Which statement describes chlorine's role in ozone depletion?

• A. It directly destroys ozone without cycling
• B. It forms a barrier preventing UV light
• C. It acts as a catalyst, repeatedly breaking down ozone molecules
• D. It increases ozone formation

Answer: (C)

Chlorine drives ozone loss through a catalytic cycle in the stratosphere. When UV light frees chlorine atoms from human-made compounds, these atoms initiate reactions that destroy ozone but are then regenerated to continue the process. A chlorine atom reacts with ozone to form chlorine monoxide and O2, and that ClO then reacts with another oxygen atom to release chlorine again as O2. The chlorine atom is not consumed in the overall reaction, so one chlorine atom can destroy many ozone molecules over time. Because chlorine can persist for years in the stratosphere, this catalytic cycling leads to substantial ozone depletion rather than a one-time destruction, a barrier to UV light, or an increase in ozone formation.