IB DP Environmental Systems & Societies — Topic 6: Atmosphere

Stratospheric Ozone

Subtopic 6.4 — New Syllabus (First Assessment 2026)
Standard Level + Higher Level

9 SL Points
6.4.1 – 6.4.9
4 HL Extensions
6.4.10 – 6.4.13
Overview

What You Need to Know

This subtopic examines the UV radiation spectrum, the stratospheric ozone layer ($\text{O}_3$), natural formation and destruction equilibrium, catalytic breakdown by Ozone-Depleting Substances (ODSs like CFCs), the Montreal Protocol success, and — at HL — halogen chemistry, polar stratospheric clouds (PSCs), HFCs, and air conditioning impacts.

A. UV Shield & Biological Impacts
UVA, UVB, UVC absorption, skin cancer, cataracts, phytoplankton decline
B. Ozone Equilibrium & CFCs
Natural $\text{O}_2/\text{O}_3$ equilibrium disrupted by Chlorine radicals from CFCs
C. Montreal Protocol & HFCs (HL)
Boundary NOT crossed (~2060 recovery), Kigali Amendment phase-down of HFCs

Guiding Questions

Time allocation: minimum 2 hours SL + additional 4 hours HL. Final subtopic in Topic 6: Atmosphere.
6.4.1 – 6.4.4

The Electromagnetic Spectrum & UV Shield

Key UnderstandingShorter wavelengths (high frequency UV radiation) carry higher energy and present severe risks to living organisms. The stratospheric ozone layer acts as a critical biological shield.
UV Radiation Spectrum and Atmospheric Shielding Diagram
UV BandWavelength RangeAtmospheric Stratospheric AbsorptionBiological Danger & Health Impacts
UVC100 – 280 nm100% Absorbed by O₂ and $\text{O}_3$Lethal / Mutagenic; zero reaches Earth's surface
UVB280 – 315 nm95% Absorbed by Ozone Layer ($\text{O}_3$)Skin cancer (melanoma), eye cataracts, DNA mutations, phytoplankton damage
UVA315 – 400 nm0% Absorbed (Passes through)Premature skin aging, low-level DNA damage; stimulates Vitamin D synthesis
6.4.5 & 6.4.6

Ozone Equilibrium & CFC Catalytic Destruction

Key UnderstandingStratospheric ozone exists in a natural dynamic equilibrium. Ozone-Depleting Substances (ODSs) like CFCs release chlorine radicals ($\text{Cl}^\bullet$) that destroy ozone in a continuous catalytic cycle.
Stratospheric Ozone Cycle Formation and CFC Catalytic Destruction Diagram

⚖️ Natural Dynamic Equilibrium

Formation: $\text{O}_2 + \text{UV-C} \rightarrow 2\text{O}$; $\text{O} + \text{O}_2 \rightarrow \text{O}_3$
Breakdown: $\text{O}_3 + \text{UV-B/C} \rightarrow \text{O}_2 + \text{O}$
Rates of formation and breakdown remain equal.

🧪 Catalytic Destruction by $\text{Cl}^\bullet$

1. $\text{CF}_2\text{Cl}_2 + \text{UV-C} \rightarrow \text{CF}_2\text{Cl}^\bullet + \text{Cl}^\bullet$
2. $\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2$
3. $\text{ClO}^\bullet + \text{O} \rightarrow \text{Cl}^\bullet + \text{O}_2$
Single $\text{Cl}^\bullet$ destroys ~100,000 $\text{O}_3$ molecules!

6.4.7 – 6.4.9

The Montreal Protocol & Planetary Boundary

Key UnderstandingThe 1987 Montreal Protocol is the most successful international environmental treaty in history. Universal ratification has prevented the stratospheric ozone planetary boundary from being crossed.

