Between May and September a continuous decrease of ozone concentrations is observed in the lower stratosphere at high latitudes in the northern hemisphere. Low Local ozone concentrations are correlated with high concentrations of NO and NO2, and HCl. A detailed photochemical box model and a two-dimensional chemical model initialized by the Halogen Occultation Experiment (HALOE) data are used to calculate ozone destruction rates between 20 and 31 km altitude for differnet situations during the observational periods in mid and late summer. The largest ozone destruction rates are computed for ozone-rich midlatitude air masses that are transported to high latitudes reaching low Sun, but 24 hours per day sunlight conditions. It is shown that the observed summertime low ozone concentrations and much of the seasonal course of ozone is due to catalytic ozone destruction by NO and NO2, which become the main odd nitrogen compounds under these conditions.