Ozone layer

Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 7 "Composition and Structure of Atmosphere"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

The ozone layer is the part of the stratosphere (the calm, dry layer of air above the troposphere) that holds most of the ozone (O3) in the atmosphere. About 90% of all atmospheric ozone is here, mostly at about 15–35 km above the ground (NCERT Class 11 gives 10–50 km). It absorbs the Sun's harmful ultraviolet (UV) radiation, mainly UV-B and UV-C.

Without this shield, life on land and in the upper ocean would face much more UV damage. Man-made chemicals such as CFCs thinned the layer. The Montreal Protocol (1987) is now helping it recover.

Explanation

How ozone forms and why it heats the stratosphere

• Ozone (O3): a molecule made of three oxygen atoms. It is only a minor gas in the air, but it is vital for life.
• The ozone cycle in the stratosphere:
• Strong sunlight splits an oxygen molecule (O2) into two single oxygen atoms (O).
• Each single atom joins another O2 molecule to make O3 (ozone).

• Ozone absorbs UV light and breaks apart again. Then the cycle repeats.

• The result: a warm stratosphere

• This non-stop cycle soaks up UV energy and heats the air.

• So in the stratosphere, temperature rises with height. In the troposphere below, it falls with height.

• What it protects:

• UV-C is almost fully blocked. UV-B is partly blocked.
• Humans get fewer cases of skin cancer and cataracts (clouding of the lens of the eye).
• Crops suffer less damage to their growth.
• In the oceans it protects plankton (tiny drifting plants and animals at the base of the marine food chain).

Measuring ozone and the "ozone hole"

• Dobson unit (DU): the unit for total column ozone. This means all the ozone in a column of air from the ground to the top of the atmosphere.
• 1 DU = a layer of pure ozone 0.01 mm thick at standard temperature and pressure (0°C, 1 atmosphere).

• The global average is about 300 DU. If you squeezed all the ozone above a place down to ground-level pressure, it would be only about 3 mm thick.

• Ozone hole: an area where total ozone falls below 220 DU.

• It is not a real hole. It is a region where the ozone has thinned badly.

What destroys ozone: the chemicals

• Ozone-depleting substances (ODS): man-made chemicals that carry chlorine or bromine up into the stratosphere.

Chemical	Main use	Harmful element
CFCs (chlorofluorocarbons)	Refrigerators, ACs, aerosol sprays, foams	Chlorine
Halons	Fire extinguishers	Bromine
Carbon tetrachloride (CTC)	Solvent, cleaning agent	Chlorine
Methyl bromide	Fumigant (pest-killing gas) for soil and stored grain	Bromine
HCFCs (hydrochlorofluorocarbons)	Stop-gap replacements for CFCs in ACs and foams	Chlorine (weaker effect)

• Why they reach so high:
• These gases are very stable, so they do not break down in the lower air.
• They drift slowly up to the stratosphere.
• There, strong UV breaks them apart and frees chlorine and bromine.

How the Antarctic ozone hole forms, step by step

• Step 1: Polar winter traps the air
• A polar vortex (a ring of strong west winds circling the pole in winter) traps very cold air over Antarctica.

• Temperatures drop below about −78°C.

• Step 2: Polar stratospheric clouds form

• Polar stratospheric clouds (PSCs): thin clouds of ice and nitric acid crystals. They form at about 15–25 km, and only in this extreme cold.

• Reactions on the cloud surfaces turn harmless "stored" chlorine into active chlorine, which is ready to attack ozone.

• Step 3: Spring sunlight starts the damage

• When the Sun returns in September, its light frees single chlorine atoms.

• Catalytic cycle (the chlorine is not used up, so the reaction repeats):

  • First, Cl + O3 → ClO + O2.
  • Then ClO reacts again and frees the Cl atom.
  • So one chlorine atom can destroy ozone again and again.

• Step 4: The hole closes

• By November–December, the vortex weakens. Warmer air that is rich in ozone mixes in, and the hole fills up.

Where it occurs

• Worldwide: the ozone layer covers the whole globe in the stratosphere, mostly at about 15–35 km.
• Antarctica (the ozone hole):
• The hole forms every southern spring (September–October).
• It was first reported in 1985 by British Antarctic Survey scientists Joseph Farman, Brian Gardiner and Jonathan Shanklin.
• 2025 season: the average size from 7 September to 13 October 2025 was about 18.71 million km². This made it the 5th smallest since 1992 [6][7].
• Its largest one-day size was 22.86 million km² on 9 September 2025. That is about 30% smaller than the record hole of 2006, which averaged 26.60 million km² [6].
• In 2025 the hole broke up nearly three weeks earlier than the average of the past ten years. A weaker-than-normal polar vortex in August 2025 kept temperatures higher [6].

• Ozone-depleting substances over Antarctica peaked around 2000. Since then they have fallen by about one-third compared with levels before the ozone hole [6].

