The largest scientific investigation ever mounted into Britain's air pollution is under way in London - both on the ground and in the skies above the capital.

Researchers are trying to understand the chemistry and weather patterns that can reduce air quality to potentially hazardous levels.

London is frequently in breach of European and World Health Organisation standards on air pollution - a failing made more pressing when there are thousands of athletes in the UK for the Olympic Games.

The project - known as ClearFlo - is designed to improve pollution forecasting.

The BBC's science editor David Shukman was given exclusive access to the research team as they gathered data just before the start of the Olympics.