Greenland’s ice sheet is like a time capsule that chronicles the recent history of Earth’s atmosphere, including the role humans have played in changing it. Scientists who analyzed core samples from the ice found they contained a record of chemical shifts in the air, including how efforts by the U.S. government to rein in air pollution helped improve the environment.

According to a new study published this week in the journal Proceedings of the National Academy of Sciences, researchers discovered a link between air acidity and how nitrogen is stored in layers of snow. In particular, scientists noted a clear distinction in nitrogen levels before and after the impact of the U.S. Clean Air Act of 1970, a landmark piece of legislation that strictly capped sulfur emissions from coal power plants and other polluters.

"The isotope records really closely follow the atmospheric acidity trends," Becky Alexander, a University of Washington associate professor of atmospheric sciences and a co-author of the study, said in a statement. "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.”

Sulfur is a component of acid rain, a phenomenon that plagued the East Coast of the U.S. in the mid-20th century following the manufacturing boom of the Industrial Revolution. Before 1970, acid rain, the main cause of which is human enterprise, was dissolving stone architecture and killing fish. The Clean Air Act significantly lowered the amounts of acid rain-causing compounds sulfur and nitrogen being pumped into the air.  

The core samples tested in the new study were collected in 2007. Researchers measured the total amounts of nitrate, a water-soluble molecule that is the result of nitrogen breaking down into more stable components, within the snow for each year. Nitrogen is the main element of smog. Its origins include smokestacks and vehicle tailpipes, as well as wildfires and natural sources like soil microbes.

Airborne nitrate exists as both a gas and a particle. How much nitrate exists in the atmosphere as a gas and how much is present as a particle, researchers found, depends on the acidity of the atmosphere. More acidic air causes more nitrate to exist as a gas, meaning there is less nitrate present in ice core samples during more acidic years.  

“How much the nitrate concentrations in ice core records can tell about [nitrogen] and the chemistry in the past atmosphere is a longstanding question in the ice-core community,” Lei Geng, a UW postdoctoral researcher in atmospheric sciences and lead author of the study, said in a statement.

Researchers say that by using nitrogen as a kind of geological record, scientists could create a clearer picture of the history of Earth’s atmosphere.