In the stratosphere, what happens when chlorine is released from CFCs?

• A. They have no effect on ozone
• B. They convert ozone into CO2
• C. They release chlorine that breaks ozone into O2
• D. They cause ozone to thicken

Answer: (C)

Chlorine released from CFCs acts as a catalyst in stratospheric ozone chemistry. When UV light frees chlorine atoms, they enter a cycle that destroys ozone and produces oxygen. A typical sequence is O3 + Cl -> ClO + O2, then ClO + O -> Cl + O2, with chlorine regenerated at the end. The net result is O3 + O -> 2 O2. Because the chlorine atom is not consumed, a single chlorine atom can destroy many ozone molecules, leading to thinning of the ozone layer. The other ideas—no effect, conversion of ozone to CO2, or ozone thickening—do not match the actual chemical process.