How Do CFCs and Other ODSs Release Halogens That Break Down Ozone?

HL only; Subtopic 6.4 Stratospheric ozone: CFCs and other ozone-depleting substances (ODSs) reach the stratosphere, where high-energy ultraviolet radiation breaks their chemical bonds and releases reactive halogen radicals. These radicals catalytically convert ozone into oxygen.

The Mechanism

CFCs are stable in the troposphere, so they are not easily removed by chemical reactions or rainfall. Over time, atmospheric circulation transports them into the stratosphere, where shorter-wavelength ultraviolet radiation provides enough energy for photodissociation.

For example, UV radiation can break a carbon to chlorine bond in the CFC trichlorofluoromethane:

$$\text{CCl}_3\text{F} + h\nu \rightarrow \text{CCl}_2\text{F}^{\bullet} + \text{Cl}^{\bullet}$$

The released chlorine radical, $\text{Cl}^{\bullet}$, has an unpaired electron and is highly reactive. It removes an oxygen atom from ozone:

$$\text{Cl}^{\bullet} + \text{O}_3 \rightarrow \text{ClO}^{\bullet} + \text{O}_2$$

The chlorine monoxide radical then reacts with an oxygen atom, regenerating the original chlorine radical:

$$\text{ClO}^{\bullet} + \text{O} \rightarrow \text{Cl}^{\bullet} + \text{O}_2$$

The overall reaction is:

$$\text{O}_3 + \text{O} \rightarrow 2\text{O}_2$$

Halons release bromine radicals through the same broad process. Bromine is more effective per atom at destroying ozone, although atmospheric abundance differs among compounds. Temporary reservoir compounds can store reactive halogens and slow the cycle; under suitable stratospheric conditions, radicals are released again. This helps explain severe seasonal polar ozone depletion.

Because the halogen radical is regenerated, it acts as a catalyst and can destroy many ozone molecules before being removed into a reservoir compound.

A common misconception is that intact CFC molecules directly attack ozone. They do not: UV radiation must first release reactive halogen radicals. Also, ozone depletion is distinct from climate change, although some ODSs are greenhouse gases.

Exam Technique

For an explain question, give the full causal sequence: atmospheric stability, transport to the stratosphere, UV photodissociation, radical release, catalytic ozone destruction and radical regeneration. Include equations if space permits and identify this mechanism as HL-only content.