In this paper we investigate the evolution of the northern polar vortex during the winter 2002–2003 in the lower stratosphere by using assimilated fields of ozone (O3) and nitrous oxide (N2O). Both O3 and N2O used in this study are obtained from the Sub-Millimetre Radiometer (SMR) aboard the Odin satellite and are assimilated into the global three-dimensional chemistry transport model of Météo-France, MOCAGE. O3 is assimilated into the ‘full’ model including both advection and chemistry whereas N2O is only assimilated with advection since it is characterized by good chemical stability in the lower stratosphere. We show the ability of the assimilated N2O field to localize the edge of the polar vortex. The results are compared to the use of the maximum gradient of modified potential vorticity as a vortex edge criterion. The O3 assimilated field serves to evaluate the ozone evolution and to deduce the ozone depletion inside the vortex. The chemical ozone loss is estimated using the vortex-average technique. The N2O assimilated field is also used to substract out the effect of subsidence in order to extract the actual chemical ozone loss. Results show that the chemical ozone loss is 1.1 ± 0.3 ppmv on the 25 ppbv N2O level between mid-November and mid-January, and 0.9 ± 0.2 ppmv on the 50 ppbv N2O level between mid-November and the end of January. A linear fit over the same periods gives a chemical ozone loss rate of ∼18 ppbv day−1 and ∼9.3 ppbv day−1 on the 25 ppbv and 50 ppbv N2O levels, respectively. The vortex-averaged ozone loss profile from the O3 assimilated field shows a maximum of 0.98 ppmv at 475 K. Comparisons to other results reported by different authors using different techniques and different observations give satisfactory results. Copyright © 2008 Royal Meteorological Society