When particles and gases fall from the air to the earth with rain, snow, clouds, and fog, atmospheric deposition forms. While this form of wet deposition (often referred to as acid rain) is more widely understood and monitored, dry deposition from the atmosphere also occurs and is the result of complex atmospheric processes such as “settling”, “impaction”, and “absorption”. Wet deposition is the major removal process for most particles and may be a factor in the elimination of gaseous contaminants as well.  Wet removal may involve in-cloud capture of gases or particles, called “rainout” or below-cloud capture (washout).  In washout, raindrops or snowflakes strike particles and carry them to the surface; gases are removed by absorption.

While carbon dioxide (CO2), chlorofluorocarbons (CFCs), and a few other chemical compounds do accumulate in the atmosphere, particulates are removed, eventually, from the atmosphere by these processes of natural deposition. 

Wet sulfate deposition research shows that the highest levels in the U.S. are emitted from the heavily industrialized Ohio River Valley. In spite of recent reductions of deposition across the eastern U.S., wet sulfur is still higher than the ecosystems of the Appalachian states can tolerate. Nitrogen deposition, like sulfur, is highest in the Midwest, lower in the western states, but on the rise - some high elevation ecosystems in the Rocky Mountains are experiencing subtle changes in aquatic and terrestrial ecosystems from atmospheric deposition.

Acid Rain

By John Gordon, Mark Nilles and LeRoy Schroder.  Excerpted from USGS Fact Sheet FS-183-95.

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

Enormous quantities of manmade and natural material are added to the air every day. Most of the materials added to the atmosphere return to the ground through a process known as deposition. Deposition occurs when it rains and snows, but it can also occur when dust settles out of the atmosphere during dry periods. The earth's gravity works to continually pull dry particles back to earth. Uneven heating of the earth results in global winds in the atmosphere. Global winds provide the energy for long-range travel of the gases, liquids, and dust in the atmosphere, which can travel great distances before falling back to the ground. The burning of fossil fuels like coal and oil products by automobiles and power plants 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 clouds. When sulfur dioxide and nitrogen oxides gases and particles come in contact with water droplets in clouds, chemical reactions can occur, resulting in acid rain. Additional processes called rainout and washout mix these acidic gases, liquids, and particles into raindrops and snowflakes and carry them to the ground.

Because of global winds and mixing in the atmosphere, every country's air pollution contributes to some degree to the earth's problem of acid rain. In 1993, the United States released approximately 90 billion pounds of sulfur dioxide and nitrogen oxides into the air. Rain and snow that falls in the Eastern U.S. typically has a much lower pH than precipitation 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.

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.

National Acid Precipitation Assessment Program Report Findings

·         We have seen reductions in SO₂ emissions since 1980 and especially in 1995, the first year of Title IV*.

·         The acidity of and sulfate concentrations in precipitation have decreased in the Midwest, Mid-Atlantic, and the Northeast United States.

·         Since 1980, lakes and streams throughout many areas of the United States have experienced decreases in sulfate concentrations. Although there is evidence of recovery from acidification in New England lakes, additional reductions in sulfur and nitrogen deposition would be required to fully recover sensitive Adirondack lakes.

·         Sulfur and nitrogen deposition have caused adverse impacts on certain highly sensitive forest ecosystems in the United States, especially high-elevation spruce-fir forests in the eastern United States. Most forest ecosystems are not currently known to be adversely impacted by acid deposition. However, if deposition levels are not reduced in areas where they are presently high, adverse effects may develop in more forests due to chronic, multiple-decade exposure.

·         Reduced SO₂ emissions are expected to reduce sulfate concentrations and, in turn, their contribution to haze.

·         Decreased emissions are expected to reduce fine-particulate sulfate and nitrate concentrations in air, possibly leading to reductions in adverse health effects.

·         Quantifiable economic benefits could be relatively large in the areas of human health and visibility and exceed the costs of reducing emissions.

·         The market-based approach to reducing emissions of SO₂ has reduced compliance costs for utilities below those of a traditional command-and-control approach.

* Clean Air Act, Title IV, SEC. 401. FINDINGS AND PURPOSES. …the purpose of this title is to reduce the adverse effects of acid deposition through reductions in annual emissions of sulfur dioxide of ten million tons from 1980 emission levels, and, in combination with other provisions of this Act, of nitrogen oxides emissions of approximately two million tons from 1980 emission levels, in the forty-eight contiguous States and the District of Columbia. It is the intent of this title to effectuate such reductions by requiring compliance by affected sources with prescribed   emission limitations by specified deadlines, which limitations may be met through alternative methods of compliance provided by an emission allocation and transfer system. It is also the purpose of this title to encourage energy conservation, use of renewable and clean alternative technologies, and pollution prevention as a long-range strategy, consistent with the provisions of this title, for reducing air pollution and other adverse impacts of energy production and use. [42 U.S.C. 7651]

Encapsulated findings from the 1996 NAPAP Biennial Report to Congress: An Integrated Assessment

Toxic Pollutants in Atmospheric Deposition

Toxic air contaminants, like the toxic metal mercury, are deposited on ecosystems primarily by coal-fired utilities where they can bioaccumulate in fish and other wildlife. Mercury is a potent neurotoxin. Thirty states have consumption advisories for certain lakes and streams, warning of mercury-contaminated fish and shellfish. High concentrations of mercury are measured in sediments and fish tissue, even in remote areas of the Arctic.

Toxic organics include persistent organic pollutants, such as pesticides, dioxins, and PCBs that “mimic” estrogens and can affect reproductive systems in wildlife and humans. Deposition of organochlorine compounds is shown to increase with altitude, and concentrations of more volatile compounds are enhanced by cold-condensation. Much of our National Forests, particularly our wilderness areas, reach to higher elevations.

Ammonia

Emissions of ammonia form in the atmosphere from primary pollutants, and often originate from agricultural sources such as animal waste and volatilization of fertilizer applied to crops. Ammonia can also come from motor vehicles and industrial sources.