Categories of Air Pollutants
From the above list you can see that the air pollutants can be broadly classified into the following two categories:
Pollutants released into the environment as a result of some natural and/or human activity, e.g., carbon dioxide, carbon monoxide, oxides of nitrogen, sulphur dioxide, suspended particulate matter and hydrocarbons.
Pollutants formed by the chemical interaction of primary pollutants with atmospheric gases and moisture, often catalysed by sunlight are known secondary pollutants e.g. ozone (O3), peroxyacyl nitrates (PAN), aldehyde sulphuric acid and nitric acid.
Figure: Major air pollutants and their Sources