Question
Explain the formation of ozone in the stratosphere and the mechanism by which CFCs cause ozone layer depletion. Why is the ozone hole more prominent over Antarctica?
(Based on NEET 2024 pattern — this concept appears almost every year in both NEET and JEE Main as a 1-mark factual or 2-mark mechanism question.)
Solution — Step by Step
UV radiation (wavelength below 240 nm) splits O₂ molecules into two highly reactive oxygen atoms. Each free oxygen atom immediately attacks another O₂ molecule to form O₃.
This is why the ozone layer exists only in the stratosphere (15–50 km) — that’s exactly where UV intensity is high enough to split O₂ but the atmosphere is still dense enough for the three-body collision to form O₃.
Ozone simultaneously absorbs UV-B and UV-C radiation and breaks back down:
Under natural conditions, the rate of formation equals the rate of destruction. The steady-state concentration remains roughly constant. CFCs destroy this balance.
Chlorofluorocarbons (Freons — CCl₂F₂, CCl₃F, etc.) are chemically inert at ground level. This stability is exactly what makes them dangerous — they don’t break down in the troposphere and slowly diffuse up into the stratosphere over decades.
Once there, UV radiation cleaves the C–Cl bond:
The chlorine radical is the actual culprit. It attacks ozone in a catalytic cycle:
Notice: Cl· is regenerated at the end. One Cl atom can destroy up to 100,000 ozone molecules before it’s eventually deactivated. This is what makes CFCs so catastrophically effective.
In the Antarctic winter, temperatures drop below −78°C. Polar Stratospheric Clouds form from ice crystals. These cloud surfaces act as reaction sites where inactive chlorine reservoirs (like HCl and ClONO₂) convert back to active Cl₂.
When spring arrives and sunlight returns, Cl₂ splits immediately:
This sudden burst of active chlorine in spring causes rapid O₃ destruction — the ozone “hole” we measure each September–October.
Why This Works
The key conceptual point: ozone depletion is a catalytic process, not a stoichiometric one. Cl· is a catalyst — it speeds up O₃ destruction without being consumed itself. This is why trace amounts of CFCs have such a disproportionate effect.
The Montreal Protocol (1987) banned CFC production globally. Because CFCs already in the atmosphere persist for 50–100 years, the ozone layer is recovering slowly — models predict recovery to 1980 levels by around 2065. This is a standard data-interpretation point in board exams.
Notice also that the mechanism involves free radical chain reactions — the same concept tested in organic chemistry (halogenation of alkanes). Examiners love connecting these themes.
Alternative Method — Ozone Number Approach
For MCQs asking about “ozone-depleting potential (ODP),” rank by chlorine and bromine content:
- CCl₃F (CFC-11) — 3 Cl atoms, ODP = 1.0 (reference standard)
- CCl₂F₂ (CFC-12) — 2 Cl atoms, ODP = 0.82
- Halons (CBrClF₂) — Br is ~50× more destructive than Cl per atom
For NEET MCQs: Bromo-compounds > Chloro-compounds in ozone depleting power, atom for atom. If an MCQ asks you to arrange by ODP and a bromine compound is listed, it ranks highest.
Common Mistake
Students write that “CFCs directly react with ozone.” Wrong. CFCs are inert — they do nothing until UV radiation cleaves the C–Cl bond in the stratosphere. It is the Cl· radical (not the CFC molecule itself) that attacks O₃. In a mechanism question, if you skip this photodissociation step, you lose marks. Write the UV step explicitly:
…then show the catalytic cycle.
Weightage note: In NEET, this topic appears under Environmental Chemistry (Class 12, Chapter 16) as 1–2 marks. In JEE Main, it comes under p-Block Elements or Environmental Chemistry. The catalytic mechanism and the “100,000 molecules” fact are the two most-tested specifics — memorise both.