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examBoard: Cambridge
examType: IGCSE
lessonTitle: Ozone Layer Depletion Processes
The ozone layer is a region of the Earth's stratosphere that contains a high concentration of ozone (O₃) molecules. Located approximately 15-35 kilometres above the Earth's surface, this layer acts as a natural shield, protecting life on Earth from harmful ultraviolet (UV) radiation from the sun.
Key Definitions:
The ozone layer absorbs about 97-99% of the sun's medium-frequency ultraviolet light, which otherwise would potentially damage life on Earth. Without the ozone layer:
In a healthy atmosphere, ozone is constantly being created and destroyed in a natural cycle:
This natural cycle maintains a stable concentration of ozone in the stratosphere.
Since the 1970s, scientists have discovered that human activities are disrupting the natural balance of ozone in the stratosphere, leading to ozone depletion. This process occurs when certain chemicals released into the atmosphere interfere with the natural ozone cycle.
The main chemicals responsible for ozone depletion are:
Sources: Refrigerants, aerosol propellants, foam blowing agents and solvents.
Examples: CFC-11, CFC-12, CFC-113
CFCs are very stable compounds that can remain in the atmosphere for 50-100 years.
Sources: Fire extinguishers and fire suppression systems.
Examples: Halon-1211, Halon-1301
Halons contain bromine, which is even more destructive to ozone than chlorine.
Carbon tetrachloride: Used in solvents and cleaning agents
Methyl chloroform: Industrial solvent
Hydrochlorofluorocarbons (HCFCs): Transitional replacements for CFCs
Methyl bromide: Agricultural pesticide
Ozone depletion occurs through a catalytic process where one ODS molecule can destroy thousands of ozone molecules. Here's how it happens:
Step 1: UV radiation breaks down a CFC molecule, releasing a chlorine atom (Cl).
Step 2: The chlorine atom reacts with an ozone molecule:
Cl + O₃ → ClO + O₂
Step 3: The chlorine monoxide (ClO) reacts with an oxygen atom:
ClO + O → Cl + O₂
Step 4: The chlorine atom is now free to destroy another ozone molecule, repeating the cycle. A single chlorine atom can destroy up to 100,000 ozone molecules before it's removed from the stratosphere.
Ozone depletion is most severe over Antarctica because of special conditions there:
The most dramatic evidence of ozone depletion is the "ozone hole" that forms over Antarctica each spring. This is not actually a hole, but a region of extremely low ozone concentration.
In 1985, British Antarctic Survey scientists Joe Farman, Brian Gardiner and Jonathan Shanklin published their discovery of severe ozone depletion over Antarctica. Using ground-based measurements from the Halley Research Station, they found that springtime ozone levels had dropped by 40% since the 1970s. This discovery shocked the scientific community, as computer models had predicted much smaller ozone losses. The discovery of the ozone hole was crucial evidence that human-made chemicals were damaging Earth's protective ozone layer and helped spur international action.
The unique conditions of the Antarctic stratosphere make it particularly vulnerable to ozone depletion:
The discovery of the ozone hole led to one of the most successful international environmental agreements in history: the Montreal Protocol on Substances that Deplete the Ozone Layer.
Adopted: 1987
Implementation: 1989
Signatories: All 198 UN Member States (universal ratification)
Goal: Phase out the production and consumption of ozone-depleting substances
The Protocol has been amended several times to accelerate the phase-out schedules and add more controlled substances. The Kigali Amendment (2016) added hydrofluorocarbons (HFCs) to the list of controlled substances, even though they don't deplete ozone, because they are powerful greenhouse gases.
Thanks to the Montreal Protocol:
The Montreal Protocol is considered a model for international cooperation on environmental issues.
Scientists continue to monitor the ozone layer using a combination of ground-based instruments, weather balloons, aircraft and satellites. The main measurement used is "Dobson Units" (DU), named after G.M.B. Dobson who developed the first instrument to measure ozone from the ground.
Current data shows that while the ozone layer is beginning to recover, it remains vulnerable. Continued compliance with the Montreal Protocol is essential to ensure the ozone layer's full recovery.
If the Montreal Protocol had not been implemented, the effects of ozone depletion would have been catastrophic. Models suggest that by 2065, ozone depletion would have led to:
The Protocol has not only protected the ozone layer but also helped combat climate change, as many ozone-depleting substances are also powerful greenhouse gases.
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