Polar Stratospheric Ozone Depletion (HL) (DP IB Environmental Systems & Societies (ESS)): Revision Note

Polar stratospheric ozone depletion

• Polar stratospheric ozone depletion mainly occurs during spring over the poles

• Causes of depletion include unique chemical and atmospheric conditions in the polar stratosphere

• This process leads to the thinning of the ozone layer, commonly referred to as the 'ozone hole'

  • An ozone hole is most commonly observed over Antarctica during the Southern Hemisphere's spring (September to November)

  • This hole reached record sizes in the early 2000s, drawing attention to global ozone depletion

  • Although the Montreal Protocol reduced CFC emissions, the Antarctic ozone hole persists and is monitored yearly

Key chemical and atmospheric conditions

Polar Stratospheric Clouds (PSCs)

• Extremely low temperatures in the polar stratosphere (particularly in the Antarctic) during winter create conditions that favour the formation of polar stratospheric clouds (PSCs)

  • The clouds provide "active surfaces" where chlorine compounds are converted into forms that can destroy ozone

  • When sunlight returns in spring, UV radiation breaks down chlorine-containing compounds on the surface of PSCs

  • This releases chlorine radicals that rapidly deplete ozone

  • The presence of PSCs amplifies ozone depletion within the polar vortex

Polar vortex

• The polar vortex is a persistent, large-scale circulation pattern that forms over the polar regions during winter

  • This vortex isolates the air within it

  • This prevents exchange with air from lower latitudes

  • This traps ozone-depleting substances within the vortex, increasing their concentration

• The Antarctic polar vortex is stronger and more stable than the Arctic vortex due to geographic and temperature differences

  • This leads to more severe ozone depletion in Antarctica

Volcanic aerosols

• Volcanic eruptions release aerosols that reach the stratosphere and enhance ozone depletion

  • These aerosols contribute additional surfaces for ozone-destroying reactions

  • Their effect is amplified in polar regions due to :

    • The presence of PSCs

    • The isolated polar vortex