Trends of surface maximum ozone concentrations in Switzerland based on meteorological adjustment for the period 1990–2014

We investigate the temporal trends of peak ozone in Switzerland for the 1990–2014 time period. The meteorological conditions have a large influence on ozone formation and drive a large part of the variability in ozone observations. Therefore, the influence of meteorology on ozone was estimated using generalized additive models and removed from the ozone observations. A variable selection method was used for model building allowing the detection of the meteorological variables that have the largest effect on the variability of daily maximum ozone at each considered station. It was found that peak concentrations of ozone have been reducing in most of the stations, indicating a positive effect of implemented air pollution control measures on locally produced ozone. In the remote, high alpine site of Jungfraujoch a small upward trend of peak ozone was observed, most likely due to influence of hemispheric background ozone. In the most polluted traffic sites, peak ozone has for a different reason also been increasing until around 2003, when this trend started to level off. In traffic sites the increasing ozone concentrations due to reduced titration by nitrogen monoxide was the dominating process. One of the advantages of meteorological correction of ozone observations for trend estimation is that the uncertainty in the calculated trends is reduced. In addition, trend estimation based on meteorologically corrected ozone is less influenced by exceptional meteorological events during a specific time period, such as heat waves or by temporal changes in meteorological variables.