The number and severity of ozone episodes that are encountered in any particular year depends crucially on what type of weather conditions prevail over the summer. 1995 was an exceptionally hot summer - at Heathrow, for instance, there were 39 hours where a temperature of 30°C or over was recorded, compared with 11 hours in 1994 and none in 1993. These high temperatures and associated levels of sunlight led to a number of ozone episodes, and UK air pollution standards were regularly exceeded.

The summer of 1998 was significantly less sunny than 1995, hence fewer episodes of high ozone levels occurred. The major exception to this was on 10th and 11th August 1998, as illustrated in Figure 11. The origins of the episode can be seen in the trajectory plot, in which the lines indicate the history of the air mass that was over the UK on 10th August. It had travelled over N. Europe, picking up ozone-producing pollutants from the industrial areas, hence giving the elevated ozone concentrations seen in the UK.

The episode is limited in extent to South East England - as can be seen from the time-series. Ozone levels in Wales, Scotland and N. Ireland remained around normal values.

On 11th August, the air stagnated over the UK. Ozone concentrations remained high in the London area, East Anglia and Sussex, but decreased considerably over the rest of the country.

---

Site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions