In atmospheric chemistry, a null cycle is a catalytic cycle that simply interconverts chemical species without leading to net production or removal of any component. In the stratosphere, null cycles are very important in contrasting series of reactions that lead to net depletion in the ozone layer.
One such cycle involves the nitrogen oxide species, which are the most responsible for ozone depletion in the stratosphere. The catalytic cycle is:
NO + O3 → NO2 + O2
NO2 + O → NO + O2
Net: O + O3 → 2O2
while the corresponding null cycle competes due to the possible photolysis of NO2 which allows conservation of the odd oxygen species:
NO + O3 → NO2 + O2
NO2 + hν → NO + O
Net: O3 + hν → O + O2
Since O and O3 can exchange rapidly, the last cycle does not affect the rate of consumption of ozone which thus decreases during the day when photolysis can occur. NO can also react with other free radicals, such as chlorine and bromine, providing pathways that lead to null cycles:
Cl + O3 → ClO + O2
ClO + NO → Cl + NO2
NO2 + hν → NO + O
Net: O3 + hν → O + O2
Other null cycles, also termed holding cycles, produce reservoirs, effectively holding up the reactive species. An example is the formation of dinitrogen pentoxide:
NO2 + NO3 → N2O5
This can lock up about 10% of the NOx family of species present in the atmosphere, limiting their ability to participate in the ozone-destructing catalytic cycles.