December 8, 2012
Nitrogen Decrease In Greenland Ice Blamed On Atmospheric Acidity
redOrbit Staff & Wire Reports - Your Universe Online
Researchers from the University of Washington (UW) have linked decreasing levels of a nitrogen isotope in Greenland ice sheet samples to increased acidity levels in the atmosphere.
Previously, the decrease in levels of the isotope nitrogen-15 had been linked to a correlating increase in nitrates linked with the burning of fossil fuels, the university explained in a December 7 report. However, the new research suggests sulfur dioxide, which becomes sulfuric acid in the atmosphere, is actually to blame, according to UW research associate Lei Geng.
The declining nitrogen-15 levels date back to the Industrial Revolution in the late 1800s, and around that time, emissions of sulfur dioxide began to increase steadily because of the increased amount of coal burning. Those emissions alter the chemical properties of the lower troposphere, Geng told those in attendance at the American Geophysical Union's (AGU) fall conference in San Francisco.
"The gradual buildup of acidity in the atmosphere over a century got a boost around 1950 with a sharp increase in nitrogen-oxygen compounds, referred to as NOx, mainly produced in high-temperature combustion such as occurs in coal-fired power plants and motor vehicle engines," the university explained. "NOx is easily converted to nitric acid in the atmosphere, further increasing the acidity.
"NOx carries a chemical signature — the abundance of nitrogen-15, one of two nitrogen isotopes — which changes depending on the source," they added. "That means it is possible to distinguish NOx that came from a forest fire from NOx produced as a result of lightning, soil emissions, car exhaust and power plant emissions. The level of nitrogen-15 can be measured in nitrates that formed from NOx and were deposited in ice sheets such as those in Greenland."
Based on the evidence, the researchers say there is most likely more nitrogen-15 in NOx that originates from coal-fired power plant and automobile exhaust fumes than in naturally-produced versions of the compound. Theoretically, that means the isotope levels found in deposited nitrate should go up, but Geng said those levels actually decreased following the Industrial Revolution.
"That´s because increasing sulfuric acid levels in the atmosphere triggered chemical and physical processes that allowed less nitrogen-15 to remain in vaporized nitrate, which can be carried to remote places such as Greenland," the university explained. "The growing acidity in the atmosphere was occurring decades before acid rain was recognized as a threat, particularly in industrial areas of North America."
Geng reports core samples obtained from ice sheets in the Greenland region reflected a link between the nitrogen-15 levels and the acidity levels of the atmosphere and the sample reflected a decline for both NOx and sulfur emissions during the Great Depression, followed by an increase afterwards, and another decrease in the early 1970s," when Western nations experienced an economic downturn and an oil shortage."
The research was funded by the National Science Foundation (NSF).