Acid Rain What Is Acid Rain? 'Acid rain' is a broad term used to describe several ways that acids fall out of the atmosphere. A more precise term is acid deposition, which has two parts: wet and dry. Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depends on many factors, including how acidic the water is, the chemistry and buffering (隔离) capacity of the soils involved, and the types offish, trees, and other living things that rely on the water. Dry deposition refers to acidic gases and particles. About half of the acidity in the atmosphere falls back to earth through dry deposition. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. Dry-deposited gases and particles can also be washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acres to the acid rain, making the combination more acidic than the falling rain alone. How Do We Measure Acid Rain? Acid rain is measured using a scale called 'pH'. The lower a substance's pH, the more acidic it is. Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the U.S. has a pH of about 4.3. Effects of Acid Rain Acid rain causes acidification (酸化) of lakes and streams and contributes to damage of trees at high elevations (for example, red spruce trees above 2,000 feet) and many sensitive forest soils. In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation's cultural heritage. Prior to failing to the Earth, SO2 and NOx gases and their particulate matter derivatives, sulfates and nitrates contribute to visibility degradation and harm public health. What Society Can Do About Deposition? There are several ways to reduce acid deposition, ranging from societal changes to individual action. •Understand Acid Deposition's Causes and Effects To understand acid deposition's causes and effects and track changes in the environment, scientists from EPA, state governments, and academies study acidification processes. They collect air and water samples and measure them for various characteristics like pH and chemical composition, and they research the effects of acid deposition on human-made materials such as marble and bronze. Finally, scientists work to understand the effects of sulfur dioxide (SO2) and nitrogen oxides (NOx)—the pollutants that cause acid deposition and fine particles—on human health. To solve the acid rain problem, people need to understand how acid rain causes damage to the environment. They also need to understand what changes could be made to the air pollution sources that cause the problem. The answers to these questions help leaders make better decisions about how to control air pollution and therefore how to reduce—or even eliminate—acid rain. Since there are many solutions to the acid rain problem, leaders have a choice of which options or combinations of options arc best. The next section describes some of the steps that can be taken to reduce, or even eliminate, the acid deposition problem. •Clean Up Smokestacks (烟囱) and Exhaust Pipes Almost all of the electricity that powers modem life comes from burning fossil fuels like coal, natural gas, and oil. Acid deposition is caused by two pollutants that are released into the atmosphere, or emitted, when these fuels are burned: sulfur dioxide (SO2) and nitrogen oxides (NOx). Coal accounts for most U.S. sulfur dioxide (SO2) emissions and a large portion of NOx emissions. Su