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What is Acid Rain?

Acid rain (pH less than 5.0) is rain that is more acidic than normal (pH greater than
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or equal to 5.0). Rising to prominence in the 1960s when Scandinavian lakes became too acidic resulting in fish deaths, acid rain was traced to pollutant emissions from western and central Europe. Today, acid rain is a ubiquitous dilemma that is a serious issue in portions of North America and eastern Canada.

Acid rain is a complicated problem, but scientists have discovered that air pollution from the burning of fossil fuels is the major cause of acid rain. Power plants and factories burn coal and oil to produce the electricity we need to heat and light our homes and to run our electric appliances. We also burn natural gas, coal, and oil to heat our homes. Cars, trucks, and airplanes use gasoline, another fossil fuel.

The smoke and fumes from burning fossil fuels undergo complex chemical reactions and become distributed in the environment. The main chemicals in air pollution that create acid rain are sulfur dioxide and nitrogen oxides. Acid rain usually forms high in the clouds where sulfur dioxide and nitrogen oxides react with water, oxygen, and oxidants. This forms a mild solution of sulfuric acid and nitric acid. Sunlight increases the rate of most of these reactions. Rainwater, snow, fog, and other forms of precipitation containing those mild solutions of sulfuric and nitric acids can all result from this chemical reaction.

Acid Precipitation
Water moves through every living plant and animal, streams, lakes, and oceans in the hydrologic cycle. In that cycle, water evaporates from the land and sea into the atmosphere. Water in the atmosphere then condenses to form clouds. Clouds release the water back to the earth as rain, snow, or fog. When water droplets form and fall to the earth they pick up particles and chemicals that float in the air. Even clean, unpolluted air has some particles such as dust or pollen. Clean air also contains naturally occurring gases such as carbon dioxide. The interaction between the water droplets and the carbon dioxide in the atmosphere gives rain a pH of 5.6, making even clean rain slightly acidic. Other natural sources of acids and bases in the atmosphere may lower or raise the pH of unpolluted rain. However, when rain contains pollutants, especially sulfur dioxide and nitrogen oxides, the rain water can become very acidic.

Isopleth Maps
Rainfall is collected all across the world to measure pH and determine general effects of acid rain, as well as other factors. Maps of precipitation pH are called isopleth maps. The National Atmospheric Deposition Program, a cooperative research program of federal, state and private organizations, collects data on the chemistry of precipitation for monitoring of geographical and temporal long-term trends. Below are isopleth maps from 1998 displaying pH, SO4 concentrations and NO3 concentrations around the United States.

 
 
Isopleth maps courtesy of the National Atmospheric Deposition Program (NRSP-3)/National Trends Network. (2000). NADP Program Office, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820.

Dry Deposition
Acid rain does not account for all of the acidity that falls back to earth from pollutants. About half the acidity in the atmosphere falls back to the earth through dry deposition as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes and trees. In some instances, these gases and particles can eat away the things on which they settle. Dry deposited gases and particles are sometimes washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone. The combination of acid rain plus dry deposited acid is called acid deposition.

Acid Rain is a Problem That Can Travel
The chemical reactions that change air pollution to acid rain can take from several hours to several days. Years ago, when smokestacks were only a few stories high, pollution from smokestacks usually stayed near the ground and settled on land nearby. This caused unhealthy conditions for plants and animals near the smokestacks. To reduce this pollution, the government passed a law permitting the construction of very tall smokestacks. At that time, people thought that if the pollution were sent high into the air it would no longer be a problem. Scientists now know that this is incorrect. Sending pollution high into the sky increases the time that the pollution stays in the air. The longer the pollution is in the air, the greater are the chances that the pollutants will form acid rain. In addition, the wind can carry these pollutants for hundreds of miles before they become joined with water droplets to form acid rain. For that reason, acid rain can also be a problem in areas far from the polluting smokestacks. Dry deposition is usually more abundant near the cities and industrial areas where the pollutants are released.

Legislation
The 1990 Clean Air Act Amendments, signed into law as P.L. 101-549 on November 15, 1990, mandated substantial reductions in pollutant discharge of both sulfur and nitrogen acid-forming compounds. These amendments will effect business production, but will have many benefits to our environment.


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