The thinning of the stratospheric ozone over the poles during spring is primarily caused by which chemicals?

• A. CO2
• B. CFCs
• C. Methane
• D. Nitrous oxide

Answer: (B)

Ozone depletion in the polar stratosphere is driven by halogen chemicals that release ozone-destroying radicals when exposed to UV light. Chlorofluorocarbons (CFCs) are stable in the troposphere, but when they reach the stratosphere they are broken apart by ultraviolet radiation, freeing chlorine atoms. Those chlorine atoms catalytically destroy ozone through cycles that convert O3 into O2 without being consumed, so a single chlorine atom can break up many ozone molecules.

In polar regions, very cold temperatures form polar stratospheric clouds, which convert reservoir chlorine into reactive forms. When spring sunlight returns, these reactive chlorine species are released, triggering rapid ozone loss and the characteristic ozone hole over the poles. Other gases like CO2, methane, and nitrous oxide can influence ozone chemistry, but they are not the primary drivers of this dramatic spring thinning—the chlorine released from CFCs is.