The ozone layer is not a shield sitting still up there. It is a balance — ozone constantly being destroyed and constantly being rebuilt, at rates that happen to match. Ozone depletion is what happens when something tips that balance, and a very small amount of the wrong chemical is enough to do it.
📚 What you need to know
Ozone (O3) is a molecule of three oxygen atoms, found mainly in the stratosphere, roughly 10 to 50 km above the surface.
It protects life by absorbing UV radiation before it reaches the ground.
Ozone is destroyed and reformed continuously. The two rates match, giving a dynamic equilibrium that keeps the amount roughly steady.
Ozone-depleting substances (ODSs) speed up destruction beyond the natural rate, so the equilibrium is disrupted.
Sources of ODSs: aerosol propellants, gas-blown plastics, some pesticides, flame retardants and refrigerants — CFCs being the best known.
More UVB then reaches the surface, raising rates of skin cancer and cataracts and reducing productivity. Ozone holes appear each spring, most strongly over the poles.
What ozone does up there
Ozone is chemically the same element as the oxygen you breathe, just arranged differently: three atoms instead of two. That third atom is loosely enough held that UV radiation can knock it off — and that is precisely what makes ozone useful. Absorbing UV is not a side effect of the layer sitting there; it is what happens when ozone is broken apart.
The UV is absorbed in the breaking step. So a thinner ozone layer does not just mean less ozone; it means less UV being intercepted on the way down.
Definition to learn
dynamic equilibrium = destruction and formation happen continuously at equal rates, so the overall amount stays roughly constant
“Dynamic” is doing real work in that phrase. Nothing is static: every ozone molecule up there is being destroyed and rebuilt on a timescale of minutes. What stays constant is the total, not the individual molecules.
Ozone-depleting substances
ODSs are chemicals that push the destruction side of the balance faster than the formation side. They do not create a new process — they accelerate the natural one beyond natural levels, and the layer thins as a result.
Source
How the substance is released
Refrigerants
CFCs were used widely in fridges and air conditioning. They escape when systems leak or when equipment is scrapped without proper recovery
Aerosol propellants
CFCs pushed the contents out of sprays, foams and deodorants, releasing the gas directly into the air with every use
Gas-blown plastics
ODSs were used as blowing agents to make foamed plastics light, and are released during manufacture or disposal
Pesticides
Some soil fumigants, such as those containing methyl bromide, vaporise after application and drift upwards
Flame retardants
Halogen-containing compounds used to make products less flammable, released as those products degrade or are thrown away
Why so little does so much damage
Here is the part that surprises people. The total mass of CFCs ever released is tiny compared with the atmosphere. So how could it thin an entire global layer?
The answer is that chlorine acts as a catalyst. A CFC molecule is stable enough to drift upwards for years without breaking down, which is exactly why it survives long enough to reach the stratosphere. There, strong UV knocks a chlorine atom loose. That single atom destroys an ozone molecule — and then is released again, free to destroy another. And another. One chlorine atom can work its way through thousands of ozone molecules before it is finally removed.
The stability that made CFCs useful — non-flammable, non-toxic, unreactive — is exactly what let them reach the stratosphere intact.
When the balance breaks
Once destruction outpaces formation, the amount of ozone falls and the equilibrium is disrupted. The consequences follow directly from the last page:
More UVB reaches the Earth’s surface.
Rates of skin cancer and cataracts rise.
Terrestrial and marine productivity fall, as crops and phytoplankton are damaged.
Ecosystems and human health are both affected, worldwide.
Ozone holes
Depletion affects the whole stratosphere, but it shows up most dramatically at the poles, where areas of very low ozone appear each spring. Two things combine there: the extreme cold of the polar winter creates conditions in which chlorine becomes far more reactive, and the returning spring sunlight then provides the UV needed to set the destruction going. The result is a seasonal collapse in ozone concentration over the polar regions rather than an evenly spread thinning.
Careful with the word “hole”. It is not a gap with nothing in it. It is a region where ozone concentration has dropped sharply — thinning, not absence.
Worked examples
WORKED EXAMPLE
Explain what is meant by a dynamic equilibrium in the context of stratospheric ozone. [3]
Step 1: the destruction
UV radiation is absorbed by ozone, which splits into an oxygen molecule and a free oxygen atom.
Step 2: the formation
That free atom combines with another oxygen molecule, reforming ozone.
Step 3: the balanceBoth processes run continuously. Because the rates are equal, the total amount of ozone stays roughly constant over long periods, even though individual molecules are constantly changing.Constant total, constant turnover
WORKED EXAMPLE
A small quantity of CFCs was released compared with the mass of the atmosphere. Explain why the effect on ozone was nevertheless large. [4]
Step 1: they survive the journey
CFCs are very stable and unreactive, so they are not broken down in the troposphere and drift up to the stratosphere over years.
Step 2: UV frees the chlorine
Strong UV in the stratosphere breaks the molecule, releasing a chlorine atom.
Step 3: the catalytic effect
The chlorine destroys an ozone molecule and is then released unchanged, so it can repeat the process thousands of times.
Step 4: the balance tipsDestruction now outpaces formation, so the equilibrium shifts and ozone concentration falls.Stable, long-lived and catalytic — a bad combination
💡 Exam tip
You do not need the chemical equations. You do need the words: absorbed, splits, reforms, equal rates, equilibrium.
Say that UV is absorbed when ozone is broken apart. That connects the layer’s protection to the cycle itself.
Use catalyst when explaining CFCs. It is the single word that explains the scale of the damage.
Give the height of the stratosphere — about 10 to 50 km — and name UVB as the band that increases.
Name specific ODS sources: refrigerants, aerosol propellants, foam blowing agents, methyl bromide.
⚠ Common mix-up
Ozone depletion is not global warming. Different gases, different layer, different radiation. Never use one to explain the other.
Ozone in the stratosphere is helpful; at ground level it is a pollutant. Same molecule, opposite verdict, depending on where it is.
An ozone hole is thinning, not a gap.
CFCs do not destroy ozone directly. UV must first free the chlorine atom, and that atom does the work.
Ozone destruction is not itself unnatural. It happens constantly; the problem is destruction running faster than formation.
Up next: The Montreal Protocol and Ozone Recovery — the one global environmental agreement that genuinely worked, and what made it different.
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