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° N, 21° E) 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 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 NOy 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 N2O values were <120 ppbv, and the observed NOy 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) diabatic 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.
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