Definition. According to Sayala (2001), the definition of acid precipitation is “any rainfall/precipitation that has an acidity level beyond what is expected in non-polluted rainfall [or] any precipitation that has a pH value of less than 5.6” (p. 6). This is because theoretically, a pH of 5.6 is representative of the naturally-occurring atmospheric concentrations of carbon dioxide normally present in “pure” rainfall; nearly all rainfall is acidic to some degree because of an imbalance between hydrogen and hydroxyl ions, but a pH of 5.6 appears to be the benchmark in the definition of acid rain, give or take (Schnabel, Lamb, Pionke, & Genito, 2000, p.1).

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It is important to note, however, that rainfall is only one of the physical forms that acid precipitation can take. Acids can be deposited in our environment in the form of fog, snow, mist, dew, and even general particulate fallout carried on prevailing winds; in other words, acids in the atmosphere can be deposited in the form of both wet acidic solutions and dry acidic particles in the air (Cunningham & Saigo, 1999, chap. 18, pp. 398-399). In general, acid deposition in aqueous form is what the term acid precipitation refers to, and acid deposition in dry or gaseous form is what the term dry deposition refers to (Manahan, 2000, chap. 14, p. 413). This paper will deal primarily with the aqueous form of acid deposition.

2.2   Common pollutants and reactions involved. There are several pollutants that can contribute to the formation of acid precipitation. These include sulfur dioxide, nitrogen oxides, chlorine, hydrogen chloride, hydrogen sulfide, carbonyl sulfide, and carbon dioxide (Sayala, 2001, p. 6). With regard to the distribution of these pollutants, it can be noted that “sulfates account for about two-thirds of the acid deposition in eastern North America and most of Europe, while nitrates contribute most of the remaining one-third. In urban areas…nitric acid is equal to or slightly greater than sulfuric acids in the air” (Cunningham & Saigo, 1999, chap. 18, p. 399). However, perhaps more important than the chemicals themselves are the ways in which theyinteract with other substances in the atmosphere and the environment in general. This concept is poignantly illustrated by Manahan (2000):

Page 2 Acid Precipitate Essay

Although acid rain can originate from the direct emission of strong acids, such as HCl gas or sulfuric acid mist, most of it is a secondary air pollutant producedby the atmospheric oxidation of acid-forming gases…As the result of such reactions the chemical properties (acidity, ability to react with other substances) and physical properties (volatility, solubility) of acidic atmospheric pollutants are altered drastically. (chap. 14, p. 414)

Thus, it can be seen that secondary pollutants, so altered in physical and chemical constituency by these atmospheric processes, can play altogether different and many times very unexpected roles in the transport and deposition of acid precipitation. This reinforces Sayala’s assertion that “chemical reactions such as oxidation and hydrolysis play a major role in the formation of acid rain” (p. 6). Sayala also notes that some of these reactions, such as the formation of nitric acid as a secondary pollutant in the atmosphere, can be made to occur even faster by the presence of catalysts, such as iron, manganese, ammonia, and hydrogen peroxide.

However, it is important to remember that regardless of the level of acidity that these chemical reactions induce in the atmosphere, “the rate of wet deposition is…governed by a complicated set of meteorological and microscale processes working in harmony to effect a general cleansing of the atmosphere” (Schnabel, Lamb, Pionke, & Genito, 2000, p. 4). So, in effect, acidic pollutants in the atmosphere may not be deposited at any specific time or place, but in a more haphazard manner entirely dependent on the conditions present in the atmosphere at any given point.

2.3   Sources of acid precipitation pollutants. The pollutants that cause acid precipitation can have both natural and industrial sources. On the natural side, for example, “volcanic emissions, biological decomposition, and chlorine and sulfates from ocean spray can drop the pH of rain well below 5.6,” thereby inducing acid precipitation (Cunningham & Saigo, 1999, chap. 18, p. 399). Even livestock manure can combine with moisture in the atmosphere and contribute to the formation of acid rain (“Hard Rain,” 2001, n.p.).

