Acid Deposition

Schematic of acid deposition sources, transport, and receipt.

EPA Acid Rain website

What is Acid Rain and What Causes It?

Scientists discovered and have confirmed, that sulfur dioxide (SO₂) and nitrogen oxides (NO_x) are the primary causes of acidic conditions in the atmosphere. In the United States, about 2/3 of all SO₂ and 1/4 of all NO_x comes from electric power generation that relies on burning fossil fuels like coal (EPA Acid Rain website).

Acidic conditions develop when these gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. Sunlight increases the rate of most of these reactions. The result is a mild solution of sulfuric acid and nitric acid. The acidic compounds then fall to or are deposited on the earth’s surface.

"Acid rain" is a broad term used to describe several ways that acids fall out of the atmosphere. Acid rain is usually used in reference to rain, snow, sleet, and hail. A more precise term is acid deposition, which includes all forms of precipitation but also includes deposition of dry particles and gases as well as occult or cloud, fog water, and rime ice deposition. Rime ice is most frequently found in the freezer compartment of a refrigerator but under certain conditions also occurs naturally outside. Thus, acid deposition has three components: wet, dry and occult deposition. Occult deposition is very difficult and expensive to measure. However, in the few places where occult deposition has been monitored, it typically has much higher concentrations of acidic compounds than either wet or dry deposition.

Dry deposited gases and particles fall onto buildings, cars, homes, trees, and the ground. They can be washed from trees and other surfaces by rainstorms. When that happens, the dry deposition adds to the acids in the precipitation, making runoff even more acidic. Acidic water flows over and through the ground, affecting soil and water chemistry and a variety of plants and animals. The strength of the effects depend on many factors, including how acidic the water is, the chemistry and buffering capacity of the soils and surface waters involved, and the types of fish, trees, and other living things that rely on the water.

Prevailing winds blow the compounds that contribute to acid deposition across state and national borders, and sometimes over hundreds of miles.

Normally rain water is slightly acidic, with a pH of between 4.9 and 6.0, because in the atmosphere, water reacts with carbon dioxide to form a weak carbonic acid. Pure water would have a neutral pH of 7. It is important to point out that the “normal” range cited here must be evaluated in the context of site-specific circumstances and acid conditions. In certain situations, even slight changes in acidity can have serious environmental consequences.

Effects of Acid Deposition

Acid rain causes acidification of lakes, streams, and sensitive forest soils and can damage trees and other plants. The presence of sulfur on and in the soil and water releases aluminum, making it available to plants, soil microbes, fish and other organisms. Acid particles also alter the nutrient make-up of forest soils. This chemical alteration may, at first, result in fertilization which can cause changes in plant growth, species composition, and pathogen development. A pathogen is a plant or animal that is capable of producing disease. Eventually, however, the soil becomes acidified. This can result in impairment of plant growth and development because nutrients that would otherwise be available to plants are diminished. These chemical alterations are particularly acute at high elevation sites because nitrogen is limited in those soils to begin with. Nutrient deficiency can lead to susceptibility to disease.

In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation's cultural heritage.

Acid Deposition at Shenandoah

Acid deposition is a particular concern at Shenandoah National Park for several reasons. First, acid deposition levels occurring within the park are amongst the highest when compared to other parks that collect deposition information. Second, roughly 60% of the watersheds within the park include bedrock types that have a low acid buffering capacity. This allows chemical interactions between soil, bedrock, and surface waters with acid depositions to proceed without neutralization or buffering. Third, streams within the park provide important habitat to fish and other aquatic organisms that are particularly sensitive to the acidic condition of the water in which they live. Fourth, forested areas within the park are subjected to various forms of stress including drought, disease, and insect damage. In some cases, the diseases and insects are not native to the park. Acid deposition builds on these conditions causing direct and indirect damage to forest vegetation.

Park staff members are working to improve the situation by monitoring acid deposition conditions, by supporting research to better understand the effects of deposition and predict the consequences, as well as by working on reducing emissions both inside and outside of the park. To read a fact sheet on this topic click here.

Related Information

Helpful literature related to acid deposition includes:

Driscoll, C.T., G.B.Lawrence, A.J. Bulger, T.J. Butler, C.S. Cronan, C. Eagar, K.F. Lambert, G.E. Likens, J.L. Stoddard, K.C. Weathers, 2001. Acid Rain Revisited: Advances in scientific understanding since the passage of the 1970 and 1990 Clean Air Act Amendments. Hubbard Brook Research Foundation. Science Links^TM Publication. Vol. 1, no. 1.

Driscoll, C.T. 2003. Nitrogen Pollution: From the Sources to the Sea. Hubbard Brook Research Foundation. Science Links^TM Publication. Vol. 1, no. 2.

Garner, J.H.B., T. Pagano, and E.B. Cowling. 1989. An evaluation of the role of ozone, acid deposition, and other airborne pollutants in the forests of eastern North America. General Technical Report SE-59. Southeastern Forest Experiment Station, Forest Service, U.S. Department of Agriculture.

Hakkarinen, C. and M.A. Allan. 1986. Forest health and acidic deposition. Electric Power Research Institute, Palo Alto, California.

Kingston, E., V. Bowersox, and G. Zorrilla eds. 2000. Nitrogen in the Nation’s Rain. NADP Brochure 2000-01. NADP Program Office, Champaign, Illinois.

Weathers, K.C. and G.E. Likens. 1998. Acid rain In Environmental and Occupational Medicine. Lippincott-Raven Publishers, Philadelphia. Pennsylvania.

The following websites provide helpful information related to acid deposition:

The USGS: Acid Depositon & Precipitation Chemistry

Environmental Protection Agency(EPA) acid rain website

Effects of Acid Rain (EPA) website

National Oceanic & Atmospheric Administration (NOAA) AIRMoN Deposition Program

National Atmospheric Deposition Program

Listing of these websites does not and is not intended to imply endorsement by the National Park Service of commercial services or products associated with the sites.