What Is the Ozone-Oxygen Cycle?

The ozone-oxygen cycle is the process by which the ozone layer absorbs UV-B radiation from the sun.

The process starts when UV radiation from the sun splits an oxygen molecule (O2) into two oxygen atoms.

O2 → O + O

A free oxygen atom (O) then reacts with an oxygen molecule (O2), forming ozone (O3).

O2 + O → O3

Ozone molecules (O3) absorb UV radiation, which splits the ozone into an oxygen molecule (O2) and an oxygen atom (O).

O3 + UV radiation → O2 + O

The free oxygen atom (O) reacts with another oxygen molecule (O2), forming a new ozone molecule.

O + O2 → O3

Ozone continually regenerates in the stratosphere by the cycle outlined above. It absorbs UV radiation and keeps most of the harmful UV-B radiation from reaching Earth’s surface.

If the free oxygen atom (O) reacts with an ozone molecule (O3), it will take one of its oxygen atoms to form two oxygen molecules (O2 + O2).

O + O3 → O2 + O2

The free oxygen atom (O) could also react with another oxygen atom (O) to form an oxygen molecule.

O + O → O2

The oxygen molecule(s) may react with the UV radiation to start the cycle over again.

Other Substances Inhibit the Ozone-Oxygen Cycle

Chemicals like chlorine and nitrogen react with ozone too, but these reactions destroy ozone. This destruction of ozone is natural. Such reactions balance stratospheric ozone by removing ozone as the ozone-oxygen cycle produces new ozone.

But manmade substances like chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODSs) are tipping this natural balance.

How CFCs Interrupt the Ozone-Oxygen Cycle

CFCs are unreactive in the troposphere. That is part of what made them so popular in refrigeration and other applications in the 20th century.

But air currents eventually carry CFCs into the stratosphere where UV radiation breaks them apart, releasing chlorine atoms that can react with ozone to form chlorine monoxide and an oxygen molecule.

Cl + O3 → ClO + O2

The chlorine monoxide then reacts with a free oxygen atom, stripping chlorine monoxide of its oxygen atom and reforming chlorine.

ClO + O → Cl + O2

This catalytic reaction is how CFCs deplete the ozone layer.

A catalytic reaction is one where a catalyst – in this case chlorine – is unchanged at the end of the reaction. Ozone, however, is destroyed.

We start with a chlorine atom and ozone molecule and end with a chlorine atom and an oxygen molecule. The chlorine molecule can then react with another ozone molecule and continue the process.

The chlorine atom may eventually react with other chemicals in the stratosphere and make its way back into the troposphere where, no longer in the form of stable chlorofluorocarbon, it can be removed from the atmosphere.

This is a decades-long process.

Ozone Layer Repair

The Montreal Protocol regulated ozone-depleting substances (ODSs) like CFCs. The treaty took effect in the late 1980s and over the decades has phased out many ODSs.

But many ODSs remain in the atmosphere for decades. So even as the Montreal Protocol has effectively reduced and even phased out these substances, it will take decades more before the ozone fully recovers.

Scientists estimate that the ozone layer will recover its 1960 levels in the 2060s and the ozone hole over Antarctica may not return to its 1960 levels until the end of the century.

But as CFCs are finally filtered out of the atmosphere, the ozone-oxygen cycle continues producing more ozone that stops UV radiation from reaching the earth.