In the 1970s, acid rain caused by sulfur emissions killed trees and even dissolved statues. But the US Clean Air Act dialed back the deadly rainfall. Until now. Why has the rain gone acidic again?

The main culprit now is nitric acid, which is caused by emissions from power plants and cars, as well as fertilizer use on factory farms.

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According to Scientific American:

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.

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In other words, farms are deadly polluters of the environment. The good news is that sulfur emissions, which caused the last round of acid rain, have been reduced by 70 percent since 1990. It's likely that nitrogen emissions can be controlled too.

via Scientific American