National Air Quality:
National Air Quality:
Status and Trends

  Published Annually by the U.S. EPA Office of Air & Radiation
December 1998

Acid Rain

Nature and Source of the Problem: Acidic deposition or "acid rain" occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (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 (rain, snow, and fog). Some are carried by the wind, sometimes hundreds of miles, across state and national borders. In the United States, about 64 percent of annual SO2 emissions and 26 percent of NOx emissions are produced by electric utility plants that burn fossil fuels.

Health and Environmental Effects: Before falling to the Earth, SO2 and NOx gases and related particulate matter (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 in soils and water bodies (making the water unsuitable for some fish and other wildlife), and damages trees at some high elevations. It also speeds the decay of buildings, statues, and sculptures that are part of our national heritage.

Program Structure: The goal of EPA's Acid Rain Program, established by the Clean Air Act, is to improve public health and the environment by reducing emissions of SO2 and NOx. The program is being implemented in two phases: Phase I began in 1995 for SO2 and targets the largest and highest-emitting power plants (boilers). Phase I for NOx began in 1996 and targets coal-fired power plants. Phase II for both pollutants begins in 2000 and will set restrictions on smaller coal-, gas-, and oil-fired plants.

The Acid Rain Program will reduce annual SO2 emissions by 10 million tons between 1980 and 2010. The program sets a permanent cap on the total amount of SO2 that may be emitted by power plants nationwide at about half of the amount emitted in 1980. An emissions trading program is in effect to achieve the required emission reduction more cost effectively. This approach gives utilities the flexibility and incentive to reduce emissions at the lowest cost, while ensuring that the overall emission limit is met.

The NOx component of the Acid Rain Program establishes an emission rate limit for all affected utilities, resulting in a 2 million ton reduction compared to 1980 levels. Under this program, the industry can choose to over-control at units where it is technically easier to control emissions, average these emissions with those at their other units, and thereby achieve overall emissions reductions at lower cost.

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.

Emissions and Atmospheric Trends: SO2 emissions reductions have been significant in the first 3 years of compliance with EPA's Acid Rain Program. As shown in the graph below, the 263 core Phase I utility units continued to emit well below the allowable emission levels required by the Clean Air Act. Additional units elected to participate early, bringing the total number of Phase I units to 423 in 1997. Total Phase I units emitted 5.5 million tons, which was still well below the 1997 allowable emissions level for SO2.

As shown in the chart below, actual NOx emissions decreased by approximately 400,000 tons (32 percent) compared to 1990 levels. NOx emissions in 1997 increased slightly from 1996, attributable to greater electrical production.

SO2 Emissions from
263 Highest-Emitting Phase I Unit

In 1997, emissions at the Phase I Units were 1.2 million tons below their allowed level.

NOx Emissions from 263 Phase I Units

1997 emissions from NOx Phase I Units decreased 32 percent from 1990 levels.

In 1995 and 1996, concentrations of sulfates in precipitation over a large area of the eastern United States exhibited a dramatic and unprecedented reduction. Sulfates have been estimated to be 10 to 25 percent lower than they would have been if the trend from 1983 through 1994 had continued (see figure below). These reductions in acid precipitation are directly related to the large regional decreases in SO2 emissions resulting from Phase I of the Acid Rain Program. The largest reductions in sulfate concentrations occurred along the Ohio River Valley and in states immediately downwind. Reductions in the East in hydrogen ion concentrations, the primary indicator of precipitation acidity, were similar to those of sulfate concentrations, both in magnitude and location. Nitrate concentrations were not appreciably different in 1995 to 1996 from historical levels.

Percent Change in Sulfate Levels Occurring in 1996 Precipitation
(compared to 1983-1994 predicted levels)

The level of sulfates in rain is an indicator of acidity. A 10 to 25 percent decrease in sulfate levels in rainfall was observed in 1996, particularly in some of the most acid-sensitive regions of the United States.

Earth Day
Mike M.