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Stratospheric ozone is created and destroyed primarily by ultraviolet radiation.
The air in the stratosphere is bombarded continuously with ultraviolet radiation from the Sun. When
high energy ultraviolet rays strike molecules of ordinary oxygen (O2), they split the molecule into two
single oxygen atoms. 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. When an ozone molecule is exposed to ultraviolet energy it may
break back into O2 and O. During dissociation the atomic and molecular oxygens gain
kinetic energy,
which produces heat and causes an increase in atmospheric temperature.
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 wavelenghts 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, (15k jpeg) 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 troposhere 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 (61k jpeg) 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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