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The Plain English Guide to the Clean Air Act

Reducing Acid Rain

You have probably heard of "acid rain." But you may not have heard of other forms of acid precipitation such as acid snow, acid fog or mist, or dry forms of acidic pollution such as acid gas and acid dust. All of these can be formed in the atmosphere and fall to Earth causing human health problems, hazy skies, environmental problems and property damage. Acid precipitation is produced when certain types of air pollutants mix with the moisture in the air to form an acid. These acids then fall to Earth as rain, snow, or fog. Even when the weather is dry, acid pollutants may fall to Earth in gases or particles.

How Acid Rain is Formed

Burning fuels release acid pollutants. These pollutants are carried far from their sources by wind. Depending on the weather, the acid pollutants fall to Earth in wet form (acid rain, snow, mist or fog) or in dry form (acid gases or dusts).

Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the principal pollutants that cause acid precipitation. SO2 and NOx emissions released to the air react with water vapor and other chemicals to form acids that fall back to Earth. Power plants burning coal and heavy oil produce over two-thirds of the annual SO2 emissions in the United States. The majority of NOx (about 50 percent) comes from cars, buses, trucks, and other forms of transportation. About 40 percent of NOx emissions are from power plants. The rest is emitted from various sources like industrial and commercial boilers.

Heavy rainstorms and melting snow can cause temporary increases in acidity in lakes and streams, primarily in the eastern United States. The temporary increases may last for days or even weeks, causing harm to fish and other aquatic life.

The air pollutants that cause acid rain can do more than damage the environment-they can damage our health. High levels of SO2 in the air aggravate various lung problems in people with asthma and can cause breathing difficulties in children and the elderly. In some instances, breathing high levels of SO2 can even damage lung tissue and cause premature death.

Acid Rain's Harmful Effects

Acid lakes and streams have been found all over the country. For instance, lakes in Acadia National Park on Maine's Mt. Desert Island have become acidic due to pollution from the midwest and the east coast. Streams in Maryland and West Virginia, as well as lakes in the Upper Peninsula of Michigan, have been damaged by acid rain. Since the wind can carry pollutants across the country, the effects of acid rain can be seen far from the original source of the acid forming pollutant.

Acid rain has damaged trees in the mountains of Vermont and other states. Red spruce trees at high altitudes appear to be especially sensitive to acid rain. The pollutants that cause acid rain can make the air hazy or foggy; this occurs in the eastern United States in areas like the Great Smokies and Shenandoah National Park, areas where vacationers go to enjoy the beautiful scenery and awe-inspiring views. In addition to damaging the natural environment, acid rain can damage manmade objects such as stone statues, buildings, and monuments.

The 1990 changes to the Clean Air Act introduced a nationwide approach to reducing acid pollution. The law is designed to reduce acid rain and improve public health by dramatically reducing emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx). Using a market-based cap and trade approach, the program sets a permanent cap on the total amount of SO2 that may be emitted by electric power plants nationwide. As of 2005, emission reductions were more than 7 million tons from power plants, or 41 percent below 1980 levels.

The initial phase of EPA's Acid Rain Program went into effect in 1995. The law required the highest emitting units at 110 power plants in 21 Midwest, Appalachian, and Northeastern states to reduce emissions of SO2. The second phase of the program went into effect in 2000, further reducing SO2 emissions from big coal-burning power plants. Some smaller plants were also included in the second phase of the program. Total SO2 releases for the nation's power plants are permanently limited to the level set by the 1990 Clean Air Act - about 50 percent of the levels emitted in 1980.

Each allowance is worth one ton of SO2 emissions released from the plant's smokestack. Plants may only release the amount of SO2 equal to the allowances they have been issued. If a plant expects to release more SO2 than it has allowances, it has to purchase more allowances or use technology and other methods to control emissions. A plant can buy allowances from another power plant that has more allowances than it needs to cover its emissions.

There is an allowances market that operates like the stock market, in which brokers or anyone who wants to take part in buying or selling allowances can participate. Allowances are traded and sold nationwide.

EPA's Acid Rain Program has provided bonus allowances to power plants for installing clean coal technology that reduces SO2 releases, using renewable energy sources (solar, wind, etc.), or encouraging energy conservation by customers so that less power needs to be produced. EPA has also awarded allowances to industrial sources voluntarily entering the Acid Rain Program.

The 1990 Clean Air Act has stiff monetary penalties for plants that release more pollutants than are covered by their allowances. All power plants covered by the Acid Rain Program have to install continuous emission monitoring systems, and instruments that keep track of how much SO2 and NOx the plant's individual units are releasing. Power plant operators keep track of this information hourly and report it electronically to EPA four times each year. EPA uses this information to make sure that the plant is not releasing quantities of pollutants exceeding the plant's allowances. A power plant's program for meeting its SO2 and NOx limits will appear on the plant's permit, which is filed with the state and EPA and is available for public review.

You can also help to reduce SO2 and NOx emissions from power plants by conserving energy and promoting conservation and renewable energy efficiency in your community. See Ways to Reduce Air Pollution for energy conservation tips.

Market Approaches and Economic Incentives

Besides the ground-breaking features in the Acid Rain Program, the 1990 Clean Air Act encouraged other innovative approaches that spur technology. These approaches allow businesses greater flexibility in how they comply with the law, and thus clean up air pollution as efficiently and inexpensively as possible. For example:

  • EPA's new cleaner vehicle standards include an averaging system that allows manufacturers to choose how to produce a mix of more- or less- polluting vehicles, as long as the overall fleet average is lower.
  • Gasoline refiners can receive credits if they produce cleaner gasoline than required, and they use those credits when their gasoline does not quite meet the clean-up requirements.

For more information on EPA's Acid Rain program, visit www.epa.gov/acidrain.


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