Acid rain is the popular term for precipitation acidified by atmospheric pollutants. It is caused by pollutants such as sulfur dioxide and nitrogen oxides, which are converted chemically to sulfuric acid and nitric acid in the atmosphere. Diluted forms of these acids fall to earth as rain, hail, drizzle, freezing rain or snow (wet deposition), or as acid gas or dust (dry deposition). Normal rain is slightly acidic, but acid rain can be as much as 100 times more acidic (Acid Rain, 2000). Acid rain is scientifically referred to as acid deposition.

       Acid deposition can form as a result of two processes. In some cases, hydrochloric acid can be expelled directly into the atmosphere. More commonly it is due to secondary pollutants that form from the oxidation of nitrogen oxides or sulfur dioxide gases that are released into the atmosphere (Long Range, 1997). The process of altering these gases into their acid counterparts can take several days, and during this time these pollutants can be transferred hundreds of miles from their

     Emissions of sulfur dioxide are responsible for 60-70% of the acid deposition that occurs globally. More than 90% of the sulfur in the atmosphere is of human origin. The main source of sulfur include coal burning, the smelting of metal sulfide ores to obtain pure metals, volcanic eruptions and organic decay. After being released into the atmosphere, sulfur dioxide can either be deposited on the Earth's surface in the form of dry deposition or it can undergo reactions to produce acids that are incorporated into the products of wet deposition (National, 1996). Some 95% of the elevated levels of nitrogen oxides in the atmosphere are the result of human activities. The remaining 5% comes from several natural processes. The major sources of nitrogen oxides include combustion of oil, coal, gas, bacterial action in soil, forest fires, volcanic activity and lightning (National, 1996).

     Acid rain has many effects on human health. One of the most serious side effects of acid pollution on human beings is respiratory problems. Also, an indirect effect of acid precipitation on humans is due to toxic metals, which are dissolved in the water, absorbed by fruits, vegetables and in animal tissues that that can be harmful if consumed (Acid Rain, 1999). For example, mercury that accumulates in the organs and tissues of animals has been linked with brain damage in children as well as nerve disorders. Another metal, aluminum, present in the organs of the animals, has been associated with kidney problems and most recently is suspected to be related to Alzheimer's.

     There are also many ecological effects caused by acid rain. These effects are most clearly seen in aquatic, or water environments, such as lakes, streams and marshes. Acid rain primarily affects sensitive bodies of water, located in watersheds whose soils have a limited ability to neutralize acidic compounds. Lakes and streams become acidic when water itself and its surrounding soil cannot buffer the acid rain enough to neutralize it.

     High pH levels in a body of water can have a number of effects that harm or kill individual fish, reduce fish population numbers, completely eliminate fish species, and decrease bio-diversity. As acid rain flows through soils in a watershed, aluminum is released from soils into the lakes and streams located in that watershed. As the pH level in a lake or stream decreases, aluminum levels increase. Both low pH and increased aluminum levels are extremely toxic to fish. In addition, low pH and increased aluminum levels cause chronic stress that may not kill individual fish, but may lead to lower body weight and a decrease in size and make the fish less able to compete for food and habitat.

     Some types of plants and animals are able to tolerate acidic waters. Others, however, are acid-sensitive and will be lost as the pH declines. Generally, the young of most species are more sensitive to environmental conditions than adults. At a pH of 5, most fish eggs cannot hatch. At lower pH levels, many adult fish die.

     Another environmental harmful effect caused by acid rain is the damage it causes trees and plants. Acid rain weakens trees by damaging their leaves, limiting the nutrients available to them, and exposes them to toxic substances slowly released from the soil, such as aluminum. Quite often, injury or death of trees is a result of these effects in combination with an additional threat. Acidic water also dissolves nutrients and helpful minerals in the soil and then washes them away before trees and other plants can use them to grow (Krajick, 2003).

     Acid rain is a global problem, effecting every part of the world. Some places, however, are affected more then others. Acid rain is a major problem in North America and is also a major concern in parts of Europe, particularly Scandinavia, and is a developing issue in China and other industrializing areas around the world. Because the emissions that cause acid rain often cross national borders, acid rain is an important issue on the international agenda (National, 1996).

     Airborne acidic pollutants are often transported by large scale weather systems thousands of km's from their point of origin before being deposited. In eastern North America, weather systems generally travel from southwest to northeast. This means pollutants emitted from sources in the industrial zones of the midwestern states and central Canada regularly fall on the more rural areas of northeastern US and southeastern Canada (National, 1996).

