Reactive nitrogen species HNO3, ClONO2, NO, NO2, and N2O5 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 NOy deficit and partitioning are investigated here. Most of the available NOy in the polar vortex was in the form of HNO3 and NOx in the lower stratosphere except for the period 22--27 September when NOy was mostly in the form of HNO3 and ClONO2 between the 400 K and 475 K levels. The dominant process throughout the lower stratosphere was enhanced photolysis of HNO3 resulting in a steady increase of NOx 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 HNO3 after this period in mid-October following the vortex repositioning on the pole. N2O5 inside the vortex increased above the 625 K level during the 22--27 September period following the enhancement of NOx from HNO3 photolysis. On the 475 K level, the NOy 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 NOy 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 4>:2O> vortex scatterplots to estimate the mixing induced NOy deficit to be 55--60% before, and 62% after, the vortex split.