During the week leading up to the Olympic Games, London experienced its worst ozone episode for six years. Fortunately fresh westerly winds began to clear Londons air pollution just before the opening ceremony.On 21 July high pressure moved over the UK and air began to slowly circulate over southern England and the near continent, one of the most densely populated and industrial areas of Europe. Ground-level ozone slowly built up due to the action of sunlight and high temperatures on pollution emitted from vehicles and industry.

via Pollutionwatch: Londons ozone level cleared just before Olympic start | Environment | The Guardian.