Ozone time scale decomposition and trend assessment from surface observations in Switzerland

Regulation for ozone (O3) precursor emissions began in Europe around 1990 with control measures primarily targeting industries and traffic. To understand how these measures have affected air quality, it is important to investigate the temporal evolution of tropospheric O3 concentrations in different types of environments. In this study, we analyze long-term trends of the concentrations of O3 and the sum of oxidants Ox (O3 + NO2) in Switzerland for the last 25 years. Statistical decomposition of the observed time series is used to extract the underlying time scales, i.e. the long-term, seasonal and short-term variability. This allows subtraction of the seasonal variation of O3 and Ox from the observations and estimation of long-term changes of de-seasonalized O3 and Ox with reduced uncertainties. A two-regime trend calculation based on the long-term variability accounts for non-monotonic temporal evolution of O3. In addition, adjustment of the higher frequency meteorological influence was applied, based on the time series containing the short-term variability. This led to an uncertainty reduction in the trend estimation, but only by a small factor. We observe that, despite the implementation of regulations and reduction of nitrogen oxides concentrations, for all studied sites daily mean O3 values increased until mid-2000s. Afterwards, a decline or a leveling-off in the concentrations is observed. The start of change in the trend depends on the site type; the more polluted the site, the later is the onset of the change in trend behavior. At locations close to sources, the observed trend can mainly be explained by the reduced titration of O3 by NO due to the strong reductions in nitrogen oxides emissions. At remote locations (such as the high alpine station in Jungfraujoch) that are influenced by hemispheric transport of O3 an increase during 1990s and a decline after early 2000s is observed. The calculated temporal trends exhibit distinct differences depending on the characteristics and pollution burdens of the measurement sites; such differences have become smaller following emission reductions.