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 y r data sets under selected meteorological conditions. In the region un der study, the emissions of precursor substances are considerably lowe r on weekends than on weekdays, as can be derived from the Swiss emiss ion 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 emiss ions. Therefore, in Switzerland, several distinct patterns of the week ly cycle of mean daily ozone peak concentrations can be detected. When meteorology is not favourable to ozone production, weekends show gene rally higher ozone peaks than weekdays. Favourable meteorology (i.e. h igh solar radiation, high temperatures, low wind speeds) produces an i nverse pattern, the mean ozone peaks being 10-15% lower on Sundays tha n on Thursdays or Fridays. Differences in emission structures slightly modify the patterns and can delay the effects. Threshold values to se parate favourable conditions can be estimated for radiation and temper ature. In Switzerland, ''favourable'' meteorology is achieved on about 30-50 days per year, corresponding well with ''summer smog days''. (C ) 1997 Elsevier Science Ltd.