Ozone trend profiles in the stratosphere: combining ground-based data over Central Europe to consider uncertainties

Observing stratospheric ozoneis essential to assess if the Montreal Protocolhas succeeded to save the ozone layerby banning ozone depletingsubstances. Recent studies have reported positive trends indicating that ozoneis recovering in the upper stratosphereat mid-latitudes, but the trend magnitudes differ and uncertainties are still high. Trends and their uncertainties are influenced by factors such as instrumental drifts, sampling patterns, discontinuities, biases, or short-term anomalies that all might mask a potential ozonerecovery. The present study investigates how anomalies, temporal measurement sampling rates 5 and trend period lengthsinfluence resulting trends. We present an approach for handling suspicious anomalies in trend estimationsto improve the derived trend profiles. The approach was applied to data from a Ground-based Millimetre-wave OzoneSpectrometer (GROMOS) located in Bern, Switzerland. We compare our improved GROMOS trend estimatewith results from other ground stations (lidars, ozonesondes, and microwave radiometers) in Central Europe. The data indicate positive trends of 1 to 3 % per decade at an altitude of about 40 km (3 hPa), providing a confirmation of ozonerecovery in the upper stratosphere10 in agreement to satellite observations. At lower altitudes, the ground station data show inconsistent trend results, which emphasize the importance of ongoing research on lower stratospheric ozonetrends. Our presented method of a combined analysis of ground station data provides a useful approach to recognize and to reduce uncertainties in stratospheric ozonetrends by considering anomalies in the trend estimation. We conclude that stratospheric trend estimationsstill need improvement and that our approach provides a tool that can also be useful for other data sets.