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.
Understanding Environmental Science: The Ozone Layer
Ozone is part of a moving chemical system, not a fixed blanket. Sunlight breaks some oxygen molecules apart high in the atmosphere. The loose oxygen atoms can join other oxygen molecules, making ozone.
Other sunlight then breaks ozone apart again. This repeating cycle absorbs energy from ultraviolet light and warms the stratosphere. That warming helps create the stratosphere’s temperature pattern, where air becomes warmer with height for a while.
This pattern limits vertical mixing with the lower atmosphere. The ozone layer therefore affects both radiation reaching the ground and the structure of the atmosphere above us.
The Antarctic ozone hole becomes severe because several conditions occur together during the Southern Hemisphere winter and spring. The polar stratosphere becomes extremely cold. Special clouds can form there, even though this part of the atmosphere is usually very dry.
Chemical reactions on the surfaces of these cloud particles change chlorine compounds into forms that can react easily when sunlight returns in spring. A strong ring of winds traps this cold air over Antarctica.
When the Sun rises after the polar winter, chlorine reactions rapidly remove ozone. As the polar winds weaken later in the season, air from lower latitudes mixes in and ozone levels rise again.
The major human cause came from chemicals once used in refrigerators, air conditioners, spray cans, and foam production. These chemicals were stable near the ground, which made them useful in products. Their stability created a problem because they could survive long enough to reach the stratosphere.
There, ultraviolet light released chlorine atoms. Chlorine can take part in a cycle that destroys ozone, then becomes available to repeat the cycle. This is why a small amount of chlorine can have a large effect.
International rules called the Montreal Protocol greatly reduced production of the most damaging chemicals. Ozone recovery is expected to take decades because many older chemicals remain in the atmosphere for a long time.
Students should separate stratospheric ozone from ground-level ozone. High-altitude ozone protects living things from intense ultraviolet radiation. Ground-level ozone is a pollutant made when sunlight drives reactions involving vehicle emissions, industrial gases, and other pollutants.
It can irritate lungs and damage plants. The same molecule has very different effects depending on where it is found. Pay attention to the ultraviolet index in weather reports, especially during outdoor activities.
Clouds reduce some ultraviolet radiation, but they do not block it completely. Scientists track ozone using satellites, balloons, and ground instruments. Their measurements show natural seasonal changes, long-term damage from past emissions, and the slow signs of recovery.
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.