Stratospheric ozone absorbs ultraviolet radiation when ozone molecules take in its energy and undergo photodissociation. This prevents most damaging UV-B and UV-C radiation from reaching Earth's surface.
Level: SL and HL; Subtopic 6.4 Stratospheric ozone
The Absorption Mechanism
The Sun emits electromagnetic radiation across a range of wavelengths. Shorter-wavelength ultraviolet radiation has a higher frequency and therefore carries more energy than visible or infrared radiation.
When an ozone molecule absorbs a sufficiently energetic UV photon, the molecule splits into an oxygen molecule and an oxygen atom:
The free oxygen atom can then combine with an oxygen molecule to reform ozone:
Together, ozone formation and destruction maintain a steady-state equilibrium in the stratosphere. The absorbed UV energy is ultimately converted into thermal energy, warming the stratosphere while reducing the amount of biologically damaging radiation transmitted toward the surface.
| UV type | Interaction with stratospheric ozone |
|---|---|
| UV-C | Almost completely absorbed by oxygen and ozone before reaching the surface |
| UV-B | Mostly absorbed by ozone, although some reaches the surface |
| UV-A | Weakly absorbed, so most reaches the surface |
This protection reduces DNA damage, mutation, skin cancer and cataracts in humans. It also protects phytoplankton and terrestrial plants from reduced photosynthesis.
A common misconception is that ozone depletion causes global warming. Stratospheric ozone depletion primarily increases UV-B transmission, whereas climate change is mainly caused by the enhanced greenhouse effect. These are distinct environmental issues, although some ozone-depleting substances are also greenhouse gases.
Exam Technique
For an explain question, link absorption to molecular change: ozone absorbs UV energy, undergoes photodissociation, and is reformed. Do not merely state that ozone “blocks” UV. The Montreal Protocol is the key named example showing how international regulation of ozone-depleting substances protected the stratospheric ozone planetary boundary.