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Polar
Ozone Depletion
Total ozone in
polar regions is measured by well-calibrated satellite instruments. Shown
here is a comparison of average springtime total ozone values found
between 1970 and 1982 (solid and dashed red lines) with those in later
years. Each point represents a monthly average in October in the Antarctic
or in March in the Arctic. After 1982, significant ozone depletion is
found in most years in the Arctic and all years in the Antarctic. The
largest average depletions have occurred in the Antarctic since 1990. The
ozone changes are the combination of chemical destruction and natural
variations. Variations in meteorological conditions influence the
year-to-year changes in depletion, particularly in the Arctic. Essentially
all of the decrease in the Antarctic and usually most of the decrease in
the Arctic each year are attributable to chemical destruction by reactive
halogen gases. Average total ozone values over the Arctic are naturally
larger at the beginning of each winter season because more ozone is
transported poleward each season in the Northern Hemisphere than in the
Southern Hemisphere.
The
stratospheric ozone layer resides between about 10 and 50 kilometers (6 to
31 miles) above Earth’s surface over the globe. Long-term observations
of the ozone layer with balloonborne instruments allow the winter
Antarctic and Arctic regions to be compared. In the Antarctic at the South
Pole, halogen gases have destroyed ozone in the ozone layer beginning in
the 1980s. Before that period, the ozone layer was clearly present, as
shown here using average ozone values from balloon observations made
between 1962 and 1971. In more recent years, as shown here for 2 October
2001, ozone is destroyed completely between 14 and 20 kilometers (8 to 12
miles) in the Antarctic in spring. Average October values in the ozone
layer now are reduced by 90% from pre-1980 values. The Arctic ozone layer
is still present in spring as shown by the average March profile obtained
over Finland between 1988 and 1997. However, March Arctic ozone values in
some years are often below normal average values as shown here for 30
March 1996. In such years, winter minimum temperatures are generally below
PSC formation temperatures for long periods. Ozone abundances are shown
here with the unit “milli-Pascals” (mPa), which is a measure of
absolute pressure (100 million mPa = atmospheric sea-level pressure).
Satellite
instruments monitor ozone and reactive chlorine gases in the global
stratosphere. Results are shown here for Antarctic winter for a narrow
altitude region within the ozone layer. In winter, chlorine monoxide (ClO)
reaches high values (1500 parts per trillion) in the ozone layer, much
higher than observed anywhere else in the stratosphere because ClO is
produced by reactions on polar stratospheric clouds. These high ClO values
in the lower stratosphere last for 1 to 2 months, cover an area that at
times exceeds that of the Antarctic continent, and efficiently destroy
ozone in sunlit regions in late winter/early spring. Ozone values measured
simultaneously within the ozone layer show very depleted values.
Values are
shown for key parameters of the Antarctic ozone hole: the area enclosed by
the 220-DU total ozone contour and the minimum total ozone amount, as
determined from space-based observations. The values are averaged for each
year near the peak of ozone depletion, as defined by the dates shown in
each panel. The ozone hole areas are contrasted to the areas of continents
in the upper panel. The intensity of ozone depletion gradually increased
beginning in 1980. In the 1990s, the depletion reached fairly steady
values, except for the anomalously low depletion in 2002. The intensity of
Antarctic ozone depletion will decrease as part of the ozone recovery
process
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