alkalinity lakes have the greatest potential for damage, since their neutralizing minerals can be quickly depleted.

Vegetation is exposed to wet acidic deposition through rain, snow, and by direct contact with low, acid-laden clouds. There is currently no widespread forest or crop damage in the United States related to these possible pathways. However, cloud acidity, together with a complex combination of other factors (e.g., ozone, soil acidification, climate) contribute to reduced cold tolerance in high-elevation spruce in the eastern United States and in Europe. This can contribute to damage to trees above cloud level during winters with particularly low temperatures.

Adverse effects on forests in other regions of the world are associated with ozone, as is the case with high-elevation pines in California, or they are closely related to localized soil nutrient deficiencies, as is the case with sugar maples in eastern Canada. Acidic deposition may increase leaching rates of important base cations, principally magnesium and calcium, in forest solids and may be a contributing factor in sugar maple decline in some areas.

Generally, controlled experiments on trees and crops have indicated that ozone, at concentrations near ambient levels, adversely affects forests and crops primarily by growth reduction. Other controlled experiments have demonstrated that normal levels of atmospheric sulfur and nitrogen deposition cause no negative direct effects. Some areas actually may benefit through nutrient enrichment by nitrogen and sulfur deposition.

Computer models project that continued acidic deposition could result in long-term deficiencies of nutrients in some soils. However, currently, there is no evidence to indicate that forest health in general is currently affected by nutrient deficiency or will be affected in the next half century.

Air pollution and acidic deposition contribute to the corrosion of metals and deterioration of stone in buildings, statues, and other cultural resources. Although air pollution is an important concern for cultural objects, the magnitude of its effect

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on construction materials has been difficult to assess. Many construction materials have protective coatings such as paints; therefore, maintenance and service of protective coatings have an important role in determining the ultimate impact of air pollutants. Paints may also be affected by ambient levels of air pollution.

Another related side effect of acid rain is visibility degradation. Fine particles in the atmosphere containing sulfate, nitrate, and other chemical constituents, which when deposited are associated with acid deposition, cause visibility degradation while in the air. These fine particles have been the major factor in the reduction of visibility in rural and urban areas in the eastern United States since the beginning of the century. In the U.S. West, visibility degradation is being reported in major urban areas and in national parks and wilderness areas.

Direct adverse effects of these pollutants on humans occur largely through the respiratory system. Sensitive populations with existing respiratory or cardiovascular problems, such as those with asthma, are especially susceptible. These effects have been mainly associated with the acid precursor gases and ozone. Studies of the effects of acidic aerosols, composed primarily of nitric acid, ammonium bisulfate, and sulfuric acid, are still relatively new. It has been found that on rare occasions acid levels approach 10 times the long-term mean levels for sulfuric acid. Although substantial uncertainty exists, the body of data raises concern that acidic aerosols alone, or in concert with other pollutants, may be contributing to health effects in exposed populations at current concentration levels.

Human health also can be affected indirectly by pollutants related to acid deposition. People who eat large amounts of fish from acidic lakes or streams may experience exposure to methylmercury in some regions of the country. Drinking water from acidic sources may contain significantly elevated levels of lead. It is unlikely, however, that exposure to humans by this pathway is important, except in isolated cases.


Historically, toxic effects have been observed in populations acutely exposed to high concentrations of air pollutants. As early as the Middle Ages in London prohibitions on coal burning were instituted in response to perceived health effects of mixtures of dust, soot, and fog. The industrial revolution brought the air pollution issue to the United States, where air quality management continued to be considered local in nature into the twentieth century.

The severity of air pollution impacts became very obvious during the London "killer fog" of 1952, when a mixture of particulates, sulfur dioxide, and acidic fog was associated with severe respiratory effects and approximately 4000 deaths. Emergence of air pollution as a public health issue in the 1950s, as a result of this and other deadly episodes, led to the development of federally funded research programs, culminating in the Clean Air Act and in the establishment of the Environmental Protection Agency (EPA) in 1970. These were major stimuli for the establishment of the U.S. National Ambient Air Quality Standards (NAAQS) that today restrict the atmospheric concentration of pollutants such as sulfur dioxide, nitrogen oxides, and ozone.

Other countries around the world have been developing institutional responses to the threat to human health of air pollution. Air pollution effects on the environment, however, had been slower to be recognized as a serious issue. Acid rain and its ecological effects were first documented in England at the end of the nineteenth century and became regional issues in northwestern Europe and the northeastern United States and eastern Canada only recently—in the late 1960s. During this period and into the 1970s, the mounting anecdotal evidence of the effects of acid rain on aquatic and terrestrial ecosystems launched acid rain as perhaps the first pollution threat to the environment to receive international attention.

The origins of the pressures to regulate acid rain in the United States were primarily twofold. First, Canada protested, lobbied, and publicized its contention that major environmental damage was occurring in its eastern provinces because of acid deposition, and that the major sources of acid precursors were in the United States Second, elected officials and citizens in the eastern and New England states echoed the same concerns, elevating the acid rain controversy to the level of a growing interregional conflict between receptor states and polluting states (Rhodes and Middleton, 1983).

The U.S. responses to these concerns took the form of federal research and eventually control programs. The first step was the Acid Precipitation Act of 1980, which created the National Acid Precipitation Assessment Program (NAPAP). During its first 10 years, the research and periodic assessments conducted by NAPAP improved the understanding of the scientific processes and effects of acid deposition. The monitoring and research conducted in the 1980s and the subsequent integrated assessment completed in 1990 provided the scientific knowledge base for Title iy the Acid Deposition Control Program, of the 1990 Clean Air Act Amendments.

Title IV is designed to reduce the adverse effects of acid deposition through the reductions in annual emissions of sulfur dioxide (S02) and nitrogen oxides (NOJ, the precursors to acid rain. Recognizing that the principal sources of acid rain precursors in the atmosphere are emissions from the combustion of fossil fuels, control measures were initiated to reduce emissions from electric utilities. However, rather than the traditional command-and-control approach to regulation, alternative methods of compliance were allowed. These methods included technological adaptation (e.g., scrubbers, higher-efficiency boilers), fuel-switching, and an innovative S02 emissions allowance trading program. This represented the first national effort to use market-based incentives to achieve environmental goals.

Due to the innovative nature of using market-based incentives for environmental regulation, Congress set up a mechanism for checking how well trading was working. As part of this activity, Congress asked NAPAP to assess the costs and economic impacts of the acid deposition control program as well as the effectiveness and benefits associated with the various human health and welfare effects. The effects included visibility, materials, and cultural resources damages and ecosystem effects. NAPAP was also asked to consider the deposition levels needed to protect sensitive ecosystems. The results of the assessment of Title IV are to be reported to Congress quadrennially, beginning with the 1996 Report to Congress (NAPAP, 1998).

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