After analyzing samples from the Greenland ice sheet, it has become crystal clear that the US Clean Air Act had an impact on climate. The discovery by University of Washington atmospheric scientists also found a link between air acidity and the way nitrogen is preserved in layers of snow.

The US Clean Air Act of 1970 came into effect after it was made certain that acid rain was killing fish and causing damages to stone monuments on the East Coast. The Industrial Revolution helped in improving the economy, but the price it charged began to outweigh its benefits for the society. Then, the US government intervened and brought into effect the US Clean Air Act of 1970 to force coal power plants and other polluters to scrub sulfur out of their smokestacks.

UW researchers have now discovered a link between the two forms of pollution in the geologic record. The researchers found that nitrogen is emitted as a short-lived compound, NOx.

NOx was found to be the main ingredient in smog and it causes ground-level ozone. It is related to the compounds that are considered as the detergent of the atmosphere. Smokestacks, vehicle tailpipes, wildfires, soil microbes or reactions triggered by lightning strikes are sources of NOx.

It is not possible to measure NOx directly from air bubbles trapped in ice cores. Most of the NOx changes into nitrate within a day or two. Nitrate is water-soluble molecule essential to life that gets deposited in soil and snow.

A previous research has showed that the emission sources of NOx could be pinpointed after comparing amounts of the two stable forms of nitrogen, nitrogen-15 and nitrogen-14, in nitrate.

The nitrogen-15 ratio was previously noticed as decreasing by analysis of ice cores from Greenland and North American lake sediments. The decrease in the nitrogen-15 ratio since 1850 suggested a corresponding rise in human emissions. But the new research has said that the detailed measurements of nitrate, NOx and sulfur showed the leveling of the nitrogen isotope ratio in 1970. It suggested that ratio is sensitive to the same chemicals responsible for acid rain.

"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", said Eric Steig, a UW professor of Earth and space sciences.