There are various types of pollutants that keep entering the atmosphere and polluting it. At some places, concentration of certain pollutants is greater compared to the others.
A pollutant can be defined as a substance or energy introduced into the environment or ecosystem that has undesired or adverse affects to the usefulness of a resource. Some substances may not be pollutants at the start but when they come in contact with other substances, they could become one.
Based on this, air pollutants can be classified as primary and secondary pollutants.
Primary pollutants are those pollutants that get added directly to the air while secondary pollutants are formed as result of chemical and physical reactions between primary pollutants and other atmospheric components.