Geoscience Reference
In-Depth Information
were outside of the
range of values. It was in this way, through the use
of multiple observing systems, that the world
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normal
'
first became aware of the Antarctic
ozone hole.
How does the ozone hole develop and why is it only observed over the Antarctic?
Before answering these questions we should
first explain what is meant by the
term
. The ozone hole represents a near total depletion of ozone within a
portion of the stratosphere and develops each year during the Antarctic spring
months of September and October.
In order to understand why the ozone hole forms over the Antarctic it is useful
'
ozone hole
'
to
first understand how ozone can be destroyed in the atmosphere. Ozone molecules
break down naturally when they absorb ultraviolet radiation, but they can also be
destroyed by chemical reaction with atoms of chlorine. It has been estimated that
one atom of chlorine is capable of destroying 100 000 ozone molecules. One
atmospheric source of chlorine is chloro
uorocarbons (CFCs), previously in wide
use as a propellant for aerosol spray cans and in refrigerants up to the 1990s.
CFCs have a lifetime in the atmosphere of approximately 100 years and though
released in the lower portion of the atmosphere will slowly mix upward, eventually
reaching the stratosphere. In the presence of ultraviolet radiation CFCs are broken
down, freeing chlorine atoms, which can then destroy ozone. The process described
above occurs in the stratosphere all around the world. However, a unique
combination of events takes place over the Antarctic stratosphere to produce
the rapid spring depletion of ozone.
The stratosphere is normally heated by the absorption of ultraviolet radiation,
but during the Antarctic winter no solar radiation, and thus no ultraviolet radiation,
is present and the stratosphere becomes quite cold. In addition, the lower portion
of the atmosphere is also cold at this time of the year due to longwave radiation
losses at the surface. These very low temperatures throughout the atmospheric
column cause the formation of the polar vortex, a band of very strong westerly
winds that encircle the Antarctic and limit the amount of mixing of air between
Antarctica and lower latitudes. This lack of mixing drives temperatures down further
so that the temperature in the Antarctic stratosphere can drop to
90
C. At
-
85
C polar stratospheric clouds, also known as nacreous
clouds, can develop. Unlike
temperatures below
-
tropospheric clouds, which are composed of
liquid water and ice, polar stratospheric clouds are composed of ice and nitrogen
compounds. Chemical reactions on the particles which make up these clouds lead
to the breakdown of CFCs and the formation of molecular chlorine. Unlike
atomic chlorine, molecular chlorine does not destroy ozone. The presence of
polar stratospheric clouds during the Antarctic winter leads to a destruction of
CFCs and an increase in the amount of molecular chlorine in the Antarctic
stratosphere.
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normal
'
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