= Solution
Lower winter temperatures favour formation and persistence of <polar stratospheric clouds>, providing particle surfaces on which reservoir <chlorine> can be activated, for example
$$
\mathrm{HCl+ClONO_2\longrightarrow Cl_2+HNO_3},\qquad
\mathrm{HCl+HOCl\longrightarrow Cl_2+H_2O}.
$$
These 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, $\mathrm{Cl_2+h\nu\to2Cl}$ provides active <chlorine>. In the <chlorine monoxide dimer cycle>,
$$
\begin{aligned}
2(\mathrm{Cl+O_3}&\mathrm{\longrightarrow ClO+O_2}),\\
\mathrm{ClO+ClO+M}&\mathrm{\rightleftharpoons ClOOCl+M},\\
\mathrm{ClOOCl}+h\nu&\mathrm{\longrightarrow Cl+ClOO},\\
\mathrm{ClOO+M}&\mathrm{\longrightarrow Cl+O_2+M}.
\end{aligned}
$$
The net reaction is $\mathrm{2O_3\to3O_2}$, 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.<Production of chlorine oxide (Cl2O2) from the self-reaction of the chlorine oxide (ClO) radical by L. T. Molina and M. J. Molina>[Journal of Physical Chemistry 91 (1987), pp. 433–436; NASA record 19870061046, flow-system generation and spectral identification]{ref}
\b[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.<Scientific Assessment of Ozone Depletion 2022 executive summary>[Polar <ozone> depletion discussion: low temperatures, heterogeneous activation and sunlight-dependent chemistry]{ref}
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