It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen oxides which react with the water molecules in the atmosphere to produce acids. Measurable improvements in air quality and visibility, human health, and water quality in many acid-sensitive lakes and streams, have been achieved through emissions reductions from electric generating power plants and resulting decreases in acid rain. These are some of the key findings in a report to Congress by the National Acid Precipitation Assessment Program, a cooperative federal program.
"Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog, cloudwater, and dew) and dry (acidifying particles and gases) acidic components. Distilled water, once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline. Clean or unpolluted rain has a slightly acidic pH of over 5.7, because carbon dioxide and water in the air react together to form carbonic acid, but unpolluted rain also contains other chemicals.
The report shows that since the establishment of the Acid Rain Program, under Title IV of the 1990 Clean Air Act Amendments, there have been substantial reductions in sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions from power plants that use fossil fuels like coal, gas and oil, which are known to be the primary causes of acid rain. As of 2009, emissions of SO2 and NOx declined by about two-thirds relative to levels in the 1990s. These emissions levels declined even further in 2010, according to recent data compiled by the U.S. Environmental Protection Agency.
Despite these emission reductions, the report also indicates that full recovery from the effects of acid rain is not likely for many sensitive forests and aquatic ecosystems. For example, in the Adirondack Mountains of New York, an especially sensitive region, 30 percent of the lakes were receiving acid rain during 2006-08 in excess of the level needed to prevent harm.
Based on models which analyze various emission scenarios, the report concludes that beyond current SO2 and NOx emission levels, future emission reductions would likely promote additional and more widespread recovery as well as to prevent further acidification in some U.S. regions.
"The principal message of this report is that the Acid Rain Program has worked. The emissions that form acid rain have declined and some U.S. areas are beginning to recover," said Doug Burns, lead author and director of the NAPAP and also a U.S. Geological Survey hydrologist. "However, some sensitive ecosystems are still receiving levels of acid rain that exceed what is needed for full and widespread recovery. We have every reason to believe that recovery will continue with further decreases in emissions which is why further emission reductions would be beneficial."
Acid rain occurs when emissions of SO2 and NOx react in the atmosphere with water, oxygen, and oxidants to form acidic compounds. These emissions may be transported hundreds of miles away from their emitting sources, and have the potential to impact large areas and populations.
Together these acidic compounds can damage human health, and in addition to degrading air quality and visibility, can cause further environmental damage, including acidification of lakes and streams, harm to sensitive forests and coastal ecosystems, and accelerate the decay of building materials. Adverse ecological impacts from acid rain include reductions in biodiversity, an increased risk of damaging forest fires, and increased susceptibility of trees to pests, disease, and winter temperatures.
Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain.
Other plants can also be damaged by acid rain, but the effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands.
The report also highlights the need for better information including the costs and benefits to ecosystems from emission reductions, consideration of the role of climate change, and the interactions of multiple pollutants.
This report, "National Acid Precipitation Assessment Program Report to Congress 2011: An Integrated Assessment," is available online. (http://ny.water.usgs.gov/projects/NAPAP/)
January 25, 2012.