Atmospheric Chemistry: Ozone Production and Destruction

Stratospheric ozone is created (1 MB Quicktime)and destroyed (1 MB Quicktime) primarily by ultraviolet radiation as molecules of ordinary oxygen (O2) are split and the free oxygen atoms can then combine with oxygen molecules (O2) to form ozone (O3) molecules.

O2 + UV light -> 2 O
O + O2 + M -> O3 + M (where M indicates conservation of energy and momentum)

The same characteristic of ozone that makes it so valuable, its ability to absorb a range of ultraviolet radiation, also causes its destruction.

Ozone production is driven by UV radiation of wavelengths less than 240 nm. Ozone dissociation typically produces atomic oxygen that is stable when exposed to longer wavelengths, up to 320 nm, and shorter wavelengths of 400 to 700 nm. Longer wavelength photons penetrate deeper into the atmosphere, creating regions of ozone production and destruction. When an ozone molecule absorbs even low energy ultraviolet, it splits into an ordinary oxygen molecule and a free oxygen atom.

O3 + UV, visible light -> O + O2

The free oxygen atom may then combine with an oxygen molecule, creating another ozone molecule, or it may take an oxygen atom from an existing ozone molecule to create two ordinary oxygen molecules.

O + O2 -> O3 or O3 + O -> O2 + O2

Processes of ozone production and destruction, initiated by ultraviolet radiation, are often referred to as "Chapman Reactions."

Most O3 destruction takes place through catalytic processes rather than Chapman Reactions. Ozone is a highly unstable molecule that readily donates its extra oxygen molecule to free radical species such as nitrogen, hydrogen, bromine, and chlorine. These compounds naturally occur in the stratosphere, released from sources such as soil, water vapor, and the oceans.

O3 + X -> XO + O2 ( where X may be O, NO, OH, Br or Cl)

Anthropogenic Destruction Manufactured compounds are also capable of altering atmospheric ozone levels. Chlorine, released from CFCs,   and bromine (Br), released from halons, are two of the most important chemicals associated with ozone depletion. Halons are primarily used in fire extinguishers. CFCs are used extensively in aerosols, air conditioners, refrigerators, and cleaning solvents. Two major types of CFCs are trichlorofluorocarbon (CFCl3), or CFC-11, and dichlorodifluoromethane (CF2Cl2), or CFC-12. Trichlorofluorocarbon is used in aerosols, while dichlorodifluoromethane is typically used as a coolant.

CFCs were originally created to provide a substitute for toxic refrigerant gases and reduce the occupational hazard of compressor explosions. Near Earth's surface, chloroflourocarbons are relatively harmless and do not react with any material, including human skin. For 50 years they appeared to be the perfect example of a benign technical solution to environmental and engineering problems, with no negative side effects. While CFCs remain in the troposphere they are virtually indestructible. They are not water soluble and cannot even be washed out of the atmosphere by rain. We now understand that the very quality that made them seem so safe, their stability, is what makes them so dangerous. CFCs remain in the troposphere for more than 40 years before their slow migration to the stratosphere is complete. Even if we were to end their production and use at this very moment, they would continue to contribute to ozone destruction far into the future.

In the stratosphere, high energy ultraviolet radiation causes the CFC molecules to break down through photodissociation. Atomic chlorine, a true catalyst for ozone destruction, is released in the process. Chlorine initiates and takes part in a series of ozone destroying chemical reactions and emerges from the process unchanged. The free chlorine atom initially reacts with an unstable oxygen containing compound, such as ozone, to form chlorine monoxide (ClO).

Cl + O3 -> ClO + O2

The chlorine monoxide then reacts with atomic oxygen to produce molecular oxygen and atomic chlorine. The regenerated chlorine atom is then free to initiate a new cycle.

ClO + O -> Cl + O2

This destructive chain of reactions will continue over and over again, limited only by the amount of chlorine available to fuel the process.

Chlorine occurs naturally in the oceans. However, the majority of chlorine in the atmosphere has originated with man-made chemicals. Without the breakdown of manufactured chlorofluorocarbons, there would be almost no chlorine in the stratosphere. CFC-12 concentrations were less than 100 parts per trillion by volume when they were first measured in the 1960s. Between 1975 and 1987, concentrations more than doubled from less than 200 parts per trillion by volume to more than 400 parts per trillion by volume. The amount of chlorine in the stratosphere increased by a factor of 2 to 3. Scientists believe that continued buildup of CFCs could lead to severe ozone loss  worldwide. Ongoing studies are essential to provide the necessary understanding of the causes of ozone depletion. The history of CFCs demonstrates that human activities can have an unexpected long-term effect on the environment.

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