<Li> Cl + O 3 → ClO + O A chlorine atom removes an oxygen atom from an ozone molecule to make a ClO molecule </Li> <Li> ClO + O 3 → Cl + 2 O This ClO can also remove an oxygen atom from another ozone molecule; the chlorine is free to repeat this two - step cycle </Li> <P> The overall effect is a decrease in the amount of ozone, though the rate of these processes can be decreased by the effects of null cycles . More complicated mechanisms have also been discovered that lead to ozone destruction in the lower stratosphere . </P> <P> A single chlorine atom would continuously destroy ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO 2). Bromine is even more efficient than chlorine at destroying ozone on a per atom basis, but there is much less bromine in the atmosphere at present . Both chlorine and bromine contribute significantly to overall ozone depletion . Laboratory studies have also shown that fluorine and iodine atoms participate in analogous catalytic cycles . However, fluorine atoms react rapidly with water and methane to form strongly bound HF in the Earth's stratosphere, while organic molecules containing iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities . </P>

Where was the ozone hole observed in 1985