Acid rain is generally considered any precipitation with a pH value lower than 5.6. Normal rain has a value of 5.6. It is slightly acidic due to the presence of carbonic acid formed by the mixture of carbon dioxide and rainwater. Ironically, 5.6 is the pH value of carbon dioxide in equilibrium with distilled water. The pH scale is based on a scale of one to seven, one being the most acidic and seven being the most basic. The pH scale is logarithmic, which means that a pH of six is ten times more acidic than a pH of seven.
Acid rain is caused when elevated levels of sulfur dioxide (SO2) and nitrogen (IV) oxide (NO2) are discharged into the atmosphere. While in the atmosphere, these chemicals dissolve and then oxidize to form ions. The ions then bond with hydrogen atoms in the air and form sulfuric acid (H2S04) and nitric acid (HNO3). This reaction is catalyzed by pollutants in clouds such as ammonia (NH3), hydrogen peroxide (H202), iron (Fe), and magnesium(Mg).
Some rain is naturally acidic because of the carbon dioxide in the air that dissolves with rain water and forms a weak acid. This is necessary to maintain the pH equilibrium of our planet. Natural sources of carbon dioxide include ocean spray, decaying plant life and plankton, other bacterial activity in the soil, and volcanic ash. However, over 90% of these emissions come from human sources, such as the burning of fossil fuels.
Since the advent of the Industrial Revolution, man has drastically increased the use of fossil fuels. Coal is used to produce electricity, and other examples include base-metal smelting and fuel combustion from automobiles. Automobiles account for about 40% of nitrogen oxides and thermoelectric generating stations account for another 25%. The other 35% comes from industrial, commercial, and residential combustion processes.
Clouds that carry acidic pollutants can be carried hundreds of miles in the wind. A good example of this is in the Adirondack Mountains in upstate New York. Although there are few pollutant-producing locations, this area is being plagued by the effects of acid rain. In 1930 testing in this area, the average pH was 6.8. Today, the acidity at the same test site is less than 5.0, even though there are no sources for acidity in the immediate area.
Acid rain has been an increasingly serious problem since the 1950s, particularly in the Northeast United States, Canada, and Western Europe, especially Scandinavia. It alters the environment in many ways. It can dissolve nutrients and bacteria that are needed in the soil. Also, sulfuric acid causes toxic metals such as aluminum and mercury to be disassociated and released from the soil into the water, harming fish, plants, and other aquatic life forms. Aluminum (Al+2) burns the gills and accumulates in the organs of fish. If the pH of a body of water is too low, the reproduction of frogs, fish, and other animals can be affected. The low pH neutralizes the calcium needed to create healthy eggs and bones. Without the proper amount of calcium, eggs become too brittle or too weak.
The acidity of rain can also affect man-made structures. One indicator of the increasing severity of this problem can be seen in some of the renowned works of art and structures in Europe. Due to acid precipitation, statues that have stood since the era of the ancient Romans have suffered more damage in the last fifty years than natural weathering had caused in thousands of years. Limestone and marble turn to a crumbling substance called gypsum upon contact with sulfur dioxide, which explains the corrosion of these buildings and statues. It also affects metals, as they corrode faster. Metals in planes, bridges, and edifices are affected. A good illustration would be the bridge over the Ohio River that collapsed in 1967, killing 46 people.
The United States government is concerned about the effects of acid rain. In 1970, the Clean Air Act was passed, establishing standards for particle matter, sulfur oxides, carbon monoxide, nitrous oxides, hydrocarbons, and photochemical oxidants. In 1990, amendments were passed that made drastic cuts in the allowable amounts of sulfur and nitrous oxides that could be released into the atmosphere. Limits were set on emissions for factories, cars, and industrial plants. The government designed agencies such as the EPA (Environmental Protection Agency) to look for ways to reduce these harmful emissions. New technologies and techniques were developed to lower the amounts emitted. Air scrubbers, which use water sprays to trap particles, have been used to remove sulfur dioxide from industrial emissions. Some of the more efficient units remove up to 95% of sulfur dioxide. However, they do produce a sulfur sludge, which presents another disposal problem. Coal can also be pre-washed, where it is put into a chemical solution. The pure coal floats while other chemicals sink to the bottom. These all help to reduce the problem, but as long as we continue to use fossil fuels we will have to continue to pursue a better solution to the problem of acid rain.