Ice cores from Greenland show definite evidence of damage caused decades ago by acid rain. The samples also clearly show the time when the United States passed the Clean Air Act. Air acidity also effects how nitrogen is stored in snow, according to the study.

Acid rain produced by coal power plants and other industries was killing off life in lakes and streams at the end of the 1960s. Stone fixtures and monuments began to dissolve from the effects of the acidic precipitation. 

The U.S. Clean Air Act of 1970 mandated industries scrub sulfur from emissions released to the atmosphere. 

University of Washington researchers set out to measure the effect of smog by looking at ice cores drilled from the Greenland Ice Sheet, and discovered the acid rain link. 

Smog is mostly composed of molecules of nitrogen and oxygen, referred to as NOx gases. These gases come from vehicle exhausts, smokestacks, microbes and are produced by the great heat in lightning strikes. These chemicals are unstable, and quickly turn into nitrates in snow and ice. 

Nitrates found in the ice cores come in two isotopes, nitrogen-14 and nitrogen-15. The first form of the gas is produced by humans, while the second is the form usually produced in nature. Investigators found concentrations of nitrogen-15 rose from the start of the industrial Revolution, and leveled off in 1970. 

Eric Steig, professor of Earth and space sciences at the University of Washington, believes this indicates that acidic air allows higher concentrations of airborne sulfur compounds. 

"This shows that the relationship between emissions and the isotopes is less direct than we thought, and the final signal recorded in the Greenland ice cores is actually not just the nitrogen emission, but the combined effect of sulfur and nitrogen emissions," Steig said. 

Nitrate particles caught in the atmosphere dissolve in clouds, and fall back to the ground as snow. Sunlight reflected off that snow can trigger reactions, causing the gas to be re-emitted back into the atmosphere as gas. 

"The isotope records really closely follow the atmospheric acidity trends. 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," Becky Alexander, associate professor of atmospheric sciences at UW, stated in a press release. 

Acidity in the air could also affect measurements of other unstable gases, such as mercury, chlorine, or organic material in other ice core samples, according to researchers. 

Investigations into the effects of acid rain decades ago was published in the journal Proceedings of the National Academy of Sciences.