Ozone layer & the Montreal Protocol

My third-year undergraduate project (on a straight chemistry BSc this would have been the final-year project) covered the chemistry of the ozone layer and the effect of the Montreal Protocol on the Antarctic ozone hole. It followed on from a bonus GCSE in geology I picked up along the way, and a few geology units taken during the degree — a small return to earth science after giving up geography at school in favour of maths, physics and chemistry A-levels. I remember this project less well than the NMR one, but the chemistry has stuck.

(A professor once told me my first draft “would have made an excellent New Scientist article” — meant as criticism, too readable and not rigorous enough for an academic report. Eighteen-year-old me took it as a compliment.)

CFCs are stable enough to survive intact all the way to the stratosphere, where UV radiation finally breaks a C–Cl bond and releases a chlorine radical. That one atom then runs a catalytic cycle: Cl• strips an oxygen from ozone (Cl + O₃ → ClO + O₂), and a free oxygen atom strips it back off again (ClO + O → Cl + O₂) — regenerating the original chlorine atom so it can do it again. A single chlorine atom can destroy tens of thousands of ozone molecules before it's eventually removed from the stratosphere: a reusable weapon, not a one-shot reaction. The cycle below runs on a loop while a running total counts the toll from one atom.

Why the hole forms specifically over Antarctica: the winter polar vortex isolates the air over the pole, and the extreme cold that builds up inside it forms polar stratospheric clouds. Reactions on the surface of those ice crystals convert otherwise-inert chlorine reservoir compounds into active, ozone-destroying forms, and when spring sunlight finally arrives it triggers rapid, localised destruction — until the vortex breaks down later in the season and lets the hole refill for the year. The chart below sweeps through the decades that followed the hole's discovery, and — genuinely, for once — policy working.

Schematic hole-area shape, not a precise dataset — but the trend (sharp growth through the 80s–90s, a plateau, then slow narrowing) and the milestone years are real.

1. Cl• + O₃ → ClO + O₂, then ClO + O → Cl• + O₂ — the chlorine atom regenerates and repeats, destroying tens of thousands of ozone molecules over its stratospheric lifetime.
2. 1975–2020: schematic ozone hole area over time.
3. 1985 — ozone hole discovered (Farman, Gardiner & Shanklin)
4. 1987 — Montreal Protocol signed
5. 1989 — Protocol enters into force
6. 1996 — CFC production phased out in developed nations
7. 2016 — first confirmed signs of recovery
8. Full recovery projected around the 2060s.