April 14, 2014
Effects Of Clean Air Act Seen In Greenland Ice Cores
Brett Smith for redOrbit.com - Your Universe Online
Over four decades ago, acid rain caused by industrial emissions was eradicating fish and dissolving stone monuments in the Eastern US. However, emissions of sulfur dioxide and nitrogen dioxide, both of which cause acid rain, dropped significantly after the passing of the Clean Air Act of 1970.
A new study from a team of French and American scientists has revealed that the rise and fall of these atmospheric contaminants has been recorded in ice cores pulled from Greenland.
The study team said they had not set out to track the levels of acid rain across time, but were looking instead to measure smog – both of which are caused by the same nitrogen compound found in industrial emissions: NOx.
"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 study author Lei Geng, an atmospheric scientist at the University of Washington.
While other gases are trapped as tiny bubbles in ice cores, NOx breaks down over the span of a couple days into nitrate. Prior studies have determined levels of NOx in ice cores by analyzing the ratio of isotopes – nitrogen-15 and nitrogen-14. Those studies have found the nitrogen-15 ratio constantly falling since 1850.
The new study, which was published in the Proceedings of the National Academy of Sciences, refutes this earlier find – instead indicating the isotope ratio plateaued in 1970 and at the same time saying this ratio is responsive to the same chemicals that create 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.
The researchers said the rise, leveling off and fall of the nitrogen-15 ratio is all related to changes in atmospheric chemistry. Airborne nitrate can exist as a gas or possibly a particle, and nitrate with lighter isotopes have a tendency to be found as a gas. However, they discovered that the overall portion of nitrate present as gas or particle may differ with the acidity of the atmosphere, and the acidic air brings about more light isotopes in a gaseous state.
"The isotope records really closely follow the atmospheric acidity trends," said study 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."
In Greenland, acid rain falls as snow that sits on the ground. Sunlight reflecting off the snow triggers chemical reactions that revert some nitrate into a gas state. Acidic air may even impact the reactivity of nitrate in snow and as a result the preservation of nitrate in ice cores.
The researchers noted that other ice core records may be affected by atmospheric acidity as well, such as the deposition and conservation of similar volatile compounds such as chlorine or mercury.