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Acid Rain

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.

Nature and Source of the Problem: Acidic deposition or "acid rain" occurs when emissions of SO2 and NOx in the atmosphere react with water, oxygen, and oxidants to form acidic compounds. These compounds fall to the Earth in either dry form (gas and particles) or wet form Acid Rain Formation (rain, snow, and fog). Some are carried by the wind, sometimes hundreds of miles, across State and national borders. In the U.S., about 70 percent of annual SO2 emissions and 30 percent of NOx emissions are produced by electric utility plants that burn fossil fuels.

Health and Environmental Effects: Before falling to Earth, SO2 and NOx gases and related PM (sulfates and nitrates) contribute to poor visibility and impact public health. Major human health concerns associated with their exposure include effects on breathing and the respiratory system, damage to lung tissue, and premature death. In the environment, acid rain raises the acid levels of lakes and streams (making the water unsuitable for some fish and other wildlife) and damages trees at high elevations. It also speeds the decay of buildings, statues, and sculptures, including those that are part of our national heritage.

Program Structure and Benefits: The overall goal of EPA's Acid Rain Program is to improve public health and the environment by reducing emissions of SO2 and NOx. In order to accomplish its goals cost-effectively, the program employs both innovative and traditional approaches for controlling air pollution and encourages energy efficiency and pollution prevention.

To achieve its goal of reducing annual SO2 emissions by 10 million tons between 1980 and 2010, the 1990 Clean Air Act Amendments require a two-phase tightening of the restrictions on fossil fuel-fired power plants. Phase I began in 1996 and affected 431 electric utility units (boilers), including 263 units specifically listed in the Act that are the biggest and dirties in the country. Phase II, which begins in the year 2000, will tighten the annual SO2 emission limits for these plants and also set restrictions on smaller, cleaner plants fired by coal, oil, and gas.

Phase I emission limitations for NOx began in 1996, with emission limitations on 239 coal-fired utility units. Some units may qualify for additional compliance options that add flexibility to the traditional rate-based emission standards. Most of the emission reductions will be achieved by requiring coal-fired utility boilers to install low-NOx burner technologies and meet new emissions standards.

Reductions in SO2 and NOx will decrease levels of sulfates, nitrates, and ground-level ozone (smog), leading to improvements in public health and other benefits such as better water quality in lakes and streams. Visibility will improve, enhancing the beauty of our country's scenic vistas, including those in national parks. Likewise, damage to the trees that populate mountain ridges from Maine to Georgia will be reduced, and deterioration of our historic buildings and monuments will be slowed.

So2 Emissions Chart

Emissions and Atmospheric Trends: Sulfur dioxide emission reductions have been significant in the first two years of compliance with EPA's Acid Rain Program. As shown below, the 263 core Phase I utility units continued to emit well below the allowable emission levels required by the Clean Air Act. In total, the 431 Phase I units emitted 5.4 million tons. These emissions were 35 percent below the 1996 allowable emissions level of 8.3 million tons, and about 50 percent below 1980 levels.

According to a study released by the U.S. Geological Survey, reductions in SO2 emissions resulted in less acidic rain in 1996. In the study, 1996 precipitation data were compared with reference data from 1983 through 1994. As shown in the map below, the study reports a 10 to 25 percent drop in wet deposition sulfate concentration and rainfall acidity, particularly at some sites located in the Midwest, Northeast, and Mid-Atlantic Regions. These areas are some of the most acid-sensitive regions of the country. Some areas on the map show increases in sulfate levels. These are attributed to drought conditions that occurred during 1996. Had rainfall conditions been more normal in 1996, even greater reductions in sulfate levels might have occurred. Preliminary data for 1996 show similar results.

Percent Departures of 1995 Annual Sulfate Ion Concentrations from Predictions of the 1983-94 Seasonalized Trend Model



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