U.S. Department of the Interior U.S. Geological Survey
What is Acid Rain? clouds. When sulfur dioxide and nitrogen oxide gases and particles come in contact with water droplets in clouds, chemical Rain and snow are naturally slightly reactions can occur, resulting in acid rain. acidic because of chemical reactions with Additional processes called rainout and carbon dioxide in the atmosphere. The washout mix these acidic gases, liquids, term "acid rain" is used to describe rain or and particles into raindrops and snow- snow that has a pH lower than what is nat flakes and carry them to the ground. The ural for a given area. pH is a measure figure below shows an overview of how ment of how acidic or basic a material is =astern U.S. Alaska South Pole Samoa acid rain forms. (Modified from Miller, 1984) and ranges from 0 to 14. The graph below depicts the pH values for some Because of global winds and mixing is harmful or fatal to fish (Walk and common substances. Precipitation with a in the atmosphere, every country's air Godfrey, 1990). Acid rain also has been pH value less than 5 is considered acid pollution contributes to some degree to found to speed up the natural decay of rain. the Earth's problem of acid rain. In 1993, stone monuments and historical build the United States released approximately ings. Other materials such as iron, steel,
The pH Scale 90 billion pounds of sulfur dioxide and zinc, and paint also can be damaged by Normal nitrogen oxides into the air. Rain and acid rain. The human-health effects of Vinegar Rain Lye snow that falls in the Eastern United acid rain also are of concern. Although Acid I Cola 4 SeaV Battery Acid States typically has a much lower pH than people aren't directly in danger from ?JJr£r4I I I I \ I ITTT precipitation in other parts of the coun exposure to acid rain, the particles in the 01 23-4567 89 10 11 12 IS- try. A comparison of the pH readings for air that lead to acid rain may be a risk to I I I I I I I 1 III the Eastern United States with parts of the human health (Semonin and Stensland, NEUTRAL world with fewer coal-burning power- 1984). pH plants and automobiles is shown in the bar graph that follows. Acid Rain Investigations
How Does Acid Rain Form? What are the Effects of Acid The U.S. Geological Survey (USGS) Rain? has been actively studying acid rain for Enormous quantities of manmade and the past 15 years. When scientists learned natural material are added to the air every More than 30 years ago, scientists that acid rain could harm fish, damage to day. Most of the materials added to the noticed that in some lakes in remote our natural environment from acid rain atmosphere return to the ground through a wilderness areas, fish populations were became a concern of the American public. process known as deposition. Deposition mysteriously declining. Some lakes that Research by USGS scientists and other occurs when it rains and snows, but it can once teemed with fish were found to con groups showed that the processes result also occur when dust settles out of the tain none at all. In their search for what ing in acid rain are very complex. Scien atmosphere during dry periods. Gravity caused the fish to die, scientists concluded tists were puzzled by the fact that, in works to continually pull dry particles that acid rain was the problem. Research some cases, it was difficult to demonstrate ers continue to document that acid rain back to Earth. Uneven heating of the that the pollution from automobiles and Earth results in global winds in the atmo sphere. Global winds provide the energy Transport for long-range travel of the gases, liquids, and Chemical and dust in the atmosphere, which can Sources »- Transformations travel great distances before falling back to the ground. The burning of fossil fuels like coal and oil products by automobiles and powerplants releases large amounts of sulfur dioxide and nitrogen oxides into the air. While being transported by winds, some of these particles get caught up in factories was causing streams or lakes Importance of Continuing ity that might be possible without stricter to become more acidic. Further experi Acid Rain Research powerplant and auto-pollution controls ments showed how the natural ability require the best possible environmental- of many soils to neutralize acids could The NADP/NTN has been in operation monitoring data. reduce the effects of acid rain in some for more than 15 years, and some might locations at least as long as the neutral wonder if that isn't long enough to answer izing ability lasted (Young, 1991). all the questions about acid rain once and for all. The atmosphere and our world The USGS has played a key role in estab are both very dynamic and in a state of 1.5 lishing and maintaining the only nation constant change. Without a continuing wide network of acid rain-monitoring acid rain network, the effects on acid stations. This program is called the rain from natural changes in weather National Atmospheric Deposition and the impacts of man's changing activi Program/National Trends Network ties could go undetected. Without the (NADP/NTN). Each week, at approxi NADP/NTN, the effectiveness of new mately 200 NADP/NTN sites across the air-pollution-control programs and other 3amoa South Pole Alaska Eastern U.J country, rain and snow samples are col activities, such as the recent Clean Air (Modified from Miller, 1984) lected for analysis. The USGS supports Act Amendments passed by Congress in about 80 of these sites. The information 1990, would be unknown. Although References stricter controls of sulfur dioxide pollu gained from monitoring the chemistry of tion are being phased in, nitrogen oxide Semonin, R.G., and Stensland, G.J., 1984, our Nation's rain and snow is important pollution is forecast to increase in the next Acid rain trends?: Weatherwise, vol. 31, for testing the results of pollution-control 20 years. The following bar graph shows no. 5, p. 250-251. laws on acid rain. that the nitrate amounts in rain and snow Miller, J.M., 1984, Acid rain: Weather- in the Eastern United States are already wise, vol. 37, no. 5, p. 241-249. Making Sure Data are much higher than in precipitation in less Accurate populated or industrialized areas. (Nitrate Walk, M.F., and Godfrey, P.J., 1990, is a chemical that is found in rain and Effects of acid deposition on surface snow as a result of chemical reactions waters: Journal of New England The USGS also has invested much involving nitrogen oxides.) Therefore, Water Works Association, vol. 104, time and effort to make sure that the data rain could become more acidic in some no. 4, p. 248-251. collected by the NADP/NTN are accurate areas despite stricter controls on sulfur Young, T.C., 1991, Method to assess and reliable. Due in part to the quality- dioxide pollution from coal-burning lake responsiveness to future acid inputs assurance experiments done by the powerplants. using recent synoptic water column USGS, the data on acid rain collected chemistry: Water Resources Research, by the NADP/NTN are regarded by scien An important reason to continue long- vol. 27, no. 3, p. 317-326. tists as some of the most reliable data of term monitoring is that climate changes tend to occur slowly over long periods any long-term environmental measure For additional information, call the of time, and year-to-year changes in ment program. The ability to detect National Water Information weather can mask long-term increases Clearinghouse at 1-800-H2O-9000 changes in rain and snow chemistry or decreases in acid rain. Decisions to (1-800-426-9000) from NADP/NTN data collected over balance the tradeoff between less acid in the years is improved by USGS quality- our rain as a result of pollution-control John Gordon, Mark Nilles, and assurance programs. laws and the higher industrial productiv LeRoy Schroder
December 1995 Fact Sheet FS-183-95