Acid Rain

Acid rain is a term that is used to describe the several ways in which acids fall out of the atmosphere and reach the earth. Acid deposition is a more precise term that can be used to describe this phenomenon, which has both wet and dry aspects. Wet deposition refers to acidic rain, fog, and snow, which fall to the ground and affect a variety of plants and animals. What effect the acid deposition has depends on many factors, including the acidity of the water, the chemistry and buffering capacity of the soils affected, and the types of fish and trees and other living things relying on the water. Dry deposition refers to acidic gases and particles that are blown about by the wind onto buildings, cars, homes, and trees. These gases and particles can also be washed from trees and other surfaces by rainstorms, adding to the acidity of runoff water.

Acid rain is traceable to the burning of fossil fuels in power plants, factories, and smelting operations and, to a somewhat lesser extent, the burning of gasoline in automobiles. It is believed to be largely a man-made problem, as the burning of fossil fuels releases sulfur dioxide, nitrogen oxides, and traces of toxic metals such as mercury and cadmium into the atmosphere to mix with water vapor. Acid rain then results from chemical reactions that follow to produce dilute solutions of nitric and sulfuric acids. Sunlight increases the rate of most of these reactions. These solutions then come down to the ground level in the form of either rain, hail, snow, or fog or as dry particles. They may travel hundreds of miles before falling to the ground and do not respect political or national boundaries.

These acid depositions are formally defined as having a pH level under 5.6, compared with a neutral solution that has a pH level of 7. Most of the acid rain falling in the United States has a pH of about 4.3. The pH levels of acid rain and the chemicals that cause it are monitored by two networks supported by the Environmental Protection Agency (EPA). The National Atmospheric Deposition Program measures wet deposition and discloses through its Web site maps of rainfall pH and other important precipitation chemistry measurements. The Clean Air Status and Trends Network measures dry deposition and discloses information about the data it collects, the measuring sites, and the kinds of equipment used.

Acid rain is believed to cause many serious environmental problems. When it enters a body of water, acid rain carries a deadly burden of toxic metals that can stunt or kill aquatic life. Many lakes that had previously supported fish life became fishless due to their high acid content. As the buffering effect of the acidneutralizing minerals in the water diminishes, lakes appear to die suddenly and turn clear and bluish. Surface waters that have a low acid-buffering capacity are unable to neutralize the acid effectively and are quickly affected.

Snowmelt in Northern areas can quickly kill a lake as all the acids accumulated in the snow are released at once. Because of the freezing point depression phenomenon combined with the recrystallization of snow after it falls, the most acidic snow crystals will melt first, thereby releasing 50% to 80% of the acids in the first 30% of snowmelt.

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