More than 15 years after the discovery of an ozone hole in the stratrosphere
over the Antarctic, the remote continent is yielding another atmospheric
surprise.
A team of researchers led by the Georgia Institute of Technology has
found a surprisingly high level of an air-purifying chemical (or 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 primary atmospheric oxidant
— known as the hydroxyl (OH) radical — at the South Pole is higher than
that estimated from OH measurements 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 to 200 meters) is a
highly oxidizing environment at the South Pole,” says Doug
Davis, one of the lead researchers and a 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.”
This finding 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, says
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. Davis
expects that scientists will soon focus more attention on this topic.
“Snow release of nitric oxide, which leads to the formation of OH, can
in principle occur anywhere globally where there are accumulations of
nitrate ions in ice and there is also solar radiation,” Davis says. “Other
researchers have found evidence of this phenomenon in Summit, Greenland,
and Alert, Canada. What makes the South Pole unique is that the levels
of nitric oxide and other nitrogen oxides are nearly an order of magnitude
higher than anywhere else.
“But any significant elevation of nitric oxide at any snow-covered location
should result in an enhancement of OH,” Davis adds. “And, anytime you
are producing higher levels of OH, it means this chemistry is having some
local or regional impact. The final global impact from this chemistry,
however, is still unknown.”
At the South Pole, researchers recorded OH radical levels over a 24-hour
period; the average measurement was about 2 X 10
molecules per cubic centimeter of air several days during their December
1998 to January 1999 expedition and again from December 2000 to January
2001. These measurements are nearly an order of magnitude higher than
what they originally expected to find based on their Antarctic coastal
measurements of nitric oxide, Davis says.
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 at Tech. Eisele is also a senior research associate
at the National Center for Atmospheric Research in Boulder, Colo.
To measure nitric oxide (NO), researchers used the well-established chemiluminescence
technique with modifications to improve its sensitivity by an order of
magnitude. Nitric oxide, also a radical, is a byproduct of internal combustion
engines. But Davis and his co-workers believe NO is formed at the South
Pole when ultraviolet radiation interacts with nitrate ions. Scientists
are not certain about the source of the nitrate, but it could originate
from stratospheric denitrification processes and the long-range transport
of nitric acid formed at low latitudes during electrical storms.
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 South Pole. This depth seems to be highly variable at the pole
and is sometimes no more than 25 meters above the surface. The Davis team’s
latest results indicate large fluctuations in atmospheric levels of NO
without major changes in NO levels within the snowpack.
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 explains. “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 to study sulfur chemistry. The project,
called ISCAT, for the Investigation of Sulfur Chemistry in the Antarctic
Troposphere, began in 1994 with an expedition to Palmer Station on the
Antarctic Palmer Peninsula. Specifically, the scientists are working 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 says.
Sulfate is a chemical signature for both southern hemispheric volcanic
activity and major fluctuations in phytoplankton populations in the Southern
Ocean that surrounds Antarctica. Phytoplankton lead to the release of
DMS from the ocean, part of which is oxidized by OH to sulfate. Methane
sulfonate is formed only from DMS.
Antarctic ice cores have revealed clear evidence of major volcanic activity
in the Southern Hemisphere, and together with methane sulfonate, evidence
of glacial and interglacial periods in the earth’s climate history. The
level of DMS is a chemical indicator of biomass production in the Southern
Ocean, which, in turn, reflects both water temperature and solar radiation,
Davis explains. So a more comprehensive understanding of DMS chemistry
around and on the Antarctic should provide valuable information in studying
past climate changes, he adds.
Based on the results of the sulfur chemistry studies led by Eisele and
former Georgia Tech Research Institute scientist Harald Berresheim at
Palmer in 1994, Davis and his colleagues moved their ISCAT research to
the South Pole. They expected to record significant atmospheric transport
of sulfate and DMS from the coast to the pole, which is 10,000 feet above
sea level.
“Well, our initial hypothesis was wrong, and we found out why when went
to the South Pole,” Davis explains. “There was very little unreacted DMS
that reached the South 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 says. 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
says. “We didn’t expect it. And, initially, it made no sense. Nobody had
the foggiest notion what was going onÖ. 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 make more sense of their data as they analyze
measurements from their 2000-01 trip during the next year. Already, Davis’
colleague, Associate Professor Greg Huey, may have identified a new atmospheric
nitrogen oxide species in the Antarctic troposphere. The research team
hopes to return to Anarctica in 2003 to continue its study. 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.
