Ozone Destruction Catalysts
Though
most of the water that evaporates from the surface of the Earth returns as
precipitation, some does escape the troposphere. The concentration of water in
the stratosphere is approximately 5 parts per million. Though small, this is
still significant because of the way the water reacts with atomic oxygen and
ozone. Once in the stratosphere, ultraviolet radiation causes the water
molecule, chemical formula H2O, to break down into two parts, H and OH. These
two parts individually react in the stratosphere to break down ozone in the
following manner.
H + O3 --->
OH + O2
OH + O ---> H + O2
When these two equations are added together, you get:
H + O3 +
OH + O ---> OH + H + 2 O2
Crossing out the terms which appear on both sides gives:
O + O3 --->
2 O2
In
this way, the water is still present at the end of the reaction and is
available to continue the cycle. It has participated in the reaction without
being consumed and thus is a catalyst. This is a natural process that is
necessary to maintain the balance of ozone in the stratosphere. Trouble arises
however, when other molecules which do not normally occur in the stratosphere
or which are usually found in lower concentration make their way into the
stratosphere due to human activity. Chlorine is one of these chemicals.