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. Observing stratospheric ozone is essential to assess whether the MontrealProtocol has succeeded in saving the ozone layer by banning ozone depletingsubstances. Recent studies have reported positive trends, indicating thatozone is recovering in the upper stratosphere at mid-latitudes, but the trendmagnitudes differ, and uncertainties are still high. Trends and theiruncertainties are influenced by factors such as instrumental drifts, samplingpatterns, discontinuities, biases, or short-term anomalies that may all maska potential ozone recovery. The present study investigates how anomalies,temporal measurement sampling rates, and trend period lengths influenceresulting trends. We present an approach for handling suspicious anomalies intrend estimations. For this, we analysed multiple ground-based stratosphericozone records in central Europe to identify anomalous periods in data fromthe GROund-based Millimetre-wave Ozone Spectrometer (GROMOS) located in Bern,Switzerland. The detected anomalies were then used to estimate ozone trendsfrom the GROMOS time series by considering the anomalous observations in theregression. We compare our improved GROMOS trend estimate with resultsderived from the other ground-based ozone records (lidars, ozonesondes, andmicrowave radiometers), that are all part of the Network for the Detection ofAtmospheric Composition Change (NDACC). The data indicate positive trends of1 % decade −1 to 3 % decade −1 at an altitude of about39  km (3  hPa ), providing a confirmation of ozone recovery inthe upper stratosphere in agreement with satellite observations. At loweraltitudes, the ground station data show inconsistent trend results, whichemphasize the importance of ongoing research on ozone trends in the lowerstratosphere. Our presented method of a combined analysis of ground stationdata provides a useful approach to recognize and to reduce uncertainties instratospheric ozone trends by considering anomalies in the trend estimation.We conclude that stratospheric trend estimations still need improvement andthat our approach provides a tool that can also be useful for other datasets.

Ground-based ozone profiles over central Europe: incorporating anomalous observations into the analysis of stratospheric ozone trends