Which of the following is a secondary pollutant?
Tropospheric ozone is formed in air rather than emitted directly in substantial amounts, so it is classified as a secondary pollutant. Sunlight photolyses nitrogen dioxide, and the liberated oxygen atom combines with molecular oxygen to produce ozone; VOC chemistry permits its net accumulation. Carbon monoxide, nitric oxide, and sulfur dioxide are commonly released directly from combustion and are primary pollutants. Exposure depends on concentration, duration, ventilation, weather, and individual susceptibility, so a pollutant's presence alone does not fully predict biological harm. Atmospher
Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology