NCEA Level 2 · Year 12 · Chemistry · 3 minutes

The ozone hole: the chemistry that saved the sky

UV frees chlorine. Chlorine is a catalyst. And the world agreed to stop it.

Key terms

Ozone
O3, three oxygen atoms; in the stratosphere it absorbs most of the Sun's harmful ultraviolet light.
CFCs
Chlorofluorocarbons, chemicals from spray cans and fridges that drift up and attack ozone.
Radical
An atom with an unpaired electron that makes it very reactive, such as chlorine freed from a CFC.
Catalyst
A substance that gives a pathway with lower activation energy and is regenerated each cycle, so chlorine can destroy many ozone molecules.
Stratosphere
The high layer of the atmosphere where the ozone layer sits.
Montreal Protocol
The 1987 agreement to phase out CFCs, ratified by every country by 2009.

Transcript

0:00

In May nineteen eighty five, British scientists in Antarctica published something alarming. Every spring, about a third of the ozone above their base was disappearing. They first suspected their instrument. But something really was eating the ozone.

0:17

Ozone is O three: three oxygen atoms. High in the stratosphere, it absorbs most of the Sun's harmful ultraviolet light. Less ozone means more UV on your skin.

0:28

The prime suspects were CFCs, chlorofluorocarbons, from spray cans and fridges. Back in nineteen seventy four, chemists Molina and Rowland had warned they could drift up and attack ozone.

0:41

Here's the chemistry. UV splits a CFC molecule, freeing a chlorine atom. It's a radical: its unpaired electron makes it very reactive. Chlorine steals an oxygen atom from ozone, making chlorine monoxide and oxygen gas. Then chlorine monoxide reacts with a free oxygen atom, making more oxygen gas and releasing the chlorine again.

1:05

That's the big idea: chlorine is a catalyst. It provides a pathway with lower activation energy, so ozone breaks down faster, and it's regenerated every cycle. So one chlorine atom can destroy up to one hundred thousand ozone molecules.

1:22

Why Antarctica? In the freezing polar winter, icy clouds form in the stratosphere, and reactions on them store chlorine in forms that sunlight breaks apart easily. So when the Sun returns in spring, ozone destruction surges.

1:37

In nineteen eighty seven, the world agreed the Montreal Protocol, to phase out CFCs. By two thousand nine, every country had ratified it, a first for any United Nations treaty.

1:49

And it's working. If countries keep their promises, Antarctic ozone should be back to nineteen eighty levels around twenty sixty six. That matters for us. New Zealand's summer UV is already about forty per cent higher than at similar northern latitudes. Without the Protocol, it would be about twenty per cent higher again.

2:10

So, for your exam: UV splits CFCs, releasing chlorine radicals. Chlorine is a catalyst, regenerated every cycle, so a little destroys a lot of ozone. And global teamwork is giving the sky a chance to heal.

Sources

Every fact in the video, and where it comes from.

  1. 1
    ukri.org/who-we-are/how-we-are-doing/research-outcomes-…
    • British Antarctic Survey scientists Joe Farman, Brian Gardiner and Jonathan Shanklin published the discovery of severe springtime ozone loss over Antarctica in Nature in May 1985 (Nature 315, 207–210).
    • Halley's Dobson spectrophotometer showed a systematic decline in spring ozone from the late 1970s; by 1984 the ozone over Halley was about two-thirds as thick as in earlier decades.
  2. 2
    ebsco.com/research-starters/environmental-sciences/resea…
    • The readings were so low that Farman first suspected the Halley instrument was malfunctioning; a new instrument gave the same results, and the team checked the data before publishing.
  3. 3
    en.wikipedia.org/wiki/Joe_Farman
    • The readings were so low that Farman first suspected the Halley instrument was malfunctioning; a new instrument gave the same results, and the team checked the data before publishing.
  4. 4
    ozone.unep.org/how-have-humans-affected-ozone-layer
    • The ozone layer in the stratosphere absorbs most of the Sun's harmful ultraviolet radiation (all UV-C and most UV-B).
    • CFCs (chlorofluorocarbons) were used in aerosol sprays, refrigerators and air conditioners.
  5. 5
    nobelprize.org/prizes/chemistry/1995/summary
    • In 1974 Mario Molina and F. Sherwood Rowland warned that CFCs could reach the stratosphere and destroy ozone; they shared the 1995 Nobel Prize in Chemistry with Paul Crutzen.
  6. 6
    science.nasa.gov/earth/earth-observatory/ozone
    • In the stratosphere strong UV light breaks CFC molecules apart, releasing chlorine atoms (radicals); Cl + O3 -> ClO + O2, then ClO + O -> Cl + O2, so chlorine is regenerated and acts as a catalyst.
  7. 7
    webarchive.library.unt.edu/web/20130224103219/http://www.epa.gov/ozone/sc…
    • One chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere (EPA estimate; UNEP says up to 100,000).
  8. 8
    britannica.com/science/catalysis
    • A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy and is not used up.
  9. 9
    csl.noaa.gov/assessments/ozone/2022/twentyquestions
    • In the cold Antarctic winter, polar stratospheric clouds form; reactions on their surfaces convert chlorine into forms that sunlight readily breaks apart in spring, causing rapid ozone loss.
  10. 10
    webarchive.library.unt.edu/web/20161225103741/http://ozone.unep.org/en/tr…
    • The Montreal Protocol was agreed on 16 September 1987 to phase out ozone-depleting substances such as CFCs; in 2009 it became (with the Vienna Convention) the first UN treaty ratified by every country.
  11. 11
    wmo.int/news/media-centre/ozone-layer-recovery-track-h…
    • The 2022 WMO/UNEP assessment projects total column ozone returning to 1980 values around 2066 over Antarctica, 2045 in the Arctic and 2040 for the near-global average, if current policies stay in place.
  12. 12
    sciencelearn.org.nz/resources/223-monitoring-uv
    • Lauder (Central Otago) receives about 40% more UV radiation in summer than places of similar latitude in the Northern Hemisphere; lower ozone overhead accounts for part (about 10%) of this, cleaner air about 20% and Earth's orbit about 7%.
  13. 13
    niwa.co.nz/news/niwa-uv-research-reinforces-success-montr…
    • NIWA (2019): without the Montreal Protocol, the UV Index would already have increased by about 20 per cent.

Image and sound credits

  • Halley station, Antarctica: NASA Goddard Space Flight Center, CC BY 2.0, via Wikimedia Commons
  • Average October ozone 1979–2000 (TOMS): NASA Scientific Visualization Studio, public domain
  • Antarctic ozone hole maximum area 1979–2025: NASA Scientific Visualization Studio, public domain
  • Top of Atmosphere: NASA Earth Observatory, public domain
  • Refrigerator advertising, 1940: Auckland Libraries Heritage Collections, CC BY
  • Antarctica from the air, 1977: Auckland Libraries Heritage Collections, CC BY
  • Nacreous clouds, Antarctica: Alan Light, CC BY 2.0, via Wikimedia Commons
  • Red Beach, 1980: Auckland Libraries Heritage Collections, CC BY
  • Sunbathing at Red Beach, 1964: Auckland Libraries Heritage Collections, no known copyright restrictions
  • Map data: Natural Earth (public domain)
  • Molecules, energy profile and chart: drawn in code
  • Paper textures: ambientCG (CC0)
  • Sound effects: Freesound contributors (CC0)
  • Spray can: generated with AI (GPT Image)
  • Music: generated with ACE-Step · Voice: Gemini TTS

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