Air pollutants that are emitted directly from a source are considered primary pollutants, as they can be released from natural ways or human action. Secondary pollutants are those that are not directly emitted from a source but form when primary pollutants react chemically in the atmosphere. There are some pollutants that are both primary and secondary; while they are emitted from a source directly, they are also made from other pollutants as well.
An example of a primary pollutant is SO2, produced by coal burning power stations. Nitrous Dioxide is a mainly a secondary air pollutant that is formed the oxidation of nitric oxide, a primary pollutant that is produced by fossil and traffic fuels released into the environment by power stations. Volcanic ash and particles are other examples of naturally released primary pollutants.
Ozone is another example of a secondary pollutant. It is formed when hydrocarbons and nitrogen oxides combine in the presence of sunlight. Secondary pollutants are harder to control because they have different ways of synthesizing; they are most often made naturally in the environment. They also cause problems like photochemical smog.
Primary pollutants in the air can cause serious environmental problems such as global warming and acid rain.