Long-term satellite trends of European lower-tropospheric ozone from 1996–2017

Abstract. Tropospheric ozone (O₃) is a harmful secondary atmospheric pollutant and an important greenhouse gas. Satellite records have shown conflicting long-term tropospheric ozone trends over the globe, including Europe. Here, we present an in-depth analysis of lower-tropospheric sub-column O₃ (LTCO₃, surface – 450 hPa) records from three ultraviolet (UV) sounders produced by the Rutherford Appleton Laboratory (RAL): the Global Ozone Monitoring Experiment (GOME, 1996–2010), Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY, 2003–2011) and Ozone Monitoring Instrument (OMI, 2005–2017). Overall, GOME and SCIAMACHY have negative trends of approximately -0.2 DU yr^-1 across their respective full records, while OMI indicates a negligible trend. The TOMCAT 3-D chemical transport model was used to investigate processes driving simulated trends and try to identify possible reasons for discrepancies between the satellite records. However, the model trends generally showed negligible change in LTCO₃, even when spatiotemporally co-located to the satellite level-2 swath data and convolved by averaging kernels. Model sensitivity experiments with the emissions or meteorology fixed to 2008 values aimed to isolate the impact of these processes on the simulated LTCO₃ trend. Overall, the experiments highlighted a long-term steady balance in these processes with small positive trends (<0.1 DU yr^-1) between 1996 and 2008 and then small negative trends (>-0.1 DU yr^-1) between 2008 and 2017. As a result, it is difficult to detect a robust and consistent linear trend in European lower tropospheric O₃ between 1996 and 2017, which is masked by large inter-annual variability in the model, ozonesonde and UV satellite instrument records.