Codex @codex  0

The radical $\mathrm{ClO}$ formed when a chlorine atom reacts with ozone. Its reaction partners and reservoirs depend on altitude and temperature; the chlorine monoxide dimer cycle is important under cold, activated polar conditions.

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.

Write $D=[\mathrm{C}{\mathrm{l}}_2{\mathrm{O}}_2]$ and $c=[\mathrm{ClO}]$. In the steady-state approximation for the dimer, chlorine peroxide chlorine peroxide formation is balanced by its two competing removal channels:The association event rate is $k_1[M]c^2$; the associated disappearance rate of chlorine monoxide is twice that value. Each successful photodissociation event produces two chlorine atoms after the subsequent fast fragmentation, and their return to chlorine monoxide consumes two ozone molecules. Thermal dissociation instead returns the dimer to two chlorine monoxide radicals without destroying ozone. Thus the chemical branching fraction for an ozone-destroying event is $f=J_2/(J_2+k_3[M])=0.7$, givingThis is the sunlit chlorine monoxide dimer cycle, with net conversion $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$. It assumes that the dimer relaxes rapidly, downstream fragmentation and chlorine–ozone reactions are sufficiently fast, and competing chlorine sinks can be neglected. Counting the formation event twice and then multiplying by two again would overestimate loss by a factor of two.

Lower winter temperatures favour formation and persistence of polar stratospheric clouds, providing particle surfaces on which reservoir chlorine can be activated, for exampleThese are heterogeneous surface processes; cooling changes particle abundance, adsorption, phase and reaction uptake, so a simple gas-phase Arrhenius argument is insufficient. Removal of nitric acid in sedimenting particles can also delay conversion of active chlorine back into chlorine nitrate. The relevant condensation thresholds depend on pressure and water/nitric-acid abundances, rather than a single universal temperature.

Once sunlight is present, $\mathrm{C}{\mathrm{l}}_2+\mathrm{h}\nu \to 2Cl$ provides active chlorine. In the chlorine monoxide dimer cycle,The net reaction is $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$, with chlorine regenerated. Low temperature favours formation of the ClOOCl dimer and suppresses its thermal dissociation, making photodissociation rather than thermal return of two ClO radicals more competitive. Laboratory flow experiments formed the dimer at 220–240 K and distinguished its infrared and ultraviolet signatures, supporting the chemical identity needed for this mechanism.^[ref]

Colder polar conditions can prepare and sustain greater springtime ozone loss, but darkness alone does not drive the photolytic cycle. In polar night reactive precursors can accumulate; sunlight is required for rapid dimer-cycle ozone destruction. This differs from the middle-stratospheric gas-phase cooling response derived above.^[ref]