London experienced its worst ozone pollution episode in six years in the days leading up to the Olympic opening ceremony.

The episode began on July 22nd with some isolated 'moderate' levels of ozone recorded across London. The situation worsened as the heatwave continued into the 23rd and persisted until a change in airflow brought cleaner air in from the Atlantic on the 27th July.

‘High’ ozone was measured at several sites in London, Kent and Sussex on 25th July, and ‘high’ was again measured at one site in Sussex on the 26th. These were the greatest widespread ground level ozone concentrations measured in south-east England since the July 2006 heatwave.

Although ozone remained ‘low’ in London on 27th, ’moderate’ particulate was measured at some London sites and ‘moderate’ ozone persisted across Sussex. All sites measured ‘low’ air pollution on the 28th July, the first full day of Olympic competition. Outside southeast England, the episode extended as far north as Manchester and as far west as Devon. ‘High’ ozone was reported in Northampton, Bournemouth and Charlton Mackrell (South Somerset) on the 26th. Elevated ozone also affected northern France, Belgium, the Netherlands and western parts of Germany.

This episode provided a strong test of the predictive trigger system, introduced as part of the UK Daily Air Quality Index at the start of the year. The predictions worked well, giving four hours advance warning of ‘high’ levels at ten sites. At four other sites where ‘high’ was measured or predicted, the predictions were within half an index (10 µg/m3) of the final readings.

The full breakdown of measured values during the episode is available here.

More Information

Item date 06/09/2012