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SCIENTISTS
DISCOVER NEW KEYS TO ARCTIC OZONE LOSS
2/8/01-Scientists have
discovered a new class of particles present in the Arctic's winter
stratosphere that likely play an important role in ozone loss in that
region, the Commerce Department's National
Oceanic and Atmospheric Administration announced today. These findings
give scientists a better understanding of the processes that set the stage
for chlorine-caused ozone depletion in the stratosphere above the Arctic.
An international team of scientists
published their findings in the Feb. 9 issue of Science
magazine. Lead author David W. Fahey, a scientist at NOAA's
Aeronomy Laboratory in Boulder, Colo., and colleagues, discovered a
new class of polar stratospheric cloud (PSC) particles, very large in
size, that had previously been undetected.
Scientists have known for some time
that during the cold Arctic winter, icy, nitric acid-containing polar
stratospheric cloud particles are formed, which enhance the destruction of
ozone by human emissions of chlorine and bromine.
The newly discovered particles have
diameters of 10-20 microns (millionths of a meter), which is about 10-20
times larger in diameter than those typically observed in PSCs. These
particles have eluded detection to date because of their unexpectedly
large size and their very small abundance in the atmosphere. The PSCs are
laden with nitric acid (HNO3), and thus serve as reservoirs for nitrogen
in the polar stratosphere. As the particles "sediment" or fall
out of the atmosphere, the stratosphere becomes "denitrified".
The discovery of this new class of large PSCs helps to explain a
longstanding mystery, namely, that the extent of denitrification observed
in the polar stratosphere could not be accounted for by the smaller, more
slowly-falling PSCs.
The researchers say that the loss of
nitrogen has important consequences for the ozone because ozone-destroying
forms of chlorine and bromine are longer-lived in a denitrified
stratosphere.
Fahey and colleagues made the
observations during the period of January-March, 2000, using instruments
placed onboard a NASA ER-2 high-altitude research aircraft. Those
instruments measured reactive nitrogen species as the aircraft traveled
toward the Pole and deep into the region of highest ozone loss.
The large particles observed in some
of the air samples contained 15-20 percent of the available reactive
nitrogen in the Arctic stratosphere, and were falling at a rate of 1-2
kilometers per day. This indicates that these particles may play an
important role in denitrifying the stratosphere. The 2000 Arctic winter
stratosphere was extensively denitrified. Large ozone losses of up to 70
percent occurred in the lower stratosphere throughout the Arctic vortex,
according to Fahey.
Cold temperatures promote the growth
of large PSCs and enhance the loss of ozone by chlorine and bromine.
Unusually cold winters, or climate shifts that reduce stratospheric
temperatures or increase the amount of water vapor in the stratosphere,
could prolong chemical ozone loss in the Arctic even as chlorine levels
fall in response to international curtailments in the use of
ozone-depleting chemicals.
"Our findings suggest that we
still need to learn more about how these large PSCs are formed, so that
we'll have a better understanding of how the ozone layer will recover in
the future," Fahey said.
For more information, visit the
Aeronomy Laboratory Web site: http://www.al.noaa.gov
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