Which of the followings is a secondary pollutant in the troposphere ?

Ground-level ozone is a secondary pollutant, formed in the troposphere rather than emitted directly.

It forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds.

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

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

Nitrogen dioxide is largely emitted directly and acts as a primary pollutant.

Tropospheric ozone harms the lungs and damages crops, unlike the protective ozone high above.