Ozone, or O₃, is contained primarily in the upper atmospheric layer called the stratosphere. The stratosphere is located approximately 15 km to 35 km above the surface of the Earth. Structurally, ozone (depicted here in VRML) is a bent molecule and due to its triatomic oxygen structure, is quite unstable.
   In general, the production of ozone in the stratosphere does not occur except in a 3-body collision. The combination of elemental and diatomic oxygen is not enough to force the creation of ozone. Thus, there needs to be a third body, M, that can push the reaction to completion and form ozone.

As the two charts show below, as the altitude increases, the temperature decreases, as expected, but then starts to increase. This is due to the exothermic, three-body collisions in the formation of ozone. When the supply of O₂ decreases, the concentration of ozone decreases and the temperature goes down again. The natural destruction of ozone occurs by two methods: the most common being the attack of the elemental oxygen and the O₃. The other method is the UV-B attacking O₃ and sending is back to the elemental stage.

    CFCs, or chlorofluorocarbons, are a class of chlorinated hydrocarbons developed by DuPont in the 1930s. It replaced ammonia and sulfur dioxide gases in the refrigeration process and eliminated the fear of dangerous toxic gas leaks and explosions. CFCs later came to be used in automobile air conditioners, propellants in aerosol cans, solvents and foam-blowing agents¹. This class of compounds was seen as a marvel of science because they were very stable and seemingly inert.
      The whole class of compounds known as chloroflurocarbons contains such species as CH₂Cl₂, CF₂Cl₂, and CHF₂Cl. To distinguish between two or more of these, a unique naming system was developed. To deduce the formula of a CFC, take the code number and add 90 to it. The resulting 3-digit number corresponds to the number of carbon, hydrogen, and fluorine atoms in the molecule. To determine the number of chlorine atoms in that molecule, use the formula 2n + 2, where n is the number of carbon atoms. Subtract the total number of hydrogens and fluorine atoms from that value and that will be the number of chlorine atoms. For example, CFC-12, adding 90 gets 102, thus there is one carbon, zero hydrogen, and two fluorines. To determine the chlorine count, subtract 2 (F) from 2(1) + 2 = 2. Thus the resulting formula is CF₂Cl₂, for CFC-12.

If all of the ozone in the stratosphere were to be compressed such that the pressure would be equal to that of the surface of the earth, the ozone layer would only be 3mm thick! Due to its physical properties, Ozone actually filters out 95-99% of the radiation in the 220 nm - 320 nm range. In this range is contained the incoming ultraviolet light, specifically that of the UV-C (200-280nm) and UV-B (280-320) type radiations. These two are the most harmful of the ultraviolet range. From long enough exposures, UV-B and UV-C can cause skin cancer, kill plankton in lakes and rivers and even cause retinal damage.
      The amount of total atmospheric ozone is measured and expressed in terms of Dobson units (DU). One DU is equivalent to a 0.01 mm thickness of pure ozone at the density it would posses if it were brought to ground level pressure (1 atm). Normal ozone levels are approximately 350 DU in temperate climates. The bulk of ozone is created in the equatorial regions, but due to stratospheric winds from the natural hot to cold temperature flow, ozone is spread away from that region, thus concentration usually average 250 DU around the equator and about 450 DU in the subpolar regions during non-hole forming times.²

   The chemistry by which CFCs break down ozone is an extremely complicated process. It was only until recently, 1990s in fact, that the chemistry of ozone creation and destruction was developed by F. Sherwood Rowland, Mario Molina (Rowland's post-doc at the time), and Paul Crutzen and in 1995 they received the Nobel Prize for their work. Molina proposed that within a short amount of time, 30% of all ozone will deplete; two years later, all chlorofluorocarbons were banned in the United States. The graphic to the left demonstrates how CFCs break down the ozone layer.

    What makes chloroflurocarbons so detrimental to the ozone layer, is there instability in the stratosphere. On the surface of the earth, CFCs are very stable molecules, but when they are released into the atmosphere and float up to the stratosphere, a different process takes place: photochemistry. What breaks down CFCs, Dr. Rowland surmised, is the same source that naturally breaks diatomic oxygen apart, UV-C radiation:

Not only do CFCs release multiple excited chlorines, but since vertical motion in the stratosphere is so slow, atmospheric lifetimes of 60 to over 100 years are possible.
      Chlorofluorocarbons are not the only ozone depleting substances. Methyl bromide, carbon tetrachloride and several other forms of CFCs located in this table are also contributors to the depletion process.

THE COMPLETE PROCESS

    The multi-directional flow chart³ above sums up all of the ozone prosses in the stratosphere, both formation and destruction by chlorinated compounds. The center chlorine radical is the product of several reactions, namely that of UV radiation and CFCs, and also the reagent of the destruction process.
    The bottom left contains the ozone cycle, containing the natural formation and destruction of O₃. The ozone comes in contact and reacts with the free chlorine atoms and produces ClO^., which reacts with itself to form Cl₂O₂. That can degrade further back into Cl^. and oxygen.
    During the polar winter of the southern hemisphere, the temperature drops considerably, creating a polar vortex about the south pole. In the atmosphere are many other chemical species present, such as methane, nitrogen dioxide, and of course, chlorinated compounds. During this time, two major species form in the ice clouds called PSCs, or Polar Stratospheric Clouds, ClONO₂ and HCl. These are called "storage compounds" because they store or tie-up the chlorine and no depletion of ozone occurs during this time. When spring-time comes, the vortex ends and the temperature rises, releasing these storage compounds and, with the aide of the sun, makes large amounts of elemental chlorine.

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