Why is "Saying the ozone layer absorbs UV-A radiation." a common mistake in Ozone Depletion?

Why it happens: Students know ozone absorbs 'UV radiation' but do not distinguish the three types. How to avoid it: The ozone layer absorbs **UV-C** (almost completely) and **UV-B** (substantially). **UV-A** is NOT absorbed by ozone — it passes through to Earth's surface. This distinction matters because UV-B causes the main harm (skin cancer, cataracts). UV-A causes tanning but is less harmful. Source: UV type distinction penalised in 0680 mark schemes for ozone questions.

Why is "Confusing ozone depletion with the greenhouse effect or climate change." a common mistake in Ozone Depletion?

Why it happens: Both are atmospheric problems and are discussed together — students conflate them. How to avoid it: Ozone depletion (CFCs, stratosphere, UV-B) is SEPARATE from climate change (CO₂/CH₄, troposphere, infrared/heat). CFC aerosol sprays cause ozone depletion — they do NOT cause climate change through the greenhouse effect (though CFCs happen to be potent GHGs as well). CO₂ does NOT deplete ozone. Source: Most commonly penalised error in 0680 atmosphere questions.

Why is "Stating that CFCs are still widely produced and used (ignoring the Montreal Protocol)." a common mistake in Ozone Depletion?

Why it happens: Students learn about CFCs as a problem and do not update their knowledge to include the successful policy response. How to avoid it: CFCs were phased out in developed countries by 1996 and in developing countries by 2010 under the Montreal Protocol. They have been largely replaced by HCFCs and then HFCs (no ozone depletion but potent greenhouse gases). Atmospheric CFC concentrations are now declining. The ozone layer IS recovering — slowly.

Why is "Failing to explain that the chlorine radical is regenerated (catalytic cycle)." a common mistake in Ozone Depletion?

Why it happens: Students write Cl• + O₃ → products but forget the second step that regenerates Cl•. How to avoid it: The two-step cycle: (1) Cl• + O₃ → ClO + O₂. (2) ClO + O → Cl• + O₂. The Cl• reappears at step 2 — this is what makes it a CATALYST. Without step 2, you have just one ozone destroyed per chlorine atom, which would not explain the devastating scale of depletion. Each Cl• destroys ~100,000 O₃ molecules BECAUSE it is regenerated.

The Atmosphere and Human ActivitiesCambridge IGCSE Environmental Management (0680)

Ozone Depletion

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Ozone Depletion — Cambridge IGCSE Environmental Management 0680

Stratospheric ozone (O₃) shields life on Earth from harmful UV-B and UV-C radiation. Human-produced chlorofluorocarbons (CFCs) and other ozone-depleting substances have thinned the ozone layer, causing the Antarctic ozone hole. The Montreal Protocol (1987) has successfully initiated recovery, though full recovery will take decades.

At a glance

Stratospheric ozone (O₃) at 20–35 km altitude absorbs UV-B and UV-C radiation, protecting life on Earth.
CFCs (chlorofluorocarbons): used in refrigerants, aerosol propellants, foam blowing; chemically inert in troposphere; rise to stratosphere.
Mechanism: UV breaks C–Cl bond → Cl• radical released → Cl• + O₃ → ClO + O₂; ClO + O → Cl• + O₂ (catalytic cycle). Each Cl• destroys ~100,000 O₃ molecules.
Antarctic ozone hole: forms September–October; polar stratospheric clouds (PSCs) accelerate reactions; discovered 1985.
Effects of increased UV-B: skin cancer (melanoma), cataracts, immune suppression; marine phytoplankton damage; reduced crop yields.
Montreal Protocol (1987): banned CFCs; 197 parties (universal ratification); most successful international environmental agreement.
Recovery: global ozone (outside polar regions) by ~2040; Antarctic hole by ~2065.
Kigali Amendment (2016): phases out HFC replacements (potent greenhouse gases, GWP 1,000–14,000) by 2047.

What you’ll learn

Mapped to the Cambridge IGCSE 0680 syllabus (2027-2029).

4.4a — Describe the role of stratospheric ozone in absorbing UV-B and UV-C radiation and its importance for life on Earth.
4.4b — Identify ozone-depleting substances (CFCs, halons, HCFCs, methyl bromide) and describe their sources and properties.
4.4c — Explain the mechanism by which CFCs deplete stratospheric ozone, including the catalytic chlorine radical cycle.
4.4d — Describe the Antarctic ozone hole — its formation, discovery, and seasonal pattern.
4.4e — Describe the effects of increased UV-B radiation on human health, marine ecosystems, and terrestrial ecosystems.
4.4f — Evaluate the effectiveness of the Montreal Protocol in managing ozone depletion, including the Kigali Amendment.
4.4g — Explain how ozone column thickness is measured using Dobson Units.

The Role of Stratospheric Ozone

The ozone layer is Earth's UV radiation shield — essential for all life on the surface.

What is ozone and where is it?

Ozone (O₃) is an allotrope of oxygen — a molecule of three oxygen atoms. In the stratosphere (approximately 20–35 km altitude), ozone is present at concentrations of approximately 10 ppm (much higher than in the troposphere where it is a pollutant at <0.06 ppm).

The total amount of ozone in a vertical column of atmosphere is measured in Dobson Units (DU):

1 DU = a layer of pure ozone 0.01 mm thick at standard temperature and pressure.
Global pre-industrial average: approximately 300 DU.
Antarctic ozone hole minimum (1990s): as low as 100 DU — one-third of normal.

How ozone forms in the stratosphere: Ozone is formed by a two-step photochemical process:

UV-C radiation splits oxygen molecules: O₂ + UV → 2O (oxygen atoms)
Each oxygen atom combines with an O₂ molecule: O + O₂ → O₃

This formation reaction also converts UV energy to heat — explaining why stratospheric temperature increases with altitude (more UV absorbed → more heating at higher levels, where ozone formation is most active).

What ozone absorbs:

UV type	Wavelength	Absorbed by ozone?	Harm if it reached surface
UV-C	100–280 nm	Almost completely	Lethal to most life
UV-B	280–315 nm	Substantially	Skin cancer, cataracts, DNA damage
UV-A	315–400 nm	Not absorbed	Tanning; minor damage

The ozone layer reduces UV-B intensity at Earth's surface by approximately 99% compared to what would be present without stratospheric ozone.

Why ozone is essential for complex life: Before cyanobacteria evolved photosynthesis (~3.5 billion years ago) and produced oxygen — which eventually created the ozone layer (~2.4 billion years ago) — high levels of UV radiation prevented complex multicellular life from colonising land. The ozone layer's formation was a prerequisite for terrestrial life as we know it.

