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Using data from the
Naval Research
Laboratory's Polar Ozone and Aerosol Measurement
(POAM
III) instrument, scientists have confirmed that despite
the
fact that this year's Antarctic ozone hole is shrinking prematurely,
the chemistry that causes the ozone depletion in the ozone hole
is unchanged from recent prior years. This corroborates the recent
findings from NASA and NOAA scientists that the unique nature
of this year's ozone hole is most probably the result of unusual
stratospheric weather patterns.
Research since the
discovery of the
ozone hole in 1985 has shown that it results
from chemical
ozone destruction by reactions involving chlorine
and bromine
compounds. The stage is set for these chemical reactions
by
high thin clouds called polar stratospheric clouds (PSCs)
that
form within the polar vortex, a very cold mass of swirling
air
located over the polar cap each winter. During the winter,
chemistry on the surface of these clouds converts chlorine into
a form that readily destroys ozone once the sun rises over the
southern polar cap in early spring, forming the ozone hole. The
ozone hole persists until the vortex begins to dissipate later
in the austral spring, letting in air that contains normal amounts
of ozone. The ozone hole, thus, requires both the presence of
chlorine and bromine compounds, and a persistent, cold, polar
vortex which allows formation of PSCs while isolating air within
the vortex.
Beginning in late
May 2002, POAM observed the amount
of ozone in the southern hemisphere
polar stratosphere from 25
km up to 60 km to be unusually variable.
This variability was
presumably due to much larger than normal
large-scale wave
disturbances that affected the stratospheric
wind flow
throughout this southern hemisphere winter, perhaps
setting the
conditions for the smaller ozone hole. The enhanced
wave
activity likely weakened and warmed the polar vortex. As
a
result, the frequency with which POAM observed PSCs this year
was smallest in the eight-year POAM record.
When the sun began
to rise over the polar stratosphere in the late winter, ozone
was observed to begin decreasing in a typical fashion. In fact,
the average ozone loss rate at altitudes near 18 km (the center
of the altitude region affected by ozone loss in the ozone hole)
from mid-August through 20 September was virtually indistinguishable
from that observed in other years. Although PSCs were less frequent
this year, it appears that they were sufficient to prime the
atmosphere for the large ozone chemical losses which occur in
the ozone hole.
On September 24th,
2002, a stratospheric wave
disturbance caused a poleward influx
of much higher ozone
abundances from mid-latitudes at altitudes
above 20 km.
However, the POAM observations show that at lower
altitudes
chemical processes caused ozone abundances to continue
to
decline to the very low typical values found in the ozone
hole
during this season. These observations confirm that unusual
stratospheric weather patterns, rather than chemistry, are
responsible
for the unusually small Antarctic ozone hole this
year.
The POAM III instrument was launched in March
1998 and is still
operational. It is the successor to NRL's
POAM II instrument,
which operated from October 1993 to
November 1996. The POAM measurement
complement includes
profiles of ozone, water vapor, nitrogen
dioxide and aerosol
particles in the polar stratosphere of both
hemispheres. POAM
is well suited for investigation of Antarctic
ozone hole and
Arctic ozone depletion events, and the POAM instruments
have
now monitored the evolution of the Antarctic ozone hole
for
eight years.
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