Ozone is never finished: High in the stratosphere, sunlight makes ozone and sunlight breaks it down, every second of the day.
The points to remember
- Made, step 1: high-energy UV splits an oxygen molecule (O₂) into two oxygen atoms.
- Made, step 2: each oxygen atom joins an oxygen molecule to make ozone (O₃).
- Broken, step 3: ozone absorbs UV and splits into an oxygen molecule and an oxygen atom.
- Broken, step 4: an oxygen atom meets an ozone molecule, and they make two oxygen molecules.
- Step 3 is how the ozone layer absorbs UV: the UV's energy is used to split the ozone.
Remember it as: UV makes it, UV breaks it.
Real example: in 1930 the British scientist Sydney Chapman worked out these steps. They are still called the Chapman cycle, and they explain why the ozone layer is high up: only there is the UV strong enough to split oxygen.
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If ozone is made and destroyed all the time, why does the ozone layer not grow or vanish? Because the two processes run at the same rate.
The points to remember
- Ozone is made and broken down at the same time, all the time.
- When the two rates are equal, the amount of ozone stays about the same.
- This is a steady-state equilibrium: things keep changing, but the total stays steady.
- If ozone builds up, more of it is there to be broken down, so it falls back: a negative feedback.
- So the concentration of ozone stayed about constant over long periods of time.
Inputs to the ozone store
- UV splits oxygen molecules
- Oxygen atoms join oxygen to make ozone
Outputs from the ozone store
- Ozone absorbs UV and splits
- Oxygen atoms and ozone make oxygen
Link 1.2: a steady state: Like a bath with the tap on and the plug out: if water flows in as fast as it drains, the level stays the same, even though the water is always changing.
Real example: at Arosa, Switzerland, ozone has been measured since 1926, the longest record in the world. It rises and falls with the seasons each year, but for decades the yearly average stayed about the same.
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How this comes up: A short Paper 2 question on how ozone forms and breaks down, or why its amount stays steady. Give each step in order, one per mark.
Ozone has been measured at Arosa, Switzerland, since 1926. For decades, the yearly average stayed about the same.
Outline why the concentration of ozone in the stratosphere stayed about the same over long periods.
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