Ozone (O₃) in stratosphere (20–35 km) absorbs UV-B and UV-C radiation.
Measured in Dobson Units (DU): pre-industrial ~300 DU; ozone hole minimum ~100 DU.
UV-C: almost completely absorbed. UV-B: substantially absorbed. UV-A: not absorbed.
Formation: O₂ + UV → 2O; O + O₂ → O₃ (also produces heat → stratospheric warming).
Without ozone layer: UV-B and UV-C would reach surface — lethal/severely harmful to most life.

Ozone-Depleting Substances and the Depletion Mechanism

CFCs release chlorine radicals in the stratosphere that catalytically destroy ozone.

Ozone-Depleting Substances (ODS):

Substance	Abbreviation	Main uses	Ozone Depletion Potential (ODP)
Chlorofluorocarbons	CFCs	Refrigerants, aerosol propellants, foam blowing	High (1.0 reference value for CFC-11)
Halons	—	Fire suppressants	Very high (up to 10×)
Hydrochlorofluorocarbons	HCFCs	CFC replacements in refrigeration	Low (~0.02–0.1)
Methyl bromide	CH₃Br	Agricultural fumigant (soil sterilant)	Moderate (~0.4)
Hydrochlorofluorocarbons	HCFCs	CFC replacements	Low (0.01–0.12)

Why CFCs are particularly dangerous: (1) Chemically inert in the troposphere — do not react with water, oxygen, or other tropospheric chemicals. No natural removal mechanism at low altitude. (2) Long atmospheric lifetime — 50–150 years depending on specific compound (CFC-11: ~55 years; CFC-12: ~100 years; CFC-113: ~85 years). Even if all production stops today, CFC molecules released decades ago are still in the atmosphere. (3) Strong UV absorption in stratosphere — UV-C breaks the C–Cl bond with high efficiency.

Step-by-step mechanism of CFC-induced ozone destruction:

Step 1 — Transport to stratosphere: CFCs are emitted at the surface and slowly diffuse upward through the troposphere over years to decades. Once in the stratosphere (20–35 km), intense UV-C radiation encounters the CFC molecule.

Step 2 — Release of chlorine radical: UV radiation breaks the carbon-chlorine bond: CF₂Cl₂ + UV → CF₂Cl• + Cl• (chlorine radical)

A radical is an atom or molecule with an unpaired electron — highly reactive.

Step 3 — Catalytic ozone destruction cycle:

Reaction A: Cl• + O₃ → ClO + O₂ (One ozone molecule is destroyed; chlorine oxide (ClO) is formed)

Reaction B: ClO + O → Cl• + O₂ (Chlorine radical is regenerated; another oxygen molecule is formed)

Net reaction: O₃ + O → 2O₂ (ozone is irreversibly converted to ordinary oxygen)

The critical feature: In Reaction B, the Cl• radical is REGENERATED — it is a catalyst. It can then immediately attack another O₃ molecule (Reaction A again), restart the cycle, and destroy another ozone molecule. This cycle repeats approximately 100,000 times before the Cl• is permanently removed (e.g. by combining with methane: Cl• + CH₄ → HCl + CH₃•, or with NO₂: ClO + NO₂ → ClONO₂).

This catalytic amplification explains why tiny amounts of CFCs (concentrations measured in parts per trillion) can cause massive ozone depletion across continental areas.

UV breaks the C–Cl bond in a CFC; the chlorine radical catalytically destroys ~100,000 ozone molecules before it is finally removed.

CFCs: synthetic compounds (refrigerants, aerosols, foam). Chemically inert troposphere → survive to stratosphere.
CFC atmospheric lifetime: 50–150 years. Long persistence → depletion continues long after emission.
UV breaks C–Cl bond → Cl• radical. Catalytic cycle: Cl• + O₃ → ClO + O₂; ClO + O → Cl• + O₂.
Cl• is regenerated → destroys ~100,000 O₃ molecules per Cl• atom.
Net: O₃ + O → 2O₂ (ozone irreversibly destroyed).

The Antarctic Ozone Hole

A seasonal region of intense ozone depletion forms annually over Antarctica each spring.

Formation of the Antarctic ozone hole:

The ozone hole does not form equally everywhere on Earth — it is most severe over Antarctica in spring (September–October) due to unique polar conditions.

Step 1 — Antarctic winter (April–August): Antarctica experiences continuous darkness (polar night) during winter. The polar vortex — a large, circular pattern of cold winds circling Antarctica — isolates polar stratospheric air from warmer air at lower latitudes. Stratospheric temperatures fall to −78°C or below.

Step 2 — Polar stratospheric cloud (PSC) formation: At these extremely low temperatures, polar stratospheric clouds (PSCs) form — composed of ice crystals containing water, nitric acid, and sulfuric acid. PSCs provide surfaces for heterogeneous (surface) chemical reactions that convert 'reservoir' forms of chlorine (HCl and ClONO₂, which do not react directly with ozone) into highly reactive molecular chlorine (Cl₂) and hypochlorous acid (HOCl).

Step 3 — Spring sunlight triggers ozone destruction (September–October): As sunlight returns in southern spring, UV radiation photolyses the reactive chlorine compounds accumulated on PSC surfaces: Cl₂ + UV → 2Cl• (two chlorine radicals released simultaneously)

With abundant Cl• radicals and a reservoir of ozone trapped within the polar vortex, ozone destruction proceeds extremely rapidly — creating the 'hole' (severe thinning).

Discovery: The ozone hole was discovered in 1985 by British Antarctic Survey scientists Joe Farman, Brian Gardiner and Jonathan Shanklin, using ground-based Dobson spectrophotometers at Halley Station, Antarctica. NASA satellite data (which had been recording low values but dismissed as instrument error) were retroactively confirmed.

The ozone hole today:

Ozone hole area peaked at approximately 29 million km² in 2000 (larger than Antarctica itself).
Since 2000, there has been a gradual trend of reduction — consistent with declining atmospheric chlorine from the Montreal Protocol phase-out.
Year-to-year variability is high: hole size depends on polar vortex strength and stratospheric temperature each year (e.g. 2019 was unusually small due to a sudden stratospheric warming event; 2023 was large partly due to water vapour injection from the Hunga Tonga underwater volcanic eruption).

2023 anomaly — Hunga Tonga volcano: The January 2022 eruption of Hunga Tonga–Hunga Ha'apai (underwater volcanic explosion, South Pacific) injected an exceptional amount of water vapour into the stratosphere — far more than typical volcanic eruptions which inject aerosols. Extra water vapour promoted PSC formation and heterogeneous ozone chemistry → contributed to an unusually large ozone hole in 2023.