     Acid rain is even a problem right here in Newfoundland. High levels of SO2 are released from Hydro's Thermal Generating Station at Holyrood and the Come By Chance oil refinery. These emissions from range from 45 to 70 kilotonnes per year and have caused a major concern about the amount of acid rain that is being created and the damage it is causing the environment. There are several acid rain monitoring sites around the province that measure the amount of acid rain in certain areas of the province.

     To solve the acid rain problem, people need to understand how acid rain causes damage to the environment. They also need to understand what changes could be made to the air pollution sources that cause the problem. Almost all of the electricity that powers modern life comes from burning fossil fuels like coal, natural gas, and oil. Coal accounts for most US sulfur dioxide (SO2) emissions and a large portion of nitrogen oxides (NOx) emissions. Sulfur is present in coal as an impurity, and it reacts with air when the coal is burned to form SO2. In addition, NOx is formed when any fossil fuel is burned. 

     There are several options for reducing SO2 emissions, including using coal containing less sulfur, washing the coal, and using devices called scrubbers to chemically remove the SO2 from the gases leaving the smokestack. Power plants can also switch fuels; for example burning natural gas creates much less SO2 than burning coal (Long Range, 1997). Certain approaches will also have additional benefits of reducing other pollutants such as mercury and carbon dioxide. Finally, power plants can use technologies that don't burn fossil fuels. Each of these options has its own costs and benefits, however; there is no single universal solution.
Similar to scrubbers on power plants, catalytic converters reduce NOx emissions from cars. These devices have been required for over twenty years in the US, and it is important to keep them working properly and tailpipe restrictions have been tightened recently. The Environmental Protection Agency has also made, and continues to make, changes to gasoline that allows it to burn cleaner (Acid Rain, 2002).

     There are other sources of electricity besides fossil fuels. They include nuclear power, hydropower, wind energy, geothermal energy, and solar energy. Of these, nuclear and hydropower are used most widely. There are also alternative energies available to power automobiles, including natural gas powered vehicles, battery-powered cars, fuel cells, and combinations of alternative and gasoline powered vehicles (Acid Rain, 1999).

     In 1985, the Canadian government and seven provinces joined forces to take action on reducing sulfur dioxide. From 1985 to 1994, sulfur dioxide emissions were lowered by 54%, better than the 50% the government had hoped for. The main action and efforts to stop acid rain are especially taking place in the eastern provinces, as they are the ones that are the hardest hit. Canada has helped the United States to try to lower emissions by a third of the 1980's level by 2000. Also, new boiler regulations require boilers to use only 1% sulfur in fuel. Ontario and Quebec account for 75% of the emission reductions that are required for Canada. The government has spent $70 million to develop innovated more cleaner ways to use coal. 

     The U.S. launched the Clean-Air Act in 1991 that stated that by 2000, US emissions would have been a third lower than they were in 1980, and by 2010, there should be a total decrease of 40% (Acid Rain, 1999).

     In Europe in 1985, the Helsinki protocol was signed by 21 countries to reduce transboundary sulfur emissions by at least 30%. In 1994, the Oslo protocol which was signed by 18 countries to reduce the area where acid rain exceeds the critical loads. Some countries do not have to reduce emissions at all, while others have to reduce by up to 80% of the 1980 levels (Acid Rain, 1999).

     It may seem like there is not much that one individual can do to stop acid deposition. However, like many environmental problems, acid deposition is caused by the cumulative actions of millions of individual people. Therefore, each individual can also reduce their contribution to the problem and become part of the solution. One of the first steps is to understand the problem and its solutions. Individuals can contribute directly by conserving energy, since energy production causes the largest portion of the acid deposition problem.
References

Acid Rain (1999). Acid Rain. [On-line]. Available:
http://www.royal.acidrain/atmoshereandclimate.html

Acid Rain (2000). Acid Rain And The Facts. [On-line]. Available:
http://www.ec.gc.ca/acidrain/acidfacts.html

Acid Rain (2002). Atmospheric Science. [On-line]. Available:
http://www.ns.ec.gc.ca/mscl.as/acidfaq.html

Krajick, Kevin. (2003). Acid Rain. Science, pp.17-19.

Long Range Transport Of Airborne Pollutants: Ecosystems Classification and Acid Rain.
[Booklet]. (1997). Environment Canada.

National Environmental Indicator Series: Acid Rain. [Booklet]. (1996) Environment Canada.

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March, 2004