Changes of daily surface ozone maxima in Switzerland in all seasons from 1992 to 2002 and discussion of summer 2003
1Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
2Swiss Federal Laboratories for Materials Testing and Research, U¨ berlandstr. 129, CH-8600 Dübendorf, Switzerland
3Institute for Atmospheric and Climate Science, ETH-Hönggerberg, CH-8093 Zürich, Switzerland
Abstract. An Analysis of Covariance (ANCOVA) was used to derive the influence of the meteorological variability on the daily maximum ozone concentrations at 12 low-elevation sites north of the Alps in Switzerland during the four seasons in the 1992–2002 period. The afternoon temperature and the morning global radiation were the variables that accounted for most of the meteorological variability in summer and spring, while other variables that can be related to vertical mixing and dilution of primary pollutants (afternoon global radiation, wind speed, stability or day of the week) were more significant in winter. In addition, the number of days after a frontal passage was important to account for ozone build-up in summer and ozone destruction in winter. The statistical model proved to be a robust tool for reducing the impact of the meteorological variability on the ozone concentrations and further ozone trend assessment. On average the explained variance ranged from 60.7% in winter to 71.8% in autumn, and the year-to-year variability of the seasonal medians of daily ozone maxima was reduced by 85% in winter, 60% in summer, and 50% in autumn and spring after the meteorological adjustment. For most of the stations, except some in the region around Zürich, no significantly negative trends (at the 95% confidence level) of either daily O3 or Ox (O3+NO2) maxima were found in summer despite the significant reduction in the precursor emissions in Central Europe. The lower effect of the titration by NO as a consequence of the reduced emissions could partially explain the significantly positive O3 trends in the cold seasons (on average 0.68 ppb yr−1 in winter and 0.58 ppb yr−1 in autumn). The increase of Ox found for most stations in autumn (on average 0.23 ppb yr−1) and winter (on average 0.40 ppb yr−1) could be due to increasing European background ozone levels, in agreement with other studies. The statistical model was also able to explain the very high ozone concentrations in summer 2003, the warmest summer in Switzerland for at least ~150 years. On average, the measured daily ozone maximum was 15 ppb (nearly 29%) higher than in the reference period summer 1992–2002, corresponding to an excess of 5 standard deviations of the summer means of daily ozone maxima in that period.