However, in most areas, industrial–or anthropogenic–sources of acid precipitation are usually much more prevalent than natural sources (Cunningham & Saigo, p. 399). Gases, chemicals, and particulates emitted from industrial processes such as fossil fuel combustion, for example, are a major source of acid precipitation (Manahan, 2000, chap. 14, p. 414). In fact, according to Sayala (2001), “industrial sources such as [the] burning of fossil fuels, e.g., coal and oil, are largely to be blamed for approximately half of the emissions of [sulfur dioxide] gas in the world” (p. 6). Sayala goes on to note that emissions from power stations and exhaust fumes can cause other major pollutants such as nitrogen oxides to rise into the atmosphere, where they can then be oxidized into nitric acid and later precipitated out as acid rain.

3   The Harmful Effects of Acid Precipitation

1   Effects on aquatic systems. Acid precipitation can have a devastating negative impact on aquatic systems, including oceans, lakes, rivers, streams, estuaries, and even groundwater. First of all, acidic water bodies can kill fish, both directly and indirectly–directly by interfering with the fish’s ability to maintain the appropriate balance, of salt, minerals, and oxygen in its tissues, and indirectly by gradually releasing heavy metals from the soils into the lakes to burn the gills of the fish and cause lethal damage to its organs (Sayala, 2001, p. 9).

For example, recent studies have shown that, due to the effects of acid rain, “forty-one percent of the lakes in the Adirondack region of New York and 15 percent of the lakes in New England are either chronically or periodically acidic…[and] show reduced aquatic life and species diversity” (“Acid Rain,” 2001, n.p.). These studies were first sparked by evidence brought to light in the early 1970s, which suggested that many of the lakes in North America were becoming acidified by air pollutant deposition; this early evidence revealed that approximately half of the high altitude lakes in the Adirondack Mountains were acidified and could not support fish (Cunningham & Saigo, 1999, chap. 18, p. 399).

However, we must also recognize that acid precipitation does not just impact fish, but also can have a detrimental effect on the other plant and animal life that rely on these aquatic systems for sustenance–including humans. In order to more fully understand this lake acidification phenomenon, we must first realize the following:

[T]he major factors influencing the impact of acid deposition on lakes and streams are (1) the amount of acid deposited; (2) the pathway and travel time from the point of deposition to the lake or stream; (3) the buffering characteristics of the soil through which the acidic solution moves; (4) the nature and amount of acid reaction products in soil drainage and from sediments; and (5) the buffering capacity of the lake or stream. (Schnabel, Lamb, Pionke, & Genito, 2000, p. 4)

More recent studies have clarified this situation further; apparently, many high altitude mountain lakes and streams have a “very low buffering capacity (ability to resist pH change) and are [more] susceptible to acidification,” including areas such as the Rocky Mountains, the Sierra Nevadas in California, and the Cascade Mountains in Washington (Cunningham & Saigo, p. 399). However, regardless of buffering capacity, according to Sayala, “lakes can almost be thought of as the ‘sinks’ of the earth, where rain that falls on land [and] is drained through the sewage systems eventually makes its way into the lakes.

Acid rain that falls onto the earth washes off the nutrients out of the soil and carries toxic metals that have been released from the soil into the lakes” (p. 8). In this way, we can see that the effects of acid precipitation on aquatic systems–through surface water and groundwater–can go hand-in-hand with the effects of acid precipitation on terrestrial ecosystems, such as forests; this is primarily because “soils are the key intermediate [between the two]. They provide the root environment for terrestrial vegetation, and also control the water quality of runoff and soil drainage, which supplies most of the water to the aquatic system[s]” (Schnabel, Lamb, Pionke, & Genito, p.4).

