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. Atmospheric inversions can trap contaminants near the surface, while wind, turbulence, precipitation, and chemical transformation determine their eventual distribution and removal. At organismal level, respiratory surfaces are especially exposed because large volumes of air contact thin, moist epithelia designed for rapid gas exchange. Sound classification links the source, atmospheric fate, exposure route, and mechanism of toxicity rather than relying only on the pollutant's name. Effective management combines cleaner fuels, emission standards, monitoring, urban planning, and protection of populations with high exposure or physiological vulnerability.
Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology