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Stratospheric ozone depletion mechanism and effects

Short answer

Stratospheric ozone forms when UV splits O2 and O combines with O2; CFCs release chlorine atoms that catalytically destroy ozone, one Cl destroying about 100,000 molecules. Polar stratospheric clouds activate chlorine, creating the Antarctic spring ozone hole below 220 DU. Effects include skin cancer, cataracts, crop damage, and harm to phytoplankton.

Explain stratospheric ozone formation and the mechanism of its depletion by chlorofluorocarbons. Describe the Antarctic ozone hole and the effects of ozone depletion.

Ozone layer

About 90% of atmospheric ozone lies in the stratosphere (10-50 km, peak near 20-25 km). It absorbs harmful solar ultraviolet radiation (UV-B 280-315 nm and UV-C) and protects life. It is formed and destroyed naturally in the Chapman cycle:

O2+hν (λ<242 nm)→2O,O+O2+M→O3+M\text{O}_2 + h\nu\ (\lambda<242\ \text{nm}) \rightarrow 2\text{O}, \qquad \text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M O3+hν→O2+O,O+O3→2O2\text{O}_3 + h\nu \rightarrow \text{O}_2 + \text{O}, \qquad \text{O} + \text{O}_3 \rightarrow 2\text{O}_2

The total ozone column is measured in Dobson Units (DU); normal is about 300 DU, and the "hole" is a region below 220 DU.

Depletion by CFCs

CFCs (e.g. CFC-11 CCl3F\text{CCl}_3\text{F}, CFC-12 CCl2F2\text{CCl}_2\text{F}_2) are stable and inert in the lower atmosphere, so they reach the stratosphere, where UV breaks them down and releases chlorine atoms:

CCl2F2+hν→CClF2+Cl\text{CCl}_2\text{F}_2 + h\nu \rightarrow \text{CClF}_2 + \text{Cl}

Catalytic cycle:

Cl+O3→ClO+O2\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 ClO+O→Cl+O2\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2 Net: O3+O→2O2\text{Net: O}_3 + \text{O} \rightarrow 2\text{O}_2

The chlorine atom is regenerated, so one Cl atom can destroy about 10510^5 ozone molecules before it is removed. Bromine (from halons) and nitrogen oxides also destroy ozone.

Antarctic ozone hole

In the polar winter, very cold air forms polar stratospheric clouds. On their surfaces, inactive chlorine (HCl\text{HCl}, ClONO2\text{ClONO}_2) is converted into reactive Cl2\text{Cl}_2. When sunlight returns in spring (September-October), Cl2\text{Cl}_2 splits into Cl atoms that rapidly destroy ozone through cycles such as the ClO dimer cycle, creating the ozone hole. The isolated polar vortex prevents ozone from being replaced.

Effects

  • Humans: skin cancer, sunburn, cataract, weakened immunity.
  • Plants: reduced growth and crop yield.
  • Aquatic life: damage to phytoplankton at the base of the food chain.
  • Materials: faster degradation of plastics and paints.
  • Increased ground-level smog and effects on climate.

✓ Answer checked by Claude Opus 5.5 (high effort)

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