Sea salt
seasons chemical brew that destroys Arctic ozone
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January 18,
2001-Purdue University WEST LAFAYETTE, Ind. –
Sunlight, snow and sea salt are sometimes used to illustrate nature at its
best. But new scientific evidence shows that, when combined, these forces
provide a potent mixture for destroying ozone.
Purdue University
researchers, working with scientists from Battelle and the University of
California, Irvine, discovered that bromine and chlorine, two chemicals
found in sea salt, may initiate a series of chemical events that destroy
ozone in the Arctic troposphere, the lowest part of the atmosphere.
The findings, published
in the Jan. 19 issue of the scientific journal Science, describe the role
that bromine and chlorine play in a complex series of chemical reactions
that occur each spring when, after several months of darkness, sunlight
interacts with the snow in the Arctic region.
The study traces the
source of those chemical precursors to the salty minerals found in sea
water that is carried into the snowpack in the form of tiny salt particles
by wind and waves.
"Bromine and
chlorine have long been suspected as major players in this series of
chemical events, but until now the source of these chemicals was
unknown," says Paul Shepson, professor of atmospheric chemistry at
Purdue. "Our findings indicate that this near-surface ozone depletion
in the Arctic is a naturally occurring event."
Ozone, a beneficial
component of the earth's upper atmosphere, is a pollutant at the ground
level but is essential to the health of the atmosphere, Shepson says.
The findings may help
scientists develop better models to simulate and predict long-term changes
in the composition of the earth's atmosphere, he says. "Because
three-quarters of the earth's surface is covered by ocean, we've uncovered
a process we need to understand much better in terms of our ability to
model ozone in the atmosphere."
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Bromine
in a Test Tube
Paul
Shepson
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Current models don't take these
interactions into account because scientists are only beginning to recognize and
document the role snow and sea salt play in atmospheric chemistry.
In 1986, scientists 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.
"This was a big surprise to
us, and it indicated that our understanding of atmospheric ozone, and the
factors that lead to its production and depletion, is poor," Shepson says.
Since then, scientists have found
evidence that a number of chemicals that lay dormant in the snow can interact
with sunlight to produce chemical "pollutants" – such as nitric
oxide, nitrogen dioxide, formaldehyde and bromine – that impact the
atmosphere's composition.
Shepson and colleague Jan
Bottenheim of Environment Canada led a research group to the Canadian Arctic
last year to measure levels of bromine and chlorine in the snowpacks and analyze
how sunlight interacts with these chemicals during the polar sunrise.
From a research site at the
Canadian Forces base in Alert, Canada, the group measured levels of bromine and
chlorine in the snow and air over a two-month period, beginning in early
February. The bromine and chlorine measurements were conducted under the
leadership of Chet Spicer of Battelle-Columbus, and Barbara Finlayson-Pitts of
the University of California, Irvine.
The measurements show that in
mid-March, when the sun began to rise over the Arctic region, these elements
increased in the air while decreasing in the snow.
Because molecular bromine is
short-lived in the atmosphere and can't be transported very far, Shepson and his
colleagues began looking for sources of bromine from the local environment.
"Sea salt is the only source of bromine in the Arctic," he says.
Bromine and chlorine also play a
role in destroying ozone in the upper atmosphere over the Arctic and Antarctic.
In those cases the sources of bromine and chloride come primarily from
human-derived pollutants, Shepson says.
Though the study indicates that
ozone-depletion is a naturally occurring process in the Arctic, the findings
suggest that mixtures of snow and salt on roads in urban areas may also affect
air quality, Shepson says.
"The ingredients from what
we observe in the Arctic are sometimes present in high concentrations on the
streets of urban areas during the winter months," says Shepson, an expert
in chlorine chemistry. "I wouldn't want to make predictions, but there
could be a variety of chemical interactions that occur in urban, road-salted
environments that we need to understand."
ABSTRACT
The Role of Br2 and BrCl in Surface
Ozone
Destruction at Polar Sunrise
Krishna L. Foster, Robert Plastridge,
Jan Bottenheim, Paul Shepson, Barbara Finlayson-Pitts and Chester Spicer
Bromine storms are believed to play a central role in
the depletion of surface-level ozone in the Arctic at polar sunrise. Br2, BrCl
and HOBr have been hypothesized as bromine atom precursors, and there is
evidence for chlorine atom precursors as well, but these species have not been
measured directly. We report here measurements of Br2, BrCl, and Cl2 made using
atmospheric pressure chemical ionization-mass spectrometry at Alert, Nunavut,
Canada. In addition to Br2, at mixing ratios up to approximately 25 parts per
trillion, BrCl was found at levels as high as approximately 35 parts per
trillion. Molecular chlorine was not observed, implying that BrCl is the
dominant source of chlorine atoms during polar sunrise, consistent with recent
modeling studies. Similar formation of bromine compounds and tropospheric ozone
destruction may also occur at mid-latitudes but may not be as apparent owing to
more efficient mixing in the boundary layer.
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