Chlorine is spread fairly evenly through the stratosphere, so why does the damage concentrate over Antarctica, and only in spring? The answer is not chemistry alone. It is chemistry plus two features of polar weather that together create the most efficient ozone-destroying machine on the planet — and switch it on the moment the Sun comes back.
📚 What you need to know
Polar ozone depletion occurs mainly during spring over the poles, and is most severe over Antarctica from September to November.
Polar stratospheric clouds (PSCs) form in extreme winter cold and provide active surfaces that convert chlorine compounds into ozone-destroying forms.
The polar vortex is a large-scale winter circulation that isolates polar air, preventing mixing with lower latitudes and concentrating ODSs inside it.
The Antarctic vortex is stronger and more stable than the Arctic one, which is why depletion is far more severe in the south.
Volcanic aerosols reaching the stratosphere provide additional surfaces for ozone-destroying reactions, with the effect amplified in polar regions.
An ozone hole is not a literal hole — it is an area where ozone concentration has fallen below a defined level.
The machine, season by season
The two-stage timing is what confuses students most. The chemistry is prepared during the dark, freezing winter, but nothing much happens until UV arrives in spring to break the stored chlorine compounds apart — all at once, in air that has been sealed off for months.
Polar stratospheric clouds
Extremely low temperatures in the polar stratosphere during winter, particularly in the Antarctic, allow polar stratospheric clouds to form. Their role is not to destroy ozone directly; it is to provide active surfaces on which chlorine compounds are converted into forms that can destroy ozone. Think of the cloud particles as workbenches. Reactions that would be far too slow in open air happen readily on a surface.
When sunlight returns in spring, UV radiation breaks down those chlorine-containing compounds sitting on the cloud surfaces, releasing chlorine radicals that rapidly deplete ozone. The presence of PSCs amplifies depletion within the polar vortex.
The polar vortex
The polar vortex is a persistent, large-scale circulation pattern that forms over the polar regions in winter. It matters because it isolates the air within it, preventing exchange with air from lower latitudes. That does three things at once: it keeps the air extremely cold, so PSCs can form; it traps ozone-depleting substances inside, raising their concentration; and it stops fresh ozone-rich air arriving to replace what is destroyed.
The geography line at the bottom is worth memorising. It is the cleanest one-sentence explanation of why the hole is an Antarctic phenomenon, and it turns a memorised fact into an understood one.
If a question asks why depletion is worse over Antarctica, do not stop at “it is colder”. Give the chain: colder means PSCs form reliably, PSCs give the surfaces that convert chlorine into destructive forms, and the stable vortex keeps that chlorine concentrated and stops replacement ozone flowing in. Three linked reasons, and each one is usually a mark.
Volcanic aerosols
Volcanic eruptions release aerosols that can reach the stratosphere and enhance ozone depletion. The mechanism is the same as for PSCs: the aerosol particles provide additional surfaces for ozone-destroying reactions. The effect is amplified in polar regions because the PSCs are already there and the vortex keeps everything isolated, so an eruption can make a bad spring noticeably worse.
Note what this means for the data. Ozone hole size varies from year to year for reasons that have nothing to do with CFC levels — how cold the winter was, how stable the vortex was, whether a large volcano erupted. When you interpret a graph of hole size, look at the long-term trend, not the year-to-year wobble.
What an ozone hole actually is
An ozone hole is not a literal hole in the atmosphere. It is an area where ozone is much less concentrated — where the concentration has fallen below a defined threshold. More harmful UV radiation reaches the surface in these areas because there is less ozone to absorb it, but there is still ozone present.
The Antarctic hole reached record sizes in the early 2000s, which drew global attention to ozone depletion. Although the Montreal Protocol reduced CFC emissions, the hole still forms and is monitored every year, because the chlorine already up there takes decades to clear.
WORKED EXAMPLE
Explain why ozone depletion is most severe over Antarctica in spring. [4]
Point 1 — the vortexThe polar vortex forms in winter and isolates the air over Antarctica, preventing mixing with lower latitudes and concentrating ozone-depleting substances within it.Point 2 — the cloudsExtremely low temperatures allow polar stratospheric clouds to form, providing active surfaces that convert chlorine compounds into ozone-destroying forms.Point 3 — the timingThese reactions need sunlight to complete, so when UV returns in spring it breaks the stored compounds apart and releases chlorine radicals rapidly.Point 4 — why the southThe Antarctic vortex is stronger and more stable than the Arctic one because Antarctica is a continent surrounded by ocean, so depletion there is far more severe.4 / 4Vortex, clouds, sunlight, geography. The third point is the one most answers leave out.
WORKED EXAMPLE
State what is meant by an “ozone hole”. [2]
Mark 1An area of the stratosphere where ozone concentration has fallen well below normal levels, rather than a physical gap in the atmosphere.Mark 2Because less ozone is present, more harmful UV radiation reaches the Earth’s surface in that region.2 / 2Short question, but the “not a literal hole” clause is what the mark scheme is looking for.
💡 Exam tip
Give three linked causes: vortex isolation, PSC surfaces, and the return of sunlight.
Explain PSCs as providing surfaces for reactions, not as destroying ozone themselves.
Name the season and hemisphere: September to November over Antarctica.
Use the geography line for the Antarctic versus Arctic contrast — continent surrounded by ocean, versus ocean surrounded by continents.
Mention volcanic aerosols as an additional natural factor providing extra reaction surfaces.
Never describe the hole as a gap. Say it is an area of low concentration.
⚠️ Common mix-up
Saying depletion happens in winter. The chemistry is prepared in winter but the ozone loss happens in spring, when sunlight returns.
Thinking PSCs destroy ozone. They provide the surfaces; chlorine radicals do the destroying.
Assuming CFCs are released over Antarctica. They are released mostly in the northern hemisphere and mix globally; the vortex concentrates them.
Treating the Arctic as identical. Its vortex is weaker and more easily disrupted, so depletion is milder and more variable.
Blaming volcanoes for the hole. They can worsen it, but the cause is human-made ODSs.
Reading a single bad year as a failure of the Montreal Protocol. Hole size varies with weather; look at the long-term trend.
Up next: CFCs, HCFCs and Their Replacements — the last page of Topic 6, and a case study in how solving one environmental problem can quietly create another.
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