📜 1987 Montreal Protocol Success

  • Universally ratified by 198 UN member states
  • Phased out 99% of global production of CFCs & Halons
  • Financial & technology transfer mechanism for developing nations
  • Precautionary principle applied before irreversible destruction occurred

🌍 Boundary Status: NOT Crossed

  • Antarctic ozone hole peaked around year 2000 (30 million $\text{km}^2$)
  • Significant healing and recovery observed since early 2000s
  • Full stratospheric recovery projected by ~2060s
  • Prevented estimated 2+ million annual skin cancer cases by 2050
HL — 6.4.10 & 6.4.11

Halogen Chemistry & Polar Stratospheric CloudsHL

Key UnderstandingSevere polar spring ozone depletion (the "Ozone Hole") is caused by extreme winter cold (-78°C) generating Polar Stratospheric Clouds (PSCs) that activate reservoir chlorine.
Polar SeasonAtmospheric & Chemical MechanismOzone State
Polar Winter (June–Aug)Polar vortex isolates air; temperatures fall below -78°C forming ice/nitric acid PSCsInactive reservoir $\text{HCl}$ & $\text{ClONO}_2$ convert to active $\text{Cl}_2$ gas on PSC surfaces
Polar Spring (Sept–Nov)Sunlight returns; UV-C photolyzes $\text{Cl}_2 \rightarrow 2\text{Cl}^\bullet$ starting massive catalytic destructionOzone Hole Forms (drop below 220 Dobson Units)
Polar Summer (Dec)Polar vortex breaks down; warmer mid-latitude air replenishes stratospheric ozoneOzone hole temporarily fills until next winter cycle
Volcanic Aerosols: Major volcanic eruptions (e.g. Mt. Pinatubo 1991) inject sulfate aerosols into the stratosphere, offering extra surfaces for chlorine activation and worsening depletion.
HL — 6.4.12 & 6.4.13

HFCs, Kigali Amendment & Air ConditioningHL

Key UnderstandingHydrofluorocarbons (HFCs) replaced CFCs to protect the ozone layer, but are potent greenhouse gases. The 2016 Kigali Amendment targets HFC phase-down to combat global warming.

❄️ The HFC Dilemma

HFCs contain no chlorine atoms ($\text{ODP} = 0$), so they do not destroy ozone. However, HFCs are super-pollutants with Global Warming Potentials up to 14,800× CO₂.

📜 2016 Kigali Amendment

Legally binding amendment to the Montreal Protocol committing countries to phase down HFC production by 80%+ over 30 years, avoiding up to 0.5°C of global warming by 2100.

Key Terms Glossary

Essential Vocabulary

TermDefinition
Ozone Layer ($\text{O}_3$)Stratospheric concentration of triatomic oxygen absorbing 95%+ of solar UV-B and 100% of UV-C
ODSsOzone-Depleting Substances (CFCs, Halons, HCFCs, Methyl Bromide) releasing reactive halogens
Dynamic EquilibriumSteady state where rate of natural ozone formation equals rate of UV-driven ozone breakdown
Catalytic CycleChain reaction where a single free chlorine radical ($\text{Cl}^\bullet$) destroys ~100,000 ozone molecules
Dobson Unit (DU)Unit measuring column ozone thickness; concentrations under 220 DU define an "Ozone Hole"
PSCsPolar Stratospheric Clouds forming below -78°C providing ice surfaces for chlorine activation
Kigali Amendment2016 Montreal Protocol extension establishing binding phase-down targets for climate-warming HFCs
Exam Preparation

Key Takeaways & Exam Tips

Must-Know Concepts

  • UV Wavelength Danger: UV-C (most energetic/dangerous, 100% absorbed) $\rightarrow$ UV-B (causes skin cancer/cataracts, 95% absorbed) $\rightarrow$ UV-A (passes through).
  • Catalytic Mechanism: $\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2$; $\text{ClO}^\bullet + \text{O} \rightarrow \text{Cl}^\bullet + \text{O}_2$. $\text{Cl}^\bullet$ is regenerated!
  • Planetary Boundary Precedent: Stratospheric ozone is the primary planetary boundary successfully protected via international governance (Montreal Protocol).

You've covered all 13 syllabus points ✅

6.4.1 – 6.4.9 (SL) + 6.4.10 – 6.4.13 (HL) · Complete Topic 6: Atmosphere & Climate Change!