- Nitrogen oxides are introduced directly into the stratosphere by the supersonic jet aircraft engines in the form of exhaust gases.
- These oxides arc released by combustion of fossil fuels and nitrogen fertilisers. Inert nitrous oxide in the stratosphere is photochemically converted into more reactive nitric oxide.
- Oxides of nitrogen catalyses the decomposition of ozone and are themselves regenerated. Ozone gets depleted as follows:
NO + O3 \(\longrightarrow\) NO2 + O2
O2 \(\overset{hv}{\longrightarrow}\) O + O
NO2 + O \(\longrightarrow\) NO + 2O\(^{\bullet}\) .
Thus NO is regenerated in the chain reaction.