Proof of the success of the U.S. Clean Air act more than 40 years on from its passing has shown up in an unexpected place, researchers say -- the Greenland ice cap.

The act, passed in 1970, mandated that coal-fueled power plants and other polluting activities take measures to scrub environment-fouling sulfur from the output of their smokestacks.

The sulfur was returning to the Earth as acid rain, killing fish and eroding buildings and statues along much of the U.S. East Coast.

Scientists from the University of Washington report they've found evidence of the decades-long success of the act in Greenland, even though acid rain evidence wasn't the target of their research.

Instead, they were retrieving ice cores from the ice cap in a study of smog, looking for nitric oxides, NOx, short-lived compounds that create smog's main ingredient, ground-level ozone.

NOx is emitted from vehicle tailpipes and smokestacks, and is also created in wildfires, lightning strikes and by soil microbes.

"How much the nitrate concentrations in ice core records can tell about NOx and the chemistry in the past atmosphere is a longstanding question in the ice-core community," says Lei Geng a UW postdoctoral researcher in atmospheric sciences and lead author of the resulting study.

What the researchers discovered in the Greenland ice cores was a link between the two forms different forms of pollution -- smog and acid rain -- in the geologic record.

While short-lived NOx can't be measured directly from ice cores because most of it changes into water soluble nitrate, the researchers determined that the ratio of two stable isotopes of nitrogen in the nitrate could pinpoint the emission sources of the NOx.

The ratio of NOx, nitrate and sulfur gave a good indication of the historic levels of both smog and acid rain, they say.

Analyzing total levels of nitrate in ice cores collected at Summit Station in Greenland in 2007, the researchers found a leveling off beginning in 1970 linked to changes in air chemistry.

"The isotope records really closely follow the atmospheric acidity trends," UW atmospheric sciences professor Becky Alexander says. "You can really see the effect of the Clean Air Act in 1970, which had the most dramatic impact on emission of acid from coal-fired power plants."