Ozone hole: forms September–October (southern spring) over Antarctica; polar vortex isolates polar air.
PSCs form at −78°C or below → provide surfaces for reactions converting reservoir Cl to reactive Cl•.
Sunlight returns → Cl₂ photolyses → 2Cl• → rapid ozone destruction within polar vortex.
Discovered 1985 (Farman, Gardiner, Shanklin — British Antarctic Survey).
Peaked ~29 million km² (2000); gradually declining since; year-to-year variability due to polar vortex strength.

Effects of Increased UV-B Radiation

Ozone depletion allows more UV-B to reach Earth's surface — damaging living organisms and ecosystems.

Effects on human health:

(1) Skin cancer:

UV-B damages DNA in skin cells — causing mutations that can lead to cancer.
Non-melanoma skin cancer (basal cell and squamous cell carcinoma): most common; rarely fatal; directly linked to cumulative UV-B exposure.
Melanoma (malignant melanoma): less common but much more dangerous; linked to intense, episodic UV-B exposure (sunburn).
Quantitative relationship: every 1% decrease in ozone column thickness causes approximately 1.5% increase in UV-B at the surface and 2–3% increase in skin cancer risk.
Australia has one of the highest melanoma rates in the world — partly attributed to its position under the edge of the Southern Hemisphere ozone hole and its high proportion of fair-skinned residents.

(2) Cataracts:

UV-B damages the protein structures in the eye lens, causing clouding (cataracts).
Cataracts are the leading cause of preventable blindness globally.
WHO estimates a sustained 10% ozone reduction would cause approximately 1.75 million additional cataract cases per year.

(3) Immune suppression:

UV-B reduces the activity and number of T-lymphocytes (immune cells) in skin.
Immune suppression increases susceptibility to infections and may reduce the effectiveness of vaccinations.
This systemic immune effect goes beyond just skin — affecting the body's ability to fight many diseases.

Effects on marine ecosystems:

(1) Marine phytoplankton:

Phytoplankton are microscopic algae in ocean surface waters — they are the foundation of marine food chains and produce approximately 50% of Earth's oxygen through photosynthesis.
UV-B damages the photosynthetic machinery (photosystem II) of phytoplankton, reducing their growth and productivity.
Reduced phytoplankton → less food for zooplankton → less food for fish → less food for marine mammals → cascading loss of marine biodiversity and fisheries.
Studies in Antarctic waters documented 6–12% reduction in phytoplankton productivity under ozone hole conditions.

(2) Other marine organisms:

UV-B damages the eggs and larvae of fish and other marine organisms developing near the ocean surface.
Coral polyps are sensitive to UV-B — ozone depletion compounds the already-stressed state of coral reefs.

Effects on terrestrial ecosystems:

(1) Crop yield reductions:

Many crop species show reduced growth, leaf damage and yield under elevated UV-B.
Sensitive crops: soybeans, rice, wheat, and some vegetables.
UV-B damages chlorophyll and reduces photosynthetic efficiency.

(2) Soil organisms:

Bacteria, fungi and other microorganisms in upper soil layers are UV-B sensitive.
Reduced microbial populations slow decomposition and nutrient cycling — affecting soil fertility.

(3) Plant DNA damage:

UV-B induces mutations in plant DNA; most plants have some repair mechanisms, but high UV-B overwhelms these.

Skin cancer: UV-B causes DNA mutations; 1% ozone decrease → 1.5% more UV-B → 2–3% more skin cancer.
Cataracts: UV-B clouds the eye lens; WHO: 10% ozone loss → 1.75 million extra cataract cases/year.
Immune suppression: fewer T-lymphocytes → increased infection susceptibility.
Marine phytoplankton: UV-B reduces productivity; cascades through marine food chains.
Crops: UV-B reduces photosynthesis in soya, rice, wheat; soil microorganisms also harmed.

Montreal Protocol and Ozone Recovery

The Montreal Protocol (1987) is one of the most successful international environmental agreements — and ozone is slowly recovering.

The Montreal Protocol (1987):

The Montreal Protocol on Substances that Deplete the Ozone Layer was signed in 1987 and entered into force in 1989. It is the most universally ratified United Nations environmental treaty in history.

Key provisions and timeline:

1989: Protocol enters into force.
1992 (Copenhagen Amendment): Accelerated CFC phase-out; added HCFCs (partial CFC replacements) to the phase-out schedule.
1996: CFCs completely phased out in developed (HICs) nations.
2010: CFCs phased out in developing (LIC) nations.
2030: HCFCs (the intermediate CFC substitutes) to be fully phased out.

Why the Montreal Protocol succeeded: (1) Universal ratification — 197 parties. Every nation on Earth has ratified. No treaty in UN history has achieved universal ratification. Universal coverage closes potential carbon leakage (no countries left with an economic advantage from continued CFC production). (2) Clear scientific consensus and industry acceptance. Rowland, Molina and Crutzen established the CFC–ozone link unambiguously in 1974 (Nobel Prize 1995). After initial resistance, major chemical companies (DuPont, ICI) accepted the science and invested in developing CFC substitutes, reducing industry opposition. (3) Substitute chemicals were available. HCFCs (less ozone-damaging) and then HFCs (no ozone effect) were available as replacement refrigerants and propellants — allowing industries to function without CFCs. Compare this to climate change, where there is no simple substitute for fossil fuels. (4) Multilateral Fund for developing countries. A financial mechanism provides ~$3 billion total to developing countries to help them transition away from ODS — addressing equity concerns and securing LIC participation. (5) Relatively small affected industries. The ODS production and use sectors (refrigerant manufacturers, aerosol companies) are far smaller and more concentrated than the fossil fuel industry — making the political battle more manageable.

Evidence of success and recovery:

Atmospheric chlorine concentrations peaked around 1993–1994 and have been slowly declining since.
Antarctic ozone hole area peaked at ~29 million km² around 2000 and shows a gradual decline.
Global ozone column outside polar regions is gradually recovering.

Recovery projections (WMO/UNEP 2022 assessment):

Global ozone (outside polar regions): projected to recover to pre-1980 values by approximately 2040.
Arctic stratosphere: recovery by approximately 2045.
Antarctic ozone hole: recovery to pre-1980 values projected by approximately 2065–2080 — slowest due to extreme polar chemistry conditions.

The Kigali Amendment (2016): HFCs — introduced as non-ozone-depleting CFC substitutes — turned out to be extremely potent greenhouse gases (GWPs of 1,000–14,000). The Kigali Amendment to the Montreal Protocol (agreed 2016, entered into force 2019) commits parties to phase down HFC production and consumption by 80–85% by 2047. Replacement substances are hydrofluoroolefins (HFOs) — with very low GWP and no ozone effect. Projected benefit: avoiding approximately 0.3–0.5°C of additional warming by 2100.

