Polar Ozone Depletion

The severe springtime loss of stratospheric ozone over the poles caused by halogen chemistry activated on polar stratospheric clouds.

Definition

Polar ozone depletion is the dramatic seasonal destruction of stratospheric ozone in the polar regions, driven by halogen radicals activated through heterogeneous chemistry on cold cloud particles.

Scope

Covers the discovery of the Antarctic ozone hole, the role of the cold polar vortex, the formation of polar stratospheric clouds, the heterogeneous reactions that convert halogen reservoirs into reactive forms, the catalytic chlorine and bromine cycles driving rapid spring ozone loss, and the interhemispheric differences between Antarctic and Arctic depletion.

Core questions

• Why does severe ozone loss occur over the poles in spring?
• How do polar stratospheric clouds activate ozone-destroying halogens?
• Why is the ozone hole far deeper over Antarctica than the Arctic?

Key theories

Heterogeneous halogen activation

Reactions on polar stratospheric cloud particles convert inert chlorine reservoirs into reactive forms that, on return of sunlight, drive catalytic ozone destruction and the ozone hole.

Mechanisms

During the polar winter the isolated, extremely cold vortex allows polar stratospheric clouds to form. Reactions on these particles convert reservoir species such as chlorine nitrate and hydrogen chloride into photolytically labile molecular chlorine and related compounds. When sunlight returns in spring, these are split into chlorine atoms and chlorine monoxide, which destroy ozone through catalytic cycles including the chlorine monoxide dimer cycle, producing the ozone hole. The Arctic vortex is warmer and less stable, so depletion there is milder and more variable.

Clinical relevance

The discovery and explanation of polar ozone depletion drove the Montreal Protocol and remains the benchmark for tracking recovery of the ozone layer that shields life from ultraviolet radiation.

History

Farman, Gardiner and Shanklin reported the Antarctic ozone hole in 1985, and Solomon and colleagues soon identified heterogeneous chemistry on polar stratospheric clouds as its cause, confirmed by aircraft campaigns later in the decade.

Key figures

• Joseph Farman
• Susan Solomon

Seminal works

• farman1985
• solomon1999

Frequently asked questions

Why is the ozone hole worse over Antarctica than the Arctic?

The Antarctic polar vortex is colder, more stable and more isolated, allowing extensive polar stratospheric clouds and sustained halogen activation, whereas the warmer, more disturbed Arctic vortex produces weaker, more variable depletion.