Find the secondary pollutant among these

The correct option is PAN.

To understand why PAN is the correct answer, we need to distinguish between primary and secondary air pollutants:
1. Primary Pollutants: These are harmful substances emitted directly from a source into the atmosphere. Examples include carbon dioxide (${\mathrm{CO}}_2$), sulfur dioxide (${\mathrm{SO}}_2$), nitrogen oxides, and carbon monoxide. In the given options, ${\mathrm{SO}}_2$, ${\mathrm{CO}}_2$, and $N_{2}O$ are released directly into the air from sources like burning fossil fuels, industrial activities, and agricultural practices.
2. Secondary Pollutants: These pollutants are not emitted directly. Instead, they form in the atmosphere when primary pollutants undergo chemical reactions with other atmospheric components, often catalyzed by sunlight (photochemical reactions).

PAN stands for Peroxyacetyl Nitrate. It is a classic example of a secondary pollutant. It is formed in photochemical smog through chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight and oxygen. Since it is not directly emitted from any chimney or tailpipe but is synthesized in the air via reactions of primary emissions, it is classified as a secondary pollutant.