Unit 9 · Topic 9.1 Beta

Stratospheric Ozone Depletion

The stratospheric ozone layer absorbs most of the Sun’s UV.

Practice 2: Visual RepresentationsPractice 5: Data Analysis

Part 1 · Hook

Why this matters

In 1985, British scientists in Antarctica published a startling result: every spring, the ozone above them was dropping far below normal. Satellites confirmed a "hole" larger than the continent. The cause was not a volcano or the Sun, but chemicals in refrigerators and spray cans, released tens of thousands of kilometers away.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. In which layer of the atmosphere is the ozone layer?

  1. Stratosphere
  2. Troposphere
  3. Mesosphere
  4. Thermosphere
Show the answer

The ozone layer is in the stratosphere, above the troposphere where weather happens.

  • Correct: Stratosphere:
  • Troposphere:
  • Mesosphere:
  • Thermosphere:

2. What does stratospheric ozone absorb?

  1. Ultraviolet radiation from the Sun
  2. Heat given off by the ground
  3. Visible light from the Sun
  4. Radio waves
Show the answer

Ozone in the stratosphere absorbs most incoming ultraviolet radiation.

  • Correct: Ultraviolet radiation from the Sun:
  • Heat given off by the ground:
  • Visible light from the Sun:
  • Radio waves:

Part 4 · See it

See it first

A three-step cycle in the stratosphere, about 15 to 35 km up. Step 1: ultraviolet light breaks a CFC molecule and frees a chlorine atom. Step 2: chlorine reacts with ozone: Cl + O3 gives ClO + O2, destroying one ozone molecule. Step 3: ClO reacts with a single oxygen atom: ClO + O gives Cl + O2, and the chlorine atom is free again, so the cycle repeats. One chlorine atom can destroy many thousands of ozone molecules. Result at the surface: less ozone lets more UV through, raising skin cancer and cataracts.
How chlorine from a CFC destroys ozone. The chlorine atom comes out unchanged, so one atom can destroy many thousands of ozone molecules. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. CFCs from refrigerants, air conditioners and spray cans are stable near the groundthey last long enough to drift up into the stratosphere
  2. Ultraviolet light in the stratosphere breaks CFC moleculeschlorine atoms are set free
  3. Each chlorine atom destroys an ozone molecule and is released againone atom can destroy many thousands of ozone molecules
  4. A thinner ozone layer absorbs less UVmore UV-B reaches the surface, raising skin cancer and cataracts and harming phytoplankton

Part 6 · Key ideas

Key ideas

  • Chlorofluorocarbons (CFCs) contain chlorine, fluorine and carbon. They were used in refrigerators, air conditioners, foams and spray cans because they are stable and nontoxic near the ground.
  • In the stratosphere, UV frees chlorine; Cl+O3→ClO+O2\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2, then ClO+O→Cl+O2\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2. The chlorine is reused.
  • Ozone depletion is worst over Antarctica each spring (the ozone hole), where extreme cold forms clouds that speed the reactions.
  • More UV-B at the surface means more skin cancer and cataracts and damage to phytoplankton and crops.

Part 7 · Misconception

A common mistake

The wrong idea: Carbon dioxide from burning fossil fuels is what makes the ozone hole.

What actually happens: Carbon dioxide traps heat but does not destroy ozone. The ozone hole is caused by chlorine and bromine from CFCs and halons.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. Which substances are the main cause of stratospheric ozone depletion?

  1. Carbon dioxide from burning fossil fuels
  2. CFCs from old refrigerants and spray cans
  3. Sulfur dioxide from coal-burning power plants
  4. Methane from cattle and landfills
Show the answer

CFCs, and halons with bromine, are the ozone-depleting substances; their chlorine and bromine destroy ozone in the stratosphere.

  • Carbon dioxide from burning fossil fuels: Carbon dioxide traps heat near the surface; it does not destroy stratospheric ozone.
  • Correct: CFCs from old refrigerants and spray cans: Right: chlorine released from CFCs destroys ozone.
  • Sulfur dioxide from coal-burning power plants: Sulfur dioxide causes acid rain; it is not the cause of the ozone hole.
  • Methane from cattle and landfills: Methane traps heat, but it is not the main ozone-depleting substance.

2. Why can one chlorine atom destroy many thousands of ozone molecules?

  1. Each chlorine atom splits into thousands of smaller atoms
  2. It absorbs ultraviolet light, which then destroys the ozone
  3. It traps heat from the ground, which breaks the ozone
  4. It is released after each cycle and reacts again
Show the answer

In Cl + O₃ → ClO + O₂ and then ClO + O → Cl + O₂, the chlorine comes out unchanged and starts again.

  • Each chlorine atom splits into thousands of smaller atoms: Chlorine atoms do not split; the same atom is reused.
  • It absorbs ultraviolet light, which then destroys the ozone: Ultraviolet frees chlorine from CFCs; the chlorine reacts with ozone directly.
  • It traps heat from the ground, which breaks the ozone: Trapping heat is a separate problem from how chlorine destroys ozone.
  • Correct: It is released after each cycle and reacts again: Right: chlorine is a catalyst; it is not used up by the reactions.