Remaining challenges:

Nitrous oxide (N₂O): Now the largest ongoing ozone-depleting substance emitted by humans (since CFCs are phased out), but NOT regulated by the Montreal Protocol (not identified in 1987). N₂O from agriculture is a growing concern.
Illegal CFC production: In 2018–2019, atmospheric measurements detected unexpected CFC-11 emissions, traced to illegal production in eastern China. Enforcement action brought emissions back down, but monitoring remains essential.
Slow ecosystem recovery: Even as the ozone layer chemically recovers, ecosystems damaged by decades of elevated UV-B (particularly some marine populations) will take much longer to fully recover.

Montreal Protocol (1987): 197 parties (universal); CFCs phased out by 1996 (HICs), 2010 (LICs).
Success factors: clear science, substitute chemicals available, Multilateral Fund for LICs, small affected industries.
Atmospheric chlorine peaked ~1993–1994 and declining; ozone hole peaked ~2000.
Recovery: global ozone by ~2040; Antarctic hole by ~2065.
Kigali Amendment (2016): phases out HFCs (GWP 1,000–14,000) — avoids ~0.3–0.5°C warming.
Remaining issue: N₂O (largest current ODS) NOT covered by Montreal Protocol.

Sources: Cambridge IGCSE Environmental Management 0680 syllabus 2027-2029, Section 4.4; 0680/22 Oct/Nov 2023 — Q10 (ozone depletion mechanism); 0680/12 Oct/Nov 2024 — Q11 (ozone recovery and Montreal Protocol); WMO/UNEP — Scientific Assessment of Ozone Depletion 2022; NASA Ozone Watch — Antarctic ozone hole data; 0680 Examiner Reports 2022-2024 — ozone questions; UNEP — Kigali Amendment to the Montreal Protocol 2016. Last reviewed 2026-05-15.

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Step-by-step worked examples — Ozone Depletion

Step-by-step solutions to past-paper-style questions on ozone depletion, written exactly the way a tutor would explain them at the board.

1Role of stratospheric ozone
Core• ozone, UV radiation, stratosphere, protection
Question
Explain the role of stratospheric ozone. What would happen to life on Earth if the ozone layer were destroyed?
Step-by-step solution
1. Step 1
  Role of stratospheric ozone: The stratospheric ozone layer is concentrated at approximately 20–30 km altitude in the stratosphere. It performs a critical function: absorbing ultraviolet (UV) radiation from the Sun, particularly UV-B (wavelengths 280–315 nm) and UV-C (100–280 nm). When ozone absorbs UV radiation, it is split: O₃ + UV → O₂ + O. The oxygen atom rapidly recombines with another O₂: O + O₂ → O₃. This cycle continuously regenerates ozone while converting UV energy to heat — which explains the temperature increase in the stratosphere (stratospheric warming).
2. Step 2
  UV radiation types and risks: UV-C (most dangerous) is almost entirely absorbed by stratospheric ozone and oxygen — none reaches Earth's surface. UV-B (moderate danger) is partially absorbed by ozone — the amount reaching Earth depends critically on ozone column thickness. UV-A (lowest energy UV) is not absorbed by ozone and always reaches Earth's surface — it causes tanning but is far less harmful than UV-B. The ozone layer reduces UV-B intensity at Earth's surface by approximately 99% compared to what would be present without it.
3. Step 3
  Consequences of ozone layer destruction: (1) Human health: Massive increase in skin cancer rates (melanoma and non-melanoma). Each 1% decrease in ozone column thickness causes approximately 1–2% increase in UV-B at the surface and 2–3% increase in skin cancer risk. Increased cataracts (UV damages the eye lens). Immune suppression — UV-B reduces the effectiveness of the human immune system. (2) Marine ecosystems: UV-B is highly damaging to phytoplankton — microscopic algae at the base of marine food chains. Reduced phytoplankton productivity would cascade through the entire marine food chain, reducing fish and marine mammal populations. (3) Terrestrial ecosystems: Many crops and plant species show reduced growth and yield under high UV-B. DNA damage in plants reduces photosynthesis efficiency. Soil microorganisms involved in nutrient cycling are UV-B sensitive.
Answer
Stratospheric ozone absorbs UV-B and UV-C radiation (O₃ + UV → O₂ + O), preventing it from reaching Earth's surface. Without it: massive increase in skin cancers and cataracts; immune suppression; marine phytoplankton loss (disrupts food chains); reduced crop yields; DNA damage to organisms.
Examiner tip
Examiners require you to specify WHICH type of UV radiation ozone absorbs (UV-B and UV-C, NOT UV-A). Saying 'ozone absorbs sunlight' is too vague. The catalytic cycle for ozone formation (O₃ + UV → O₂ + O; then O + O₂ → O₃) shows WHY stratospheric temperature increases with altitude — a common link question.
2Mechanism of CFC-induced ozone depletion
Core• Adapted from 0680/22 Oct/Nov 2023• CFCs, chlorine radicals, catalytic cycle, mechanism, ozone hole
Question
Explain how chlorofluorocarbons (CFCs) deplete stratospheric ozone. Describe the sources of CFCs and the catalytic mechanism of ozone destruction.
Step-by-step solution
1. Step 1
  Sources and properties of CFCs: Chlorofluorocarbons (CFCs) are synthetic organic compounds containing carbon, chlorine, and fluorine atoms. They were developed in the 1930s as apparently ideal industrial chemicals — chemically inert (non-toxic, non-flammable, stable), with useful physical properties. Main uses: refrigerants (in air conditioning and refrigerators — R-12, Freon), aerosol propellants (sprays, deodorants), foam-blowing agents (in polystyrene manufacturing), industrial solvents. Other ozone-depleting substances (ODS): halons (fire suppressants — very potent ODS), HCFCs (partial replacements for CFCs — less damaging but still deplete ozone), methyl bromide (agricultural fumigant).
2. Step 2
  Journey of CFCs to the stratosphere: CFCs are chemically inert in the troposphere — they do not react with water, oxygen, or other molecules. This inertness means they persist in the atmosphere for 50–150 years (depending on the specific CFC compound). Over years to decades, atmospheric circulation carries CFC molecules upward into the stratosphere (15–35 km altitude). At these altitudes, intense shortwave UV radiation (specifically UV-C) has sufficient energy to break the C–Cl chemical bond in CFC molecules.
3. Step 3
  Catalytic ozone destruction cycle: When UV breaks the C–Cl bond, a chlorine radical (Cl•) is released: CFC molecule + UV → Cl• + remaining fragment
  
  The chlorine radical then attacks an ozone molecule: Step 1: Cl• + O₃ → ClO + O₂ (chlorine oxide is formed; ozone is destroyed) Step 2: ClO + O → Cl• + O₂ (chlorine radical is regenerated)
  
  Net reaction: O₃ + O → 2O₂ (one ozone molecule and one oxygen atom are converted to two ordinary oxygen molecules — ozone is irreversibly destroyed)
  
