This refers to the types of pollutants that are either found in the atmosphere or on the ground and are emitted directly from some sources such as automobile tailpipes or smoke stacks. This is contrasted with secondary pollutants which in and of themselves are not directly emitted from a source. Instead they are created from chemical and photochemical reactions of primary pollutants. Nitrogen monoxide is an example of a primary pollutant and ozone is an example of a secondary pollutant.