Key Idea: Topic 6.4 explains the ozone layer (good ozone in the stratosphere vs bad ozone at ground level), why ozone protects life from UV, how CFCs/halons destroy ozone via catalytic cycles, and why the Montreal Protocol is a landmark success.
🟦 Good ozone vs bad ozone
Stratosphere (GOOD): Absorbs UV-B/UV-C. Protects DNA and ecosystems. We want MORE of this.
Troposphere (BAD): Smog pollutant. Respiratory irritant. Damages plants.
🧪 Ozone depletion mechanism
- ODS (CFCs/halons) reach stratosphere (very stable)
- UV breaks them → releases Cl/Br atoms
- Cl/Br catalytically destroy ozone and are regenerated
- One atom can destroy many ozone molecules
🕳️ The ozone hole (why Antarctica)
- Polar vortex isolates air
- Extreme cold forms PSCs, activating chlorine
- Spring sunlight triggers rapid ozone loss
- Seasonal: worst around Sep–Oct
🤝 Montreal Protocol (why it worked)
- Clear science + clear target (phase out ODS)
- Near-universal participation
- Funding + longer timelines for developing countries
- Regular strengthening + measurable recovery
Evaluation move: explain why ozone was easier than climate (fewer industries, clearer substitutes, smaller economic reach).
✅ Exam Checklist
- Good vs bad ozone distinction
- UV-B impacts on humans + ecosystems
- Explain catalytic destruction (Cl regenerated)
- Evaluate Montreal Protocol success + one limitation (slow recovery / illegal production / replacements)