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 (NOy) and nitrous oxide (N2O) were made up to 29 km over Kiruna, Sweden (68 degrees N, 21 degrees E) on February 10 and 25, 1997. Kiruna was located at the edge or inside of the Arctic vortex at potential temperatures between 475 (similar to 19 km) and 675 K (similar to 26 km). Below 500 K (similar to 21 km) the N2O values were >120 ppbv on both days, and the observed NOy mixing ratios agreed well with those calculated using the NOy-N2O correlation previously obtained at northern midlatitudes. An exception was a sharp dip in NO 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 (similar to 24 km) the N2O values were <120 ppbv, and the observed NO values were some 4-6 ppbv lower than those calculated using the midlatitude NOy-N2O correlation, which includes the NOy 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) diabetic descent and (2) quasi-horizontal mixing of vortex air are likely causes of the anomalous NOy-N2O correlation. The CH4-N2O correlation obtained inside the Arctic vortex in February 1997 also supports this hypothesis. A similar anomalous NOy-N2O 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.