Which statement about ozone-depleting processes involving CFCs is correct?

• A. OH radicals are the primary destroyers of ozone in this context.
• B. Cl radicals have no effect on ozone depletion.
• C. NOx is the main catalyst in ozone destruction by CFCs.
• D. Cl radicals released by CFCs catalytically destroy ozone, reducing its concentration.

Answer: (D)

The main idea here is that ozone in the stratosphere is destroyed mainly by chlorine radicals released from CFCs, not by other radical species in this context. When CFCs reach the stratosphere, UV light splits them and frees chlorine atoms. Those Cl atoms enter catalytic cycles that destroy ozone over and over without being consumed themselves. A classic cycle is Cl reacting with ozone to form ClO and O2, then ClO reacting with a free oxygen atom to regenerate Cl and produce another O2. Net, ozone and a stray O atom turn into two molecules of O2, and the chlorine catalyst is regenerated to repeat the process. Because one chlorine atom can catalyze the destruction of many ozone molecules, this mechanism effectively thins the ozone layer.

That’s why the statement about chlorine radicals released by CFCs catalytically destroying ozone and reducing its concentration is the correct description. OH and NOx do participate in ozone chemistry in other contexts, but in the specific case of CFC-related ozone depletion, chlorine catalysis is the dominant process.