LAQN Pollution Episodes

Pollution Notification released on 30/07/2012

Ozone Pollution Episode Late July 2012 Summary

Isolated ‘moderate’ ozone was measured on 22nd July. This became widespread across London and south east England on 23rd and continued until a fresher Atlantic air flow bought about an end to the episode on the 27th July. Although ozone remained ‘low’ in London on 27th ’moderate’ particulate occurred 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.

‘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. ‘Moderate’ ozone was measured across London, Kent, Essex, Sussex, Surrey and Berkshire.

These were the greatest widespread ground level ozone concentrations measured in south-east England since the July 2006 heat wave.

Unusually, ozone remained ‘moderate’ overnight between 25/26th throughout London and the south east and this pattern was repeated in coastal parts of Sussex during the night of 26/27th. Coastal areas of Sussex experienced ‘moderate’ or ‘high’ ozone from around midday on 25th right through into the 27th, leading to prolonged exposure.

Outside southeast England the episode extended as far north as Manchester and as far west as Devon in the UK. ‘High’ ozone was reported in Northampton, Bournemouth and Charlton Mackrell (South Somerset) on the 26th. 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 µgm-3) of the final readings.

Dr Gary Fuller, from King’s College London said, "This ozone episode has been caused by the action of sunlight and high temperatures on pollution emitted from vehicles and industry. These chemical reactions took place over several days as air travelled over northern France and south east England, gathering pollution, before reaching the capital. With sources spread over such a wide geographic area there is little London can do by acting alone to control this type of pollution episode. However emissions from London also affect the surrounding region and all cities must therefore take responsibility for both their local and regional air pollution impacts."

The complexity of the airflows during the episode is illustrated by the afternoon of Friday 27th. Air in London at this time had passed just north of the city on the previous evening (26th), had been over the UK before on the 23rd, and had been over Northern France and the Low Countries on 21st. In between the air had both travelled along the Channel and spent two days over the North Sea.

You can find out more about summer-time smog in our information film at:

Health advice for moderate ozone can be found at:

For more information please visit and also our dedicated web pages for the 2012 Games at

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