Step 1: Recall the cause of ozone loss.
The ozone layer is mainly destroyed by chlorofluorocarbons (CFCs). High in the air, sunlight breaks CFC molecules and frees reactive pieces.
Step 2: Form the chlorine radical.
Ultraviolet light splits a CFC like this: \[ CF_2Cl_2 \xrightarrow{h\nu} CF_2Cl^\bullet + Cl^\bullet. \] The chlorine radical $Cl^\bullet$ that forms is very reactive and starts the chain.
Step 3: Attack ozone.
The chlorine radical eats an ozone molecule: \[ Cl^\bullet + O_3 \rightarrow ClO^\bullet + O_2. \]
Step 4: Regenerate the chlorine radical.
The $ClO^\bullet$ then reacts with a free oxygen atom and gives back the chlorine radical: \[ ClO^\bullet + O \rightarrow Cl^\bullet + O_2. \]
Step 5: See why it is a chain.
Because the $Cl^\bullet$ is made again at the end, a single chlorine radical can destroy thousands of ozone molecules over and over.
Step 6: State the initiator.
The radical that starts and drives the ozone-destroying chain is \[ \boxed{Cl^\bullet} \]