Question
How do CFCs destroy the ozone layer, what are the effects of UV radiation, and what did the Montreal Protocol achieve?
Solution — Step by Step
Ozone () exists in the stratosphere (15-35 km altitude). It absorbs harmful UV-B radiation from the sun, preventing it from reaching Earth’s surface.
Without the ozone shield, UV-B would cause skin cancer, cataracts, DNA damage in organisms, and reduced crop yields.
Chlorofluorocarbons (CFCs) are stable synthetic chemicals used in refrigerators, ACs, and aerosol sprays. They drift up to the stratosphere, where UV light breaks them apart:
The free chlorine radical attacks ozone:
One chlorine atom can destroy 100,000 ozone molecules because it is regenerated in the cycle — it acts as a catalyst for ozone destruction.
graph TD
A[CFCs released from ACs, fridges, sprays] --> B[CFCs rise to stratosphere - very stable]
B --> C[UV radiation breaks CFC, releasing Cl radical]
C --> D[Cl attacks O3: Cl + O3 gives ClO + O2]
D --> E[ClO reacts with O: ClO + O gives Cl + O2]
E --> F[Cl radical regenerated - cycle repeats]
F --> D
D --> G[Ozone layer thins - ozone hole forms]
G --> H[More UV-B reaches Earth surface]
H --> I[Skin cancer, cataracts, crop damage]Signed in 1987, the Montreal Protocol is an international treaty that phased out the production of CFCs and other ozone-depleting substances.
Key achievements:
- 197 countries ratified it — the most successful environmental treaty ever
- CFC production reduced by 99% since 1987
- The ozone hole over Antarctica has been slowly recovering
- Expected full recovery by 2060-2070
CFCs were replaced by HFCs (hydrofluorocarbons), which do not destroy ozone (though they are greenhouse gases — a separate issue).
Why This Works
The key insight is that CFCs are incredibly stable in the lower atmosphere (that is why they were useful), but UV radiation in the stratosphere has enough energy to break the C-Cl bond. Once freed, chlorine is a catalyst — it participates in the reaction but is not consumed. This catalytic nature is what makes even small amounts of CFCs devastatingly effective at destroying ozone.
For CBSE and NEET, remember three things: (1) CFCs release Cl radicals in the stratosphere, (2) one Cl destroys ~100,000 ozone molecules, (3) the Montreal Protocol of 1987 banned CFCs. These three points cover most exam questions on this topic.
Alternative Method
Another way to understand ozone depletion is through the concept of dynamic equilibrium. Normally, ozone is constantly being created (by UV splitting ) and destroyed (naturally) in a balance. CFCs shift this equilibrium by adding a powerful new destruction pathway — the Cl-catalysed cycle — which destroys ozone faster than it can be replenished.
Common Mistake
Students often write that “the ozone hole means there is no ozone at all.” This is misleading. The “hole” refers to a region where ozone concentration drops dramatically (below 220 Dobson Units), not to a literal hole with zero ozone. Some ozone still exists there — just far less than normal. Use “ozone depletion” or “thinning” rather than implying complete absence.