3.2   Effects on forests and other terrestrial ecosystems. Keeping this interrelationship in mind, both the qualitative and quantitative negative impacts of acid precipitation on forests and other terrestrial ecosystems can seem overwhelming. First of all, acid precipitation has been shown to have a dramatic ability “to alter the nutritional needs of timberlands,” according to studies done in the mid-1990s (Raloff, 1995, p. 90). In addition, despite valiant government attempts over the years to lessen the amount of industrial pollutants being released into the atmosphere, researchers have recently discovered that “years of acid rain have made [many] ecosystems more sensitive to additional pollution” (“Acid Rain,” 2001, n.p.). The documented negative effects of acid precipitation on global forests in years past are sobering:

In the early 1980s, disturbing reports appeared of rapid forest declines in both Europe and North America…A 1980 survey showed that seedling production, tree density, and viability of spruce-fir forests at high elevations [on Camel’s Hump Mountain in Vermont] had declined about 50 percent in 15 years…By 1990, almost all the red spruce, once the dominant species on the upper part of the mountain, were dead or dying…European forests are also dying at an alarming rate. West German foresters estimated in 1982 only 8 percent of their forests showed air pollution damage…in 1985, more than 4 million hectares (about half the total forest) were reported to be in a state of decline. (Cunningham & Saigo, 1999, chap. 18, p. 399)

3.3   Effects on man-made structures and materials. Acid precipitation also has definite negative effects on man-made structures and materials. This can be seen in the way in which famous historical structures such as “the Parthenon in Athens, the Taj Mahal in Agra, the Colosseum in Rome, frescoes and statues in Florence, medieval cathedrals in Europe, and the Lincoln Memorial and Washington Monument in Washington, DC, are slowly dissolving and flaking away because of acidic fumes in the air” (Cunningham & Saigo, 1999, chap. 18, p. 401).

This type of disintegration happens when acid particles are deposited on the structures–which can range from bridges to railroads to airplanes–by acid precipitation, causing corrosion (Sayala, 2001, p. 11). The atmospheric acids thus deposited, most notably sulfuric and nitric acids, cause billions of dollars of damage to buildings and other man-made materials each year (Cunningham & Saigo, p. 401). They also cause a safety hazard to the public, a hazard graphically illustrated by the 1967 incident in which “the bridge over the Ohio River collapsed killing 46 people–[where] corrosion due to acid rain was the reason for [the] accident” (Sayala, p. 11).

3.4   Effects on human health. The effects of acid precipitation on human health are not always direct or obvious, but they are there nonetheless and are just as devastating in impact. According to Sayala (2001):

One of the serious side effects of acid pollution on humans is respiratory problems. The SO2 and NO2 emissions give rise to respiratory problems such as asthma, dry coughs, headaches, eye, nose and throat irritations. An indirect effect of acid precipitation on humans is that the toxic metals dissolved in the acid water are absorbed by fruits, vegetables and in the tissues of animals.

Although these toxic metals do not directly affect the animals, they have serious effects on humans when they are being consumed. For example, mercury that accumulate[s] in the organs and tissues of the animals has been linked with brain damage in children as well as nerve disorders…and death. Similarly, another metal, aluminum, present in the organs of the animals, has been associated with kidney problems and recently, was suspected to be related to Alzheimer’s disease. (p. 11)

Some other important chronic health effects of air pollutants include heart attacks and lung cancer, as well as bronchitis and emphysema, all of which taken together “can mean as much as a 5- to 10-year decrease in life expectancy if you live in the worst parts of Los Angeles or Baltimore, compared to a place with clean air” (Cunningham & Saigo, 1999, chap. 18, p. 396). However, given the demonstrated corrosive effects of acid precipitation on anthropogenic materials such as limestone, marble, and steel, it should come as no surprise that these same pollutants can have detrimental effects on the fragile human body as well.

