NOyfrom Michelson Interferometer for Passive Atmospheric Sounding on Environmental Satellite during the Southern Hemisphere polar vortex split in September/October 2002

Reactive nitrogen species HNO 3 , ClONO 2 , NO, NO 2 , and N 2 O 5 were retrieved from high resolution atmospheric limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the European Environmental Satellite (ENVISAT) during the split of the southern polar vortex in September/October 2002. The chemical and transport processes determining the NO y deficit and partitioning are investigated here. Most of the available NO y in the polar vortex was in the form of HNO 3 and NO x in the lower stratosphere except for the period 22–27 September when NO y was mostly in the form of HNO 3 and ClONO 2 between the 400 K and 475 K levels. The dominant process throughout the lower stratosphere was enhanced photolysis of HNO 3 resulting in a steady increase of NO x during the split of vortex. The enhanced photolysis was initiated following the displacement of the vortex to low and midlatitudes. This observation was confirmed by the buildup of HNO 3 after this period in mid‐October following the vortex repositioning on the pole. N 2 O 5 inside the vortex increased above the 625 K level during the 22–27 September period following the enhancement of NO x from HNO 3 photolysis. On the 475 K level, the NO y volume mixing ratio (VMR) inside the vortex is lower than the reference value derived from its proxy early winter exvortex relation by about 12.5 ppbv during the whole period. The artificial reference linear tracer method suggests that the contribution to the NO y deficit due to quasi‐horizontal mixing and denitrification before the split of vortex is approximately 25% and 75%, respectively. After the vortex split the contribution due to mixing increased to 40–45%, while that due to denitrification decreased to 55–60%. The quasi‐isentropic mixing line approach uses [CH 4 ]:[N 2 O] vortex scatterplots to estimate the mixing induced NO y deficit to be 55–60% before, and 62% after, the vortex split.