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

Observing stratospheric ozone is essential to assess if the Montreal Protocol has succeeded to save the ozone layer by banning ozone depleting substances. Recent studies have reported positive trends indicating that ozone is recovering in the upper stratosphere at 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 ozone recovery. The present study investigates how anomalies, temporal measurement sampling rates 5 and trend period lengths influence resulting trends. We present an approach for handling suspicious anomalies in trend estimations to improve the derived trend profiles. The approach was applied to data from a Ground-based Millimetre-wave Ozone Spectrometer (GROMOS) located in Bern, Switzerland. We compare our improved GROMOS trend estimate with 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 ozone recovery in the upper stratosphere 10 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 ozone trends. Our presented method of a combined analysis of ground station data provides a useful approach to recognize and to reduce uncertainties in stratospheric ozone trends by considering anomalies in the trend estimation. We conclude that stratospheric trend estimations still need improvement and that our approach provides a tool that can also be useful for other data sets.