6.4 Stratospheric ozoneIB Environmental Systems and Societies HL: Revision notes

The Sun emits electromagnetic radiation from low-frequency radio waves to high-frequency gamma radiation. Shorter wavelengths have higher frequencies and more energy, so UV is more dangerous to life than visible light.

Stratospheric ozone absorbs UV radiation, reducing the amount reaching the surface. UV reduces photosynthesis in phytoplankton and damages DNA, causing mutations and cancer. In humans it causes sunburn, premature ageing of the skin and cataracts, as well as skin cancer (Australia has one of the highest rates in the world).

Ozone is formed and destroyed continuously, and the relative concentration has stayed constant over long periods because of a steady state equilibrium. Ozone-depleting substances (ODSs), such as CFCs, destroy ozone and augment natural breakdown, allowing more UVB to reach the surface and affecting ecosystems and human health.

The Montreal Protocol (1987) regulates the production, trade and use of CFCs and other ODSs. Actions taken have prevented the planetary boundary for stratospheric ozone depletion being crossed. Because CFCs last 50 to 100 years, recovery is slow (Antarctica about 2066).

(HL) ODSs release halogens, such as chlorine (and bromine), into the stratosphere, where they break down ozone in catalytic reactions (one chlorine atom can destroy many ozone molecules).

(HL) Polar stratospheric ozone depletion occurs in spring because of the unique conditions: in the dark winter a polar vortex isolates very cold air, polar stratospheric clouds form and provide surfaces on which chlorine compounds accumulate; when sunlight returns in spring, reactions release chlorine atoms, which destroy ozone and create the ozone hole.

(HL) Hydrofluorocarbons (HFCs) were developed to replace CFCs because they can be used in similar ways and cause much less ozone depletion (no chlorine). However, they are potent greenhouse gases: HFC-134a has a global warming potential of about 1 300. The Kigali Amendment (2016) phases down HFCs.

(HL) Air conditioning units are energy-intensive, contribute to GHG emissions through electricity use and refrigerant leaks, and traditionally contained ODSs. Demand is rising in hot countries such as the UAE and India. Strategies include low-GWP refrigerants, efficiency standards, renewable power and passive cooling.