  The critical point: the Cl• radical is regenerated at Step 2 — it can attack another O₃ molecule, beginning the cycle again. Each chlorine radical can destroy approximately 100,000 ozone molecules before being permanently removed from the cycle (e.g. by combining with methane to form HCl). This catalytic amplification is why small amounts of CFCs can cause enormous ozone depletion.
4. Step 4
  The Antarctic ozone hole: The ozone 'hole' is not literally a hole but a severe thinning of the ozone layer over Antarctica, typically reaching its maximum extent in September–October (southern spring). It was discovered in 1985 by British Antarctic Survey scientists Farman, Gardiner and Shanklin, using ground-based measurements. The hole forms due to unique polar conditions: during Antarctic winter, extremely cold temperatures (−78°C or below) allow the formation of polar stratospheric clouds (PSCs) — ice crystals that provide surfaces for heterogeneous reactions that rapidly convert reservoir forms of chlorine (HCl, ClONO₂) into reactive ozone-destroying forms. When sunlight returns in spring, UV radiation triggers intense ozone destruction — forming the hole.
Answer
CFCs: synthetic compounds used in refrigerants, aerosols, foam blowing. Chemically inert in troposphere — persist 50–150 years. Rise to stratosphere; UV breaks C–Cl bond → Cl• radical. Catalytic cycle: Cl• + O₃ → ClO + O₂; ClO + O → Cl• + O₂. Net: O₃ + O → 2O₂. Cl• regenerated — each destroys ~100,000 O₃ molecules. Antarctic hole: September–October, polar stratospheric clouds accelerate reactions; discovered 1985.
Examiner tip
The two-step catalytic cycle (Cl• + O₃ → ClO + O₂; ClO + O → Cl• + O₂) is the most precisely tested mechanism in this topic. The key insight is that Cl• is a catalyst — it is regenerated. 'Ozone is destroyed by CFCs' is too vague; the mark scheme requires the radical mechanism. The number ~100,000 ozone molecules per chlorine atom is a useful quantitative detail for Extended answers.
3The Montreal Protocol and recovery of the ozone layer
Core• Montreal Protocol, CFCs, HFCs, recovery, international agreements
Question
Describe the Montreal Protocol and explain why it has been successful in addressing ozone depletion. What is the projected timeline for ozone layer recovery?
Step-by-step solution
1. Step 1
  The Montreal Protocol (1987): The Montreal Protocol on Substances that Deplete the Ozone Layer was signed in 1987 and entered into force in 1989. It is considered one of the most successful international environmental agreements in history. It committed signatory nations to progressively phase out the production and consumption of ozone-depleting substances (ODS). Key milestones: CFCs phased out in developed countries by 1996; developing countries given additional time (phased out by 2010); halons phased out; HCFCs (partial substitutes) scheduled for phase-out by 2030.
2. Step 2
  Why the Montreal Protocol succeeded: (1) Universal ratification: The Montreal Protocol has been ratified by 197 parties — every nation on Earth. This is the only UN environmental treaty to achieve universal participation, providing global coverage. (2) Clear scientific consensus: The chemistry linking CFCs to ozone depletion was established unambiguously by Rowland, Molina and Crutzen (Nobel Prize in Chemistry, 1995). Industry accepted the evidence relatively quickly. (3) Available substitutes: Replacement substances (initially HCFCs, then HFCs — hydrofluorocarbons) were available from the chemical industry, making the transition technically feasible. This reduced industry opposition. (4) Multilateral Fund: A financial mechanism was established to help developing countries meet their phase-out obligations — ensuring LICs did not face an unfair burden.
3. Step 3
  Recovery timeline and ongoing challenges: The stratospheric ozone layer has been showing signs of recovery since the 2000s — stratospheric chlorine concentrations have been slowly declining as CFCs disperse and are removed. Projections:
  
  Global ozone (outside polar regions): projected to recover to pre-1980 levels by approximately 2040.
  
  Antarctic ozone hole: projected to close (recover to pre-1980 values) by approximately 2065–2080 — recovery is slower over Antarctica due to the extreme polar conditions that accelerate reactions.
  
  Kigali Amendment (2016): HFCs (the CFC replacements that became widely used) have no effect on ozone but are extremely potent greenhouse gases (HFC-134a: GWP ~1,430). The Kigali Amendment to the Montreal Protocol committed nations to phase down HFCs by 80–85% by 2047, switching to more climate-friendly alternatives (HFOs — hydrofluoroolefins).
Answer
Montreal Protocol (1987): phased out CFCs globally — developed nations by 1996, developing nations by 2010; universal ratification (197 parties). Successful because: clear science, substitutes available, Multilateral Fund for LICs, universal participation. Recovery: global ozone by ~2040; Antarctic hole closure ~2065. Kigali Amendment (2016): phases out HFC substitutes (potent greenhouse gases, GWP ~1,430) by 2047.
Examiner tip
The Montreal Protocol is frequently cited as the international environmental agreement that actually worked — contrast with the Paris Agreement (climate) which has not yet achieved its targets. The Kigali Amendment is a useful detail showing that solving ozone depletion created a new climate problem (HFCs). Recovery timescales (2040 global, 2065 Antarctic) are specific and expected in Extended answers.
4Analyse quantitative data on UV-B increase and health impacts
Extended• UV-B, skin cancer, data analysis, ozone thickness, Dobson units
Question
Data: In the 1990s, Antarctic ozone column thickness dropped to as low as 100 Dobson Units (DU) during spring, compared to the pre-1980 baseline of approximately 300 DU. Research shows that a 1% decrease in ozone column thickness causes approximately 1.5% increase in UV-B intensity at the surface. (a) Calculate the percentage decrease in ozone column thickness between baseline and the 1990s minimum. (b) Using the relationship given, calculate the approximate increase in UV-B intensity at the surface. (c) Evaluate the health implications for humans exposed to this increased UV-B over Antarctica and nearby regions.
Step-by-step solution
1. Step 1
  Percentage decrease in ozone column thickness: Percentage decrease = (300 − 100) ÷ 300 × 100 = 66.7% decrease in ozone column thickness compared to the pre-1980 baseline. The ozone column over Antarctica in the 1990s spring was approximately one-third of its natural thickness — a catastrophic reduction. Note: Dobson Units (DU) measure the total amount of ozone in a vertical column of atmosphere — 300 DU means the ozone, if compressed to standard temperature and pressure, would form a layer 3 mm thick.
2. Step 2
  Increase in UV-B intensity at the surface: Using the relationship: 1% decrease in ozone → 1.5% increase in UV-B. For a 66.7% decrease in ozone: UV-B increase = 66.7 × 1.5 = 100% increase (doubling) in UV-B intensity. This means UV-B intensity at the Antarctic surface during ozone hole conditions in spring is approximately twice what it was before ozone depletion. In absolute terms, UV-B levels under the ozone hole can exceed UV-B levels measured at the equator during the same period — extraordinary for a region not known for sunny conditions.
3. Step 3
  Health implications for humans: Direct Antarctic exposure: Antarctica itself has a very small permanent human population (research station personnel — approximately 1,000 in winter, up to 5,000 in summer). Research workers stationed at Antarctic bases face significantly elevated skin cancer and cataract risk during spring. All Antarctic field workers now use protective measures: high-SPF sunscreens, UV-blocking goggles, protective clothing.
  
