More than 15 years after the discovery of an ozone hole in the stratosphere over the Antarctic, the remote continent is yielding another atmospheric surprise.A team of scientists led by the Georgia Institute of Technology has found a surprisingly high level of an air-purifying oxidizing agent in the near-surface atmosphere over the South Pole. The finding has implications for interpreting historical global climate records stored in Antarctic ice cores.
The summertime 24-hour average value of the atmospheric oxidant known as the hydroxyl (OH) radical is higher than that recorded at the equator. The researchers will report their findings this fall in the journal Geophysical Research Letters.
The OH radical is widely recognized as vital to scrubbing pollution and naturally occurring chemicals from the air throughout the globe; it prevents a buildup of toxic levels of these substances.
"What we now know is that the near-surface atmospheric zone called the mixed layer (from the surface upward to between 20 and 200 meters) is a highly oxidizing environment at the South Pole," said Doug Davis, a lead researcher and professor in the Georgia Tech School of Earth and Atmospheric Sciences. "Equally exciting, we are beginning to see evidence that a lot of this oxidizing chemistry is also occurring down in the snowpack. Thus, once things get buried in the snow, there continues to be active chemistry -- including oxidation -- that could further modify chemical species before they are trapped in the ice in their final chemical forms."
These findings suggests that glacio-chemists -- who study climate change based on an analysis of trace chemicals trapped in polar ice -- have to be far more careful in their interpretation of Antarctic ice cores, said Davis, whose research team is funded by the National Science Foundation. Changes in some chemical species buried may continue for another five to 10 years after they are trapped in the snowpack.
It is the presence of elevated nitric oxide (NO) that causes the high levels of OH, Davis explained. The NO is being released from the snowpack.
"Snow release of nitric oxide can in principle occur anywhere there are accumulations of nitrate ions in ice and there is also solar radiation," Davis said. "… The South Pole is unique because the levels of NO and other nitrogen oxides are nearly an order of magnitude higher than anywhere else." Researchers know this chemistry is having a local or regional impact wherever it occurs. The global impact is still unknown, Davis added.
At the South Pole, researchers recorded OH radical levels over a 24-hour period; the average measurement was about 2 X 106 molecules per cubic centimeter of air over several days of sampling during their December 1998 to January 1999 expedition and again from December 2000 to January 2001. To measure OH, the scientists used the selected-ion chemical-ionization mass spectrometer (SICIMS) technique, which in the early 1990s became the first sensitive method for measuring this radical. Georgia Tech Adjunct Professor Fred Eisele, the other lead researcher for this project, developed the SICIMS technique. Eisele is also a senior research associate at the National Center for Atmospheric Research in Boulder, Colo.
To measure NO, researchers used the well-established chemiluminescence technique with modifications to improve its sensitivity by an order of magnitude.
Although the factors that cause NO levels at the South Pole to exceed 550 parts per trillion by volume of air (pptv) are still under investigation, Davis believes the most important factor is the atmospheric mixing depth at the pole. It seems to be highly variable and is sometimes no more than 25 meters above the surface.
Elevated levels of NO (20 to 550 pptv) in the near-surface atmosphere react with the hydroperoxyl radical -- a less reactive oxidizing agent than OH -- and are converted to OH and nitrogen dioxide. The latter reacts with OH to produce nitric acid, which can return to the snow, thus forming a closed cycle.
"It's not that this is new chemistry," Davis explained. "Most of the time in the background remote atmosphere where NO levels are typically less than 10 pptv, a large fraction of the hydroperoxyl radical reacts with itself and creates hydrogen peroxide, which is lost to the surface. But at the South Pole, in the presence of this large source of nitric oxide, the hydroperoxyl radical predominantly reacts with NO to generate the more reactive OH radical. Everybody tends to associate nitric oxide levels with combustion, thus the South Pole is one of the last places on Earth that you might expect to find nitric oxide in such large concentrations."
Davis and his colleagues discovered the high NO and OH radical levels in their funded research project called ISCAT, for the Investigation of Sulfur Chemistry in the Antarctic Troposphere. Researchers hope to more fully understand the oxidation of dimethyl sulfide (DMS) under the cold conditions and high latitudes of Antarctica. This information will also help glacio-chemists better interpret sulfate and methane sulfonate concentrations incorporated into the continent's 400,000-year-old ice records, Davis said.
Based on their 1994 studies near the Antarctic coast, the team moved ISCAT research to the South Pole. They expected to record significant atmospheric transport of sulfate and DMS from the coast to the pole.
"Well, our initial hypothesis was wrong…." Davis explained. "There was very little unreacted DMS that reached the pole because of the very high levels of OH in the near-surface air at the South Pole -- and perhaps more importantly -- over the entire polar plateau."
Elevated NO maintains a highly oxidizing environment on the polar plateau 24 hours a day, Davis said. The OH radical oxidizes most of the DMS before it reaches the South Pole. "The oxidizing environment at the South Pole is truly astounding," Davis said. "…. Initially, it made no sense. It was like finding some distant planet's atmosphere plugged into Earth's atmosphere, but having it limited to only the Antarctic polar plateau."
The researchers hope to learn more as they analyze data from their 2000-01 trip and return to Antarctica in 2003.
Other institutions represented in the ISCAT team are the National Center for Atmospheric Research, New Mexico State University, the University of California at Irvine, Drexel University, the University of Minnesota, the University of New Hampshire and Arizona State University.
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