One molecule of chlorine can destroy how many ozone molecules?

• A. 1
• B. 10
• C. 100,000
• D. 1,000,000

Answer: (C)

Chlorine acts as a catalyst in ozone destruction, meaning one chlorine atom can keep destroying many ozone molecules without being consumed itself. In the stratosphere, Cl atoms are released from chlorine-containing compounds and participate in catalytic cycles: Cl reacts with ozone to form ClO and O2, then ClO reacts with atomic oxygen to release another O2 and regenerate the chlorine atom. Net effect: O3 plus an extra O goes to two O2, while the chlorine atom is reused again and again. Because the chlorine atom is regenerated, it can destroy a huge number of ozone molecules before it’s sequestered into stable forms, a typical estimate being about 100,000 ozone molecules per chlorine atom. The other numbers are not consistent with these catalytic cycles and observed depletion rates, which is why 100,000 is the commonly cited count.