The ozone layer is not a solid shield sitting up there doing nothing. It is a chemical process running constantly — ozone being torn apart by ultraviolet light and put back together again, millions of times over, in a balance that has held for a very long time. Understanding it as a balance rather than a barrier is what makes ozone depletion make sense.
📚 What you need to know
Ozone is a molecule of three oxygen atoms, written O3.
It is mainly found in the stratosphere, roughly 10 to 50 km above the Earth’s surface.
It protects life by absorbing a significant portion of the Sun’s harmful UV radiation before it reaches the ground.
Ozone concentration stays fairly constant because formation and destruction sit in a dynamic equilibrium — a steady state.
Ozone-depleting substances (ODSs) speed up the destruction side, so the balance tips and the layer thins.
An ozone hole is an area of unusually low stratospheric ozone, most prominent over the poles each spring.
Where the layer sits, and what it filters
The dashed line is the important one. A small amount of UVB always reaches the surface, and it is that amount which grows when the layer thins — which is why ozone depletion translates so directly into skin cancer and cataract rates.
The equilibrium that keeps it stable
The amount of ozone in the stratosphere stays relatively constant over long periods because it exists in a steady state. Ozone is being destroyed all the time and formed all the time, and while those two rates match, the concentration holds.
Destruction happens naturally. When UV radiation strikes an ozone molecule, the molecule absorbs that energy and breaks apart, producing an oxygen molecule (O2) and a free oxygen atom (O). Under normal conditions that free atom then combines with another oxygen molecule to form ozone again. The cycle runs continuously, which is why this is called a dynamic equilibrium: nothing is static, but the totals stay the same because the forward and backward rates are equal.
The middle line is the one students find surprising. Ozone shields us precisely because it absorbs UV energy and breaks apart doing so. A molecule that simply sat there would protect nothing.
Good news for your revision: you are not required to learn the chemical equations for ozone formation and destruction at this level. Describe the process in words. What you do need is the idea of equal rates — and the best way to show it is a sentence like “the rate of destruction equals the rate of reformation, so the concentration of ozone remains relatively constant”. That phrasing maps straight onto the mark scheme.
Where ozone-depleting substances come from
ODSs are chemicals that cause stratospheric ozone depletion by enhancing the natural breakdown process beyond its natural level. They were not exotic laboratory chemicals; they were in ordinary products in ordinary homes, which is part of why the problem grew so quickly.
Source
How the substance escapes
Aerosols
CFCs were used as propellants in sprays, foams and deodorants; released during spraying, they eventually reach the stratosphere
Gas-blown plastics
ODSs were used as blowing agents to create air pockets in foamed plastics, and escape during manufacture or disposal
Pesticides
Some, such as those containing methyl bromide, were used for soil fumigation and vaporise into the atmosphere after application
Flame retardants
Some contain halogen atoms; the halogenated compounds are released when the products degrade or are thrown away
Refrigerants
CFCs were widely used in fridges and air conditioners, and escape through leaks or improper disposal
Notice the common thread. In almost every case the chemical escapes during use or disposal, not during manufacture. That is why the solution had to be banning the substances themselves rather than regulating factories — there was no chimney to fit a filter to.
When the balance tips
When formation and destruction rates become unequal, the equilibrium is disrupted and the layer thins. The consequences run in a chain: more UVB reaches the surface, which affects ecosystems and human health, raising rates of skin cancer and cataracts and reducing both terrestrial and marine productivity.
Ozone holes are areas of unusually low stratospheric ozone. Depletion affects the entire stratosphere, but the holes are most prominent at the poles, where they appear each spring because of a combination of ODSs and seasonal weather patterns. An ozone hole is not an actual gap — it is a region where the concentration has fallen below a defined level, letting more UV through than it should.
WORKED EXAMPLE
Explain how a dynamic equilibrium maintains ozone concentration in the stratosphere. [3]
Point 1 — destructionOzone molecules absorb UV radiation and break apart, forming an oxygen molecule and a free oxygen atom.Point 2 — reformationThe free oxygen atom combines with another oxygen molecule, reforming ozone.Point 3 — equal ratesBecause the rate of destruction equals the rate of reformation, the concentration of ozone remains relatively constant over time even though both processes run continuously.3 / 3The third point carries the concept. Describing only the two reactions would score two.
WORKED EXAMPLE
Identify two sources of ozone-depleting substances and state how each reaches the atmosphere. [4]
Source 1Refrigerants in fridges and air conditioning systems, historically CFCs, which escape through leaks or improper disposal of the units.Source 2Aerosol propellants in sprays and foams, which are released directly into the air each time the product is used.4 / 4Two marks per source: one for identifying it, one for the release mechanism. Read the command term carefully.
💡 Exam tip
Quote the range: ozone sits in the stratosphere, roughly 10 to 50 km above the surface.
Use the words dynamic equilibrium and steady state. Both appear in mark schemes.
You do not need chemical equations for the natural cycle. Describe it in words instead.
Say that ODSs increase the rate of destruction, disrupting the balance — rather than saying they “destroy the ozone layer”.
Learn the five ODS sources. Aerosols and refrigerants are the safest two to name.
Define an ozone hole as an area of low concentration, not a gap. Examiners look for this.
⚠️ Common mix-up
Thinking the ozone hole is a physical hole. It is a region where concentration has dropped below a threshold.
Confusing stratospheric and tropospheric ozone. High up it protects us; at ground level it is a pollutant that harms lungs and crops.
Saying ozone reflects UV. It absorbs it, and breaks apart in the process.
Treating the layer as unchanging. It is constantly being destroyed and reformed.
Blaming ozone depletion for global warming. They are different problems, though some chemicals contribute to both.
Forgetting natural destruction exists. The problem is the rate increasing, not destruction happening at all.
Up next: The Montreal Protocol and Ozone Recovery — how the world found out, agreed to act, and produced the one environmental treaty that clearly worked.
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