Polar stratospheric clouds

PSCs form in the lower stratosphere when polar air becomes exceptionally cold, most often inside the winter polar vortex. In the Antarctic, observations commonly place them around 15–25 kilometres altitude, with formation favoured by temperatures near or below about 195 kelvin, though thresholds depend on cloud composition and atmospheric conditions. Type I clouds contain nitric-acid-bearing liquid droplets or particles; Type II clouds are made mainly of water-ice crystals and require colder conditions. Some particles are too small or optically faint to be obvious to the naked eye, while ice-rich clouds can produce striking nacreous colours.

The clouds are not themselves ozone-destroying pollutants. Instead, chemical reactions on or within their particles transform chlorine stored in comparatively stable compounds, such as hydrogen chloride, into reactive chlorine species. During polar darkness, these activated compounds can accumulate. When sunlight returns in spring, photochemical reactions produce chlorine monoxide and related forms that participate in catalytic cycles: chlorine is regenerated and can react again, destroying many ozone molecules. Bromine chemistry also contributes to polar ozone loss. The combination of cold temperatures, PSC surfaces, isolation of polar air, and returning sunlight makes the seasonal process especially intense over Antarctica.

A strong winter polar vortex helps keep cold air isolated and supports PSC formation. As spring sunlight returns, ozone-depleting chemistry intensifies. Later warming weakens the vortex and clouds disappear; mixing with ozone-richer air from lower latitudes helps end the seasonal depletion episode. The Arctic also experiences PSCs and ozone loss, but its stratosphere is generally warmer and more variable, so the conditions for extensive cloud formation and severe depletion occur less consistently than in the Antarctic.

Scientists study PSCs using satellite instruments, ground-based lidar and atmospheric measurements. Observations of cloud particles, temperature, nitric acid, water vapour and chlorine compounds help explain where clouds form and how they influence ozone chemistry. A major review of satellite observations and atmospheric processes notes that PSC characteristics vary between years and that uncertainties in their distribution and microphysics affect ozone modelling. Some larger particles can settle, removing nitric acid and water from the surrounding air and thereby influencing how long ozone-depleting conditions persist.