Out of $S{O}_2,CO,O_3,NO$ and Peroxy acetyl nitrate (PAN) primary and secondary pollutants respectively are

         

Objectional materials that are emitted directly into the atmosphere by identifiable sources are called primary pollutants as  $S{O}_2,S{O}_3,CO,NO$.

Secondary pollutants are those which are formed in the atmosphere as a result of some chemical reactions between matter already present $O_{3,}HCHO,PAN$.