From AMS Glossary
(Also known as odd nitrogen species.) Reactive forms of nitrogen comprising nitric oxide, NO, and nitrogen dioxide, NO2; usually designated by [[NOx]].
These oxides of are responsible for ozone formation in the troposphere and play a major part in ozone loss in the stratosphere. The major sources of active nitrogen in the atmosphere are combustion, soil emissions, lightning, and the reaction of nitrous oxide with excited oxygen atoms in the stratosphere.