  Nearby regions — South America, New Zealand, Australia: The ozone hole extends beyond Antarctica and shifts with polar vortex dynamics. Sub-Antarctic and southern tip South America (Patagonia, Tierra del Fuego), southern New Zealand, and southern Australia have all recorded elevated UV-B levels during ozone hole season. Australia already has one of the world's highest melanoma rates globally (due to fair-skinned population, UV climate, outdoor lifestyle). Elevated UV-B from ozone depletion compounds this existing risk. New Zealand has implemented aggressive UV awareness campaigns ('Slip, Slop, Slap') in response.
4. Step 4
  Epidemiological evidence: Studies in southern Chile showed measurable increases in UV-B during ozone hole events and associated research documenting higher non-melanoma skin cancer rates in the region. The WHO estimates that a sustained 10% reduction in global ozone would cause approximately 300,000 additional non-melanoma skin cancers and 4,500 additional melanomas globally per year. Immune suppression is a less visible but significant health impact — UV-B reduces T-lymphocyte function, increasing susceptibility to infections. Vaccine effectiveness may also be reduced in populations with high UV-B exposure.
Answer
Percentage decrease: (300 − 100) ÷ 300 × 100 = 66.7%. UV-B increase: 66.7% decrease × 1.5 = 100% increase (doubling). Health implications: Antarctic research personnel directly exposed; protective measures required. Southern South America, New Zealand, Australia: elevated UV-B during spring ozone hole; increased melanoma risk (Australia already highest in world); WHO: 10% ozone reduction → 300,000 extra non-melanoma cancers/year globally; immune suppression reduces T-lymphocyte function and vaccine effectiveness.
Examiner tip
Data analysis questions require all working to be shown. The 66.7% × 1.5 = 100% calculation is straightforward but must be presented clearly. The health impact section should extend beyond 'skin cancer' to include cataracts and immune suppression — these are three separate categories of UV-B health impact. Geographic knowledge of which regions are affected by the Antarctic ozone hole (southern South America, New Zealand, Australia) adds important spatial context.

5Compare ozone depletion and climate change as atmospheric problems

Extended• compare, ozone depletion, climate change, greenhouse gases, CFCs, Montreal Protocol, Paris Agreement

Question

Both ozone depletion and climate change are atmospheric environmental problems caused by human activities. (a) Compare the causes, mechanisms, effects, and management of ozone depletion and climate change. (b) Explain why the Montreal Protocol has been more successful than climate change agreements. (c) Describe the link between ozone depletion and climate change.

Step-by-step solution

Step 1

Comparison — Causes and Mechanisms:

Feature	Ozone Depletion	Climate Change
Primary cause	CFCs (refrigerants, aerosols, foam)	CO₂ (fossil fuels), CH₄ (agriculture), N₂O (fertilisers)
Atmospheric region affected	Stratosphere (20–35 km)	Troposphere (0–12 km)
Mechanism	Cl• radicals catalytically destroy O₃	Greenhouse gases absorb long-wave IR radiation → warming
Primary effect	Increased UV-B at Earth's surface	Rising global temperatures, sea level rise
Scale	Regional/global (polar most severe)	Global (tropics and poles differently)

Key contrast: ozone depletion is about one class of specific industrial chemicals (ODS) with clear substitutes available; climate change involves fundamental energy sources (fossil fuels) that power entire economies — far harder to replace.

Step 2
Why Montreal Protocol succeeded better than climate agreements: (1) Substitute availability: When CFCs were banned, drop-in replacement chemicals (HCFCs, then HFCs) were ready. The refrigeration and aerosol industries could switch without losing their core function. For climate change, there is no 'drop-in substitute' for fossil fuels in all uses — the entire energy infrastructure must be redesigned. (2) Smaller number of industries affected: Ozone-depleting substances were produced by a relatively small number of large chemical companies (DuPont, ICI, Hoechst). These companies were persuaded that developing ODS substitutes would create profitable new markets — reducing industry opposition. For climate, virtually every sector of every economy (energy, transport, agriculture, industry, construction) must change. (3) Faster consensus: The science linking CFCs to ozone depletion was established relatively quickly and was not significantly contested by industry (after initial resistance). Climate science faced and continues to face well-funded campaigns of doubt and denial. (4) Equity aspects manageable: The Multilateral Fund transferred relatively modest sums to developing countries. Climate finance requirements are orders of magnitude larger. (5) Regional vs. global costs: Ozone protection benefits everyone, including industrialised countries (their own populations get skin cancer). The politics of cost-sharing were more tractable.
Step 3
Links between ozone depletion and climate change: (1) CFCs are potent greenhouse gases: CFCs have Global Warming Potentials of 5,000–14,000 (molecule for molecule, far more potent than CO₂). The phase-out of CFCs under the Montreal Protocol therefore had a significant beneficial effect on climate — it is estimated that the Protocol prevented warming of approximately 0.5°C by 2100 compared to a world without it. (2) HFC replacements are powerful greenhouse gases: The HFCs introduced to replace CFCs have no ozone-depleting effect but GWPs of 1,000–14,000. The Kigali Amendment (2016) to the Montreal Protocol phases them out — preventing an estimated 0.3–0.5°C of warming by 2100. (3) Climate change affects ozone recovery: Cooling of the stratosphere caused by increased CO₂ in the troposphere (a counterintuitive effect: more CO₂ traps heat in the troposphere, less escapes to the stratosphere → stratosphere cools) can increase the formation of polar stratospheric clouds, potentially slowing ozone recovery over Antarctica. (4) Ozone recovery affects climate: A thicker ozone layer absorbs more UV radiation → more stratospheric warming → potential changes to stratospheric circulation that interact with surface climate patterns.

Answer

Causes: CFCs → Cl• destroys O₃ in stratosphere; vs. CO₂/CH₄/N₂O absorb IR in troposphere → warming. Montreal success vs. climate: substitutes immediately available for CFCs; small number of industries; science not effectively disputed; manageable climate finance. Links: CFCs are potent GHGs (GWP 5,000–14,000) — Montreal prevented ~0.5°C warming; HFC replacements are also GHGs (Kigali Amendment); CO₂-induced stratospheric cooling may slow Antarctic ozone recovery.

