Stratospheric Ozone Layer
Depletion
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The
Destruction of Ozone
There
are many ways that ozone can be destroyed. As was shown in the Chapman cycle,
ozone is transformed into a molecule and an atom of oxygen by ultraviolet
light. However, this is not the only way that destruction of ozone occurs. When
molecules of other elements make their way into the stratosphere, they can
interact with the highly reactive molecule of ozone to destroy it. Many of these molecules act as catalysts
and thus accelerating the destruction of ozone by their presence. A catalyst is
a chemical that participates in a reaction without being consumed in the
reaction itself. Catalysts increase the speed of reactions or reduce the amount
of energy required to allow the reaction to occur. In the case of ozone
destruction, atoms of hydrogen, nitrogen, or other elements can act as
catalysts.
More About the Ozone Catalysts
One of the most effective and thus dangerous catalysts is chlorine. Chlorine is released by CFCs when they reach the stratosphere. Once in the stratosphere CFC molecules no longer are shielded from ultraviolet radiation by the ozone layer. Once exposed to the sun's radiation, CFC molecules release a chlorine atom or free radical. The chlorine then can react with ozone molecules, taking one oxygen atom to form chlorine monoxide and leaving an ordinary oxygen molecule behind.
See the Rowland/Molina Reaction Pathway
If each
chlorine atom released from a CFC molecule destroyed only one ozone molecule,
CFCs probably would pose very little threat to the ozone layers. However, when
a chlorine monoxide molecule encounters a free atom of oxygen, the oxygen atom
bonds with the chlorine monoxide, forcing it to release the oxygen atom. As a
result, the chlorine atom is released back into the stratosphere where it can
attack another ozone molecule. This same reaction is repeated thousands of
times in the stratosphere before the chlorine bonds to another atom that will
not release it back into the atmosphere.
Fortunately,
chlorine atoms do leave the atmosphere or there would be no ozone left. When a
free chlorine atom reacts with gases such as methane (CH4), it is bound up into
a molecule of hydrogen chloride (HCl), which can be carried from the
stratosphere into the troposphere, where it can be washed away by rain. This
removal process is important because it means that if humans stop adding
compounds to the stratosphere, eventually, they will wash themselves out and
everything will return to normal.
The
reaction pathway by which CFC molecules are photo-decomposed and work to
destroy stratospheric ozone was first theorized by Rowland and Molina. After
many years of presentations, papers, disputes, discussions, and debates, the
theory was accepted. By that time, a great deal of evidence had been gathered
to support the Rowland and Molina theory as well as show that there was indeed
a hole in the ozone layer above the continent of Antarctica.
The Ozone
Hole
In the area over Antarctica, clouds hold ice
particles that are not present at warmer latitudes. Reactions occur on the
surface of the ice particles that accelerate the ozone destruction caused by
stratospheric chlorine. This phenomenon has caused documented decreases in
ozone concentrations over Antarctica. In fact, ozone levels drop so low in
spring in the southern hemisphere that scientists have observed what they call
a "hole" in the ozone layer. At first this was not that horrifying a
discovery because there were no people living on the continent. Unfortunately,
the conditions worsened and spread. Also, at the end of spring, the hole lost
its integrity and shifted to more populated areas such as Australia and
southern Chile. Scientists began to see a global dilution of ozone as more and
more ozone was destroyed in the ozone hole.
In
addition, scientists have observed declining concentrations of ozone over the
whole globe. In the second half of 1992, for example, worldwide ozone levels
were the lowest ever recorded.
Since the 1920's, ozone has been measured
from the ground. Scientists place instruments at locations around the globe to
measure the amount of ultraviolet radiation getting through the atmosphere at
each site. From these measurements, they calculate the concentration of ozone
in the atmosphere above that location. These data, although useful in learning
about ozone, are not able to provide an adequate picture of global ozone
concentrations.
