The rise and fall of acid rain is a global experiment whose results are preserved in the geologic record and in Greenland ice sheet samples, which discovered a link between air acidity and how nitrogen is preserved in layers of snow, University of Washington atmospheric scientists say the U.S. Clean Air Act worked.

Writing in the Proceedings of the National Academy of Sciences, the authors say that 45 years ago, acid rain was implicated in dead fish and dissolving stone monuments on the East Coast. Air pollution rose beginning with the Industrial Revolution and started to improve when the U.S. Clean Air Act of 1970 required coal power plants and other polluters to scrub sulfur out of their smokestacks. In this case, the researchers began their study of ice cores interested in smog, not acid rain, but they discovered a link between the two forms of pollution in the geologic record.

Nitrogen is emitted as a short-lived compound, NOx, which causes ground-level ozone, the main ingredient in smog, and relates to compounds that are the "detergent" of the atmosphere. Sources of NOx include smokestacks and vehicle tailpipes, as well as wildfires, soil microbes or reactions triggered by lightning strikes.


Ice core drilling at Summit, Greenland in June 2007. Credit: Joel Savarino / Laboratory of Glaciology and Geophysics of the Environment (LGGE)

"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 lead author Lei Geng, a UW postdoctoral researcher in atmospheric sciences.

Unlike other gases, short-lived NOx can't be measured directly from air bubbles trapped in ice cores. Within a day or two most of the NOx changes into nitrate, a water-soluble molecule essential to life that gets deposited in soil and snow.



South Dakota State University's Jihong Cole-Dai is logging an ice-core sample at Summit, Greenland. Credit: Joel Savarino, Laboratory of Glaciology and Geophysics of the Environment (LGGE)



"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."


