9.1 Stratospheric Ozone Depletion

Ozone's protective role

Stratospheric ozone (O3) absorbs harmful UV radiation from the sun, protecting living things from DNA damage, skin cancer, cataracts, and crop damage. It forms naturally when UV light splits O2 molecules, which recombine with O2 to form O3, in an ongoing cycle of formation and breakdown.

CFCs and the ozone hole

Chlorofluorocarbons (CFCs), once used in refrigerants, aerosols, and solvents, rise into the stratosphere and are broken apart by UV light, releasing chlorine radicals. Each chlorine atom catalytically destroys thousands of ozone molecules without being consumed, thinning the ozone layer and creating the seasonal ozone hole over Antarctica.

Global response: the Montreal Protocol

The Montreal Protocol (1987) is an international treaty that phased out production of CFCs and other ozone-depleting substances (ODS). It's considered one of the most successful environmental agreements, and the ozone layer is slowly recovering, though full recovery is expected to take decades.

Key terms

Stratospheric ozone

A layer of O3 in the stratosphere that absorbs harmful UV radiation, protecting life on Earth.

CFCs (Chlorofluorocarbons)

Man-made chemicals once used in refrigerants/aerosols that release chlorine radicals, destroying ozone.

Ozone hole

A seasonal area of severely thinned ozone, most notably over Antarctica, caused by chlorine-catalyzed destruction.

Chlorine radical catalytic cycle

A single chlorine atom breaks down thousands of ozone molecules without being consumed itself.

Montreal Protocol

1987 international treaty that phased out production of CFCs and other ozone-depleting substances; considered highly successful.

Try an AP-style question

Which chemicals are mainly responsible for stratospheric ozone depletion?

1. Nitrogen gas
2. CFCs
3. CO₂
4. Methane