Answer: Sulphur dioxide (SO 2 ).
- A Ozone (O<sub>3</sub>)
- B Peroxyacetyl nitrate (PAN)
- C Sulphur dioxide (SO<sub>2</sub>)
- D Sulphuric acid aerosol
Correct answer: C. Sulphur dioxide (SO<sub>2</sub>)
Explanation: Primary pollutants are emitted directly from identifiable sources. SO<sub>2</sub> is directly released from burning sulphur-containing coal and petroleum. Ozone, PAN, and sulphuric acid aerosol are secondary pollutants formed in the atmosphere by reactions among primary pollutants.
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.