Ozone

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Earth's ozone layer plays a critical role in protecting Earth's surface from the Sun's harmful ultraviolet (UV) radiation. Every ozone molecule, which consists of three oxygen atoms, has the ability to absorb a certain amount of UV radiation. Under normal circumstances, the ozone layer, which is located in the stratosphere between 15 and 50 kilometers (9 and 31 miles) above Earth, remains in a continuous balance between natural processes that both produce and destroy ozone.

Ozone is produced in the upper atmosphere through a two-step chemical process that involves oxygen and UV radiation.

O + UV radiation → O + O

O + O ₂ → O ₃

The process begins with UV radiation breaking apart molecular oxygen (O ₂ ), thus producing two oxygen (O) atoms. In the second step, an oxygen atom (O) recombines with an oxygen molecule (O ₂ ) to form an ozone (O ₃ ) molecule.

Ozone can also be naturally destroyed through reactions with chlorine, nitrogen, and hydrogen. For example, chlorine can be a very effective destroyer of ozone via the following set of reactions.

Cl + O ₃ → ClO + O ₂

ClO + O → Cl + O ₂

In this process, a chlorine atom (Cl) reacts with ozone (O ₃ ) to produce chlorine monoxide (ClO) and an oxygen molecule (O ₂ ). ClO can then combine with an oxygen atom (O) to reform Cl and O ₂ . In this reaction set, because chlorine is reformed after destroying ozone, the cycle can repeat itself very quickly.

In recent years global chlorine levels have increased due to the use of chlorofluorocarbons (CFCs) , a large class of chemicals useful in a variety of industries. Under certain circumstances, even a single chlorine atom released from a CFC's molecule can destroy many thousands of ozone molecules through a chemical chain reaction. Current declines in global ozone levels and the development of the Antarctic ozone hole have both been linked to CFC use.

Although ozone concentrations in the upper atmosphere play an important role in protecting Earth's surface from harmful UV radiation, ozone at its surface is a pollutant harmful to human health. Enhanced levels of surface ozone are often the result of automobile exhaust and pose a serious health risk. Fortunately, current levels of surface ozone (also known as smog) over most major cities have declined to healthier levels due in part to domestic and international governmental regulations.

SEE ALSO Atmospheric Chemistry .

Eugene C. Cordero

Bibliography

Graedel, T. E., and Crutzen, Paul J. (1993). Atmospheric Change: An Earth System Perspective. New York: W. H. Freeman.

Internet Resources

Stratospheric Ozone: An Electronic Textbook. Available from http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm .