When pollution from burning fossil fuel enters the atmosphere, it can react to create acids, which then return to the earth in precipitation, having environmental consequences. Environmentalists and those living in affected communities have challenged industry and government alike over the issue of acid rain.
The results show that under a scenario of future climate change, snowfall at Hubbard Brook will begin later in winter, snowmelt will happen earlier in spring, and soil and stream waters will become acidified, altering the quality of water draining from forested watersheds.
In this study, a laboratory leaching column experiment with acid forest soil was set up to investigate the responses of soil and soil solution chemistry to simulated acid rain (SAR).
Acid rain was first identified in North America at Hubbard Brook in the mid-1960s, and later shown to result from long-range transport of sulfur dioxide and nitrogen oxides from power plants.
Long-term biogeochemical measurements, for example, have documented a decline in calcium levels in soils and plants over the past 40 years. Calcium is leaching from soils that nourish trees such as maples. The loss is primarily related to the effects of acid rain (and acid snow).
All forms of precipitation are naturally acidic because of naturally occurring carbon dioxide; human activities tend to add to the acidity. Non-polluted rain is assumed to have a pH of 5.6. This is the pH of distilled water. Natural sources of these environmentally regulated chemicals may be significant. Emissions due to human activity tend to be concentrated near historic industrial sites and older population centers. The presence of other naturally occurring substances can produce pH values ranging from 4.9 to 6.5. This scientific dynamic has kept other debates alive regarding whether government has an effective role in environmental policy if the sources are natural. pH levels are among the many factors that are monitored.
Part of the controversy about the sources of acid rain has revolved around the question of whether environmental policy could really affect the acidity of rain. Scientific debate about natural, human, and industrial causes engulfed much of the political battleground. Although the policy question was answered in the affirmative—that, yes, environmental policy can make the air cleaner—the debate about sources continues.
The sulfur and nitrogen oxides are the common pollutants from coal-burning activities such as power generation. Many if not most of these emissions are legal in that they are within the terms of their permits from the Environmental Protection Agency (EPA) or state environmental agency. Legal or not, these pollutants are oxidized in the atmosphere and converted into sulfuric and nitric acids. These acids are then absorbed by clouds laden with raindrops. As they become heavier, they fall to the earth. This process is called acid deposition. Acidic fog, snow, hail, and dust particles also occur. The acidity of these different forms of precipitation can vary greatly.
Unpolluted rain is normally slightly acidic, with a pH of 5.6. Carbon dioxide from the atmosphere dissolves to form carbonic acid, which is why normal rain is slightly acidic. When acidic pollutants combine with the rain, the acidity increases greatly. The acidity of rainfall over parts of the United States, Canada, and Europe has increased over the past 40 years. This is primarily due to the increased emission of sulfur and nitrogen oxides that accompanies increased industrialization.
The term acid deposition is used to encompass both the dry and wet deposition of acidic compounds in acid precipitation. The most recent term used in place of acid rain is atmospheric deposition, which includes acidic compounds as well as other airborne pollutants. The term reflects the recognition that air pollution involves the complex interaction of many compounds in chemical stew within the atmosphere.
In the United States, limestone is the second most commonly used building stone. It was widely used before Portland cement became available. Limestone was preferred because of its uniform color and texture and because it could be easily carved. Limestone from local sources was commonly used before 1900. Nationwide, marble is used much less often than other stone types. Granite is primarily composed of silicate minerals, which are resistant to acid rain. Sandstone is also composed of silica and is resistant to most types of acid rain. Limestone and marble are primarily composed of calcium carbonate, which dissolves in weak acid. Depending on the building materials, many older U.S. cities are suffering damage due to acid rain.
The relationship between pollutant emission sources and the acidity of precipitation at affected areas has not yet been determined. More research on tracing the release of pollutants and measuring their deposition rates to evaluate the effects on the environment is under way. This is an area of much scientific and legal controversy. If it were possible to show that a given emission definitely came from a given plant, then government would be able to assign liability to the polluter. Governments would also be able to locate sources of acid rain that comes from other countries.
Boric acid does not emit vapors into the atmosphere. Particles that get intothe air do not break down. They settle to the ground or are removed by rain.