Atmospheric chemicals seen in new light
A student collects a sample of snow and ice crystals to measure formaldehyde levels
March 23, 1999
Web posted at: 11:00 AM EST
University researchers studying natural processes that influence ozone in the Arctic atmosphere have discovered that snowpacks not only absorb chemicals from the atmosphere, but also can help produce them.
The findings cast new light on scientists' perceptions of how atmospheric gases are processed, said Paul Shepson, a professor of atmospheric chemistry at Purdue University.
The findings may also affect the way that scientists view data from ice core studies, because researchers have assumed that the air trapped in ice provided representative samples of atmospheric conditions at the time the ice was formed.
"Ice core studies designed to look at reactive species such as nitrates may have to be revisited, as the air bubbles found in these ice cores may not be the mirrors of atmospheric composition that we suspected they were," said Shepson.
However, this is not a concern for more stable greenhouse gases such as carbon dioxide and methane, which have been extensively studied in ice cores, because these stable gases are less likely to react with other compounds in snow or ice.
Last winter, Shepson led a research group to the Canadian Arctic to observe how sunlight interacts with various gases in the atmosphere to reduce near-surface ozone levels.
Ozone is a beneficial component of the Earth's upper atmosphere, but is a pollutant at the ground level.
"It has recently been observed that, at polar sunrise, which occurs in March or April after several months of complete darkness, ozone in a thin layer of air over the Arctic Ocean is completely removed," said Shepson. "This was a big surprise to us, and it indicates that our understanding of atmospheric ozone is poor."
One of the gases Shepson's team observed was formaldehyde. Formaldehyde is an important part of the atmosphere's self-cleaning mechanism because it is a major source of free radicals, said Shepson.
"The atmosphere acts to clean itself of pollutants through reactions involving free radicals. When formaldehyde absorbs light, it falls apart to produce these free radicals."
Previous studies of formaldehyde in the Arctic had shown concentrations up to 10 times higher than expected, so graduate student Ann Louise Sumner spent two months at the Alert laboratory in the Canadian Arctic measuring formaldehyde in the snowpack and in the atmosphere.
These measurements 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," said Shepson.
In another study, led by Richard Honrath, a professor of civil and environmental engineering at Michigan Technological University, it was found that nitrate ions in the snow can interact with sunlight to produce nitric oxide and nitrogen dioxide, pollutants derived largely from the combustion of fossil fuels and critical precursors to the production of ozone in the atmosphere, said Shepson.
"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," he said. "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."
These findings, said Shepson, suggest that models used to predict long-term changes in the composition of the atmosphere need to be fine-tuned to take into account the interaction of gases with surfaces.
"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," said Shepson.
Articles on these research efforts were published in the March 18 issue of the journal Nature and the March 15 issue of Geophysical Research Letters.
Copyright 1999, Environmental News Network, All Rights Reserved
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Paul B. Shepson
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