Examiner tip

Comparison questions require structured parallel contrast — ideally a table or organised by theme (causes, mechanisms, effects, management). The links section is an Extended-level insight that few students know: CFCs being potent GHGs, and Montreal Protocol having inadvertently helped the climate problem. The Kigali Amendment (2016) linking ozone and climate policy is a high-quality detail. Never confuse the stratosphere (ozone depletion) with the troposphere (greenhouse warming) — examiners penalise this conflation.

6Evaluate management options for ozone recovery using real-world data
Extended• Adapted from 0680/12 Oct/Nov 2024• Montreal Protocol, HFCs, recovery, evaluate, scenario
Question
Scientific data shows: (1) Atmospheric chlorine concentrations peaked around 1993–1994 and have been slowly declining since. (2) The Antarctic ozone hole area peaked at approximately 29 million km² in 2000 but shows high variability year to year (e.g. 2019 was very small, 2023 was large). (3) Ozone column thickness globally (outside polar regions) shows gradual recovery. (a) Interpret these data — what do they tell us about the effectiveness of the Montreal Protocol? (b) Explain why ozone hole size varies significantly year to year despite declining chlorine levels. (c) Evaluate whether any additional policy actions are needed to accelerate ozone recovery.
Step-by-step solution
1. Step 1
  Interpretation of the data — Montreal Protocol effectiveness: (1) Declining atmospheric chlorine (since 1993–1994): This is the clearest evidence of Montreal Protocol success. Chlorine levels peaked approximately 5 years after the Protocol entered into force — reflecting the long atmospheric lifetime of CFCs (50–150 years). The subsequent slow decline confirms that CFC emissions have been substantially cut and the atmosphere is gradually purging itself of existing CFC molecules. However, CFC concentrations will not return to pre-industrial levels for many decades due to long atmospheric lifetimes. (2) Ozone hole area peaked in 2000 and shows gradual trend of decline: This is consistent with declining chlorine — less reactive chlorine means less ozone destruction annually. The 2000 peak corresponds to the time of maximum chlorine loading in the stratosphere. (3) Global ozone (outside polar regions) gradually recovering: Confirms that the Montreal Protocol is working at the global scale, not just over Antarctica.
2. Step 2
  Why does ozone hole size vary year to year? Despite declining chlorine, the ozone hole size varies because the Antarctic ozone hole formation is driven by polar stratospheric clouds (PSCs), which form only when stratospheric temperatures fall below approximately −78°C. PSC formation is controlled by stratospheric meteorology — specifically the strength and stability of the polar vortex (a large-scale pattern of winds circling Antarctica in winter).
  
  Strong, cold polar vortex (e.g. 2006, 2023): Temperatures fall below −78°C over a larger area for longer → extensive PSC formation → intense ozone chemistry → large hole.
  
  Weak, disturbed polar vortex (e.g. 2019): Warmer stratospheric temperatures → less PSC → smaller ozone hole.
  
  2019 anomaly: 2019 had an unusually small ozone hole due to a sudden stratospheric warming event in September — a rare meteorological event that broke down the polar vortex, warmed the stratosphere, and suppressed PSC formation. 2023 large hole: Partly attributed to the eruption of Hunga Tonga–Hunga Ha'apai underwater volcano in 2022, which injected exceptional quantities of water vapour into the stratosphere — promoting PSC formation and heterogeneous ozone chemistry. This illustrates that natural variability and volcanic events can significantly modulate ozone hole size from year to year.
3. Step 3
  Additional policy actions to accelerate recovery: (1) Full enforcement of the Kigali Amendment (HFC phase-down): HFCs cause powerful greenhouse warming that contributes to tropospheric warming, which indirectly cools the stratosphere — potentially slowing ozone recovery. Full implementation of HFC phase-down by all parties is needed. Evaluate: 145+ parties have ratified; developing countries have longer timelines; industry has developed HFO alternatives. Effective, but enforcement in some nations needs strengthening. (2) Eliminating nitrous oxide (N₂O) emissions: N₂O is now the largest remaining ozone-depleting substance emitted by humans (since CFCs are phased out). It is produced mainly from agriculture (synthetic fertilisers, livestock). Evaluate: N₂O is NOT regulated by the Montreal Protocol (was not identified as a concern in 1987). Regulating agricultural N₂O is politically very difficult — affects global food production; no easy substitute for synthetic nitrogen fertilisers. This is a significant gap in the current policy framework. (3) Monitoring and enforcement of illegal CFC-11 production: In 2019, investigations revealed illegal production of CFC-11 (trichlorofluoromethane) in Eastern Asia — primarily China — despite the global phase-out. This delayed recovery by years. Evaluate: UNEP monitoring and pressure from the Montreal Protocol Secretariat led to enforcement action; emissions subsequently declined. But monitoring gaps remain a concern. (4) Accelerating transition from HCFCs (still used in developing countries): Ensuring all developing nations meet the 2030 HCFC phase-out deadline requires continued technical and financial support through the Multilateral Fund.
4. Step 4
  Overall evaluation: The Montreal Protocol has successfully initiated ozone recovery — atmospheric chlorine is declining, global ozone outside polar regions is recovering, and the Antarctic hole shows a long-term trend of improvement despite year-to-year variability. However, recovery will take until approximately 2065 (Antarctic) and 2040 (global) at current rates. Additional actions are needed to avoid delays: particularly addressing N₂O (unregulated but significant), enforcing Kigali HFC phase-down, and policing illegal CFC production. The Montreal Protocol framework remains the right institutional vehicle for these actions — its success gives it authority and institutional capacity that other agreements lack.
Answer
Interpretation: declining chlorine (since 1993–1994) shows CFC emission cuts effective; ozone hole peaked 2000, gradually declining; global ozone recovering — all confirm Protocol success. Year-to-year variability: polar vortex strength controls PSC formation and ozone chemistry; 2019 small (stratospheric warming event); 2023 large (Hunga Tonga water vapour injection). Additional actions: Kigali Amendment enforcement (HFCs); regulating N₂O from agriculture (now largest ODS, not covered by Montreal Protocol); monitoring illegal CFC-11 production; HCPh phase-out in developing nations. Full recovery: ~2040 global, ~2065 Antarctic.
Examiner tip
This high-level Extended question tests interpretation skills (using specific data), process understanding (polar vortex / PSC), and policy evaluation. The most impressive detail is N₂O — it is now the largest unregulated ozone-depleting substance, and most students are unaware of this. The 2019 illegal CFC-11 production story is real and shows that monitoring matters even after agreements are reached. Examiners reward candidates who can explain year-to-year variability without attributing it to human emissions — it is natural meteorological variability.

