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A primary air pollutant is directly from the source and secondary pollutants are omitted from two or more different primary pollutants interact with each other in the atmosphere.
EX of primary: carbon monoxide, nitrogen oxide, sulfur oxid
EX of secondary: NO2 acid rain
Think of it this way:
DNA inside cells store all the genetic information on a person's looks, eye colour, hair colour etc. It makes them unique. The analogy of being like books in a library basically suggests that in a library there is so much information and every book is different with different knowledge... That's basically what DNA is; millions of different information things.