Ozone layer: depletion and recovery – UV, CFCs, the ozone hole and the Montreal Protocol
GeographyWeather & ClimateAges 15–16
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Four linked views of the ozone layer. UV: sunlight passes through an ozone column you choose; UV-C is absorbed completely, UV-B partly and UV-A hardly at all, and the ground spectrum and UV index change with ozone and the Sun's height. Chlorine: in a box of stratospheric air, UV splits CFC molecules and each chlorine atom destroys ozone again and again (Cl + O₃ → ClO + O₂, ClO + O → Cl + O₂) until it is locked away in a reservoir such as HCl; compare with nitrogen oxide from N₂O. Ozone hole: a polar map and a season graph show the Antarctic hole opening in spring after polar stratospheric clouds release chlorine, and the much smaller Arctic loss. Montreal Protocol: chlorine, ozone and UV from 1950 to 2100 with the protocol and its amendments, the 1987 protocol alone, or no protocol.
Lesson: Ozone layer: absorption of UV, catalytic destruction by chlorine from CFCs, the polar ozone hole and recovery under the Montreal Protocol
What it shows
Ozone in the stratosphere absorbs almost all UV-C and most UV-B, so very little of the most harmful sunlight reaches the ground. Chlorine atoms released from CFCs by UV act as a catalyst: each cycle destroys ozone and gives the chlorine atom back, so one atom can remove thousands of ozone molecules. Over Antarctica, clouds in the very cold polar night free extra chlorine, and spring sunlight makes the ozone hole. The UV model uses real ozone cross-sections; chlorine and ozone trends are calibrated to WMO assessments but simplified. The molecule box is a scaled model with far fewer molecules and speeded-up time.
How to use
On UV, move Ozone column and Sun height above horizon, then press Record to add the UV index to the table and graph. On Chlorine, choose Cl from CFCs or NO from N₂O, set the number of source molecules, tick Reservoirs, and press Run for 60 model seconds. On Ozone hole, pick a hemisphere, year and scenario and press Run to play the season, or drag the date. On Montreal Protocol, choose a Scenario and Graph and drag the year line.
Parameters you can change
- View UV absorption, Chlorine, Ozone hole, Montreal Protocol
- Ozone column 100–500 DU
- Sun height above horizon 15–90 °
- Catalyst Cl from CFCs, NO from N₂O
- Source molecules released 0–6
- Reservoirs (HCl, ClONO₂, HNO₃)
- Hemisphere Antarctic, Arctic
- Year 1960–2100
- Date in the season (days after 1 June or 1 December) 0–213
- Scenario Protocol + amendments, 1987 protocol only, No protocol
- Graph Emissions, Chlorine, Ozone, UV index
Questions to explore
- Why does halving the ozone column more than double the UV index on a summer day?
- Why can a few chlorine atoms destroy many more ozone molecules than there are chlorine atoms?
- Why is the ozone hole much deeper over Antarctica than over the Arctic, and why does it appear in spring?
