Ozone Depletion

The ozone layer sits in the stratosphere and acts like a sunscreen for the planet: it absorbs much of the Sun's harmful ultraviolet (UV) radiation before it reaches the surface. Ozone depletion is the thinning of this protective layer, which lets more UV through.

The cause — chlorofluorocarbons (CFCs). CFCs were widely used in aerosols (spray cans) and as refrigerants (in fridges and air conditioners). The problem is that CFCs are very unreactive (stable) compounds.

Outline the steps in order — this is a frequent multi-mark question:

1. CFCs are unreactive, so they are not broken down in the troposphere (the lower atmosphere where we live).
2. Because they are not destroyed, CFCs slowly move up into the stratosphere.
3. In the stratosphere, CFCs break down in the presence of ultraviolet (UV) light, releasing a chlorine atom.
4. There are rapid reactions between chlorine atoms and ozone that break down ozone (O₃) into oxygen (O₂), causing ozone depletion.
5. The chlorine atoms remain in the stratosphere and can continue to destroy ozone — a single chlorine atom can destroy many ozone molecules.

The syllabus does not require the detailed chemical equations — but it does want the steps in the right order, with the key terms (unreactive, troposphere, stratosphere, UV light, chlorine atom, ozone → oxygen).

Scientists need a way to measure how much ozone is in the layer so they can track whether it is being depleted.

The Dobson Unit (DU). Ozone concentration is measured using the Dobson Unit. You simply need to state that ozone is measured in Dobson Units — it is the standard unit for the amount of ozone overhead.

The ozone hole. An ozone hole is an area where the average concentration of ozone is below 100 Dobson Units. Learn this figure exactly — the number (below 100 DU) is creditworthy.

Term	What it means
Dobson Unit (DU)	The unit used to measure ozone concentration in the atmosphere.
Ozone hole	An area where the average ozone concentration is below 100 Dobson Units.

A key idea to write carefully: the ozone "hole" is not a literal hole or empty space in the sky. It is a region where the ozone layer is severely thinned — the concentration has dropped below 100 DU — so much more UV can pass through there.

The largest ozone hole forms over Antarctica. Three linked conditions explain why depletion is greatest there — name and link all three.

Factor	Why it increases ozone depletion
Temperature	The Antarctic stratosphere becomes extremely cold during the polar winter. These very low temperatures allow special clouds (PSCs) to form and create the conditions for chlorine to destroy ozone rapidly.
Polar vortex	A circulating mass of cold air (a strong wind that spins around the pole) that isolates the Antarctic air through winter. This traps the cold air and the ozone-destroying chemicals over Antarctica instead of mixing them away.
Polar stratospheric clouds (PSCs)	Clouds that form only in the very cold stratosphere. Their surfaces enable reactions that convert chlorine into forms that rapidly destroy ozone when sunlight returns in spring.

Putting it together. The extreme cold lets polar stratospheric clouds form; the polar vortex traps this cold air and the chlorine over the pole all winter; then, when sunlight returns in the Antarctic spring, the reactions speed up and ozone is destroyed rapidly — producing the deepest ozone hole over Antarctica.

Because the thinned ozone layer lets through more ultraviolet (UV) radiation, the impacts all stem from extra UV reaching the surface. The syllabus groups them into four areas — describe each with a clear consequence.

Affected area	Impact of increased UV
Human health	More UV increases cataracts (clouding of the eye lens, damaging eyesight) and skin cancer.
Crops	Increased UV damages plants, causing decreased crop yields (threatening food supply).
Biodiversity	UV harms organisms in both terrestrial and aquatic ecosystems — for example damaging plankton in oceans (the base of marine food webs), reducing biodiversity.
Materials	Increased UV causes degradation of materials used in clothing and construction — fabrics, plastics and building materials break down and weaken faster.

A connected picture. Every impact traces back to the same root cause: a thinner ozone layer lets through more UV. Linking the cause ("more UV reaches the surface") to each specific effect is what earns the marks — not just listing problems.

International agreements. Ozone-depleting substances (mainly CFCs) have been reduced and phased out through international agreements. The syllabus does not require detailed knowledge of specific agreements, but you should be able to evaluate them — weigh strengths against limitations:

• Strengths: because CFCs spread worldwide and the stratosphere is shared by all countries, only a global agreement can work; international agreements led countries to phase out CFCs, and ozone-layer recovery is now being observed — often cited as one of the most successful global environmental responses.
• Limitations: agreements rely on every country joining and enforcing them; CFCs persist for decades, so recovery is slow even after the ban; and the replacement substances chosen brought new problems of their own (below).

Alternatives to ozone-depleting substances — and their impacts. CFCs were replaced with alternatives, but these have impacts too:

• Hydrochlorofluorocarbons (HCFCs): introduced as a transitional replacement; they are less damaging to ozone than CFCs but still deplete some ozone, so they are themselves being phased out.
• Fluorinated gases (F-gases): do not deplete ozone, but many are powerful greenhouse gases that contribute to global warming.

So replacing CFCs solved most of the ozone problem but created a trade-off — evaluating that trade-off is exactly the kind of "evaluate the agreements / the alternatives" thinking examiners reward.

The Rowland-Molina hypothesis — the importance of experimental evidence (links to 2.1). In the 1970s, Rowland and Molina proposed the hypothesis that CFCs destroy ozone. This is a textbook example of how science actually works:

• Initially the main hypothesis was not accepted — it was a new and surprising idea.
• Some auxiliary (supporting) hypotheses were not backed up by experimental evidence at first.
• The hypothesis led to further research and data collection by other scientists, which eventually confirmed that CFCs are ozone-depleting.

The lesson (and the link to the scientific method in 2.1): a hypothesis must be supported by experimental evidence and replicated by other scientists before it is accepted. Evidence and independent confirmation — not the status of the person proposing it — are what turn a hypothesis into accepted knowledge.