Ozone-depleting substances at Higher Level: The same ideas as SL, with the chemistry: the equations for how chlorine destroys ozone, bromine from halons and methyl bromide, and the lifetimes that keep ODSs in the air for a century.
Practise this as you read
- Write the chlorine cycle as equations and say why chlorine is a catalyst.
- Explain why depletion goes on long after a ban.
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Useful gases with a hidden cost: ozone-depleting substances (ODSs) are gases people made for useful jobs: cooling, spraying, putting out fires, killing pests. High in the stratosphere, they release chlorine or bromine, which destroy ozone.
The points to remember
- ODSs are man-made gases that release chlorine or bromine in the stratosphere.
- CFCs: coolants in fridges and air conditioners, spray cans (aerosols), foam.
- Halons: fire extinguishers. Methyl bromide: a pesticide sprayed into soil.
- Carbon tetrachloride: a cleaning solvent. HCFCs: replacement coolants that still destroy some ozone.
- Name a real chemical in answers (CFCs, halons): 'ODSs' alone, or HFCs, gets no credit.
Real example: farmers in California once sprayed methyl bromide into the soil of strawberry fields to kill worms and fungi before planting. It was phased out in the United States by 2005, with a few farms allowed to use it until the mid-2010s while they found other methods.
Name the chemical and its use: 'ODSs' alone is too vague, and HFCs are not ODSs. Write, for example: 'CFCs used as coolants in fridges' or 'halons used in fire extinguishers'.
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CFCs do no harm near the ground. The damage starts years later, high in the stratosphere, where UV light breaks them apart. Chlorine then acts as a catalyst.
The points to remember
- CFCs are very stable, so they last for decades and drift up to the stratosphere.
- There, strong UV light breaks the CFC apart and frees a chlorine atom.
- The chlorine atom breaks an ozone molecule into ordinary oxygen.
- Then the chlorine is released again, so it can break another ozone molecule.
- Chlorine is a catalyst: one atom can destroy about 100,000 ozone molecules.
- Bromine (from halons and methyl bromide) works the same way, and is even more destructive.
Remember it as: UV frees the chlorine, chlorine breaks the ozone, chlorine is freed again.
- UV breaks the CFC
- CFCl3 → CFCl2 + Cl (a free chlorine atom)
- Chlorine breaks ozone
- Cl + O3 → ClO + O2
- Chlorine is freed
- ClO + O → Cl + O2
- Overall
- O3 + O → 2O2 (chlorine is not used up)
Real example:
A balance that tipped: Ozone in the stratosphere is always being made and broken down. For millions of years the two rates were equal, an equilibrium. ODSs speed up the breaking, so the balance tips.
The points to remember
- Ozone is made (UV splits oxygen) and broken down (UV splits ozone) at the same time.
- For millions of years the two rates were equal: an equilibrium, so the amount of ozone stayed steady.
- ODSs add an extra way of breaking ozone down, on top of the natural one.
- Now destruction is faster than formation, so the balance tips: the amount of ozone falls.
- Cut the ODSs and the balance tips back: formation wins and ozone recovers.
Real example: satellites show the balance tipping over the Antarctic: the mean of the lowest daily ozone each spring fell from 225 Dobson units (DU) in 1979 to 92 DU in 1994, as chlorine in the stratosphere kept rising.
Ozone depletion does not cause global warming: Thinner ozone lets in more UV, but that does not warm the planet. They are separate problems. The only link: CFCs are also greenhouse gases, so cutting them helped the climate too.
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Most ODSs were banned by 2010, yet ozone is still thinner than in 1980. Human activity keeps affecting ozone because of the gases already released and a few still being released. Each gas has a long lifetime.
The points to remember
- Long lifetimes: CFCs released decades ago are still in the air: a time lag before recovery.
- Chlorine and bromine are removed from the stratosphere very slowly.
- Old equipment still leaks CFCs: fridges, air conditioners, foam in buildings.
- Illegal production and a black market: some factories still make banned CFCs.
- HCFCs are weaker ODSs but still destroy ozone; and some countries do not comply fully.
- Climate change cools the stratosphere, which can make polar ozone loss worse in some years.
Already up there
- CFC-11 lasts about 52 years in the air.
- CFC-12 lasts about 102 years.
Still leaking
- Old fridges, air conditioners and building foam.
- Illegal factories and a black market.
Still allowed
- HCFCs: weaker ODSs, phased out only by 2030-2040.
- Nitrogen oxides from fertilizers and burning fuel: not controlled.
Real example: CFC-11 lasts about 52 years and CFC-12 about 102 years in the atmosphere. So CFCs released in the 1980s will still be destroying ozone in the 2040s, long after the factories stopped.
'Continue' means now: In a question on how human activity continues to affect ozone, explain why the damage goes on today: long lifetimes, leaks, illegal use, HCFCs. Without these points, answers are capped.
How this comes up: Paper 2, Section B (b) [7]: explain how ODSs destroy ozone and why the damage continues; at HL, the equations can earn the chain points.
Production of CFCs ended worldwide in 2010, but the 2020 Antarctic ozone hole was one of the largest and longest-lasting ever measured.
Explain why ozone depletion continued after the production of CFCs had stopped.
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