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See Inside September 2010

Sour Showers

Acid rain is back—this time triggered by nitrogen emissions

The acid rain scourge of the 1970s and 1980s that killed trees and fish and even dissolved statues on Washington, D.C.’s National Mall has returned with a twist. Rather than being sulfuric acid derived from industrial sulfur emissions, the corrosive liquid is nitric acid, which has resulted not just from smokestacks but also from farming.

Besides dissolving cement and limestone and lowering the pH of lakes and streams, acid rain leaches critical soil nutrients, injuring plants, and liberates toxic minerals that can enter aquatic habitats. To combat the problem the first time around, the U.S. Environmental Protection Agency passed the Clean Air Act Amendments of 1990, which cut sulfur emissions from power plants by 59 percent from 1990 to 2008. Emissions of nitrogen compounds, however, have not fallen as steeply.

Overall, coal-fired power plants and motor vehicles spew out most of the nation’s nitrogen oxides, the feedstock for nitric acid rain. But a good deal of it also comes from the agricultural sector in the form of ammonia (NH3), which bacteria can convert to nitric acid on the ground. A major culprit is fertilizer manufacture, which takes nonreactive nitrogen gas in the atmosphere and turns it into ammonia via the so-called Haber-Bosch process. Concentrated animal-feeding operations in the South also produce ammonia. “Agriculture is increasingly functioning as an intensively managed industrial operation, and that is creating serious water, soil and air problems,” says Viney P. Aneja, a professor of air quality and environmental technology at North Carolina State University.

Scientists are beginning to document the damage. Researchers at Hubbard Brook Experimental Forest in New Hampshire’s White Mountain National Forest found evidence of nitric acid rain, which appears to have originated from nitric oxides from the upper Midwest. They reported that it might reduce cold or stress tolerance in some tree species, including red spruce and sugar maple. Similarly, researchers have traced nitric oxide rising from Kentucky and Tennessee and drifting toward the Great Smoky Mountains, where researchers have observed some of the worst acid rain and forest decline, says William H. Schlesinger, president of the Cary Institute for Ecosystem Studies in Millbrook, N.Y.

Although the U.S. could tighten existing clean air rules to combat atmospheric nitrogen emissions, the nation has neither comprehensive laws nor adequate monitoring devices for such emissions by livestock and farms. Schlesinger thinks that national arguments over climate change have allowed the U.S. to ignore the nitrogen problem, which he predicts will be the next big environmental issue. “It’s another example of humans upsetting global biogeochemical cycles with unintended consequences,” he says.

Government action could help significantly: the European Union, for instance, passed an acidification abatement pact called the Gothenburg Protocol in 1999, which has decreased Europe’s nitrogen emissions by one third, during a time when U.S. emissions have remained constant. Adding insult to injury, the U.S. increased its ammonia emissions by 27 percent from 1970 to 2005, according to a 2009 paper in Environmental Science & Technology.

Without intervention, the problem will likely worsen. The growing world population, expected to increase from 6.5 billion today to nine billion by 2050, will put pressure on agricultural productivity and, subsequently, fertilizer use. The Integrated Nitrogen Committee of the EPA’s science advisory board, which held a June public teleconference on the issue of reactive nitrogen in the environment, has generated a draft report that lays out the details, including management options for nitric acid rain. It also discusses ways to monitor atmospheric emissions, currently the weak link in the nitrogen-control picture. The final report is expected be released next year.

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