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Ozone Ozone is extremely valuable since it absorbs a range of ultraviolet energy. When an ozone molecule absorbs even low-energy ultraviolet radiation, it splits into an ordinary oxygen molecule and a free oxygen atom. Usually this free oxygen atom quickly re-joins with an oxygen molecule to form another ozone molecule. Because of this "ozone-oxygen cycle," harmful ultraviolet radiation is continuously converted into heat.

How ozone is made In the first step, an ozone molecule's life begins when ultraviolet solar radiation (with high energy; less than 240 nm in wavelength) breaks apart an oxygen molecule (O2) into two oxygen atoms. These atoms react with other oxygen molecules to form ozone molecules. O2 + (radiation < 240nm) → 2 O O2 + O + M → O3 + M In the second reaction, "M" is a so-called "third body collision partner", a molecule (usually nitrogen or oxygen) which undergoes no chemical change, but carries off the excess energy of the reaction. Ozone is formed primarily in the upper stratosphere, since the radiation at wavelengths less than 240 nm is absorbed efficiently already at about 30 km height.

How the ozone layer works The ozone molecules formed by the above reaction absorb ultraviolet radiation having wavelengths between 240 and 310 nm. The triatomic ozone molecule becomes diatomic molecular oxygen plus a free oxygen atom: O3 + (radiation < 310 nm) → O2 + O The atomic oxygen produced immediately reacts with other oxygen molecules to reform ozone: O2 + O + M → O3 + M where "M" once again denotes the third body that carries off the excess energy of the reaction. In this way, the chemical energy released when O and O2 combine is converted into kinetic energy of molecular motion. The overall effect is to convert penetrating UV light into heat, without any net loss of ozone. This cycle keeps the ozone layer in a stable balance while protecting the lower atmosphere from UV radiation, which is harmful to most living beings. It is also one of two major sources of heat in the stratosphere (the other being the kinetic energy released when O2 is photolyzed into O atoms).

concentration of ozone in the stratosphere Natural reactions other than the "ozone-oxygen cycle" described above also affect the concentration of ozone in the stratosphere. Because ozone and free oxygen atoms are highly unstable, they react very easily with nitrogen, hydrogen, chlorine, and bromine compounds that are found naturally in Earth's atmosphere (released from both land and ocean sources). For example, single chlorine atoms can convert ozone into oxygen molecules and this ozone loss balances the production of ozone by high-energy ultraviolet rays striking oxygen molecules.

This movie shows what happens when excess pollution interferes with the natural ozone formation cycle. Volatile organic compound (VOC) emissions are very reactive with nitric oxide (NO). Therefore, the hydrocarbons react with NO to produce nitrogen dioxide (NO2) that yields more NO and O that react with more hydrocarbons. The excess O reacts with oxygen (O2) to form ozone (O3). The ozone is not depleted by reactions with NO because the NO has already been used by VOC's. The net result is excess ozone.

CFCs Starting in the early 1970's, however, scientists found evidence that human activities were disrupting the ozone balance. Human production of chlorine-containing chemicals such as chlorofluorocarbons (CFCs) has added an additional factor that destroys ozone. CFCs are compounds made up of chlorine, fluorine and carbon bound together. Because they are extremely stable molecules, CFCs do not react easily with other chemicals in the lower atmosphere. One of the few forces that can break up CFC molecules is ultraviolet radiation. In the lower atmosphere, CFCs are protected from ultraviolet radiation by the ozone layer itself. CFC molecules thus are able to migrate intact up into the stratosphere. Although the CFC molecules are heavier than air, the air currents and mixing processes of the atmosphere carry them into the stratosphere.

How ozone is removed If an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules: O3 + O → 2 O2 The overall amount of ozone in the stratosphere is determined by a balance between production by solar radiation, and removal by recombination. The removal rate is slow, since the concentration of O atoms is very low. Certain free radicals, the most important being hydroxyl (OH), nitric oxide (NO), and atoms of chlorine (Cl) and bromine (Br), catalyze the recombination reaction, leading to an ozone layer that is thinner than it would be if the catalysts were not present. Most of the OH and NO are naturally present in the stratosphere, but human activity, especially emissions of chlorofluorocarbons (CFCs) and halons, has greatly increased the Cl and Br concentrations, leading to ozone depletion. Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.

ozone depletion Human activity is by far the most prevalent and destructive source of ozone depletion, while threatening volcanic eruptions are less common.  Once in the stratosphere, the CFC molecules are no longer shielded from ultraviolet radiation by the ozonelayer. Bombarded by the suns ultraviolet energy, CFC molecules break up and release chlorine atoms. Human activity, such as the release of various compounds containing chlorine or bromine, accounts for approximately 75 to 85 percent of ozone damage. Perhaps the most evident and destructive molecule of this description is chloroflourocarbon (CFC). 

ozone depletion CFCs were first used to clean electronic circuit boards, and as time progressed, were used in aerosols and coolants, such as refrigerators and air conditioners.  When CFCs from these products are released into the atmosphere, the destruction begins.  As CFCs are emitted, the molecules float toward the ozone rich stratosphere.  Then, when UV radiation contacts the CFC molecule, this causes one chlorine atom to liberate.  This free chlorine then reacts with an ozone (O3) molecule to form chlorine monoxide (ClO) and a single oxygen molecule (O2).  This reaction can be illustrated by the following chemical equation:  Cl + O3 --> O2 + ClO. 

ozone depletion Then, a single oxygen atom reacts with a chlorine monoxide molecule, causing the formation of an oxygen molecule (O2) and a single chlorine atom O + ClO --> Cl + O2 This threatening chlorine atom then continues the cycle and results in further destruction of the ozone layer .  Measures have been taken to reduce the amount of CFC emission, but since CFCs have a life span of 20-100 years, previously emitted CFCs will do damage for years to come.

ozone depletion

ozone-destroying chemicals If each chlorine atom released from a CFC molecule destroyed only one ozone molecule, CFCs would pose very little threat to the ozone layer. However, when a chlorine monoxide molecule encounters a free atom of oxygen, the oxygen atom breaks up the chlorine monoxide, stealing the oxygen atom and releasing the chlorine atom back into the stratosphere to destroy more ozone. This reaction happens over and over again, allowing a single atom of chlorine to act as a catalyst, destroying many molecules of ozone.

ozone-destroying chemicals Fortunately, chlorine atoms do not remain in the stratosphere forever. When a free chlorine atom reacts with gases such as methane (CH4), it is bound up into a molecule of hydrogen chloride (HCl), which can be carried downward from the stratosphere into the troposphere, where it can be washed away by rain. Therefore, if humans stop putting CFCs and other ozone-destroying chemicals into the stratosphere, the ozone layer eventually may repair itself.

REFERENCES http://earthobservatory.nasa.gov/Library/Ozone/ozone_2.html http://www.ucar.edu/learn/1_6_1.htm http://svs.gsfc.nasa.gov/vis/a000000/a000800/a000825/index.html http://www.atmosphere.mpg.de/enid/2__Ozone_hole/__Worksheet_1_1c5.html http://www.outdoors.org/conservation/mountainwatch/ozone1-polluted.cfm http://en.wikipedia.org/wiki/Ozone-oxygen_cycle