Weekend-weekday differences of near-surface ozone concentrations in Switzerland for different meteorological conditions

Mean weekly cycles of daily ozone peak levels are extracted out of 8 yr data sets under selected meteorological conditions. In the region under study, the emissions of precursor substances are considerably lower on weekends than on weekdays, as can be derived from the Swiss emission inventory or from traffic frequencies. Chemical production as well as destruction of ozone are affected differently by sudden changes in emissions, depending on meteorology and on the structure of the emissions. Therefore, in Switzerland, several distinct patterns of the weekly cycle of mean daily ozone peak concentrations can be detected. When meteorology is not favourable to ozone production, weekends show generally higher ozone peaks than weekdays. Favourable meteorology (i.e. high solar radiation, high temperatures, low wind speeds) produces an inverse pattern, the mean ozone peaks being 10–15% lower on Sundays than on Thursdays or Fridays. Differences in emission structures slightly modify the patterns and can delay the effects. Threshold values to separate favourable conditions can be estimated for radiation and temperature. In Switzerland, “favourable” meteorology is achieved on about 30–50 days per year, corresponding well with “summer smog days”.