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[1] We present vortex-averaged calculations of ozone loss rates between late December 1999 and mid-March 2000 derived from data collected by the Radiometer for Atmospheric Measurements (RAM). The ground-based microwave radiometer RAM is operated at the Arctic station of the Network for the Detection of Stratospheric Change (NDSC) at Ny-Ålesund, Spitsbergen (79°N, 12°E). Simultaneous observations of stratospheric ozone and chlorine monoxide (ClO) profiles were performed by the RAM during winter and spring 1999/2000. Ozone was monitored continuously and ClO was observed between late February and early April. Maximum ozone loss rates of 35 ppb/d were obtained in mid-February and in the beginning of March. The ozone loss accumulated to 41% in a layer with a thickness of approximately 10 km centered at the 475 K isentropic level. Ozone loss estimates are compared to results obtained by other groups and to model calculations showing good agreement when all results are adjusted to the same vertical resolution. Simultaneously, we have observed record ClO mixing ratios in the lower stratosphere with a peak mixing ratio of 1.25 ppb on 1 March, corresponding to the strong chemical ozone loss at the same time. The ClO observations are compared to model calculations indicating that the SLIMCAT model underestimates the amount of chlorine activation at the end of the winter in Ny-Ålesund, in contrast to the good agreement between modeled and observed ozone loss.