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

Tropospheric ozone (O3) is a harmful secondary atmospheric pollutant and an important greenhouse gas. Multiple satellite records have shown conflicting long-term O3 trends across regions of the globe, including Europe. Here, we investigate 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), the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY, 2003–2011) and the Ozone Monitoring Instrument (OMI, 2005–2017). GOME and SCIAMACHY detect negative trends of approximately −0.2 DU yr−1, while OMI indicates a negligible trend. The TOMCAT 3-D chemical transport model was used to investigate processes driving simulated trends and to identify possible reasons for satellite trend discrepancies. The simulated LTCO3 trends were negligible (consistent with ozonesonde trends), even when spatiotemporally co-located to the satellite level-2 swath data and convolved by averaging kernels (i.e. a measure of the satellite retrieval vertical sensitivity). Model sensitivity experiments with the emissions or meteorology fixed to 2008 also showed negligible LTCO3 trends between 1996 and 2018, indicating that changes in emissions and meteorology had a limited impact on LTCO3 temporal evolution. Given the substantial decrease in air pollutant emissions, this was unexpected, while year-to-year variability dominated the meteorological influence on LTCO3. Finally, we find a negligible trend in the long-term stratosphere O3 flux into the free troposphere over this period arriving over Europe. Overall, our observational and modelling analysis indicates that European LTCO3 trends have been stable between 1996 and 2018.