Which of the following is NOT a primary air pollutant?

Ozone at ground level is a secondary pollutant because it forms in the air from reactions of other pollutants rather than being emitted directly.

Carbon monoxide is a primary pollutant emitted straight from incomplete combustion.

Nitric oxide is a primary pollutant released directly by high-temperature burning.

Sulphur dioxide is a primary pollutant discharged directly from burning sulphur-rich fuels.

Ground-level ozone is produced when nitrogen oxides and volatile organics react under sunlight.

Primary pollutants enter the air directly, while secondary ones are made within the atmosphere.