4   Global Implications of the Acid Precipitation Problem

4.1  Air pollution migration. In studies of the movement of air pollution and the transport and deposition of pollutants through acid precipitation, it has become obvious that there are no clear boundaries in which air pollution can be contained. In other words, “political boundaries often cross regional airsheds” (Brazel & Fitch, 2000, p. 1), so, in effect, one country’s pollution may become another country’s acid precipitation problem.  This is because industrial and other pollutants can be transported over long distances by the wind, becoming chemically altered by reactions in the atmosphere along the way, and ultimately being deposited over another country’s land (Cunningham & Saigo, 1999, chap. 18, p. 394). There are two main problems that arise because of this migration phenomenon:

[First, t]racing the sources of these chemically altered pollutants can be difficult. Lakes and forests in Sweden were showing evidence of sulfuric acid contamination years before the source of the acidity was traced to Germany, England, and other distant parts of Europe. [Second, c]ontrolling long-range pollutants is a highly political process. Germany and England were not very sympathetic about acid precipitation until their own forests began to die. In another case, 90 percent of the pollution falling into Lake Superior originates thousands of kilometers away in the United States, Canada, and even Mexico. Farms and industries in these distant regions resist spending money, however, to reduce emissions to protect someone else’s environment. (Cunningham & Saigo, chap. 18, pp. 394-395)

Other phenomena also illustrate the way in which airshed regions do not match up with politically contiguous areas–for instance, the phenomenon of the Arctic Haze, which has been shown to have its source in the widespread industrialization efforts of North America and Europe (Brazel & Fitch, p. 1).

4.2   The international challenge. The global implications of air pollution migration–as they relate to acid precipitation–are a constant challenge for government officials and policymakers to overcome as they attempt to implement environmental air-quality regulations. As an example of one recently emerging international challenge:

With the enactment of the North American Free Trade Agreement (NAFTA), there is a rapidly emerging economic and political focus on the United States-Mexico border. On the United States side, national air-quality legislation has been enacted, albeit differentially at local levels. On the Mexican side, however, little or no major environmental legislation and controls on pollution have been developed. Border air pollution problems are exacerbated by rapid population growth in the region, large population differences on opposite sides of the border, and disparate economic levels of twin towns and cities along the border. (Brazel & Fitch, 2000, pp. 1-2)

This example illustrates one of the key international environmental concerns today–the unfairness of air pollution dispersion. This dispersion is primarily a function of wind and topography, both of which act without regard for which side of the border has pollution controls or which side emits the largest proportion of pollutants into the atmosphere.

5   Conclusion

All in all, it is easy to see why acid precipitation is an important problem in today?s society. Acid precipitation “washes nutrients out of soils, stresses trees, poisons lakes and harms wildlife…caus[ing] a series of harmful and lingering changes in the soil and water” (“Hard Rain,” 2001, n.p.). Acid precipitation is responsible, either directly or indirectly, for the deaths of plant, animal, and human life worldwide. However, perhaps most importantly, acid precipitation respects no political boundaries. It can affect us even if we are following air pollution regulations and even if we seem to live in an area with cleaner air.

With all of the far-reaching detrimental effects that acid precipitation is having on our environment, our society, and ourselves, we cannot ignore the presence of this growing ecological problem. We must use our knowledge about the causes, effects, and implications of this global phenomenon to come up with a coordinated, practical–and global–solution to the acid precipitation situation before this state of affairs gets so out of control that the choice to act is no longer in our hands.


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2001, from the Expanded Academic Index ASAP database at

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Cunningham, W. P., & Saigo, B. W. (1999). Environmental science: A global concern (5th ed.).

Boston: McGraw-Hill Companies, Inc.

Hard rain keeps falling. (2001, August-September). National Wildlife, n.p. Retrieved November

            6, 2001, from the Expanded Academic Index ASAP database at

Manahan, S. E. (2000). Environmental chemistry (7th ed.). New York: Lewis Publishers.

Raloff, J. (1995, February 11). When nitrate reigns: Air pollution can damage forests more than

            trees reveal. Science News, 147, 90. Retrieved November 6, 2001, from the Expanded

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