Sour Showers: Acid Rain Returns--This Time It Is Caused by Nitrogen Emissions
Acid rain is now caused by nitric rather than sulfuric acid--and it comes from more sources than the earlier acidic precipitation
By Michael Tennesen
The acid rain scourge of the '70s and '80s that killed trees and fish and even dissolved parts of statues on Washington, D.C.'s National Mall is back. But unlike the first round, in which sulfur emissions from power plants mixed with rain to create sulfuric acid, the current problem stems primarily from nitrogen emissions mixed with rain to create nitric acid.
"Both are strong acids, and both create serious problems for the environment," says William Schlesinger, president of the Cary Institute for Ecosystem Studies in Millbrook, N.Y. Acid rain degrades cement and limestone as well as leaches critical soil nutrients, which injures plants. It also liberates toxic minerals from the ground that flow into stream runoff where they can kill fish.
Sulfur emissions from power plants were one of the primary motivations for the U.S.'s Clean Air Act Amendments of 1990, which set reduction targets for both sulfur dioxide (SO2) and nitrogen oxides (NOx). However, whereas sulfur dioxide emissions decreased almost 70 percent from 1990 to 2008, emissions of one NOx—nitrogen dioxide (NO2)—went down only 35 percent for that same period, and amendment targets have yet to be made, according to the U.S. Environmental Protection Agency (EPA). "This comes as scientists have grown increasingly aware of the consequences of the remaining nitric acid deposition," Schlesinger says.
Schlesinger is one of a number of scientists calling attention to the problem. On June 8 the Integrated Nitrogen Committee of the EPA's Science Advisory board held a public teleconference to discuss a draft report of possible solutions to nitrogen problems, including acid rain. A final report is pending.
Nitric acid rain is derived primarily from power plant, car and truck emissions as well as from gases released by fertilizer use.
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