What is the difference between a primary and secondary pollutant?
A primary pollutant is an air pollutant emitted directly from some source. A secondary pollutant is formed when primary pollutants react with other compounds in the atmosphere. Some primary pollutants include carbon monoxide (CO) and sulphur dioxide (SO2) emitted from the burning of gas and oil. Sulphuric acid is a secondary pollutant, formed from sulphur dioxide reacting with the oxygen and water vapor found in the atmosphere:
2 SO2 + O2 --> 2 SO3 (sulphur trioxide)
SO3 + H2O --> H2SO4 (sulphuric acid)