Stratospheric ozone (O3) is produced by the combination of an oxygen atom (O) with an oxygen molecule (O2). The basic steps to formation are:
In the above diagram, oxygen atoms are represented as dark blue circles. This reaction is written in chemical equations as
O2 + UV => O + O
2 O + 2 O2 + third molecule => 2 O3 + third molecule
Net Reaction: 3 O2 + UV => 2 O3
UV radiation is also involved in the destruction of O3.
This destruction is expressed as
O + O3 => 2 O2
Net Reaction: 2 O3 + UV => 3 O2
UV radiation plays a crucial role in the formation and destruction of ozone. UV amounts are greatest in the tropical regions, thus it is not suprising that most of the destruction and production of ozone occurs in the tropical stratosphere.
In 1970 Dr. P. Crutzen proposed the following catalytic reaction that results in the destruction of O3.
O3 + UV => 2 O2
O + XO => X + O2
Net Reaction: 2 O3 + UV => 3 O2
In this sequence of reactions, X is an atom or molecule that acts as a catalyst to convert O3 to O2. Note that X does not change in the net reaction and so it can continue to destroy O3 molecules. The important radicals represented by X include chlorine (Cl), hydroxyl (OH), nitric oxide (NO), and bromine (Br). There is a delicate balance between the production and destruction of O3, resulting in what is referred to as an O3 shield that protects us from high energy UV radiation. This natural balance has recently been disrupted by human activities.
One molecule that can serve as the catalyst molecule X is chlorine (Cl). But how does Cl get into the stratosphere? In the 1930s useful chemical compounds known as chlorofluorocarbons (CFCs) were produced for use in refrigeration, air conditioning, solvents, aerosol spray cans, and Styrofoam™ puffing agents. They are very stable in the troposphere with lifetimes of approximately 100 years. This long lifetime allows CFCs that are emitted near the surface to be carried by the winds upward. In the stratosphere CFCs are dissociated by UV light producing chlorine atoms. The destruction of O3 then follows with the following chemical reactions:
Fortunately chlorine atoms (represented as light blue circles) do not remain in the stratosphere for centuries. If the use of CFCs and other ozone-destroying chemicals are banned, it is hoped that ozone depletion may be reduced. The term ozone depletion means that the destruction of O3 exceeds the creation of O3.
When present together in the stratosphere, chlorine (Cl) and ozone quickly react to produce chlorine oxide. Bromine can also act as a catalyst to destroy stratospheric ozone.
In 1995 Drs. Paul Crutzen, Mario J. Molina and F. Sherwood Rowland won the Nobel prize in chemistry for their work concerning the formation and decomposition of ozone.