---
title: "Stratospheric ozone depletion mechanism and effects"
subject: "Environment and Pollution Control"
chapter: 7
key: "semester_7/environment_and_pollution_control:ozone-depletion-mechanism-effects"
source: https://www.bctengineer.com/q/semester_7/environment_and_pollution_control/ozone-depletion-mechanism-effects
site: BCT Engineering
---

# Stratospheric ozone depletion mechanism and effects

Environment and Pollution Control · Chapter 7: Global issues and responsible development practices

- Asked in: year not given, 8 marks
- Video: [Stratospheric Ozone Depletion](https://www.youtube.com/watch?v=7juQgrnFxHI) — Kate the Chemist
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/environment_and_pollution_control/ozone-depletion-mechanism-effects
- Whole chapter: https://www.bctengineer.com/md/semester_7/environment_and_pollution_control/chapter-7

## Question

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

## 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.

## Answer

#### 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:
$$
\text{O}_2 + h\nu\ (\lambda<242\ \text{nm}) \rightarrow 2\text{O}, \qquad \text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M
$$
$$
\text{O}_3 + h\nu \rightarrow \text{O}_2 + \text{O}, \qquad \text{O} + \text{O}_3 \rightarrow 2\text{O}_2
$$
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 $\text{CCl}_3\text{F}$, CFC-12 $\text{CCl}_2\text{F}_2$) are stable and inert in the lower atmosphere, so they reach the stratosphere, where UV breaks them down and releases chlorine atoms:
$$
\text{CCl}_2\text{F}_2 + h\nu \rightarrow \text{CClF}_2 + \text{Cl}
$$
Catalytic cycle:
$$
\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2
$$
$$
\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2
$$
$$
\text{Net: O}_3 + \text{O} \rightarrow 2\text{O}_2
$$
The chlorine atom is regenerated, so one Cl atom can destroy about $10^5$ 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 ($\text{HCl}$, $\text{ClONO}_2$) is converted into reactive $\text{Cl}_2$. When sunlight returns in spring (September-October), $\text{Cl}_2$ 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is stratospheric ozone formed?
   A. O2 absorbs UV (<242 nm) and splits to O atoms, which combine with O2 to form O3
   B. CFCs react with oxygen
   C. Lightning splits O3
   D. O3 forms from N2 and O2 at ground level
   Answer: A. In the Chapman cycle UV splits O2 into 2O and O + O2 + M gives O3. (Key point: Ozone formation)

2. Why can one chlorine atom destroy about 10^5 ozone molecules?
   A. It is consumed with each ozone
   B. Cl is regenerated in the catalytic cycle (Cl + O3 -> ClO + O2; ClO + O -> Cl + O2)
   C. It is replaced by new CFC each time
   D. It converts O3 to O2 only once
   Answer: B. Cl is released again in the second step, so it acts as a catalyst until removed. (Key point: Catalytic destruction)

3. Why do CFCs reach the stratosphere?
   A. They are lighter than air and burn
   B. They are stable and inert in the lower atmosphere, then UV releases Cl there
   C. They are carried by rain
   D. They are made in the stratosphere
   Answer: B. CFCs resist breakdown below, reach the stratosphere and are photolysed by UV releasing chlorine atoms. (Key point: Why CFCs reach stratosphere)

4. What causes the Antarctic ozone hole each spring?
   A. Polar stratospheric clouds convert inactive chlorine to Cl2, which splits in returning sunlight and destroys ozone
   B. Volcanic dust in summer
   C. Warm air entering the polar vortex
   D. Ozone moving to the equator
   Answer: A. On PSC surfaces HCl and ClONO2 form Cl2; sunlight in September-October releases Cl atoms, and the polar vortex stops replacement. (Key point: Antarctic ozone hole)

## Related in Environment and Pollution Control

- [Photochemical smog and its reactions](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aphotochemical-smog-reactions) — Environment and Pollution Control, chapter 1

## Related in other subjects

- [CFCs in ozone depletion and harmful effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acfc-ozone-harmful) — Engineering Chemistry, chapter 3
- [Short note: Ozone depletion](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aozone-depletion) — Engineering Chemistry, chapter 3, asked 2×
- [Photochemistry behind ozone layer depletion](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aozone-photochemistry) — Engineering Chemistry, chapter 3