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The
Ozone Hole of 2008
NASA Earth
Observatory-On September 12, 2008, the Antarctic ozone hole reached its
maximum size for the year. Represented by blues and purples in this image
from the Ozone Monitoring Instrument (OMI)
on NASA’s Aura
satellite, the ozone hole covered about 27 million square kilometers,
making it larger than North America, which is about 25 million square
kilometers. Though larger than it was in 2007, the 2008 ozone hole was
still smaller than the record
set in 2006.
The ozone hole is an area
of the stratosphere where the concentration of ozone drops dramatically.
The average amount of ozone in the atmosphere is roughly 300 Dobson Units;
compressed into a single layer, it would be about 3 millimeters (0.12
inches) thick—the height of 2 pennies stacked together. Any place where
the concentration drops below 220 Dobson Units is considered part of the
ozone hole. Average ozone concentrations in the ozone hole are around 100
Dobson Units—about the height of a dime.
Stratospheric ozone absorbs
ultraviolet (UV) light that can be dangerous to living things. A thinner
ozone layer increases humans’ and other creatures’ exposure to harmful
UV light. In the stratosphere, ozone blocks ultraviolet light that could
cause skin cancers, cataracts, and other damage to animals and plants if
it reached the surface. (In contrast, ozone that forms near the ground
from air pollution is harmful. It can aggravate respiratory problems like
asthma, bronchitis, and emphysema, and it can damage crops and other
plants.)
The ozone hole does not
persist year round. It forms every spring in the Southern Hemisphere
(August and September), when sunlight begins to return to Antarctica after
the months of polar darkness. During the cold, dark winter, a vortex of
winds encircles Antarctica. This vortex isolates the air in the
stratosphere from mixing with warmer air at lower latitudes. In the
extreme cold, unusual clouds, called polar stratospheric clouds, form,
even though the air is very dry. Within these clouds, chemically stable
forms of chlorine and bromine (put into the atmosphere by people) are
converted into less stable gases.
When the sunlight returns
in the spring, ultraviolet light breaks the less-stable chlorine and
bromine gases into free chlorine and bromine atoms that catalyze ozone
destruction. Ozone concentrations thin throughout August and into
September or October. As Southern Hemisphere spring progresses,
temperatures climb, and the vortex of winds that isolated the Antarctic
stratosphere weakens. Ozone-destroying chlorine and bromine gases disperse
into the rest of the atmosphere. Ozone destruction ceases for the year,
and ozone levels begin to rebuild throughout the summer.
Although the ozone hole is
not the cause of global warming, the two man-made climate changes are
related in other ways. The destruction of ozone has caused the Antarctic
stratosphere to be cooler than it would normally be in the summer and
fall. Models and observations indicate that the cooling intensifies the
vortex of winds that isolate frigid Antarctic air from warmer air from
lower latitudes. Ozone-hole-related cooling may be the reason why interior
Antarctica has not warmed up as fast as most of the rest of the planet due
to global warming. NASA scientists are using data from satellites like
Aura along with computer simulations of atmospheric chemistry and global
climate to predict how recovery of the ozone hole could accelerate
Antarctic warming.
NASA
image courtesy Ozone Hole
Watch. Caption by Holli Riebeek.
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