Summer nocturnal ozone maxima in Göteborg, Sweden

Abstract

The magnitude and frequency of nocturnal ozone maxima in a high mid-latitude city (Göteborg, Sweden) has been analysed. Nocturnal ozone maxima have been reported from cities in Europe and North America and can be explained by vertical mixing of high ozone concentrations from higher levels or horizontal transportation from rural areas through local and mesoscale wind systems. Data from four summer months (May–August) in 1994 were used to analyse the relative importance of local- and mesoscale wind systems and vertical mixing in Göteborg during clear and calm weather conditions. Results show that nocturnal ozone maxima frequently occur during these conditions, with a magnitude up to 104 μg m −3. The nocturnal ozone maxima were positively correlated to both situations with a well-developed land breeze and situations with vertical mixing. During the period investigated, in total 17 nights with secondary ozone maximum occurred. The majority of the secondary ozone maxima (80%) appeared early in the night, i.e. an ozone increase within the first 3 h after sunset and sometimes even two peaks occurred. Seven of these occasions can be explained by horizontal advection, eight by vertical mixing and five cannot by certainty be explained to be due to horizontal or vertical transportation only. During the measurement period the Swedish guideline of 80 μg m −3 (for 1-h value) was exceeded 55% of the days (i.e. 68 days, 557 h) and 33% of the nights (i.e. 41 nights, 103 h) in the central parts of Göteborg. The results thus show that in Scandinavia nocturnal ozone concentrations must be considered.