Nocturnal secondary maximum of atmospheric ozone in urban areas during stable atmospheric weather conditions has been reported by several authors. As no O3 is produced during the night the nocturnal ozone maximum is explained as an effect of vertical mixing of high concentrations of ozone from higher levels or horizontal transport from rural areas through local wind systems.

In this study, nocturnal urban ozone maximum during a summer with high frequency of clear and calm weather was investigated together with meteorological data in order to describe · The magnitude, frequency and timing of nocturnal ozone maximum · The relative importance of local wind systems and vertical mixing

The Göteborg urban district, is with it’s 700 000 inhabitants, the second largest city in Sweden. The city is located on the Swedish westcoast at 57° 42´ N, 11°58´E. The climate is a marine West Coast climate associated with alternating low pressure westerly flows, blocking highs in the eastern Atlantic and high pressures over Russia. In contrast to many other parts of Europe the pollution pattern in Scandinavia is influenced by long-transported pollutants from the continent brought to Scandinavia by the dominating regional south-westerly winds.

The analysis of ozone concentrations and meteorological data was focused on four summer months, May, June, July and August in 1994. Anticyclonic weather conditions prevailed during this period. Ozone data was collected from one urban and one rural meteorological station and meteorological data was gathered from another eight meteorological stations within the area. From the database all nights with a mean wind speed of maximum 2 ms-1 and maximum cloud cover of 2 octas was selected. Altogether 34 such clear and calm nights were identified. The diurnal ozone concentrations for a 24 hour period during these nights were analysed and those with an ozone increase of 10 µgm-3 or higher was selected. Meteorological data for these 34 nights were analysed with focus on the horizontal (and vertical) wind field, and air stability.

Nocturnal urban ozone maximum occurred at 62% of the clear and calm nights in the summer of 1994. The preliminary results showed that the ozone peak was observed both early and late in the night and at some nights two peaks were recorded. The timing of the nocturnal ozone maximum was related to the air stability and the interpretation was that both horizontal and vertical transport of ozone is important.