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Acid Rain

Avoiding Acid Rain

Neutralization of acidic ecosystems treats the symptoms, but not the sources of acidification. Clearly, large reductions in emissions of the acid-forming gases SO2 and NOx are the ultimate solution to this widespread environmental problem. However, there is controversy over the amount that the emissions must be reduced in order to alleviate acidic deposition, and about how to pursue the reduction of emissions. For example, should large point sources such as power plants and smelters be targeted, with less attention paid to smaller sources such as automobiles and residential furnaces? Not surprisingly, industries and regions that are copious emitters of these gases lobby against emission controls, for which they argue the scientific justification is not yet adequate.

In spite of many uncertainties about the causes and magnitudes of the damage associated with acid rain and related atmospheric depositions, it is intuitively clear that what goes up (that is, the acid-precursor gases) must come down (as acidifying depositions). This common-sense notion is supported by a great deal of scientific evidence, and because of public awareness and concerns about acid rain in many countries, politicians have began to act effectively. Emissions of sulfur dioxide and oxides of nitrogen are being reduced, especially in western Europe and North America. For example, in 1992 the governments of the United States and Canada signed an air-quality agreement aimed at reducing acidifying depositions in both countries. This agreement calls for large expenditures by government and industry to achieve substantial reductions in the emissions of air pollutants during the 1990s. Eventually, these actions should improve environmental conditions related to damage caused by acid rain.

However, so far the actions to reduce emissions of the precursor gases of acidifying deposition have only been vigorous in western Europe and North America. Actions are also needed in other, less wealthy regions where the political focus is on industrial growth, and not on control of air pollution and other environmental damages that are used to subsidize that growth. In the coming years, much more attention will have to be paid to acid rain and other pollution problems in eastern Europe and the former USSR, China, India, southeast Asia, Mexico, and other so-called "developing" nations. Emissions of important air pollutants are rampant in these places, and are increasing rapidly.

See also Sulfur dioxide.



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Bill Freedman


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Acid mine drainage

—Surface water or groundwater that has been acidified by the oxidation of pyrite and other reduced-sulfur minerals that occur in coal and metal mines and their wastes.

Acid shock

—A short-term event of great acidity. This phenomenon regularly occurs in freshwater systems that receive intense pulses of acidic water when an accumulated snowpack melts rapidly in the spring.

Acidic rain (acidic precipitation)

—(1) Rain, snow, sleet or fog water having a pH less than 5.65. (2) The deposition of acidifying substances from the atmosphere during a precipitation event.


—An increase over time in the content of acidity in a system, accompanied by a decrease in acid-neutralizing capacity.

Acidifying substance

—Any substance that causes acidification. The substance may have an acidic character and therefore act directly, or it may initially be non-acidic but generate acidity as a result of its chemical transformation, as happens when ammonium is nitrified to nitrate, and when sulfides are oxidized to sulfate.


—The ability of a solution to neutralize an input of hydroxide ion (OH). Acidity is usually measured as the concentration of hydrogen ion (H+), in logarithmic pH units (see also pH). Strictly speaking, an acidic solution has a pH less than 7.0.


—Refers to organisms that only occur in acidic habitats, and are tolerant of the chemical stresses of acidity.

Conservation of electrochemical neutrality

—Refers to an aqueous solution, in which the number of cation equivalents equals the number of anion equivalents, so that the solution does not have a net electrical charge.


—Abbreviation for mole-equivalent, and calculated as the molecular or atomic weight multiplied times the number of charges of the ion. Equivalent units are necessary for a charge-balance calculation, related to the conservation of electrochemical neutrality (above).


—The movement of dissolved chemicals with water percolating through soil.


—The negative logarithm to the base 10 of the aqueous concentration of hydrogen ions in units of moles per liter. An acidic solution has pH less than 7, while an alkaline solution has pH greater than 7. Note that a one-unit difference in pH implies a 10-fold difference in the concentration of hydrogen ions.

Additional topics

Science EncyclopediaScience & Philosophy: 1,2-dibromoethane to AdrenergicAcid Rain - Atmospheric Deposition, Chemistry Of Precipitation, Spatial Patterns Of Acidic Precipitation, Dry Deposition Of Acidifying Substances