Which of the following is a primary air pollutant?
Nitric oxide is emitted directly during high-temperature combustion when atmospheric nitrogen and oxygen react, making it a primary pollutant. Once released, it is oxidised to nitrogen dioxide and participates in photochemical cycles. Ozone and peroxyacyl nitrate, including PAN, arise through atmospheric reactions involving nitrogen oxides, volatile organic compounds, and sunlight, so they are secondary pollutants. Exposure depends on concentration, duration, ventilation, weather, and individual susceptibility, so a pollutant's presence alone does not fully predict biological harm. Atmospheric
Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology