Air pollutants are divided into primary pollutants and secondary pollutants. After being discharged into the atmosphere from the pollution source, direct air pollution is called primary pollutants, mainly including sulfur dioxide, carbon monoxide, nitrogen oxides, nitrogen dioxide, particulate matter (floating dust, falling dust, oil fume, etc.), nitrogen, oxygen-containing, nitrogen, chlorine, sulfur organic compounds and radioactive substances. Secondary pollutants are pollutants exposed to sunlight, The primary pollutants discharged into the environment change under the action of physical, chemical or biological factors, or react with other substances in the environment to form new pollutants with different physical and chemical properties from the primary pollutants.
According to the above definition, particulate matter is solid and liquid particles floating in the atmosphere does not mention secondary pollutants.