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.

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.

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.

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.

Bibliography:

1) Environment Canada. 2002. "Acid Rain." Retrieved December 3, 2007.

2) Hubbard Brook Research Foundation. "Acid Rain." Retrieved December 3, 2007.

3) U.S. Environmental Protection Agency. 2007. "Acid Rain." Retrieved December 3, 2007.

4) U.S. Geological Survey. "Acid Deposition." Retrieved December 3, 2007.

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