Causes of ozone depletion
Big idea: Ozone-depleting substances (ODS) like CFCs release chlorine atoms that catalytically destroy ozone molecules — one chlorine atom can destroy thousands of ozone molecules.
Main ozone-depleting substances
- CFCs (chlorofluorocarbons): Used in refrigerators, air conditioners, aerosol sprays, foam
- Halons: Used in fire extinguishers; contain bromine (even more destructive than chlorine)
- HCFCs: Transitional replacements for CFCs; less harmful but still damaging
- Carbon tetrachloride: Industrial solvent
- Methyl bromide: Agricultural fumigant
The catalytic destruction process
- CFCs are very stable and rise slowly to the stratosphere
- UV radiation breaks down CFCs, releasing chlorine atoms (Cl)
- Chlorine reacts with ozone: Cl + O₃ → ClO + O₂
- ClO reacts with oxygen atom: ClO + O → Cl + O₂
- The chlorine atom is released and can destroy MORE ozone
- One Cl atom can destroy ~100,000 ozone molecules before being removed
CFCs are catalysts — they speed up ozone destruction without being used up. Thats why even small amounts are so damaging.
Exam tip: Be able to explain the catalytic cycle — how chlorine destroys ozone AND is regenerated to destroy more.
The ozone hole
Big idea: The ozone hole forms over Antarctica each spring due to unique polar conditions that accelerate ozone destruction.
Why Antarctica?
- Polar vortex: Circular wind pattern isolates Antarctic air during winter
- Extreme cold: Temperatures below -78°C allow polar stratospheric clouds (PSCs) to form
- PSCs: Ice crystals provide surfaces for chemical reactions that release active chlorine
- Spring sunlight: When sun returns, UV triggers rapid ozone destruction
- Seasonal cycle: Hole forms in September-October, shrinks by December
Trends in ozone depletion
- 1985: Ozone hole first discovered by British Antarctic Survey
- 1987–2000s: Hole expanded, reaching maximum size ~25 million km²
- Since ~2000: Stabilisation and early signs of recovery
- Projection: Full recovery expected by ~2066 for Antarctic, sooner elsewhere
- Time lag: CFCs persist in atmosphere for 50–100 years
Recovery is slow because CFCs have long atmospheric residence times. Even though we stopped producing them, theyre still in the atmosphere destroying ozone.
Exam tip: Questions often ask you to describe trends from ozone data. Note the increase (1980s–2000s), stabilisation, and early recovery.
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IB-style question — how CFCs deplete ozone [4]
An aerosol spray and an old fridge both once released chlorofluorocarbons (CFCs) into the air. These gases slowly drift up to the stratosphere.
Explain how CFCs cause the depletion of ozone in the stratosphere. [4]
How to answer it, step by step
- CFCs release chlorine
• UV light hits the CFC molecule and breaks a chlorine atom free.
• This free chlorine is very reactive. - Chlorine destroys ozone again and again
• The chlorine reacts with ozone and breaks it into oxygen.
• One chlorine atom can destroy many ozone molecules, so the ozone layer gets thinner.
Final answer
The mark for 'one Cl destroys many O₃ molecules' is the one students forget — it explains why a small amount of CFC does so much damage.
IB-style question — reading an ozone-thickness graph [3]
Ozone thickness is measured in Dobson Units (DU). Over Antarctica in spring, average ozone fell from 300 DU in 1980 to 120 DU in 1995.
(i) Calculate the percentage decrease in ozone thickness over this period. [2]
(ii) State one likely consequence of this thinning for organisms below. [1]
How to answer it, step by step
- (i) Percentage decrease
• Formula: (change ÷ original) × 100.
• ((300 − 120) ÷ 300) × 100 = (180 ÷ 300) × 100 = 60%. - (ii) Consequence below
• More UV now reaches the surface.
• This causes more harm such as skin cancer, eye damage or harm to plants/plankton.
Final answer
Always divide the change by the ORIGINAL (1980) value, not the new one — and remember to put the % sign in your final answer.