Which one of the following is not a primary pollutant?

مندرجہ ذیل میں سے کون سا بنیادی آلودگی نہیں ہے؟

NO₂

SO₄

None of these

Primary pollutants are directly emitted from sources like vehicles and industries (e.g., NO₂, CO, SO₃).

SO₄ (Sulfate) is a secondary pollutant, formed through chemical reactions in the atmosphere.