The ozone layer is a region of the stratosphere that contains a higher concentration of ozone gas, O3, than the rest of the atmosphere. It matters because ozone absorbs much of the Sun’s harmful ultraviolet radiation before it reaches Earth’s surface. Without this shield, life would face higher risks of DNA damage, skin cancer, cataracts, and harm to crops and ocean plankton.
The famous ozone hole is not an empty gap, but a seasonal thinning of ozone, especially over Antarctica.
Key Facts
- Ozone is O3, a molecule made of three oxygen atoms.
- Most protective ozone is in the stratosphere, about 15 km to 35 km above Earth’s surface.
- O2 + UV light -> 2O and O + O2 -> O3 describe the basic natural formation of ozone.
- O3 + UV light -> O2 + O shows how ozone absorbs ultraviolet radiation.
- CFCs release chlorine in the stratosphere, and one chlorine atom can destroy many ozone molecules.
- Total ozone is measured in Dobson units, where 1 DU = 0.01 mm of pure ozone at standard pressure and temperature.
Vocabulary
- Ozone layer
- The ozone layer is a region of the stratosphere that absorbs much of the Sun’s harmful ultraviolet radiation.
- Stratosphere
- The stratosphere is the atmospheric layer above the troposphere, where most protective ozone is found.
- Ultraviolet radiation
- Ultraviolet radiation is high-energy light from the Sun that can damage DNA and living tissues.
- CFC
- A chlorofluorocarbon is a human-made chemical once used in sprays and refrigerants that can destroy stratospheric ozone.
- Dobson unit
- A Dobson unit is a measurement of the total amount of ozone in a vertical column of the atmosphere.
Common Mistakes to Avoid
- Calling the ozone hole a literal empty hole is wrong because the ozone hole is a region where ozone concentration becomes much lower than normal.
- Confusing ozone with oxygen gas is wrong because oxygen gas is O2, while ozone is O3 and has different chemical behavior.
- Thinking all ozone is helpful is wrong because stratospheric ozone protects life, but ground-level ozone is a harmful air pollutant.
- Blaming the ozone hole mainly on carbon dioxide is wrong because ozone depletion is driven mostly by chlorine and bromine compounds such as CFCs and halons.
Practice Questions
- 1 A region of the atmosphere has total ozone of 300 DU. What thickness of pure ozone would this equal at standard pressure and temperature, in millimeters?
- 2 If normal Antarctic spring ozone is 300 DU and an ozone hole measurement is 120 DU, what percent of the normal ozone remains?
- 3 Explain why CFCs can be harmless near the ground for many years but still become dangerous to the ozone layer after they reach the stratosphere.