Stratospheric Ozone Depletion

You’ll be able to

Explain the protective role of stratospheric ozone and how CFCs destroy it
Describe the effects of increased UV radiation reaching Earth’s surface
Identify the Montreal Protocol as the response to ozone depletion

The ozone layer as a shield

High in the stratosphere, a layer of ozone (O₃) absorbs most of the Sun’s harmful ultraviolet (UV) radiation before it reaches the surface. This is the "good" ozone — protective because it is high up (in contrast to harmful ground-level ozone). Without it, far more UV-B would reach living things, damaging DNA. The layer is naturally maintained by a balance in which UV both creates and breaks apart ozone molecules; human-made chemicals upset that balance.

How CFCs destroy ozone

The main culprits are chlorofluorocarbons (CFCs) — stable, human-made chemicals once used in refrigerants, aerosol sprays, and foam. Being inert, CFCs drift intact up to the stratosphere, where intense UV finally breaks them apart, releasing chlorine atoms. Each chlorine atom acts as a catalyst, destroying ozone molecules over and over — a single chlorine atom can break down tens of thousands of ozone molecules before it is removed. This catalytic destruction thinned the layer worldwide and opened the seasonal "ozone hole" over Antarctica, where cold-cloud chemistry makes losses most severe.

Effects and the Montreal Protocol

A thinner ozone layer lets more UV-B radiation reach the surface, increasing skin cancer and cataracts in humans, suppressing immune systems, and harming phytoplankton (the base of marine food webs) and crops. The global response is a rare environmental success: the Montreal Protocol (1987) phased out CFCs worldwide, replacing them first with less-damaging HCFCs and then HFCs. Because CFCs are so long-lived, recovery is slow, but the ozone layer is now measurably healing — proof that coordinated international action can reverse a global problem.

Catalytic ozone destruction

CFC + UV → free Cl· ; then Cl· destroys ozone repeatedly (one Cl → tens of thousands of O₃)

Chlorine is a catalyst: it is not used up, so a small amount of CFC destroys a vast amount of ozone. This is why even low CFC concentrations were so damaging.

Worked example

A CFC molecule releases chlorine atoms in the stratosphere, and each chlorine atom can catalytically destroy about 100,000 ozone molecules. If a leak releases enough CFCs to free 5 chlorine atoms, roughly how many ozone molecules could be destroyed, and why is chlorine so damaging?

1.Identify the catalytic capacity: each chlorine atom destroys about 100,000 ozone molecules.
2.Multiply by the number of chlorine atoms: 5 × 100,000 = 500,000 ozone molecules.
3.Recognize why: chlorine is a catalyst, so it is regenerated and reused rather than consumed in each reaction.
4.A tiny amount of CFC therefore causes enormous ozone loss — the key reason CFCs were so destructive.

Answer: About 500,000 ozone molecules. Because chlorine acts as a catalyst (not consumed), a small quantity of CFCs destroys a disproportionately large amount of ozone.

Checkpoint

What is the primary human-caused reason for the thinning of the stratospheric ozone layer?

Watch out

Do not confuse ozone depletion with climate change — a top exam trap. Ozone depletion is caused by CFCs and lets in more UV radiation; climate change is caused by greenhouse gases like CO₂ and traps heat. They are different problems with different chemicals, even though both are global atmospheric issues.

Checkpoint

The Montreal Protocol is widely regarded as a successful international environmental agreement. What did it accomplish?

On the exam

Anchor the ozone story: cause = CFCs → chlorine → catalytic O₃ destruction; effect = more UV-B → skin cancer, cataracts, harm to phytoplankton; solution = Montreal Protocol. Keep it firmly separate from the greenhouse-gas/climate story in Lessons 2–3.

Answer the 2 checkpoints as you read.