Which of the following is considered a secondary air pollutant?

Ozone at ground level is a secondary pollutant, since it is not emitted directly but forms when sunlight acts on other pollutants.

Nitric oxide is emitted straight from combustion sources and so is a primary pollutant.

Sulphur dioxide is released directly by burning sulphur bearing fuels, making it primary.

Carbon monoxide comes straight out of incomplete combustion and is likewise a primary pollutant.

Ground level ozone forms when nitrogen oxides and volatile organic compounds react in sunlight.

This photochemical ozone is a key component of urban smog and harms lungs and crops.