The thinning of the ozone layer over the poles in spring is mainly caused by chlorofluorocarbons. Which option lists the correct pollutant?

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

Answer: (B)

Ozone thinning in the polar spring is driven by chlorine released from human-made chlorofluorocarbons. These compounds are very stable in the lower atmosphere, so they travel up to the stratosphere. Ultraviolet radiation breaks CFCs apart, freeing chlorine atoms. In the stratosphere, especially over polar regions, chlorine acts as a catalyst in reactions that destroy ozone (O3) molecules, turning them into ordinary oxygen (O2) without consuming the chlorine in the initial step. The situation is intensified in winter and early spring by polar stratospheric clouds, which convert chlorine into more reactive forms; when sunlight returns, those reactive chlorine species rapidly destroy significant amounts of ozone, creating the seasonal thinning or ozone holes.

Other listed pollutants don’t drive this specific catalytic destruction of ozone. CO2 and methane are mainly discussed as greenhouse gases affecting climate and atmospheric chemistry in different ways, not as the primary agents breaking down stratospheric ozone. Nitrous oxide can contribute to ozone depletion through nitrogen oxide chemistry, but the dominant pollutant responsible for the characteristic polar thinning is chlorofluorocarbons.