Chlorine monoxide dimer cycle ($\mathrm{ClO}+\mathrm{ClO}+\mathrm{M}\rightleftharpoons \mathrm{ClOOCl}+\mathrm{M}$)

A low-temperature catalytic ozone destruction pathway involving ClO association to ClOOCl, photodissociation of the dimer and regeneration of chlorine atoms. Combined with two Cl + O3 reactions its net effect is $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$. Low temperature suppresses thermal dimer dissociation, while sunlight is needed for the photolytic step. Polar stratospheric clouds can activate reservoir chlorine before the sunlit cycle becomes rapid.

For dimer photodissociation frequency $J$ and competing thermal dissociation frequency $k_d[M]$, the successful branch fraction is $f=J/(J+k_d[M])$. With steady dimer abundance and rapid subsequent recycling, ozone destruction is $2f{k}_a[M][\mathrm{ClO}{]}^2$. The factor two counts ozone molecules consumed per productive dimer event.

The peroxy-linked chlorine oxide $\mathrm{ClOOCl}$, often written $\mathrm{C}{\mathrm{l}}_2{\mathrm{O}}_2$ in a chlorine monoxide dimer cycle. Thermal return to two chlorine monoxide radicals competes with a photolytic pathway that ultimately yields two chlorine atoms. The formula alone should not be taken as specifying every possible dichlorine-dioxide isomer.