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. Part of the problem dates back to WWI, when two German scientists invented the Haber–Bosch process, which took nonreactive nitrogen from the air (N2) and converted it into reactive, usable ammonia (NH3). Most of the nitrogen harvested via this process has been used in fertilizers, and the runoff from farms has created dead zones in Chesapeake Bay and at the mouths of the Columbia and Mississippi rivers. Some efforts have been made to regulate the agricultural nitrogen runoff, but atmospheric emissions of agricultural ammonia remain virtually unrestricted.
Agri-ammonia vapors also derive from concentrated animal feeding operations in the U.S. South. The gas rises into the air and is deposited dry or in rainfall where in the ground bacteria breaks it into nitrogen and nitric acid, which can kill fish and plants. "Agriculture is increasingly functioning as an intensively managed industrial operation, and that is creating serious water, soil, and air problems," says Viney Aneja, a professor at North Carolina State University in Raleigh. Aneja says that state's concentrated animal feeding operations may also emit particulate matter from swine and chicken manure into the atmosphere, which can carry diseases.
NOx escapes from power plants as a by-product of coal combustion, whereas vehicular engines run at high enough pressures and temperatures to combine nitrogen and oxygen in the air. "Though catalytic converters have decreased the amount of pollution per vehicle, there are more vehicles on the road and more miles driven," Schlesinger says. Emissions from fertilizers are the chief source of atmospheric nitric oxide, but motor vehicles have now overtaken coal power plants as the secondary most critical source of this problem.