Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2021 / Interdisciplinary paper IDP1 / 1 / d

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.

Outside the cold polar activation regime, cooling generally slows temperature-activated gas-phase ozone destruction and can increase ozone if other conditions are held fixed. This is a weaker, less universal conclusion in the lower stratosphere because transport and imported ozone are important. Enhanced sulfate-surface activation or unusually cold cloud-forming air can instead activate chlorine and reverse the purely gas-phase tendency. Distinguish a local chemical cooling effect from the dynamical response of the midlatitude ozone-column photolysis shielding.

In the upper stratosphere, cooling slows temperature-dependent odd-oxygen destruction, including $\mathrm{O}+{\mathrm{O}}_3\to 2{\mathrm{O}}_2$ and important catalytic reactions. With photochemical production otherwise fixed, the local steady ozone abundance tends to rise. This chemical response is one reason increasing carbon dioxide can enhance upper-stratospheric ozone while cooling that region. It is compatible with enhanced chemical depletion in cold activated polar air; the controlling regimes differ.^[ref]