The accumulation of acids or acidic compounds on the surface of the Earth, in lakes or streams, or on objects or vegetation near the Earth's surface, as a result of their separation from the atmosphere. Acid deposition can harm the environment in a variety of ways, as by causing the acidification of lakes and streams, the leaching of minerals and other nutrients from soil, and the inhibition of nitrogen fixation and photosynthesis in plants.♦ The accumulation of acids that fall to the Earth dissolved in water is known as wet deposition. Wet deposition includes all forms of acid precipitation such as acid rain, snow, and fog.♦ The accumulation of acidic particles that settle out of the atmosphere or of acidic gases that are absorbed by plant tissues or other surfaces is known as dry deposition.
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A Closer Look
Acid deposition-usually referred to simply as acid rain-actually includes two forms of pollution, wet and dry. When fossil fuels such as coal, gasoline, and oil are burned, they release the gases sulfur dioxide and nitrogen oxide. In the wet type of acid deposition, these compounds combine with water vapor in the atmosphere to form highly corrosive sulfuric and nitric acids. Prevailing winds carry the acids away from the industrial areas where they originate, and they fall to earth as rain, snow, or fog. In the dry type, the prevailing winds deposit acidic gases and particulate matter on objects in the open such as buildings, vehicles, and trees. When rain washes away this acidic matter, the runoff is even more acidic than the rainwater. Acid deposition is a serious environmental problem in parts of the world with a high density of factories, power plants, and automobiles, including much of the United States and Canada, as well as areas of Europe and Asia. It harms forests and soils and pollutes lakes and rivers, killing fish and other aquatic life. It also affects human health, contributing to respiratory diseases such as asthma and emphysema. Many scientists believe that some environmental damage caused by acid deposition could take years, even decades or centuries, to repair. Acid deposition can also damage historic buildings and monuments by corroding the stone and metal of which they are constructed. Individual and societal efforts to reduce acid deposition involve many interrelated social, economic, and political factors.
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