The pollutants which come directly in the air from sources are called primary pollutants. Primary pollutants are sometimes converted into secondary pollutants. Which of the following belongs to secondary air pollutants?

1.  CO

2. Hydrocarbon

3.  Peroxyacetyl nitrate

4.  NO

Hydrocarbons present in atmosphere combine with oxyen atom produced by the protolysis of NO2 to form highly reactive intermediate called free radical. Free radical initiates a series of reaction.
Peroxyacetyl nitrates are formed, which can be said as secondary pollutants.
Hydrocarbon+ORCO*(freeradicals)
RCO*+O2RCO3
RCO3*+NO2RCO3NO2Peroxyacetylnitrate