Acid Rain Program Benefiting Environment, Human Health

The National Acid Precipitation Assessment Program (NAPAP) is a Federal interagency program that coordinates acid rain research and reports to Congress on the effects of acid rain on sensitive ecosystems and progress towards minimizing those impacts. The new report concludes that the Acid Rain Program—created by bipartisan mandate under Title IV of the 1990 Clean Air Act Amendments—has not only successfully reduced emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from power plants, but also far exceeded its emissions-reduction goals.  A creative, market-based cap-and-trade approach has been critical to the success of the SO2 emissions-reductions efforts. The approach, which combines the best of American science, government action, and innovation, significantly reduced compliance costs by giving utilities flexibility in how they achieved SO2 emissions reductions.

Acid rain is formed when air pollutants like SO2 and NOx react with water and other compounds in the atmosphere. A large percentage of SO2 and NOx emissions come from man-made sources such as electric power generation.  Decades of science has shown that acid rain can damage lakes, streams, and forests; degrade air quality; impair visibility; and negatively affect human health. Since its formation in 1995, the Acid Rain Program has worked to protect Americans from these harmful effects by gradually reducing SO2 and NOX emissions from power plants. 

In this latest report, numbers tell the story of the Acid Rain Program’s success. U.S. SO2 emissions, for example, decreased by 64% between 1990 and 2009 to a level substantially below the 8.95 million ton statutory limit set by Congress.  NOx emissions in the United States decreased by 67% between 1995 and 2009—exceeding the goal set by Congress by more than three-fold.

The estimated cost of complying with Title IV has amounted to a fraction of initial estimates, and the benefits have been enormous, including benefits to human health and the environment. The report concludes that the innovative cap-and-trade system used to reduce SO2­ under Title IV was key to achieving these benefits economically. 

While the NAPAP report presents evidence of significant improvement, it also points to recent science that suggests that many sensitive ecosystems will be unable to fully recover from the effects of acid rain.  Further emissions reductions from additional sources will be necessary to protect these sensitive ecosystems. The report notes that additional reductions projected to occur under the Cross-State Air Pollution Rule, finalized by the U.S. Environmental Protection Agency ,would reduce the number of lakes being harmed by acid rain by more than half relative to current levels.

Environmental Problems Related to Electric Power Plant Emissions

Electric power plant emissions are factors in three major environmental issues: acid rain, urban air quality, and global climate change. These issues are discussed below.

Acid rain refers to rain, fog, mist, or snow that is more acidic than normal. The acidity of precipitation is stated in terms of its pH level, which describes the concentration of hydrogen ions along a scale (from 0 to 14) that defines the continuum from acid to base. The pH scale is logarithmic; pH levels of 4.0 and 3.0, for example, are 10 and 100 times more acidic, respectively, than a pH level of 5.0. Although a pH level of 7.0 is neutral, unpolluted rainfall is normally slightly acidic (pH=5.6). Acid rain is defined as any precipitation with a pH of 5.5 or less.

Chemical analysis of data collected by means of cloud sampling and experimentation reveals the presence of sulphuric acid and nitric acid in precipitation in the United States .Sulphur dioxide and nitrogen oxides in the air, partly the result of emissions from electric power plants, gradually react with water vapor and become acids. Precipitation becomes acidic by mixing with these acids. The acidity of the precipitation depends upon the amount of acid in the atmosphere and the amount of water in which it is dissolved. Undissolved acids may also fall to Earth by themselves or in combination with dust particles.

The most severely acidic conditions are found in the eastern United States. EPA believes that acid rain has been the primary cause of the acidification of hundreds of streams in the mid-Atlantic highlands and the New Jersey Pine Barrens and of many lakes in the Adirondack Mountains of New York.The National Acid Precipitation Assessment Program (NAPAP) identified acid rain as one of several possible causes of increased nitrate leaching and acidification of surface waters in several northeastern watersheds. Episodes of acidification are believed to harm populations of fish and invertebrates in small streams and lakes.

Field studies have implicated acid rain in observed damage to high-elevation red spruce forests in the northeastern United States. Nutrient leaching and changes in soil chemistry due to acid deposition have also been detected in forests south of the Great Lakes. In general, NAPAP concluded that acid deposition, among other stressors, threatens the long-term structure, function, and productivity of many sensitive ecosystems.

Some research suggests that emissions of sulphates and other pollutants from the combustion of fossil fuels may be linked to abnormally high mortality rates in humans.Clinical studies have shown lung irritation and impaired lung cleansing in human subjects exposed to acidic aerosols.

Urban ozone - Electric power plants contribute heavily to NOx emissions, which are precursor chemicals that (along with VOCs) react in the atmosphere in the presence of sunlight to form ozone. Strong concentrations of ozone often occur in and downwind of large urban areas.

During cardiovascular exercise, human exposure to ozone at concentrations both above and below the 120-part-per-billion maximum allowed under the NAAQS has been shown to result in transient respiratory problems.Ozone can also seriously irritate the eyes and mucous membranes. The effects of elevated ozone levels are not known for all types of vegetation, but such levels are harmful to many types of trees and crops. High ozone concentrations seem to be more detrimental than low-level extended exposure.

The assessment of the impact of NOx controls on ozone concentrations is complex and must be studied carefully in developing ozone abatement strategies, according to a 1992 report from a National Research Council committee. The committee found that ambient measurements of VOC/NOx ratios--which, as they vary, have different effects on ozone formation--were larger than expected from an assessment of emission inventories. The committee also determined that the effectiveness of efforts to control VOC and NOx emissions depends on ambient VOC/NOx ratios. Generally, at ratios of 10 or less, VOC control is more effective and NOx control may be counterproductive. At ratios greater than 20, NOx control is generally more effective. Hence, if VOC emission inventories have been understated, past ozone control strategies may have been misdirected. Tighter controls on NOx may be more effective in controlling ozone under certain circumstances.

The committee also found that combinations of biogenic VOCs and anthropogenic NOx can significantly affect ozone formation in some urban and rural regions of the United States and concluded, again, that the appropriate strategy may be to monitor and control NOx emissions.