1. CFCs are not broken down in the Troposphere because they are too unreactive
2. CFCs are broken down by the sun's high energy UV radiation in the stratosphere via photodissociation
3. This releases Chlorine radicals
4. Chlorine radicals are highly reactive and (react with/break down) ozone to form O2 and O
5. Chlorine radical act as catalysts because they can be re-used to break down more ozone
Iodine radicals are reactive and tend not to reach the stratosphere
Fluorine has a lower ozone depletion potential than Chlorine because Fluorine radicals tend to react with H2O and methane in the stratosphere to form HF which has a strong covalent bond which is not broken down easily by photodissociation
Bromine has a lower ozone depletion potential than Chlorine (but Bromine is more efficient than Chlorine at ozone depletion) because there is less Bromine in the stratosphere because the bond in a bromoalkane (C-Br) is weaker than the bond in a Chloroalkane (C-Cl), therefore bromoalkanes tend to get broken down in the troposphere
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