In the lower stratosphere over Antarctica atomic oxygen is scarce, so the step $\mathrm{ClO}+\mathrm{O}\to \mathrm{Cl}+\mathrm{O}{}_2$ is slow there. The chlorine cycle can still continue through

1. Athe reaction $\mathrm{ClO}+\mathrm{NO}\to \mathrm{ClONO}$ only
2. Bthe reaction of $\mathrm{ClO}$ with $\mathrm{CF}{}_2\mathrm{Cl}{}_2$
3. Cdirect reaction of $\mathrm{HCl}$ with $\mathrm{O}{}_3$ giving $\mathrm{Cl}{}_2$
4. Dthe reaction of two $\mathrm{ClO}$ molecules to form $\mathrm{Cl}{}_2\mathrm{O}{}_2$, which is photolysed to regenerate Cl atoms