Dry air consists of about 78.08% nitrogen by volume, approximately 20.95% oxygen, 0.93% argon, and only about 0.04% carbon dioxide. Nitrogen therefore dominates the atmosphere quantitatively. Its abundance reflects the stability of molecular nitrogen, whose strong triple bond limits rapid biological or chemical removal, although nitrogen-fixing organisms make part of it biologically available. 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. The distinction is ecologically important because emission control must target either the directly released precursor or the product formed after atmospheric transport and reaction.
Ref:
Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology
