Ozone Levels, Ice Cores Impacted By Chemistry At Earth's Surface

Researchers studying natural processes that impact ozone levels in the Arctic atmosphere have discovered that snowpacks not only absorb chemicals from the atmosphere, but also produce them. The research, conducted by a team from Purdue University (West Lafayette, IN; 765-494-7441), casts a new light on the theory of how atmospheric gases are processed. The findings also question the validity of ice-core studies traditionally used to reveal atmospheric conditions at the time the ice was formed.

Research Methods
Findings
Ice Core Studies
Future Research

Research Methods (Back to Top)
At polar sunrise, which occurs in the Arctic in March or April after several months of complete darkness, ozone is completely removed from a thin layer of air over the Arctic Ocean. To investigate this recently discovered phenomenon, a Purdue research team studied natural processes that impact ozone in the Arctic atmosphere. During this research, the team observed that snowpacks not only absorb chemicals from the atmosphere, but also produce them as well. The research was conducted at the Environment Canada research site at the Canadian Forces base (Alert, Canada), located in the Canadian Arctic.

Led by Paul Shepson, professor of atmospheric chemistry at Purdue, the research team studied the chemistry of ozone in the troposphere—the lowest part of the atmosphere. Although it is a beneficial component of the earth's upper atmosphere, ozone is a pollutant at the ground level. Shepson's group observed how sunlight interacts with various gases in the atmosphere to reduce near-surface ozone levels.

Previous studies showed that formaldehyde levels were up to 10 times higher than expected in the Arctic. To investigate these findings, Purdue graduate student Ann Louise Sumner spent two months at the Alert laboratory measuring formaldehyde and other atmospheric compounds in both the snowpack and the atmosphere. Purdue graduate students Bryan Splawn and Brian Michalowski also worked on the project.

Formaldehyde is an important part of the atmosphere's self-cleaning mechanism because it is a major source of free radicals, Shepson says. "The atmosphere acts to clean itself of pollutants through reactions involving free radicals," Shepson says. "When formaldehyde absorbs light, it falls apart to produce these free radicals."

Findings (Back to Top)
Sumner's measurements, published in the March 18, 1999, issues of Nature, suggest that formaldehyde is produced through photochemical reactions at the snow surface.

"The data account for much of the discrepancy between the high concentrations of formaldehyde found in the Arctic and the amounts predicted by our models," Shepson says.

More stable greenhouse gases such as carbon dioxide and methane, which have been extensively studied in ice cores, are less likely to react with other compounds in snow or ice, he says.

Ice Core Studies (Back to Top)
In a similar research project, Richard Honrath of Michigan Technological University (Houghton, MI) studied ice cores in Summit, Greenland. The Purdue research team also participated in this work. The findings were published in the March 15, 1999, issues of Geophysical Research Letters.

The researchers found that concentrations of nitric oxide and nitrogen dioxide—collectively known as Nox—were actually higher within the snowpack than in the atmosphere. The findings suggest that nitrate ions in the snow can interact with sunlight to produce NOx, a pollutant derived largely from the combustion of fossil fuels and a critical precursor to the production of ozone in the atmosphere, Shepson says.

"This observation changes the way we look at atmospheric chemistry in a fundamental way, in that deposition of nitric acid to the snow was previously regarded as the final fate of NOx," Shepson says. "Now it appears that nitric acid in the snow can be reprocessed by interactions with light, causing re-release of a variety of pollutants back into the atmosphere."

The research suggests that air bubbles in the ice cores may not be the mirrors of atmospheric composition that researchers originally suspected. Because of this, ice-core studies designed to look at reactive species such as nitrates may have to be revisited, Shepson says. Ice core samples will still accurately report the history of more stable greenhouse gases such as carbon dioxide and methane. This is because stable gases are less likely to react with other compounds in snow or ice, Shepson says.

In addition to forcing a re-evaluation of data from many ice core studies, the new findings call into question some models that are used to predict long-term changes in the composition of the atmosphere. "Specifically, models of atmospheric chemistry need to do a better job of treating the interaction of gases with surfaces," Shepson says. "Although we are starting to do better with atmospheric particles, it is important to remember that a potentially important atmospheric surface is the surface of the earth."

Future Research (Back to Top)
Shepson and his group are now working with another group at Purdue to develop new computer models that incorporate the chemical reactions that occur in snowpacks into the current models of atmospheric chemistry and transport.

The National Science Foundation and BASF funded Shepson's studies at Purdue.

For more information, call Shepson at 765-494-7441 or Sumner at 765-496-2404.