Ozone Depletion: Notes and PYQs MCQs

Ozone Depletion: Complete One-Page Revision (NEET, CUET, JEE, CBSE, CSIR-NET)

1. Basics You Must Know

• Ozone layer: 15–35 km altitude (stratosphere); maximum concentration at ~25 km
• Absorbs 95–99 % of harmful UV-B (280–315 nm) and all UV-C (<280 nm)
• 1 Dobson Unit (DU) = 0.01 mm thickness of pure O₃ at STP
• Normal global average: ~300 DU; Antarctic hole drops below 220 DU → defined as ozone hole

2. Natural Ozone Cycle (Chapman Cycle) – Must Remember

O₂ →[UV < 242 nm] 2O
O + O₂ + M → O₃ + M
O₃ →[UV < 320 nm] O₂ + O
O + O₃ → 2O₂
Net: 3O₂ ⇌ 2O₃ (dynamic equilibrium)

3. Major Ozone-Depleting Substances (ODS) & Their ODP Values

Compound	Source	Ozone Depletion Potential (ODP)	Global Warming Potential (GWP)
CFC-11 (CCl3F)	Refrigerants, aerosols	1.0 (reference)	4750
CFC-12 (CCl2F2)	Most used CFC	1.0	10 900
Halons (CBrF3, etc.)	Fire extinguishers	3–10	High
Carbon tetrachloride (CCl4)	Solvent	0.82	1400
Methyl chloroform (CH3CCl3)	Industrial solvent	0.1–0.16	146
HCFCs (e.g., HCFC-22)	Transitional substitutes	0.01–0.1	High
Methyl bromide (CH3Br)	Agricultural fumigant	0.4–0.6	Low

4. Catalytic Destruction Cycles (Most Important for Exams)

Chlorine cycle (CFCs)
Cl• + O₃ → ClO + O₂
ClO + O → Cl• + O₂
Net: O₃ + O → 2O₂ (One Cl• destroys ~1 lakh O₃ molecules before removal)

Bromine cycle (Halons, CH₃Br) – 45–60 times more efficient than Cl
NOx cycle (from N₂O – natural + fertilizers)
HOx cycle (from water vapour, methane)

5. Why Antarctic Ozone Hole is Most Severe?

Mnemonic: PSCs → “Polar Stratospheric Clouds = Perfect Storm for Chlorine”

• Very low winter temperature (< -78 °C) → Polar Stratospheric Clouds (Type I & II)
• PSCs provide surface for: ClONO₂ + HCl → Cl₂ + HNO₃ (reservoir → active Cl)
• Polar vortex traps Cl₂; spring sunlight → massive Cl• release → rapid O₃ loss in Sept–Oct

6. Effects of Ozone Depletion (High-Weightage)

Effect	Consequence
↑ UV-B radiation	Skin cancer (melanoma), cataracts, immune suppression
Phytoplankton damage	Disrupts marine food chain, ↓ CO2 fixation
Crop yield reduction	Soybean, wheat sensitive
Degradation of polymers/plastics	Material damage

7. International Efforts – Timeline (Very Important)

• 1974 – Molina & Rowland theory (Nobel 1995)
• 1985 – Discovery of Antarctic ozone hole (Farman et al.)
• 1987 – Montreal Protocol (first treaty signed by all UN countries)
• Phase-out: CFCs – 1996 (developed), 2010 (developing); HCFCs – 2030; HFCs (high GWP) controlled by Kigali Amendment 2016
• 2024–2025 status: Ozone layer on track to recover by ~2066 (UNEP 2023 report)

One-Liner Revision Points

• Most powerful natural ozone depleter → Atomic oxygen (O)
• Most powerful anthropogenic → Bromine from halons
• Only gas that destroys ozone in troposphere also → CH₃Br
• UV-B causes pyrimidine dimers → skin cancer
• Recovery sign: Antarctic hole smallest in 2023 since 1980s

Test Your Knowledge with PYQs (2014–2025) with Detailed Answers

(Covers NEET, JEE Main, CUET PG, CSIR NET, GATE – High-Yield for 2025 Exams)

NEET 2020 (Sep): Montreal protocol was signed in 1987 for control of
(a) Emission of ozone depleting substances
(b) Release of greenhouse gases
(c) Biodiversity conservation
(d) Control of CO₂ emission

Ans: (a) Montreal Protocol targets ODS like CFCs for ozone protection. (Direct NCERT line; 4 marks)

NEET 2018: In stratosphere, which element acts as catalyst in ozone degradation?
(a) Oxygen
(b) Hydrogen
(c) Chlorine
(d) Carbon

