There can be two kinds of pollutants that are normally found in the air: (1) Particulate contaminants and (2) Gaseous contaminants.
Particulate pollution is pollution of an environment that consists of particles suspended in some medium. Some particles are released directly from a specific source, while others form in chemical reactions in the atmosphere.
Pollutants like carbon dioxide (CO2), carbon monoxide (CO), sulphur dioxide (SO2), nitrous oxide (NO) and nitrogen dioxide (NO2) are collectively called gaseous contaminants.