How ODSs cause ozone depletion
- Ozone-depleting substances (ODSs) such as CFCs, HCFCs, carbon tetrachloride and methyl chloroform are stable in the troposphere but break down under UV radiation in the stratosphere.
- When ODS molecules reach the stratosphere, UV radiation causes halogens (especially chlorine and fluorine) to be released.
- These halogens form highly reactive radicals, which catalytically destroy ozone (O₃).
- One chlorine radical can destroy thousands of ozone molecules because it is regenerated rather than consumed during reactions.
- This reduces the concentration of ozone in the stratospheric ozone layer, lowering protection against UV-B radiation.
Major ODSs and Their Impact
| ODS | Sources | Harnful Element Released | Ozone Depletion Potential (ODP) |
|---|---|---|---|
| CFC-11 (Trichlorofluoromethane) | Refrigerators, air conditioners, foam insulation | Chlorine (Cl) | 1.0 |
| CFC-12 (Dichlorodifluoromethane) | Aerosols, propellants, solvents | Chlorine (Cl) | 1.0 |
| Halon-1301 (Bromotrifluoromethane) | Fire extinguishers | Bromine (Br) | 10.0 |
| Carbon Tetrachloride (CClâ‚„) | Industrial solvent, dry cleaning | Chlorine (Cl) | 1.1 |
| Methyl Bromide (CH₃Br) | Pesticides, fumigation | Bromine (Br) | 0.6 |
Bromine compounds (e.g., halons, methyl bromide) are even more destructive than chlorine, as one bromine atom can destroy up to 100 times more ozone molecules than chlorine.
The catalytic breakdown of ozone
- Chlorine radicals initiate a two-step cycle that converts ozone (O₃) into oxygen (O₂).
- The radical is continuously regenerated and therefore continues destroying ozone as long as it remains in the stratosphere.
- CFCs are especially harmful because they have long atmospheric lifetimes (often greater than 100 years) and spread globally.
- CFCs break down → Release chlorine (Cl)
- Cl reacts with ozone (O₃) → Forms ClO (chlorine monoxide) and O₂
- ClO reacts with oxygen atoms (O) → Cl is freed, repeats the destruction cycle
A single chlorine atom from CFC-12 can destroy up to 100,000 ozone molecules before deactivation.
Polar Stratospheric Ozone Depletion in Spring
Why ozone depletion is strongest at the poles
Definition
Polar Stratospheric Clouds
Ice-based clouds that form only at extremely low stratospheric temperatures and enable reactions that convert stored chlorine into ozone-destroying radicals.
- The most severe depletion occurs in Antarctica during Southern Hemisphere spring (September–November).
- Extremely low winter temperatures (as low as −90 °C) allow the formation of polar stratospheric clouds (PSCs) made of frozen water vapour and nitric acid.
- These clouds provide active surfaces where inactive chlorine compounds (for example, chlorine nitrates) convert into highly reactive forms.
- PSCs act like charging stations for chlorine radicals.
- They prepare inactive chlorine to become highly destructive once sunlight returns.
Role of the polar vortex and volcanic aerosols
- A polar vortex forms over Antarctica during the winter, isolating air masses and trapping ODSs and reactive halogens.
- This isolation prevents dilution with air from lower latitudes, increasing the concentration of ozone-destroying chemicals.
- Volcanic aerosols reaching the stratosphere provide additional reactive surfaces, intensifying ozone loss during PSC formation.
Why Antarctic ozone depletion is stronger than Arctic
- The Antarctic polar vortex is stronger, larger and more stable than the Arctic vortex.
- Temperatures are lower for a longer duration, enabling more PSC formation.
- Therefore, greater chlorine activation and greater seasonal ozone depletion occur over Antarctica.
The Antarctic ozone hole reached record size in the early 2000s, despite ODS phase-outs, because long-lived CFCs remained in the stratosphere.
Why depletion peaks in spring
- During winter, halogens accumulate in PSCs but little ozone is destroyed due to lack of sunlight.
- At the beginning of spring, sunlight returns and UV radiation reaches the stratosphere, triggering rapid halogen-driven ozone destruction.
- The ozone hole shrinks during summer once PSCs disappear and halogens convert back to inactive forms.
HFCs as Substitutes for CFCs and the Kigali Amendment
Why HFCs were originally adopted
- As CFCs were phased out under the Montreal Protocol, hydrofluorocarbons (HFCs) became replacements because they do not significantly deplete ozone.
- HFCs were used in the same applications as CFCs: aerosols, refrigeration, foam blowing and air-conditioning systems.
Environmental challenges of HFCs
- Although HFCs do not break down ozone, they have very high global warming potentials (GWP).
