Answer: Chlorofluorocarbons (CFCs).
- A Chlorofluorocarbons (CFCs)
- B Carbon dioxide according to standard textbooks
- C Methane in general practice
- D Water vapour as frequently described
Correct answer: A. Chlorofluorocarbons (CFCs)
Explanation: CFCs (chlorofluorocarbons, also called Freons) migrate to the stratosphere where short-wavelength UV breaks the C-Cl bond, releasing Cl radicals. Each Cl radical destroys approximately 10<sup>5</sup> ozone molecules in a catalytic cycle before being removed. CO<sub>2</sub> and CH<sub>4</sub> are greenhouse gases but do not deplete ozone.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Concept context
Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.