GATE 2021 ES – Question 47
Which are primary air pollutants?
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Correct answer: (A) CO; (B) SO₂; (D) NO₂
Explanation
CO and SO₂ are emitted directly from combustion, and NO₂ also has a directly emitted fraction, so **A, B, D**. Tropospheric ozone is primarily secondary, formed photochemically. A substance can have both primary emissions and secondary production, as NO₂ does.