Acid rain is a broad term that describes low pH precipitation that combines with acidic particles in the air, and gets deposited on land and in waters. The primary cause of acid rain is sulfur that reacts, in the atmosphere, to become sulfur dioxide. The major source of this sulfur is coal-burning power plants, while secondary acid rain causes for sulfur can be natural, such as volcanoes and bacterial activities. Nitrogen oxides are the other major cause of acid precipitation. These gases usually result from the burning of fossil fuels, particularly in power plants and automobiles.
Generally, normal rain tends to be somewhat on the acidic side, with a pH of less than 7. The introduction of acidic molecules from acid deposition, including precipitation and the fall of the particles themselves, can greatly lower the pH of the water and surrounding soils. These acid rain effects can be quite devastating to aquatic life, soil ecology, and to forests — particularly in mountainous areas. Acid rain causes accelerated the erosion of many hard substances, like metal and stone, resulting in rapid degradation of historical structures, and artworks that have withstood the elements for long periods of time.
The majority of acid rain causes come from pollution generated by the burning of fossil fuels. Much of the coal burned to generate electricity contains sulfur. When burned in a power plant, this sulfur enters the atmosphere and reacts with oxygen to form sulfur dioxide. Some municipalities have introduced large smokestacks onto their power plants to avoid pollution in their communities. This results in the pollutants entering the atmosphere at higher levels, and contributing to acid rain problems over a wider area.
There are also natural sources of sulfur that contribute to acid precipitation. Such organic acid rain causes include volcanoes, which produce about one-tenth the amount of sulfur produced by human activities. Wildfires are another source of sulfur, as are bacterial activities on land and in water. Phytoplankton produce sulfur emissions, and part of the characteristic smell of the ocean is due to such sulfur compounds.
Another major source of acid rain is nitrogen oxide compounds, which react in the atmosphere to form nitric acid. Such chemicals are becoming a greater cause for concern, as more controls are imposed on sulfur use. These compounds are produced when any fossil fuels are burned, including coal in power plants. Automobiles are also a major source of nitrogen oxides. As automobile usage increases throughout the world, the production of these compounds grows.
One of the acid rain causes is ammonia, a different nitrogen-containing compound that also ends up in the atmosphere. It is produced as a waste product of livestock production. Livestock raised by humans are also a big contributor to acid rain. Electrical activity from lightning also adds to acidic nitrogen to the atmosphere.