---
title: "Short note: Ozone depletion"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:ozone-depletion"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/ozone-depletion
site: BCT Engineering
---

# Short note: Ozone depletion

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2073 Bhadra, 2.5 marks; 2067 Chaitra, 1+2 marks (asked 2 times)
- Video: [Ozone Depletion | Chemical Reactions of Ozone Depletion |  Science Land](https://www.youtube.com/watch?v=3jAXF_BKjl0) — Science Land
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/ozone-depletion
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

Write a short note on ozone depletion.

## Short answer

Ozone in the stratosphere absorbs harmful UV radiation. CFCs release chlorine atoms upon UV photolysis, which catalytically destroy ozone (Cl• + O3 -> ClO• + O2, ClO• + O -> Cl• + O2), causing the ozone hole. Increased UV-B reaching Earth elevates skin cancer, cataracts, and harms ecosystems. Controlled by the Montreal Protocol.

## Answer

#### Ozone layer
Ozone (O₃) in the stratosphere (15 to 35 km) is formed from oxygen by UV and absorbs harmful UV-B and UV-C radiation:

```text
O₂ + hν (λ < 240 nm) → 2O
O  + O₂ → O₃
O₃ + hν (UV-B) → O₂ + O
```
The last step makes ozone a natural shield, absorbing 97 to 99 % of the sun's harmful UV.

#### Depletion
Ozone is destroyed faster than it forms, mainly by chlorine from CFCs (and bromine from halons) and, to a smaller extent, by NO from supersonic aircraft (NO + O₃ → NO₂ + O₂).

Because CFCs are so stable, they slowly rise to the stratosphere, where strong UV radiation (λ < 220 nm) breaks the C–Cl bond and releases chlorine atoms:

```text
CCl₃F  + hν → •CCl₂F + Cl•
CCl₂F₂ + hν → •CClF₂ + Cl•
```
The free chlorine atom then destroys ozone in a chain reaction:

```text
Cl• + O₃  → ClO• + O₂
ClO• + O  → Cl• + O₂
-------------------------------
Net:  O₃ + O → 2O₂        (Cl• is regenerated)
```
A single Cl atom can destroy about 10⁵ ozone molecules before it is removed (for example, as HCl by reaction with CH₄, or as ClONO₂ when ClO combines with NO₂).

Depletion is highest over Antarctica in spring, where polar stratospheric clouds help release chlorine, producing the **ozone hole**.

#### Effects
- Increased UV-B reaching the earth causes skin cancer, sunburn, cataract and weakening of the immune system.
- Damages crops and forests, reducing yield.
- Kills phytoplankton and other aquatic organisms, upsetting the marine food chain.
- Degrades plastics and paints; increases ground-level ozone and smog.
- Contributes to global warming (CFCs are greenhouse gases).

#### Control
The **Montreal Protocol (1987)** phased out CFCs, halons and carbon tetrachloride. Alternatives are HCFCs and HFCs (later restricted by the Kigali Amendment, 2016), hydrocarbons and ammonia as refrigerants. Nepal is a party to the Montreal Protocol.

## Check yourself

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

1. What does stratospheric ozone do?
   A. Absorbs infrared to warm the earth
   B. Reflects visible light
   C. Absorbs harmful UV-B and UV-C (97 to 99% of harmful UV)
   D. Produces oxygen for breathing
   Answer: C. O3 + hv (UV-B) -> O2 + O absorbs UV. (Key point: Ozone shield)

2. What is the chain reaction of chlorine with ozone?
   A. Cl. + O2 -> ClO2, then ClO2 -> Cl. + O2
   B. Cl. + O3 -> Cl2 + O2 only
   C. Cl. + O3 -> ClO3 which is stable
   D. Cl. + O3 -> ClO. + O2, then ClO. + O -> Cl. + O2
   Answer: D. Chlorine is regenerated in the second step, so it acts as a catalyst. (Key point: Chain reaction)

3. Why is depletion highest over Antarctica in spring?
   A. Polar stratospheric clouds help release chlorine
   B. Because there are no CFCs elsewhere
   C. Because Antarctica has high NO2 pollution
   D. Because ozone forms only in the tropics
   Answer: A. PSCs help release reactive chlorine, causing the ozone hole. (Key point: Ozone hole)

4. Which treaty phased out CFCs, halons and CCl4?
   A. Kyoto Protocol (1997)
   B. Montreal Protocol (1987)
   C. Paris Agreement (2015)
   D. Kigali Amendment (2016) only
   Answer: B. The Montreal Protocol of 1987 phased out these substances. (Key point: Control)

## Related in Engineering Chemistry

- [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
- [Photochemistry behind ozone layer depletion](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aozone-photochemistry) — Engineering Chemistry, chapter 3

## Related in other subjects

- [Stratospheric ozone depletion mechanism and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aozone-depletion-mechanism-effects) — Environment and Pollution Control, chapter 7