Key Definitions and Keywords — Ozone Depletion

Definitions to memorise and the exact keywords mark schemes credit for ozone depletion answers — sharpened from recent examiner reports for the 2026 0680 sitting.

Ozone layer
Examiner keyword
A region of the stratosphere approximately 20–35 km above Earth's surface where ozone (O₃) is concentrated (~10 ppm compared to ~0.04 ppm in the troposphere). Absorbs 97–99% of incoming UV-B and UV-C radiation, preventing it from reaching Earth's surface. Ozone column thickness is measured in Dobson Units (DU); pre-industrial baseline ~300 DU globally.
Related: stratosphere, UV radiation, ozone depletion, Dobson units
Chlorofluorocarbon (CFC)
Examiner keyword
A class of synthetic organic compounds containing carbon, chlorine and fluorine. Used as refrigerants, aerosol propellants and foam-blowing agents. Chemically inert in the troposphere (atmospheric lifetime 50–150 years), they rise to the stratosphere where UV radiation breaks the C–Cl bond, releasing chlorine radicals that catalytically destroy ozone. Regulated and largely phased out under the Montreal Protocol.
Related: ozone depletion, Montreal Protocol, chlorine radical, catalytic cycle
Chlorine radical (Cl•)
Examiner keyword
A highly reactive chlorine atom with an unpaired electron, released when UV radiation breaks a C–Cl bond in CFC molecules. Acts as a catalyst in ozone destruction: Cl• + O₃ → ClO + O₂, then ClO + O → Cl• + O₂. The radical is regenerated after each reaction cycle, allowing each Cl• to destroy approximately 100,000 ozone molecules before being permanently removed.
Related: CFCs, catalytic cycle, ozone depletion
Antarctic ozone hole
Examiner keyword
A region of severe ozone depletion that forms annually over Antarctica in spring (September–October). Caused by CFC-derived chlorine radicals reacting on polar stratospheric cloud (PSC) surfaces during extreme cold. Discovered in 1985 by British Antarctic Survey. The hole peaked at approximately 29 million km² in 2000 and has shown gradual reduction since.
Related: CFCs, polar stratospheric clouds, Montreal Protocol, Dobson units
Montreal Protocol
Examiner keyword
An international legally binding treaty (signed 1987, entered into force 1989) that phases out the production and consumption of ozone-depleting substances (ODS). Ratified by 197 parties — every nation on Earth. The most universally ratified UN environmental treaty. CFCs phased out in developed countries by 1996. Led to declining atmospheric chlorine and gradual ozone recovery. The Kigali Amendment (2016) extended the Protocol to phase down HFCs.
Related: CFCs, ozone layer recovery, Kigali Amendment, international agreements
Dobson Units (DU)
The standard unit for measuring the total amount of ozone in a vertical column of atmosphere. 100 DU means the ozone, if compressed to standard temperature and pressure, would form a layer 1 mm thick. Pre-industrial global average: approximately 300 DU. During Antarctic ozone hole conditions in the 1990s: as low as 100 DU over Antarctica.
Related: ozone layer, ozone hole, ozone measurement
Polar stratospheric clouds (PSCs)
Clouds of ice crystals that form in the stratosphere above polar regions when temperatures fall below approximately −78°C. Their surfaces provide sites for heterogeneous chemical reactions that convert reservoir chlorine compounds (HCl, ClONO₂) into highly reactive forms (Cl₂, HOCl). When sunlight returns in spring, these reactive chlorine compounds rapidly destroy ozone — causing the seasonal ozone hole.
Related: ozone hole, Antarctica, polar vortex

Common Mistakes and Misconceptions — Ozone Depletion

The traps other students keep falling into on ozone depletion questions — taken from recent Cambridge IGCSE 0680 examiner reports and mark schemes — and how to avoid them.

✕Saying the ozone layer absorbs UV-A radiation.
UV type distinction penalised in 0680 mark schemes for ozone questions.
Why it happens
Students know ozone absorbs 'UV radiation' but do not distinguish the three types.
How to avoid it
The ozone layer absorbs UV-C (almost completely) and UV-B (substantially). UV-A is NOT absorbed by ozone — it passes through to Earth's surface. This distinction matters because UV-B causes the main harm (skin cancer, cataracts). UV-A causes tanning but is less harmful.
✕Confusing ozone depletion with the greenhouse effect or climate change.
Most commonly penalised error in 0680 atmosphere questions.
Why it happens
Both are atmospheric problems and are discussed together — students conflate them.
How to avoid it
Ozone depletion (CFCs, stratosphere, UV-B) is SEPARATE from climate change (CO₂/CH₄, troposphere, infrared/heat). CFC aerosol sprays cause ozone depletion — they do NOT cause climate change through the greenhouse effect (though CFCs happen to be potent GHGs as well). CO₂ does NOT deplete ozone.
✕Stating that CFCs are still widely produced and used (ignoring the Montreal Protocol).
Why it happens
Students learn about CFCs as a problem and do not update their knowledge to include the successful policy response.
How to avoid it
CFCs were phased out in developed countries by 1996 and in developing countries by 2010 under the Montreal Protocol. They have been largely replaced by HCFCs and then HFCs (no ozone depletion but potent greenhouse gases). Atmospheric CFC concentrations are now declining. The ozone layer IS recovering — slowly.
✕Failing to explain that the chlorine radical is regenerated (catalytic cycle).
Why it happens
Students write Cl• + O₃ → products but forget the second step that regenerates Cl•.
How to avoid it
The two-step cycle: (1) Cl• + O₃ → ClO + O₂. (2) ClO + O → Cl• + O₂. The Cl• reappears at step 2 — this is what makes it a CATALYST. Without step 2, you have just one ozone destroyed per chlorine atom, which would not explain the devastating scale of depletion. Each Cl• destroys ~100,000 O₃ molecules BECAUSE it is regenerated.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Ozone Depletion

Detailed Notes

What you’ll learn

1Role of stratospheric ozone

2Mechanism of CFC-induced ozone depletion

3The Montreal Protocol and recovery of the ozone layer

4Analyse quantitative data on UV-B increase and health impacts

5Compare ozone depletion and climate change as atmospheric problems

6Evaluate management options for ozone recovery using real-world data

Ozone layer

Chlorofluorocarbon (CFC)

Chlorine radical (Cl•)

Antarctic ozone hole

Montreal Protocol

Dobson Units (DU)

Polar stratospheric clouds (PSCs)

✕Saying the ozone layer absorbs UV-A radiation.

✕Confusing ozone depletion with the greenhouse effect or climate change.

✕Stating that CFCs are still widely produced and used (ignoring the Montreal Protocol).

✕Failing to explain that the chlorine radical is regenerated (catalytic cycle).

Past paper style quiz