Which among the following is not a "secondary air pollutant"?

Among these options, carbon monoxide is not a secondary air pollutant. 

Carbon monoxide is a primary pollutant that is emitted directly into the atmosphere from sources like cars and industrial processes. It is a toxic gas that can cause health problems, including headaches, dizziness, and nausea, when inhaled in high concentrations.

In contrast, PANs, ozone, and secondary organic aerosol are all secondary air pollutants. PANs are formed when NOx and volatile organic compounds (VOCs) react in the atmosphere. Ozone is formed when NOx and VOCs react in the presence of sunlight. Secondary organic aerosol is formed when VOCs react with other substances in the atmosphere, such as nitrogen oxides and ozone.

Reducing emissions of primary pollutants like carbon monoxide, NOx, and VOCs can help reduce the formation of secondary air pollutants and improve air quality.