Acid rain is rain that is more acidic than normal. Acid rain is a complicated problem. Caused by air pollution, acid rain^s spread and damage involves weather, chemistry, soil, and the life cycles of plants and animals on the land and from acid rain in the water. Air Pollution Creates Acid Rain 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. Power plants use that 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 rise into the atmosphere and combine with the moisture in the air to form acid rain. 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 fall to the earth as acid rain. 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. Maps of precipitation pH in the US (called isopleth maps) can be found on the Web site for the National Atmospheric Deposition Program . 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. Natural Acids There are also natural sources of acids such as volcanoes, natural geysers and hot springs. Nature has developed ways of recycling these acids by absorbing and breaking them down. These natural acids contribute to only a small portion of the acidic rainfall in the world today. In small amounts, these acids actually help dissolve nutrients and minerals from the soil so that trees and other plants can use them for food. The large amounts of acids produced by human activities overload this natural acidity. What Can Be Done? To solve the acid rain problem, people need to understand how acid rain causes damage to the environment and what can be done to help stop acid rain. More information on the problem will help leaders make better decisions about how to control air pollution -- the cause of acid rain. Scientific Research Experts from the United States Environmental Protection Agency (EPA) have taken samples of pollution and acidity from thousands of streams and lakes in the United States. From these samples, they determine the number of streams and lakes which are now acidic and which are in danger of becoming acidic. EPA and other scientists are also studying the effects of acid rain on fish, plants, humans, and materials such as marble, brick, cement, and metal. Until we reduce air pollution, acid rain will continue to be a problem. Activities to resolve this problem include cleaning up the smokestacks and exhaust pipes that pour pollutants into the air, finding alternative sources of energy, repairing the damage already done by acid rain, and conserving our resources. Cleaning up Smokestacks and Exhaust Pipes Right now, burning fossil fuels is one of the most inexpensive ways to produce electricity for the daily activities of modern life and to power cars, buses, and airplanes. In the United States, sulfur in coal makes up the greatest part of the sulfur dioxide that becomes acid rain. When coal is burned to make electricity or heat, the sulfur goes up the smokestacks and into the atmosphere to become air pollution. There are several ways to reduce the amount of sulfur entering the air. One way is to wash the sulfur out of the coal before it is burned. Another is to wash the sulfur out of the smoke before it goes up to the smokestacks. Scrubbers remove sulfur from the smoke by spraying a mixture of water and powdered limestone into the smokestack. The mixture traps the sulfur before it can escape into the air above. Scientists and engineers are also discovering new ways to burn fossil fuels that produce much lower amounts of pollution. Nitrogen oxides from burning coal and from vehicles also contribute to acid rain. Vehicles give off nitrogen oxides and other pollutants in their exhaust fumes. Devices such as catalytic converters reduce the pollution from those exhaust fumes. All new cars sold in the United States are required to have catalytic converters. Alternative Ways of Producing Energy There are other sources of energy besides fossil fuels. These include hydroelectric power and nuclear power. Dams use the power of water to turn turbines and make electricity. People have been using this form of energy for most of this century. Nuclear power plants make electricity from the energy released by splitting atoms. A small amount of nuclear fuel can [Image]make a very large amount of electricity. There are problems with using hydroelectric and nuclear power. Hydroelectric plants require a constant source of water. Because rainfall is not always predictable, hydroelectric plants are not as reliable as those using coal or oil. Hydroelectric plants can also harm the environment. Thousands of acres of land often have to be flooded to create a reservoir, a holding place for the great amounts of water needed to power these plants. Sometimes the land that would be flooded is home to rare types of plants or animals. Nuclear power plants produce electricity cheaply. But the nuclear waste they leave remains dangerous for thousands of years. Scientists are looking at other energy sources, such as windmills and solar energy, using the power of the sun itself. In several states, there are modern windmills like airplane propellors that make energy from the wind. In other places, wind power pumps water from the ground. In Arizona and New Mexico, solar energy is at work making electric power. Each of these sources has drawbacks as well. Windmills and solar panels are reliable only where it is windy or sunny most of the time. All sources of energy have benefits and limitations, including the cost of producing the energy. All of these factors must be weighed when deciding which energy source to use. Restoring a Damaged Environment It can take years for an acidic lake or stream to recover naturally, even if the acid rain stops. People have brought some lakes and streams back to neutral or basic conditions more quickly than nature could alone. They have added powdered limestone (a natural base) to the water in a process called liming. The people of Norway and Sweden have successfully restored hundreds of lakes and streams with liming. Few lakes and streams have been limed in the United States. Liming is expensive and the effects are only temporary. As long as acid rain continues to fall, limestone must be reapplied or the water will become acidic again. Liming may be the only way to make sure that life in acid lakes or streams survives until the amount of acid rain falling on the surrounding land can be reduced. Conserving Resources It may seem like there is not much that individuals can do to stop acid rain. However, environmental problems -- including acid rain -- are caused by the combined actions of individual people. Individuals can take part in solving these problems as well. One of the first steps is to assume responsibility for the problem by finding out what can be done. Each person who turns off the lights when no one is using them and uses energy-saving appliances reduces the amount of electricity a power plant has to produce. When less power needs to be produced, pollution from power plants decreases. Car-pooling, using public transportation, and walking reduce the pollutants that come from vehicles. The sum total of all these individual actions can be very great indeed. The more informed people are about acid rain and other environmental problems, the more they can do to make the earth a cleaner, healthier place.