Early spring ozone episodes:: Occurrence and case study

Episodes of high ozone concentration in early spring are investigated with time series from different sites in Switzerland. Peaks >60 ppb occur often already in February at elevated rural sites (similar to 1000 m a.s.l.), later also in the lowland. Typically, they occur in episodes of several days during fair weather conditions. Ozone then shows a marked diurnal cycle. It is suggested that most of the peaks are due to regional scale photochemistry. A classification according to influencing parameters shows that ozone formation can be restricted by too high NOx concentrations in the lowland, while the key limitation at elevated sites is the solar radiation. A case study of a March ozone episode is presented. Data from sites in Switzerland, Austria, and Hungary show a slowly eastward moving ozone peak associated with an eastward moving ridge. The study shows that long-lasting anticyclonic conditions, which were a dominant feature in spring 1996 and 1997 in Central Europe, can promote considerable photochemical ozone formation on a regional to large scale already in early spring. (C) 1999 Elsevier Science Ltd. All rights reserved.