an air also contains naturally occurring gases such as carbon dioxide. The interaction between the water droplets and the carbon dioxide in the atmosphere, and also from chlorine, which comes from the salt in the sea, gives rain an average pH of about 5.6. This can make even clean rain slightly acidic. Other natural sources of acids and bases in the atmosphere may lower or raise the pH in clean rainwater. But when rain contains pollutants, especially sulfur dioxide and nitrogen oxides, the rainwater can become very acidic.
             Acid rain does not account for all of the acidity that falls back to earth from pollutants. About half the acidity in the atmosphere comes back to the earth as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes and trees. Sometimes 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 acids get into the runoff and adds to the acids already present, making the combination more acidic than the falling rain alone. This mixing process is called acid deposition.
             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 pollut...