The global chemical transport model Oslo CTM2, driven by European Centre for Medium-Range Weather Forecasts international Integrated Forecast System meteorology and including tropospheric and stratospheric chemistry, is described and evaluated against measurements by satellite-based instruments, ozone sondes, and aircraft. A new heterogeneous chemistry and microphysics scheme capable of reproducing severe ozone depletion events is described. The Oslo CTM2 simulates ozone measurements well, overestimating by less than 20% in the lowermost stratosphere, while underestimating slightly at higher altitudes. In the upper troposphere, the spatial and temporal variation is well simulated, although a 30–50% overestimation is seen. In order to demonstrate ozone depletion in the Arctic by heterogeneous processes, the ozone loss during the cold 2004/2005 Arctic winter stratosphere is compared to the warm 2000/2001 Arctic winter. Enhanced ozone loss is shown to have occurred at eight Arctic stations. From January to March the accumulated ozone loss amounts to up to 1 ppmv, with a maximum ozone loss rate of 32 ppb/d. Ozone sonde measurements are well reproduced for both years.