Severe and extensive denitrification in the 1999-2000 Arctic winter stratosphere

Observations in the 1999-2000 Arctic winter stratosphere show the most seve re and extensive denitrification ever observed in the northern hemisphere. Denitrification was inferred from in situ measurements conducted during hig h-altitude aircraft flights between January and March 2000. Average removal of more than 60% of the reactive nitrogen reservoir (NOy) was observed in air masses throughout the core of the Arctic vortex. Denitrification was ob served at altitudes between 16 and 21 km, with the most severe denitrificat ion observed at 20 to 21 km. Nitrified air masses were also observed, prima rily at lower altitudes. These results show that denitrification in the Arc tic lower stratosphere can approach the severity and extent of that previou sly observed only in the Antarctic.