Acid rain refers to both wet and dry deposition of atmospheric materials that contain high concentrations of nitric and sulfuric acid. The wet deposition can include fog, hail, sleet, or snow in addition to rain; the dry deposition is usually dust or smoke.
How Does Acid Rain Form?
Acid rain is a secondary air pollutant. It is not released directly into the air; rather, it forms as a result of the discharge of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere. In the atmosphere, SO2 reacts with other chemicals, primarily water and oxygen, to form sulfuric acid (H2SO4); the nitrogen oxides react to form nitric acid (HNO3). Once formed, prevailing winds can transport these compounds to distances as great as hundreds of miles, across state and national boundaries.
Although natural sources such as erupting volcanoes and decaying plant material can release these gases, most emissions result primarily from the combustion of fossil fuels. In the United States, approximately 67 percent of all the emitted SO2 and 25 percent of the emitted NOx come from electrical power plants that burn fossil fuels. Other sources for these gases are also primarily industrial in nature, including ore smelting, coal-fired generators, and combustion of fuel in motor vehicles.
How Is Acid Rain Measured?
All acids, including acid rain, are measured using the pH scale. The pH scale is based on the tendency of a substance to release hydrogen ions in solution; the more readily a substance releases hydrogen ions, the stronger an acid it is. The pH scale runs from a value of 0 for very strong acid (very weak base) to a high value of 14 for a very weak acid (very strong base). Calculating in powers of 10, water with a pH of 4 is 10 times more acidic than water with a pH of 5. Distilled water has a pH of 7, something rarely seen in nature, even with unpolluted rain. This is because naturally occurring carbon dioxide (CO2) in the atmosphere dissolves into the rainwater, forming weak carbonic acid and lowering the pH to around 5.6. According to the U.S. Environmental Protection Agency (USEPA), as of 2000, the most acidic rain falling in the United States had a pH of approximately 4.3.
Where Is Acid Rain a Problem?
In the United States, acid rain is a problem primarily in the eastern half of the country, in parts of the Northeast and the northern Midwest. The lowest pH values—the result of heavy industrialization in Pennsylvania, Ohio, and Illinois—are found in New York and central New England, as well as in Ontario, Quebec, and the Maritime Provinces in Canada. Except for some localized instances of slightly lower pH values, the problem is less pronounced in the southern and western parts of the United States.
A National Surface Water Survey conducted by USEPA in the mid-1980s investigated more than 1,000 lakes larger than 10 acres and many streams thought to be vulnerable to acidification. The survey found that many of these lakes and streams suffer from chronic acidity, with the water constantly maintaining a low pH. The survey found that of the lakes and streams surveyed, acid rain was the cause of acidity in 75 percent of the acidic lakes and 50 percent of the acidic streams. The survey identified the Adirondacks and Catskill Mountains in New York, the mid-Appalachian highlands along the East Coast, the northern Midwest, and mountainous areas of the West as areas where many of the surface waters are particularly sensitive to acidification.
Ongoing monitoring by the U.S. Geological Survey, as well as a study conducted by the Hubbard Brook Research Foundation, has found that conditions have not significantly improved. In the Northeast, where the soils have little ability to neutralize acids (known as buffering capacity), some lakes now have a pH of 5 or less, with a lowest reported pH of 4.2 in Little Echo Pond in Franklin, New York. The scope of the problem is even greater if lakes smaller than 10 acres are considered.
Eastern Canada has soil quite similar to that in the Adirondack Mountains, and its lakes are extremely vulnerable to chronic acidification. An estimated 14,000 lakes in that region are acidic, according to the Canadian government. Also susceptible to the effects of acid deposition are streams flowing over soils with little buffering capacity. The survey found that 580 streams in the Mid-Atlantic coastal plain are acidic. The highest concentration of acidic streams was found in the New Jersey Pinelands, where over 90 percent of the streams are acidic. In the Mid-Atlantic Highlands, more than 1,350 of the streams are acidic.
In addition to chronic acidification, there can be brief periods, known as episodic acidification, when pH drops because of heavy downpours of rain or runoff from snowmelt. Many lakes and streams in the United States and Canada are susceptible to this episodic effect. USEPA estimates that approximately 70 percent of lakes in the Adirondacks are at risk.
What Are the Effects of Acid Rain?
The environmental effects of acid rain are most clearly seen in surface water environments such as streams, lakes, and marshes. Acid rain falls directly on these aquatic habitats, and acidic runoff flows into them after falling on rural and urban areas. The impact can be disastrous. In the United States, many aquatic species are showing the deadly effects of prolonged exposure to acidic conditions, sometimes to such an extent that the overall populations of whole species are reduced and species that are more sensitive to low pH levels become extinct. All of these effects contribute to a reduction in the biodiversity of the affected systems. Some acid lakes no longer have fish in them.
Aquatic systems are not the only ones affected. Forest systems in Europe, North America, and Asia also show damage from acid rain, negatively affecting seedling production, tree density, and overall viability of the forests. The problem is particularly serious in high-altitude forests, where the trees are exposed to the acidic precipitation for longer periods. The most direct damage is to seedlings and to the tissues of adult trees. However, the higher acidity can leach nutrients from the soil and mobilize metals, such as aluminum, that are toxic to the plants. Furthermore, weakened trees can become vulnerable to insects and diseases.
In addition to damage done to the natural environment, acid rain also causes damage to non-natural objects. In many cities, acid precipitation is destroying numerous historic and contemporary buildings and works of art. Structures of limestone and marble— including the Parthenon, the Taj Mahal, the Washington Monument, and numerous medieval cathedrals throughout Europe—are most vulnerable because of their high reactivity with acids. Additionally, acid precipitation can corrode the steel in reinforced concrete, damaging buildings, bridges, and roads. The Council on Environmental Quality estimates that the economic losses in the United States amount to about $4.8 billion in direct costs every year.
What Can Be Done?
Because acid precipitation is a result of air pollution, the most effective strategy is to reduce emissions of the pollutants to the atmosphere. New technology has allowed factories to decrease amounts of SO2 in smokestack emissions. However, emissions of NOx have increased over the same time period, suggesting the need for more stringent air pollution regulation.
- Environment Canada. 2013. “Acid Rain.” Retrieved March 29, 2017 (https://www.ec.gc.ca/air/default.asp?lang=En&n=AA1521C2-1)
- Hubbard Brook Research Foundation. “Acid Rain.” Retrieved March 29, 2017 (http://hubbardbrookfoundation.org/acid-rain/).
- S. Environmental Protection Agency. 2007. “Acid Rain.” Retrieved March 29, 2017 (https://www.epa.gov/acidrain).
- S. Geological Survey. “Acid Deposition.” Retrieved March 29, 2017 (https://www2.usgs.gov/science/science.php?thcode=2&term=6).
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