NOy‐N₂O correlation observed inside the Arctic vortex in February 1997: Dynamical and chemical effects

Simultaneous balloon‐borne in situ measurements of total reactive nitrogen (NO y ) and nitrous oxide (N 2 O) were made up to 29 km over Kiruna, Sweden (68°N, 21°) on February 10 and 25, 1997. Kiruna was located at the edge or inside of the Arctic vortex at potential temperatures between 475 (∼19 km) and 675 K (∼26 km). Below 500 K (∼21 km) the N 2 O values were >120 ppbv on both days, and the observed NO y mixing ratios agreed well with those calculated using the NO y ‐N 2 O correlation previously obtained at northern midlatitudes. An exception was a sharp dip in NO y at 445 K (18.4 km) observed on February 25. Back trajectory analyses indicate that this layer had experienced cold temperatures close to ice saturation, i.e., favorable conditions for denitrification. Between 500 and 600 K (∼24 km) the N 2 O values were <120 ppbv, and the observed NO y values were some 4–6 ppbv lower than those calculated using the midlatitude NO y ‐N 2 O correlation, which includes the NO y reduction due to the N + NO reaction. The temperatures in the Arctic winter above 550 K were too high to cause extensive denitrification. The combined processes of (1) diabatic descent and (2) quasi‐horizontal mixing of vortex air are likely causes of the anomalous NO y ‐N 2 O correlation. The CH 4 ‐N 2 O correlation obtained inside the Arctic vortex in February 1997 also supports this hypothesis. A similar anomalous NO y ‐N 2 O correlation was observed from the ER‐2 measurements and from the atmospheric trace molecule spectroscopy ATLAS 2 measurements made inside the vortex in the winters of 1991–1992 and 1992–1993.