Ozone Layer Depletion — PYQ 2023 (10 marks)

3.1 The Ozone Layer

The ozone layer is a region of high ozone (O₃) concentration in the stratosphere at 15–35 km altitude, with peak concentration at ~23 km. It absorbs 97–99% of the Sun's harmful UV-B (280–315 nm) and UV-C (100–280 nm) radiation.

Why ozone in stratosphere? UV radiation from the sun converts O₂ to O₃ in the upper stratosphere. Chapman (1930) described the photochemical reactions:

• O₂ + UV → 2O (atomic oxygen)
• O + O₂ → O₃ (ozone)
• O₃ + UV → O₂ + O (ozone absorbs UV)

Measurement: Ozone column thickness measured in Dobson Units (DU). Normal stratospheric ozone: ~300 DU. Antarctic spring ozone hole: drops to 100–150 DU (severe depletion).

3.2 Causes of Ozone Depletion

Ozone-Depleting Substances (ODS):

ODS	Chemical	Sources	Impact
CFCs (Chlorofluorocarbons)	CCl₂F₂, CCl₃F etc.	Refrigerants (old fridges, AC), aerosol spray cans, foam blowing	One Cl atom destroys 100,000 O₃ molecules (catalytic)
HCFCs	CHClF₂ etc.	CFC replacement (also ozone-depleting but less); being phased out	Less damaging than CFCs
Halons	CF₃Br etc.	Fire extinguishers	More potent than CFCs; very long-lived
Carbon Tetrachloride (CCl₄)	CCl₄	Solvent, dry cleaning (historical)	Long atmospheric lifetime
Methyl Bromide (CH₃Br)	—	Fumigant (agriculture)	Bromine highly effective ozone destroyer
Nitrous Oxide (N₂O)	—	Fertilisers, combustion	Most significant ODS still increasing (CFCs declining)

Mechanism of ozone destruction by CFCs:

1. CFCs are released at surface and drift slowly to stratosphere (takes 7–10 years)
2. UV radiation breaks Cl-F bond: CCl₂F₂ + UV → CF₂Cl• + Cl• (chlorine radical)
3. Cl• + O₃ → ClO + O₂ (ozone destroyed)
4. ClO + O → Cl• + O₂ (chlorine regenerated — catalytic cycle)
5. One Cl atom destroys 100,000 ozone molecules before being permanently removed

Why the Antarctic Ozone Hole?

• Antarctic Polar Vortex in spring (Aug-Oct): −80°C stratospheric temperatures
• Polar Stratospheric Clouds (PSCs) form — provide surface for reactions that release reactive chlorine
• When spring sunlight returns → catalytic ozone destruction is extremely rapid
• Hole was largest on record: 2023 ozone hole peaked at ~26 million km² (nearly area of North America)

3.3 Consequences of Ozone Depletion

1. Increased UV-B at Earth's surface:

• Skin cancer: UV-B is the primary cause of melanoma and non-melanoma skin cancers. 10% reduction in ozone → 15–20% increase in UV-B → 26% increase in skin cancer. Australia has the world's highest skin cancer rate (Australians under thinned ozone layer)
• Eye damage: Increased cataracts; WHO estimates 1.6 million cataract cases/year attributable to ozone depletion
• Immune suppression: UV-B suppresses human immune responses

2. Ecosystem damage:

• Marine ecosystem: UV-B penetrates 20 m into clear ocean water; damages phytoplankton (base of marine food chain); phytoplankton produce 50% of world's oxygen
• Amphibians: Most sensitive to UV-B; deformities and population collapses in frog populations linked to ozone depletion
• Agriculture: UV-B reduces crop yields for sensitive crops (soybeans, wheat, corn); affects plant DNA

3. Air quality:

• Increased UV-B stimulates ground-level ozone formation (tropospheric smog)
• Complex interactions with climate change (stratospheric ozone cooling effect; tropospheric ozone warming effect)

3.4 Montreal Protocol — The Success Story

The Montreal Protocol on Substances that Deplete the Ozone Layer (1987):

• Adopted on 16 September 1987 in Montreal, Canada
• 16 September = World Ozone Day (celebrated globally)
• The protocol is universally ratified — the only UN environmental treaty to achieve universal ratification (197/197 countries)
• Kofi Annan (former UN Secretary-General) called it "perhaps the single most successful international agreement to date"

Phase-out schedule:

• CFCs: Developed countries phased out by 1996; developing countries by 2010
• HCFCs: Developed by 2020; developing by 2030
• Halons: Developed by 1994
• Kigali Amendment (2016): Extended Montreal Protocol to phase out HFCs (hydrofluorocarbons — CFC replacements that don't deplete ozone but are powerful GHGs)

Results:

• ODS production reduced by 99%
• Ozone layer expected to return to pre-1980 levels by 2066 over Antarctica (scientific consensus)
• Prevented approximately 2 million skin cancer deaths per year globally
• Also prevented significant climate warming (CFCs and HFCs are powerful GHGs) — estimated 0.5°C of avoided warming

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