Data

Average size of the Antarctic ozone hole, 1980 to 2020

Mean area of the Antarctic ozone hole each September to October, millions of km² (invented data for practice, shaped like the satellite record)
YearMean area (million km²)
19801.1
198512.0
199019.5
199522.5
200024.8
200524.6
201021.2
201523.1
202023.5

3. Describe the trend in the average size of the ozone hole from 1980 to 2000.

  1. It shrank steadily, from about 25 to about 1 million km²
  2. It grew from about 1 to 25 million km²
  3. It stayed about the same at about 20 million km²
  4. It grew until 1985, then shrank until 2000
Show the answer

Read the values in order: 1.1 in 1980, then 12.0, 19.5, 22.5 and 24.8 in 2000.

  • It shrank steadily, from about 25 to about 1 million km²: The values rise over 1980 to 2000, so the hole grew.
  • Correct: It grew from about 1 to 25 million km²: Right: the hole grew more than twentyfold over those 20 years.
  • It stayed about the same at about 20 million km²: It was about 1 million km² in 1980, so it did not stay the same.
  • It grew until 1985, then shrank until 2000: It kept growing through 2000.

4. By what percent did the average size of the ozone hole change from 1985 to 2000?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

(24.8−12.0)12.0×100\frac{(24.8 - 12.0)}{12.0} \times 100≈107%\approx 107\%: the hole roughly doubled.

  • Answer: 107 %

5. The hole stopped growing after about 2000, but did not shrink much by 2020. What best explains this?

  1. CFC production rose sharply after 2000
  2. Carbon dioxide began to rebuild the ozone layer after 2000
  3. The ozone hole moved from Antarctica to the Arctic after 2000
  4. Long-lived CFCs make recovery slow after cuts
Show the answer

The slow response fits long-lived CFCs already in the stratosphere; recovery is expected around the 2060s.

  • CFC production rose sharply after 2000: CFC production was being phased out by then.
  • Carbon dioxide began to rebuild the ozone layer after 2000: Carbon dioxide does not rebuild ozone.
  • The ozone hole moved from Antarctica to the Arctic after 2000: The data track the Antarctic hole each year.
  • Correct: Long-lived CFCs make recovery slow after cuts: Right: CFC molecules stay in the air for 50 to 100 years or more, so recovery lags behind the cuts.

6. Why does the ozone hole form over Antarctica each spring?

  1. Polar clouds activate chlorine; spring sun starts the loss
  2. Antarctica produces more CFCs than any other continent
  3. The Antarctic summer sun is strong enough to burn ozone away
  4. Antarctic ice releases chlorine as it melts each spring
Show the answer

Extreme cold over Antarctica forms polar stratospheric clouds, where chlorine becomes reactive; when sunlight returns in spring, ozone loss is rapid.

  • Correct: Polar clouds activate chlorine; spring sun starts the loss: Right: polar stratospheric clouds form in the extreme cold and turn chlorine into active forms; spring sunlight starts the destruction.
  • Antarctica produces more CFCs than any other continent: Few CFCs are released in Antarctica; they mix through the whole atmosphere.
  • The Antarctic summer sun is strong enough to burn ozone away: The hole forms in spring (September to October), and sunlight alone does not remove ozone.
  • Antarctic ice releases chlorine as it melts each spring: The chlorine comes from CFCs, not from ice.

7. Which effects are expected from more UV-B radiation reaching Earth’s surface? Select all that apply.

  1. More cases of skin cancer
  2. More cataracts, a clouding of the eye’s lens
  3. Damage to phytoplankton at the ocean surface
  4. Faster loss of sea ice in the Arctic Ocean
  5. A rise in global average air temperature
  6. More acid rain in the eastern United States
Show the answer

More UV-B harms skin, eyes and some organisms, including phytoplankton; it is not the cause of global warming or acid rain.

  • Correct: More cases of skin cancer: Right: UV-B damages DNA in skin cells.
  • Correct: More cataracts, a clouding of the eye’s lens: Right: UV-B damages the eye.
  • Correct: Damage to phytoplankton at the ocean surface: Right: UV-B harms phytoplankton, the base of ocean food webs.
  • Faster loss of sea ice in the Arctic Ocean: Sea ice melts with warming, not because of more UV.
  • A rise in global average air temperature: Warming is driven by heat-trapping gases, not by more UV.
  • More acid rain in the eastern United States: Acid rain comes from NOx and SOx emissions.

Part 9 · Summary

Summary

The stratospheric ozone layer absorbs most of the Sun’s UV. CFCs drift up to the stratosphere, where UV frees chlorine, and each chlorine atom destroys many thousands of ozone molecules because it is released again after each cycle. The ozone hole forms over Antarctica each spring. Thinner ozone lets more UV-B through, raising skin cancer and cataracts. Carbon dioxide is not an ozone-depleting gas.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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