The main difference between primary pollutants and secondary pollutants is that the primary pollutants are those that are directly emitted from a source and secondary pollutants are pollutants that are not directly emitted from any source, but forms when primary pollutants react chemically in the atmosphere. Primary pollutants can be the cause of severe air pollution and cause acid rains. A great example of primary pollutant is SO2 or sulfur dioxide and a good example of secondary pollution is nitrous dioxide that is formed by the oxidation of nitric oxide.