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# USGS Tracks Acid Rain

### *By John Gordon, Mark Nilles and LeRoy Schroder*     [Fact Sheet FS-183-95]

### WHAT IS ACID RAIN?

Rain and snow are naturally slightly acidic due to chemical reactions with carbondioxide in the atmosphere. The term "acid rain" is used to describe rain or snowthat has a pH lower than what is natural for a given area. pH is a measurement of howacidic or basic a material is and ranges from 0 to 14. Precipitation with a pH value lessthan 5 is considered acid rain. 

Graph depicting pH values forcommon substances.

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### HOW DOES ACID RAIN FORM?

Enormous quantities of manmade andnatural material are added to the air every day. Most of the materials added to theatmosphere return to the ground through a process known as deposition. Deposition occurswhen it rains and snows, but it can also occur when dust settles out of the atmosphereduring dry periods. The earth's gravity works to continually pull dry particles back toearth. Uneven heating of the earth results in global winds in the atmosphere. Global windsprovide the energy for long-range travel of the gases, liquids, and dust in theatmosphere, which can travel great distances before falling back to the ground. Theburning of fossil fuels like coal and oil products by automobiles and power plantsreleases large amounts of sulfur dioxide and nitrogen oxides into the air. While beingtransported by winds, some of these particles get caught up in clouds. When sulfur dioxideand nitrogen oxides gases and particles come in contact with water droplets in clouds,chemical reactions can occur, resulting in acid rain. Additional processes called rainoutand washout mix these acidic gases, liquids, and particles into rain drops and snowflakesand carry them to the ground. 

Overview of how acid rain forms.

Because of global winds and mixing in the atmosphere, every country's air pollutioncontributes to some degree to the earth's problem of acid rain. In 1993, the United Statesreleased approximately 90 billion pounds of sulfur dioxide and nitrogen oxides into theair. Rain and snow that falls in the Eastern U.S. typically has a much lower pH thanprecipitation in other parts of the country. The pH of rain and snow in the Eastern U.S.is much lower than in less populated areas. 

Graph comparing pH readings for theEastern U.S. with parts of the world with fewer coal burning powerplants and automobiles.

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### WHAT ARE THE EFFECTS OF ACID RAIN?

More than thirty years ago, scientists noticed that in certain lakes in remote   wilderness areas, fish populations were mysteriously declining. Some lakes that   once teemed with fish were found to contain none at all. In their search for   what caused the fish to die, scientists concluded that acid rain was the problem.   Researchers continue to document that acid rain is harmful or fatal to fish   (Walk and Godfrey, 1990). Acid rain also has been found   to speed up the natural decay of stone monuments and historical buildings(Edmonds, et. al.,   1997). Other materials such as iron, steel, zinc and paint also can be damaged   by acid rain. The human health effects of acid rain are also of concern. Although   people aren't directly in danger from exposure to acid rain, the particles in   air that lead to acid rain may be a risk to human health. 

### INVESTIGATIONS INTO ACID RAIN

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The U.S. Geological Survey (USGS) has beenactively studying acid rain for the past 15 years. When scientists learned that acid raincould harm fish, fear of damage to our natural environment from acid rain concerned theAmerican public. Research by USGS scientists and other groups began to show that theprocesses resulting in acid rain are very complex. Scientists were puzzled by the factthat in some cases it was difficult to demonstrate that the pollution from automobiles andfactories was causing streams or lakes to become more acidic. Further experiments showedhow the natural ability of many soils to neutralize acids would reduce the effects of acidrain in some locations--at least as long as the neutralizing ability lasted (Young, 1991).The USGS has played a key role in establishing and maintaining the only nationwide networkof acid rain monitoring stations. This program is called the National Atmospheric Deposition Program/NationalTrends Network (NADP/NTN). Each week, at approximately 220 NADP/NTN sites across thecountry, rain and snow samples are collected for analysis. NADP/NTN site in Montana.

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The USGS supports about 72 of these sites. The information gained from monitoring thechemistry of our nation's rain and snow is important for testing the results of pollutioncontrol laws on acid rain. 

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### MAKING SURE THE DATA ARE ACCURATE

The USGS has also invested much time and effort to make sure that the data collected bythe NADP/NTN are accurate and reliable. Due in part to the quality-assurance experiments done by the USGS, the dataon acid rain collected by the NADP/NTN are regarded by scientists as some of the mostreliable data of any long-term environmental measurement program. The ability to detectchanges in rain and snow chemistry from NADP/NTN data collected over the years is improvedby the USGS quality assurance programs. 

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### THE IMPORTANCE OF CONTINUING ACID RAIN RESEARCH

The NADP/NTN has been in operation for more than 15 years and some might wonder if thatisn't long enough to answer all the questions about acid rain once and for all. Theatmosphere and our world are both very dynamic and in a state of constant change. Withouta continuing acid rain network, the effects on acid rain from natural changes in weatherand the impacts of man's changing activities could go undetected. Without the NADP/NTN,the effectiveness of new air pollution control programs and other activities, such as therecent Clean Air Act Amendments that became law in December 1990, would be unknown.Although stricter controls of sulfur dioxide pollution are being phased in, nitrogen oxidepollution is forecast to increase in the next 20 years. The chart below shows that thenitrate amounts in rain and snow in the Eastern United States is already much higher thanin precipitation in less populated or industrialized areas. (Nitrate is a chemical that isfound in rain and snow as a result of chemical reactions involving nitrogen oxides).Therefore, rain could become more acidic in some areas despite stricter controls on sulfurdioxide pollution from coal-burning power plants. 

Graph comparing nitrate levels in theEastern U.S. with nitrate levels in less populated areas.

An important reason to continue long-term monitoring is that climate changes tend tooccur slowly over long periods of time, and year to year changes in weather can masklong-term increases or decreases in acid rain. Decisions to balance the trade off betweenless acid in our rain as a result of pollution control laws and the higher industrialproductivity that might be possible without stricter power plant and auto pollutioncontrols require the best possible environmental monitoring data. 

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### REFERENCES

Walk, M.F., and Godfrey, P.J., 1990, Effects of Acid Deposition on Surface Waters:Journal of New England Water Works Association, Volume 104, Number 4, p. 248-251. 

Young, T.C., 1991, Method to Assess Lake Responsiveness to Future Acid Inputs UsingRecent Synoptic Water Column Chemistry: Water Resources Research, Volume 27, Number 3., p.317-326. 

Please e-mail Greg Wetherbee: wetherbe@usgs.gov or Mark Nilles:manilles@usgs.gov 

Do you want to learn more about   acid rain? Go to the main USGS acid rain main 

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