Abbreviation for various oxides of nitrogen, including NO (nitric oxide), NO₂ (nitrogen dioxide), or N₂O (nitrous oxide). These gaseous pollutants can be transformed in the presence of water to nitric acid HNO₃, one of the acids in acid deposition. They are formed anthropogenically inside combustion engines, where the ambient nitrogen N₂ and oxygen O₂ gases in the atmosphere can react at high temperatures. They are also produced naturally by lightning and by biological activity in soils. See active nitrogen.