The concentration of atmospheric zone is small, approximately 3 molecules of ozone for every ten million air molecules. It provides a fundemental role in. Ozone primarily occurs in the stratosphere though some ozone, approximately 10% of the total amount, exists in the troposphere. Stratospheric ozone is beneficial to life because it absorbs ultraviolet radiation that is biologicaly damaging. Absorption of UV energy heats the atmosphere and is responzible for the temperature inversion observed in the stratosphere.
When atmospheric ozone is present near the surface it is considered a pollutant. Ozone is a chemically active species and . High levels of ozone can damage plant and animal tissues due to these chemical reactions. High concentrations of ozone crop production and human health. Thus, while decreased amounts of stratospheric ozone are dangerous to human health, decreased amounts of tropospheric ozone near the surface are benificial!
Stratospheric ozone (O3) is produced by the combination of an oxygen atom (O) with an oxygen molecule (O2). The basic steps to formation are:
This reaction is written in chemical equation as
O2 + UV O + O
2O + 2O2 + third molecule 2O3 + third molecule
Net Reaction: 3O2 + UV 2 O3
UV radiation is also involved in the destruction of O3.
This destruction is expressed as
O3 + UV O + O2
O + O3 2 O2
Net Reaction: 2O3 + UV 3O2
In 1970 Dr. P. Crutzen proposed the following catalytic reaction that results in the destruction of O3.
X + O3 XO + O2
O3 +UV 2O2
O + XO X + O2
Net Reaction: 2O3 + 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. 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 Styrofo am™ 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 does not remain for centuries in the stratosphere. 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.
Antarctic Ozone Hole
The total amount of ozone in the atmosphere at a given location is measured in Dobson Units, named after Dobson who developed methods of measuring atmospheric ozone. Dobson units represent how thick the . 300 Dobsons is approximately the thickness of a dime. The figure below depicts global measurements of ozone in the southern hemisphere in Dobson units.
Which month has the least amount of ozone over Antarctica?
Observations of ozone concentrations over Antarctica reveal a decreasing trend between 1955 and 1995.
Stratospheric clouds composed of ice particles exist over Antarctica but not over the mid-latitudes or tropical regions. These polar stratotspheric clouds (PSC) form in very cold air, temperatures colder that approximately -80C or -112F. Chemical reactions occur on the surface of these ice particles that accelerate the depletion of O3. Destruction is so rapid over the south pole region in the southern hemisphere spring time (e.g., October) that it has been termed a "hole in the ozone layer."
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.
Chlorofluorocarbons
Chlorofluorocarbons or CFCs do not occur naturally. CFCs were made for propellants in spray cans, Styrofoam™ puffing agents, and as coolants for refrigerators and air conditioners. The usage of two CFC compounds (CFC-11 and CFC-12) is plotted in Figure 2.7. Notice that usage decreased in the mid 1970s. When this human made, or anthropogenic, gas was found to destroy ozone, the United States banned its use, resulting in its decrease.
In the 1980s other countries increased their use of CFCs. In response to the discovery of CFCs role in destroying ozone, representatives from 23 nations met in Montreal, Canada in 1987 to address concerns of ozone depletion by CFCs. The resulting Montreal Protocol called for a 50% reduction in the usage and production of this anthropogenic gas by the year 1999. This accounts for the decline shown in Figure 2.7 in the late 1980s. While the use of these chemicals has declined, the atmospheric concentrations have not, as shown in Figure 2.8. This is because CFCs are very stable molecules and will stay in the atmosphere for nearly 100 years before finally declining.