Ozone-depleting substances (ODSs) are human-made chemicals that release ozone-destroying halogens in the stratosphere. They accelerate the conversion of protective stratospheric ozone into oxygen, allowing more harmful ultraviolet radiation to reach Earth’s surface.
How ODSs Reach the Stratosphere
At SL and HL, you should know that ODSs include chemicals such as chlorofluorocarbons and halons. They were widely used because they are stable, non-flammable and useful in refrigeration, aerosol propellants and fire suppression.
| ODS group | Former uses | Important halogen |
|---|---|---|
| Chlorofluorocarbons (CFCs) | Refrigerants, foam production and aerosol propellants | Chlorine |
| Halons | Fire extinguishers | Bromine |
| Carbon tetrachloride and methyl chloroform | Solvents and industrial processes | Chlorine |
Their stability allows them to remain intact in the troposphere and gradually reach the stratosphere. There, high-energy ultraviolet radiation breaks their chemical bonds and releases reactive halogen radicals.
Catalytic Destruction of Ozone
The detailed halogen mechanism is HL-only content in subtopic 6.4. A chlorine radical can react as follows:
The overall reaction is:
Chlorine is regenerated, so it acts as a catalyst and can destroy many ozone molecules. Polar stratospheric conditions intensify these reactions, producing severe springtime depletion such as the Antarctic ozone hole.
The Montreal Protocol regulates the production, trade and use of CFCs and other ODSs. It is a named example of successful international cooperation that prevented the planetary boundary for stratospheric ozone depletion from being crossed.
A common misconception is that the ozone hole causes global warming. Stratospheric ozone depletion increases ultraviolet exposure, whereas climate change is primarily driven by the enhanced greenhouse effect; they are distinct environmental issues.
Exam Technique
For an “explain” question, link ODS stability, stratospheric transport, ultraviolet breakdown, halogen release and catalytic ozone destruction in a clear causal sequence. State the societal consequences, such as increased skin cancer and cataracts, and use the Montreal Protocol as a named example.
