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Ozone is produced in the upper atmosphere when ultraviolet radiation from the sun is absorbed by oxygen molecules. Oxygen molecules are good at absorbing UV radiation but the energy is just the right wavelength to break the bond between the two oxygen atoms resulting in 2 free radical oxygen atoms. These oxygen radicals then react with normal oxygen atoms to produce ozone. (They may also recombine to re-form oxygen or react with existing ozone molecules to re-form normal oxygen.) Ozone is even better than normal oxygen at absorbing UV radiation and thus helps shield us from excessive UV radiation. When it does, it breaks down to form a normal oxygen atom and a new oxygen radical - which can then later react to re-form some new ozone.

Halogenated gases - hydrocarbons with one or more hydrogens replaced by halogens like chlorine and especially those with fluorine are very stable in the lower atmosphere. They have been popular as solvents, degreasers, and refrigerants because they are non-flammable and very stable. Because they are so stable, they tend to stay in the atmosphere for a long time once they evaporate. Eventually they diffuse up into the high atmosphere where the energy of the sunlight is much more intense - intense enough to start breaking them down and forming radicals from the halogen atoms.

These halogen atoms compete with the reactions that produce ozone to accelerate the breakdown of ozone and soak up oxygen radicals that would otherwise form ozone.

As an example - if the halogen is chlorine:

O2 + hv " border="0">O + O (1)O+O2" border="0">O3 (2)O+O" border="0">O2 (3)O+O3 " border="0">2O2 (4)O3 + hv " border="0">O2 + O (5)Cl+O3" border="0">O2+ClO (6)ClO+O" border="0">Cl+O2 (7)

Reaction 6 breaks down ozone like reactions 4 and 5, but unlike 5, it doesn't yield a new oxygen radical to be used to make more ozone - instead, it produces ClO which then (by way of reaction 7) competes with reaction 2 to soak up the available oxygen radicals that could form new ozone. Note that when reaction 7 occurs, it produces a new chlorine radical to promote reaction 6. Eventually the chlorine radicals may combine with something else to produce a more stable molecule - perhaps HCl - but in the meantime they can go through a lot of cycles of inhibiting ozone formation and promoting ozone destruction.

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8y ago

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