Ozone depletion

The ozone layer and the Chapman cycle

About 90% of the Earth's ozone (O₃) is in the stratosphere, roughly 15–35 km up, where it absorbs harmful ultraviolet-B radiation. Ozone amount is measured in Dobson units (DU). In 1930 Sydney Chapman explained how ozone forms and breaks down naturally (the Chapman cycle):

• O₂ + UV → O + O (oxygen molecules split)
• O + O₂ + M → O₃ + M (ozone forms; M is a third molecule that carries away energy)
• O₃ + UV → O₂ + O (ozone absorbs UV and splits)
• O + O₃ → 2O₂ (ozone is destroyed)

In a natural balance, formation and destruction are equal.

Ozone-depleting substances (ODS)

Human-made chemicals upset this balance. The main ODS are chlorofluorocarbons (CFCs) used in refrigerators, air-conditioners, aerosols and foams; halons in fire extinguishers; carbon tetrachloride; methyl chloroform; methyl bromide (a fumigant); and, to a smaller extent, HCFCs. In the stratosphere UV breaks CFCs and releases chlorine, which destroys ozone in a catalytic cycle: Cl + O₃ → ClO + O₂; ClO + O → Cl + O₂. The chlorine is regenerated, so a single chlorine atom can destroy many thousands of ozone molecules. Bromine is even more destructive per atom.

The Antarctic ozone hole was reported in 1985 by scientists of the British Antarctic Survey. It forms in the southern spring because polar stratospheric clouds in the very cold winter convert chlorine into active forms that destroy ozone when sunlight returns. Rowland, Molina and Crutzen won the 1995 Nobel Prize in Chemistry for this research. Thinner ozone means more UV-B, which causes skin cancer, cataracts, weakened immunity and harm to plankton and crops.

International response

• Vienna Convention (1985): framework for protecting the ozone layer.
• Montreal Protocol (1987, in force 1989): phases out production and use of ODS on fixed timetables, with a later schedule and a Multilateral Fund for developing countries. It has been ratified by every UN member state.
• Kigali Amendment (2016): phase-down of HFCs, which do not harm ozone but are powerful greenhouse gases.

16 September is observed as the International Day for the Preservation of the Ozone Layer.

Key points

• Stratospheric ozone absorbs UV-B; measured in Dobson units.
• Chlorine acts as a catalyst, so small amounts destroy much ozone.
• Montreal Protocol (1987) phases out ODS; Kigali (2016) targets HFCs.
• Polar stratospheric clouds explain why the hole forms over Antarctica.

Common mistakes

• Confusing the ozone hole with global warming; they are different problems, though linked by some gases.
• Thinking HFCs deplete ozone; they were controlled for their warming effect.