CFCs and the ozone layer

High above the weather, a thin layer of ozone absorbs most of the Sun's harmful ultraviolet light. In the 1970s, chemists found that a family of very unreactive molecules was breaking it down.

Ozone, O₃, is three oxygen atoms in a bent row. It forms in the stratosphere, mostly 15 to 35 km up, where strong ultraviolet light splits O₂ molecules into oxygen atoms and each atom joins an O₂ to make O₃. Ozone absorbs ultraviolet light in its turn, including most of the kind that causes sunburn. Squeezed down to the pressure at sea level, all the ozone overhead would make a layer only about 3 mm thick.

CFCs, short for chlorofluorocarbons, are small molecules of carbon, chlorine and fluorine. From the 1930s they cooled fridges and air conditioners, pushed the spray out of aerosol cans and blew the bubbles into foam. They are non-flammable, low in toxicity and very unreactive, so they were safe to use at home.

How chlorine destroys ozone

Being so unreactive, CFCs do not dissolve in rain or break down in the lower air. They last about 50 to 100 years, long enough to drift up into the stratosphere. There, ultraviolet light strong enough to break a C–⁠Cl bond splits a chlorine atom off.

A chlorine atom takes an oxygen atom from ozone, making ClO and O₂. The ClO then meets a free oxygen atom and gives up its oxygen to make another O₂, and the chlorine atom is free again. It comes out of each round unchanged, so it is a catalyst, and one chlorine atom can destroy more than 100,000 ozone molecules.

The Montreal Protocol

Mario Molina and Sherwood Rowland showed in 1974 that CFCs could destroy ozone in this way. In 1985 scientists of the British Antarctic Survey reported that the ozone over Antarctica thinned sharply every spring: the ozone hole. In 1987 countries agreed the Montreal Protocol to phase out CFCs, and every member of the United Nations has since ratified it. Nearly 99% of the ozone-destroying chemicals it controls are now phased out.

The CFCs already in the air take decades to break down, so the stratosphere's chlorine falls slowly. The 2022 assessment by the World Meteorological Organization and the UN Environment Programme expects the ozone layer to return to its 1980 levels around 2040 over most of the world, around 2045 over the Arctic and around 2066 over Antarctica.

Many fridges now use isobutane, the branched isomer of butane, which holds no chlorine. Others use HFCs, which hold fluorine and hydrogen and no chlorine, so ozone is safe from them. HFCs are strong greenhouse gases, and an addition to the protocol agreed in Kigali, Rwanda, in 2016 is phasing them down too.