Ans: (c) Cl from CFCs catalyzes O₃ destruction (~1 lakh molecules per Cl). (Mechanism-based; frequent repeat)

NEET 2014: The zone of atmosphere with ozone layer is
(a) Stratosphere
(b) Troposphere
(c) Mesosphere
(d) Ionosphere

Ans: (a) Stratosphere (15–35 km) hosts protective O₃. (Basic fact; 4 marks)

NEET 2009: Global agreement to reduce ozone-depleting substances adopted by
(a) Montreal Protocol
(b) Kyoto Protocol
(c) Ramsar Convention
(d) Johannesburg Summit

Ans: (a) 1987 Montreal Protocol phased out CFCs/HCFCs. (International treaty focus)

NEET 2023: Dobson units measure thickness of
(a) Ozone
(b) Troposphere
(c) CO₂
(d) O₂

Ans: (a) 1 DU = 0.01 mm O₃ at STP; hole <220 DU. (Units-based; analytical)

JEE Main 2021 (Feb): Assertion (A): Ozone destroyed by CFCs in upper stratosphere
Reason (R): CFCs release Cl reacting with O₃ → ClO + O₂; ClO + O → Cl + O₂
(a) Both true, R explains A
(b) Both true, R doesn't explain A
(c) A true, R false
(d) A false, R true

Ans: (a) Cl catalytic cycle explains destruction. (Assertion-Reason; 4 marks)

JEE Main 2019: Chlorofluorocarbons responsible for
(a) Ozone layer depletion
(b) Global warming
(c) Acid rain
(d) Eutrophication

Ans: (a) CFCs → Cl• radicals deplete O₃. (Direct cause-effect)

JEE Main 2024: On polar stratospheric clouds, hydrolysis of ClONO₂ gives
(a) Cl₂ and HNO₃
(b) HCl and Cl₂
(c) Cl• and O
(d) NO₂ and ClO

Ans: (a) ClONO₂ + HCl → Cl₂ + HNO₃ on PSCs. (Mechanism; advanced)

JEE Main 2016: Upper stratosphere ozone protects from sun's radiation in wavelength
(a) 0.6–0.8 μm
(b) 200–750 nm
(c) 400–550 nm
(d) <200–320 nm

Ans: (d) UV-B (280–315 nm) & UV-C (<280 nm). (Wavelength recall)

JEE Main 2022: Gases causing greenhouse effect include
(a) CO₂, H₂O, CFCs
(b) O₂, N₂
(c) O₃ only
(d) CH₄ only

Ans: (a) CFCs contribute to both GHG & ODS. (Interlinkage)

CSIR NET 2020 (Dec): Montreal Protocol amendments resulted in
(a) Reduced ozone depletion
(b) Increased UV-A only
(c) No change in Antarctic hole
(d) Faster global warming

Ans: (a) Phased out ODS; hole shrinking since 2010s. (Applied ecology; 2 marks)

CSIR NET 2018: Ozone hole over South Pole more severe due to
(a) PSCs & polar vortex
(b) Higher CFCs in tropics
(c) NOₓ from volcanoes
(d) Water vapor increase

Ans: (a) PSCs activate Cl₂; vortex isolates air. (Regional mechanism)

CSIR NET 2022: Dobson units (DU) for ozone hole threshold
(a) <220 DU
(b) >300 DU
(c) 100 DU
(d) 500 DU

Ans: (a) <220 DU defines hole. (Measurement)

CUET PG 2024: Ozone reacts with alkenes followed by Zn/H₂O gives
(a) Carbonyl compounds
(b) Alcohols
(c) Alkyl halides
(d) Carboxylic acids

Ans: (a) Ozonolysis cleaves C=C to carbonyls. (Organic reaction; analytical)

CUET PG 2023: Main cause of ozone holes
(a) Propellants, foam agents, halocarbon refrigerants
(b) CO₂ emissions
(c) Deforestation
(d) SO₂ pollution

Ans: (a) All are ODS. (Multiple choice; conceptual)

CUET PG 2022: CFC in refrigerators is
(a) Freon
(b) Ammonia
(c) Methane
(d) CO₂

Ans: (a) Freon (e.g., CF₂Cl₂) depletes O₃. (Application-based)

NEET 2024: Which of the following is NOT an ozone-depleting substance?
(a) CH₃Br (b) CFCl₃ (c) CH₄ (d) CCl₄

(c) CH₄ – Methane is a GHG but has negligible ODP.

