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

A secondary pollutant is formed in the atmosphere through chemical reactions involving primary pollutants. Ozone (O3) in the troposphere is formed by photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. In contrast, carbon monoxide and sulphur dioxide are primary pollutants emitted directly from sources like combustion engines. Therefore, Ozone is the correct classification as a secondary pollutant.