• Arctic: the ozone thins only in some very cold years. Land, sea and mountain ranges such as the Rockies and Himalaya create air waves that break up the northern vortex.

• Global treaties:
• Vienna Convention (1985): a framework treaty for research and cooperation, with no binding cuts.
• Montreal Protocol (adopted 16 September 1987): sets binding phase-out deadlines. In 2009 it became the first UN treaty ratified by every country.
• Kigali Amendment: agreed in Kigali, Rwanda, in 2016 and in force from 1 January 2019.

• About 99% of the ozone-depleting substances controlled by the Protocol have now been phased out worldwide [3].

• India:

• India joined the Montreal Protocol in 1992. The Ozone Cell of the Ministry of Environment, Forest and Climate Change (MoEFCC) runs the programme [3].
• CFCs, carbon tetrachloride and halons were all phased out by 2010 [3].
• HCFCs:
  • India follows an HCFC Phase-out Management Plan (HPMP). Its roadmap was launched in 2009 [4].
  • The 19th Meeting of the Parties (2007) moved the HCFC phase-out schedule 10 years earlier [4].
  • By 2025, India had cut HCFC production and use by 67.5% [4].
• Kigali Amendment:
  • The Union Cabinet approved ratification in August 2021 [2].
  • India's baseline is 2024–26, with a freeze in 2028 [2].
  • India's cuts come in four steps: 10% by 2032, 20% by 2037, 30% by 2042 and 85% by 2047 [2].
• India Cooling Action Plan (ICAP, 2019): a 20-year plan (2017–18 to 2037–38). It aims to cut cooling demand and refrigerant use and to move to climate-friendly refrigerants.

Don't confuse with

• Ground-level ozone: this is the same gas, but it sits near the ground in the troposphere. It forms when vehicle and factory fumes react in sunlight. It is a pollutant that harms lungs and crops and is part of urban smog. It is not a shield.
• Ozone hole vs a real "hole": the ozone hole is an area where ozone falls below 220 DU. It is severe seasonal thinning, mainly in Antarctic spring. It is not a gap where no ozone is left.
• HFCs vs CFCs: HFCs have no chlorine, so they do not deplete ozone. But they are very strong greenhouse gases. That is why they are phased down under Kigali, an amendment to the Montreal Protocol.
• Vienna Convention vs Montreal Protocol: Vienna (1985) is a framework treaty with no binding cuts. Montreal (1987) sets binding phase-out deadlines.

Prelims Hooks

• About 90% of ozone is in the stratosphere, mostly at 15–35 km. 1 DU = 0.01 mm of pure ozone at STP. The global average is about 300 DU, and an ozone hole means below 220 DU.
• The Antarctic ozone hole forms in September–October (southern spring), not in the dark polar winter. Sunlight is needed to free the chlorine, and PSCs form only below about −78°C.
• Halogen match: CFCs, CTC and HCFCs → chlorine; halons and methyl bromide → bromine.
• Chronology: Vienna 1985 → Montreal 1987 → universal ratification 2009 → Kigali 2016 (in force 2019). World Ozone Day = 16 September.
• Return to 1980 ozone levels, from earliest to latest: rest of the world ~2040 → Arctic ~2045 → Antarctic ~2066 [5].
• The 2025 Antarctic hole was the 5th smallest since 1992. The largest on record was in 2006 [6].

Mains Points

• Causes and process (GS-I):
• Man-made CFCs and halons carry chlorine and bromine into the stratosphere.
• The very cold Antarctic polar vortex forms PSCs, and spring sunlight then starts catalytic ozone loss.

• This explains why the hole is seasonal and polar, and why it is bigger over Antarctica than over the Arctic.

• Montreal Protocol as a model (GS-III):

• Its features: universal ratification (2009), clear deadlines, more time for developing countries and regular scientific reviews (2022 Assessment).
• Results: about 99% of ODS phased out [3], with recovery expected by about 2040–2066 [5].

• It is often called the most successful environmental treaty and offers lessons for climate deals.

• Ozone–climate link and new risks (GS-III):

• HFCs: these CFC substitutes swapped an ozone problem for a climate problem. Kigali fixes this and could avoid 0.3–0.5°C of warming by 2100 [5].
• India: ICAP 2019 and the Kigali steps (freeze in 2028, 85% cut by 2047) [2] matter because AC demand is rising fast in hot cities.
• Stratospheric aerosol injection (SAI): this is geoengineering, meaning deliberate large-scale change of Earth's climate. It would spray reflective sulphate particles into the stratosphere.
  • It could deepen the Antarctic ozone hole and delay recovery [5].
  • It could also disturb monsoon rainfall.
  • Termination shock: if spraying stops suddenly, the hidden warming returns very quickly.
  • So a careful, multilateral approach to governing it is needed.

Related concepts

• Dobson unit
• Ozone hole
• Polar stratospheric clouds
• Montreal Protocol
• Kigali Amendment
• Stratospheric aerosol injection

Read more

• The ozone layer, its depletion and the Montreal Protocol