A3.3 Characteristics of Pollutants


A3.3.1       Primary & Secondary Pollutants

Pollutants may be classified as either primary or secondary.

Primary:          Emitted directly from a process.

  • E.g.      ash from a volcanic eruption.
  • E.g.      carbon monoxide gas from a vehicle exhaust.


Secondary:    Formed in the air when pollutants react with each other, but are not directly emitted.

  • E.g.      Ground Level Ozone, which is formed by a chemical reaction between oxides of nitrogen and VOCs in the presence of sunlight.


Some pollutants are present as both Primary and Secondary Pollutants. This is possible when they are emitted directly and formed through a chemical reaction in the air.

A3.3.2       Physical Nature of Pollutants

Pollutants may occur in a number of physical forms. For example:

  • Gas
    • E.g. Carbon Monoxide
    • Liquid
      • E.g. Leachate from a landfill site
      • Solid
        • E.g. Waste
      • Energy
        • E.g. Heat