Background Essay: Acid Rain
Mammoth Cave National Park is a monitoring station for several types of air pollution, including acid rain. Just what is acid rain and how is it caused?
This form of air pollution has been around since the beginning of the Industrial Revolution, but it became markedly worse in the latter part of the 20th century. All rainfall is acidic, but polluted rainfall can be as much as 100 times more acidic than ordinary rainfall. Some acid rain is as acidic as lemon juice.
Acid rain is caused mainly by the burning of fossil fuels at power plants and factories. These plants and factories emit three pollutants known in the slang of air pollution monitors as rocks, SOx and NOx. The rocks are small, unburned particles like ash that escape into the air. SOx refers to sulphur dioxide, and NOx refers to nitrogen oxide. Automobile exhaust is a major contributor to nitrogen pollution, while ore smelters and petroleum refineries are sources of sulphur pollution. Natural sources of these types of pollution include volcanoes, forest fires, and lightning.
When sulphur dioxide and nitrogen oxide enter the atmosphere and react with the moisture in clouds, they form strong acids. Winds may carry these clouds thousands of miles from the source of pollution before the clouds drop their acidic moisture as acid rain, snow, fog, or sleet.
Scientists measure the acidity or alkaline content of solutions in a number of ways. The best known measure is of pH (or the power of the concentration of the hydrogen ion). In common practice, pH is measured on a scale of 0 to 14. A pH of 7 is considered neutral. Anything below 7 is acidic, anything above, alkaline. Each unit change in pH represents a tenfold increase or decrease. In other words, a solution with a pH of 4 is 10 times more acidic than one with a pH of 5, and a solution with a pH of 3 is 100 times as acidic as if the pH is 5.
The impact of acid rain
Pure water is neutral, but normal rain is acidic, with a pH of around 5.6, because when the carbon dioxide in the atmosphere dissolves in rainwater, it produces a weak carbonic acid. When gases like sulphur dioxide and nitrogen oxide are present in the atmosphere, these oxides react with rain water to lower the pH to 4.5 or even 3.0. Although acid rain looks and feels the same as normal rain, the increased acidity may be harmful. For example, researchers have documented that acid rain causes trees to grow more slowly, to lose their needles or leaves, and even to die. In many areas of the eastern United States and Canada, where acid rain is most concentrated, sections of forests have died because of acid rain. Die-off is especially dramatic in higher elevations of the Appalachian Mountains from Maine to Georgia.
Acid deposition has increased the acidity in some streams and lakes to such an extent that they can no longer support life. Swimming in acidic lakes does not seem to cause any adverse reactions in humans, but the pollutants that cause acid rain are dangerous to human health in other ways. Sulphur dioxide and nitrogen oxide interact in the atmosphere to form fine sulphate and nitrate particles, which can be transported great distances by winds. When inhaled, they penetrate deep into people's lungs, increasing illnesses like asthma, bronchitis, and heart and lung disorders.
Acid deposition also takes a toll on manmade objects such as buildings, bridges, and statues. The acidity in the rain eats away paint, stones such as marble and limestone, and metals like bronze. It even damages automotive paints.
Factories and coal burning plants are required to take steps to remove pollutants, such as installing scrubbers in their smokestacks. However, some facilities are allowed to exceed pollution limits through the Acid Rain Program, a federally regulated cap-and-trade system. Here is how it works: The federal Environmental Protection Agency sets a limit on the amount of each kind of pollutant that a facility can emit. If a facility produces less pollution than the limit, it can sell its leftover allowance to another company on the open market. This allows the company that buys the pollution credits to exceed its pollution limits. So, one company can pollute more because another company polluted less.
Cap-and-trade works well in theory, but its practice raises ethical questions. For example, under cap-and-trade, some areas of the country become more polluted than others. Is this fair to the people who live in the more highly polluted areas?
The Acid Rain Program, however, has helped reduce air pollution overall. It has reduced power plant emissions by over 40% nationwide and decreased acid deposition in the eastern United States by 35%. It has had the largest human health benefit of any federally regulated program in the last ten years. The annual benefit exceeds the cost of the program by more than 40 to 1. You can see the impact of the Cap and Trade program by accessing the interactive maps for emission reduction at the Environmental Protection Agency web site.
Another factor that would significantly reduce sulphur dioxide and nitrogen oxides in the atmosphere would be cleaner automobiles that recirculate automobile exhaust back into the engine to burn extra NOx. Many people think that catalytic converters do this, but they do not (although they do remove hydrocarbons that are thought to cause cancer). Increasing the number of electric vehicles on the road would greatly reduce emissions. California is currently requiring that 10% of all new cars sold in the state produce zero emissions.
Acid rain in the National Parks
The incidence and effects of acid deposition have been monitored closely since the beginning of the Acid Rain Program. Understanding the movement of prevailing air currents and storm systems helps scientists discover where the acid rain is coming from and where it is falling to earth. Numerous state and federal agencies have formed a partnership called the National Atmospheric Deposition Program to share and compare their research. The National Park Service is a partner in this effort.
Why? Because the National Park Service is charged with protection of the unique natural resources of our national parks. There is a lot of evidence that suggests that acid rain is damaging many of these unique features. The National Park Service would like to understand more fully the effects of acid rain and ways to minimize these effects.
- Find general information on acid rain, what causes it, its effects, how it is measured, and what is being done to reduce it, at the United States Environmental Protection Agency web site on acid rain .
- Find information, diagrams, videos, links, and photos of effects from at the How Stuff Works web site.
- General information is also found at the Environment Canada web site.