nitrogen oxide vs pollutant

nitrogen oxide and pollutant both are nouns.

Word	Noun	Adjective	Verb	Adverb
nitrogen oxide	Yes	No	No	No
pollutant	Yes	No	No	No

As nouns, pollutant is a hypernym of nitrogen oxide; that is, pollutant is a word with a broader meaning than nitrogen oxide:

nitrogen oxide: any of several oxides of nitrogen formed by the action of nitric acid on oxidizable materials; present in car exhausts
pollutant: waste matter that contaminates the water or air or soil

Other hypernyms of nitrogen oxide include oxide.

nitrogen oxide (noun)	pollutant (noun)
any of several oxides of nitrogen formed by the action of nitric acid on oxidizable materials; present in car exhausts	waste matter that contaminates the water or air or soil