Changes of daily surface ozone maxima in Switzerland in all seasons from 1992 to 2002 and discussion of summer 2003

[摘要] An Analysis of Covariance (ANCOVA) was used to derive theinfluence of the meteorological variability on the daily maximumozone concentrations at 12 low-elevation sites north of the Alpsin Switzerland during the four seasons in the 1992–2002 period.The afternoon temperature and the morning global radiation werethe variables that accounted for most of the meteorologicalvariability in summer and spring, while other variables that canbe related to vertical mixing and dilution of primary pollutants(afternoon global radiation, wind speed, stability or day of theweek) were more significant in winter. In addition, the number ofdays after a frontal passage was important to account for ozonebuild-up in summer and ozone destruction in winter. Thestatistical model proved to be a robust tool for reducing theimpact of the meteorological variability on the ozoneconcentrations. The explained variance of the model, averaged overall stations, ranged from 60.2% in winter to 71.9% inautumn. The year-to-year variability of the seasonal medians ofdaily ozone maxima was reduced by 85% in winter, 60% insummer, and 50% in autumn and spring after the meteorologicaladjustment. For most stations, no significantly negative trends(at the 95% confidence level) of the summer medians of dailyO₃ or O_x (O₃+NO₂) maxima were founddespite the significant reduction in the precursor emissions inCentral Europe. However, significant downward trends in the summer90th percentiles of daily O_x maxima wereobserved at 6 sites in the region around Zürich (on average−0.73 ppb yr^-1 for those sites). The lower effect of thetitration by NO as a consequence of the reduced emissions couldpartially explain the significantly positive O₃ trends in thecold seasons (on average 0.69 ppb yr^-1 in winter and 0.58 ppbyr^-1 in autumn). The increase of O_x found for moststations in autumn (on average 0.23 ppb yr^-1) and winter (onaverage 0.39 ppb yr^-1) could be due to increasing Europeanbackground ozone levels, in agreement with other studies. Thestatistical model was also able to explain the very high ozoneconcentrations in summer 2003, the warmest summer in Switzerlandfor at least ~150 years. On average, the measured dailyozone maximum was 15 ppb (nearly 29%) higher than in thereference period summer 1992–2002, corresponding to an excess of5 standard deviations of the summer means of daily ozone maxima inthat period.

[学科分类] 大气科学