Example of an ozone-depleting substance?

• A. CO2, CH4, N2O
• B. CFCs, Halons, HCFCs
• C. O3, O2, N2
• D. H2O, CO

Answer: (B)

Ozone depletion happens when certain chlorine- and bromine-containing compounds release reactive halogen radicals in the stratosphere under UV light. These radicals participate in catalytic cycles that destroy ozone molecules: for example, chlorine radicals convert ozone into ordinary oxygen, and the chlorine is regenerated to destroy more ozone. Because one halogen atom can catalytically destroy many ozone molecules, compounds that release chlorine and bromine are the classic ozone-depleting substances.

CFCs, halons, and HCFCs are precisely those substances. They are stable enough to rise to the stratosphere intact, where UV light breaks them apart to release chlorine and bromine radicals, driving ozone loss. HCFCs are milder in impact than CFCs because they break down more quickly, but they still contribute to ozone depletion. That makes them the best example among the options.

The other groups listed aren’t the archetypal ozone-depleting substances. The first set are greenhouse gases, not the primary agents known for sustained stratospheric ozone destruction; the third set lists ozone or common atmospheric gases that don’t cause catalytic ozone loss in the same way; and water vapor and carbon monoxide don’t drive the ozone-depleting chemistry in the stratosphere like halogen-containing compounds do.