Question
Explain the mechanism of ozone depletion by CFCs. What are the harmful effects of UV radiation on living organisms? How has the Montreal Protocol helped address this problem?
(CBSE 10 + NEET pattern)
Solution — Step by Step
Chlorofluorocarbons (CFCs) — used in refrigerators, ACs, and aerosol cans — are stable at ground level but break down in the stratosphere under UV radiation:
The free chlorine radical attacks ozone:
The chlorine atom is regenerated — one Cl atom can destroy approximately 100,000 ozone molecules before being deactivated. This catalytic cycle is why even small amounts of CFCs cause massive damage.
Without the ozone shield, UV-B radiation reaches Earth’s surface:
- Skin cancer (melanoma) — DNA damage in skin cells
- Cataracts — damage to eye lens proteins
- Immune suppression — weakened immune response
- Harm to phytoplankton — reduces marine food chain productivity
- Crop damage — reduces yields of sensitive crops like soybeans
Signed in 1987, the Montreal Protocol is an international treaty that phased out production of CFCs and other ozone-depleting substances. Key outcomes:
- CFC production dropped by over 99% globally
- The Antarctic ozone hole is slowly recovering
- Expected to return to pre-1980 levels by approximately 2066
- Considered the most successful international environmental treaty ever
flowchart TD
A["CFCs released from ACs, fridges"] --> B["Rise to stratosphere"]
B --> C["UV breaks CFC → free Cl radical"]
C --> D["Cl + O₃ → ClO + O₂"]
D --> E["ClO + O → Cl + O₂"]
E -->|"Cl regenerated"| D
D --> F["Ozone layer thins"]
F --> G["More UV-B reaches Earth"]
G --> H["Skin cancer, cataracts, crop damage"]
I["Montreal Protocol 1987"] --> J["CFC phase-out"]
J --> K["Ozone layer slowly recovering"]Why This Works
Ozone () in the stratosphere absorbs harmful UV-B radiation, preventing it from reaching the surface. The ozone layer is constantly being formed and broken down naturally, maintaining a balance. CFCs disrupt this balance by adding a powerful catalytic destruction pathway.
The reason one chlorine atom causes so much damage is that it is not consumed in the reaction — it acts as a catalyst. After destroying one ozone molecule, the Cl atom is freed to destroy another, and another. This catalytic cycle continues until the Cl atom is eventually removed by reacting with methane or nitrogen dioxide.
Alternative Method — Dobson Unit Measurement
Ozone concentration is measured in Dobson Units (DU). Normal levels are around 300 DU. The Antarctic ozone hole is defined as the area where ozone drops below 220 DU. The “hole” is not a literal hole — it is a region of severely depleted ozone concentration.
For NEET, remember the specific numbers: one Cl atom destroys ~100,000 O molecules, the Montreal Protocol was signed in 1987, and ozone is measured in Dobson Units. These factual details appear in MCQs. Also know that the ozone hole is worst over Antarctica due to unique meteorological conditions (polar vortex and polar stratospheric clouds).
Common Mistake
Students confuse ground-level ozone (a pollutant and component of smog, harmful when breathed in) with stratospheric ozone (the protective shield). The same molecule O plays opposite roles at different altitudes. Ground-level ozone is bad for health; stratospheric ozone is essential for life. “Ozone depletion” refers to stratospheric ozone loss, not ground-level ozone removal.