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Cooling can increase springtime ozone destruction by extending cold conditions favourable for polar stratospheric clouds, stratospheric chlorine activation and dimer stability. Removal of reactive nitrogen into settling particles can delay chlorine nitrate reformation. With sunlight and substantial available chlorine, the active period can become longer or stronger. Vortex breakup, transport and the declining halogen inventory also matter, so a colder year does not determine the ozone loss by temperature alone.

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]

Conversion of relatively unreactive chlorine reservoirs into forms that produce active radicals in sunlight. Heterogeneous reactions on appropriate particle surfaces can turn hydrogen chloride and chlorine nitrate into photolabile species. Available surface composition, temperature, reservoir supply and subsequent illumination all matter; adding arbitrary particles does not guarantee activation.

Removal of reactive nitrogen from an air mass through formation and sedimentation of nitric-acid-containing particles. This can delay reformation of chlorine nitrate and prolong active chlorine chemistry. It is a physical/chemical atmospheric removal process, distinct from biological denitrification.