Samples from the Greenland ice sheet suggest the US Clean Air Act had a clear effect on conditions in the Earth's atmosphere. The discovery by University of Washington atmospheric scientists also alludes to a link between air acidity and nitrogen preservation in snow layers, according to a release by the school.

While the research team had been interested in studying smog, it discovered a link in the geologic record between smog and acid rain. The US Clean Air Act of 1970 was a reaction to air pollution increases and incidents of acid rain and required coal power plants and other polluters to scrub sulfur out of their smokestacks.

The main ingredient in smog is ground-level ozone caused by nitrogen, which is emitted as a short-lived compound known as NOx. Researchers said they may be able to measure global human emissions by determining the chemical composition of the atmosphere of the past.

"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," said Lei Geng, a University of Washington researcher in atmospheric sciences, and lead author of the new research published in Proceedings of the National Academy of Sciences

Earlier research by co-author Eric Steig, a UW professor of Earth and space sciences, suggested that comparing amounts of the two stable forms of nitrogen - nitrogen-15 and nitrogen-14 - in nitrate could pinpoint the emission sources of NOx. The detailed measurements of nitrate, NOx, and sulfur show the nitrogen isotope ratio leveling off in 1970, suggesting the ratio is sensitive to the same chemicals that cause acid rain.

Geng's work showed the long-term decrease in the nitrogen-15 isotope since 1850, and its leveling off in 1970, are linked to changes in air chemistry. Airborne nitrate can exist as a gas or a particle, and nitrate with lighter isotopes tends to exist as a gas. But he found that the total fraction of nitrate present as gas or particle varies with the acidity of the atmosphere, and the acidic air causes more of the light isotopes to exist as a gas.

"The isotope records really closely follow the atmospheric acidity trends," said co-author Becky Alexander, a UW associate professor of atmospheric sciences. "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."