NEET 2023: The gas released from rice fields that indirectly contributes to ozone depletion is
(a) CO₂ (b) CH₄ (c) N₂O (d) SO₂

(c) N₂O → stratospheric NOx cycle.

NEET 2022: Ozone hole was first discovered over
(a) Antarctica (b) Arctic (c) Amazon (d) Sahara

(a) Antarctica (Farman et al., 1985).

JEE Main 2025 (Jan): The compound with highest ODP among the following is
(a) CFC-11 (b) CFC-12 (c) Halon-1211 (d) HCFC-22

(c) Halon-1211 (ODP ≈ 7.1; bromine is 45–60× more destructive).

JEE Main 2023: Polar Stratospheric Clouds are mainly composed of
(a) Ice only (b) HNO₃·3H₂O (c) NaCl (d) Dust particles

(b) HNO₃·3H₂O (Type I PSC).

CSIR NET June 2024: The reaction ClONO₂ + HCl (heterogeneous) → Cl₂ + HNO₃ occurs on the surface of
(a) Cirrus clouds (b) Polar stratospheric clouds (c) Cumulonimbus (d) Tropospheric aerosols

(b) Polar stratospheric clouds.

CSIR NET 2021: Which cycle contributes maximum to ozone depletion in mid-latitudes?
(a) HOx (b) NOx (c) ClOx (d) BrOx

(b) NOx cycle dominates outside polar regions.

GATE Chemistry 2023: The lifetime of CFC-12 in atmosphere is approximately
(a) 1 year (b) 10 years (c) 100 years (d) 1000 years

(c) ~100 years.

NEET 2021: Bad ozone is formed photochemically in
(a) Stratosphere (b) Troposphere (c) Mesosphere (d) Ionosphere

(b) Troposphere (photochemical smog).

JEE Main 2024: The wavelength range responsible for skin cancer is
(a) 100–280 nm (b) 280–315 nm (c) 315–400 nm (d) >400 nm

(b) 280–315 nm (UV-B).

NEET 2019: Which is responsible for depletion of ozone layer?
(a) CO (b) SO₂ (c) Freons (d) Smog

(c) Freons (CFCs).

CSIR NET Dec 2023: The Kigali Amendment (2016) to Montreal Protocol deals with phase-down of
(a) CFCs (b) HCFCs (c) HFCs (d) Methyl bromide

(c) HFCs (high GWP, zero ODP).

NEET 2025 (Predicted): The expected year for complete recovery of ozone layer is
(a) 2030 (b) 2040 (c) 2066 (d) 2100

(c) 2066 (UNEP/WMO 2023 report).

Ozone Depletion: Extra Questions

1. The primary reason why the ozone hole is most pronounced over Antarctica and not the Arctic is
(a) Higher CFC concentration in southern hemisphere
(b) Presence of polar vortex and extremely low winter temperatures leading to PSCs
(c) More UV radiation in southern hemisphere
(d) Higher volcanic activity in Antarctica

Answer: (b) Presence of polar vortex and extremely low winter temperatures leading to PSCs

2. Which of the following statements is correct regarding Type-I and Type-II Polar Stratospheric Clouds (PSCs)?
(a) Type-I are ice clouds; Type-II are nitric acid trihydrate clouds
(b) Type-I are nitric acid trihydrate (NAT); Type-II are water-ice clouds
(c) Both are composed only of sulfuric acid
(d) Type-II clouds do not participate in chlorine activation

Answer: (b) Type-I → Nitric acid trihydrate (NAT); Type-II → Water ice

3. As per the latest UNEP/WMO Scientific Assessment of Ozone Depletion (2022–2023), the ozone layer is expected to return to 1980 levels (full recovery) over Antarctica around the year
(a) 2040 (b) 2050 (c) 2066 (d) 2100

Answer: (c) 2066

4. Which gas, although having zero Ozone Depletion Potential (ODP), is being phased down globally under the Kigali Amendment (2016) to the Montreal Protocol?
(a) CO₂ (b) CH₄ (c) HFCs (d) N₂O

Answer: (c) HFCs (Hydrofluorocarbons) – high Global Warming Potential

5. In the catalytic ozone destruction cycle, the net reaction for the chlorine-catalyzed mechanism is
(a) 2O₃ → 3O₂ (b) O₃ + O → 2O₂ (c) O₃ → O + O₂ (d) Cl + O₃ → ClO + O₂ (only)

Answer: (b) O₃ + O → 2O₂ (One Cl atom regenerates and destroys ~1,00,000 ozone molecules)