During 2003, the UK experienced a series of pollution episodes that caused the Air Quality Strategy (AQS) Objectives to be exceeded throughout London and South East England.

The purpose of this preliminary report is to review air quality in London during 2003. This report is based on measurements that are subject to further ratification. As such, reported values may be subject to change.

South East England experienced a series of PM10 incidents during 2003. The incident based AQS Objective for PM10 was exceeded at kerbside and roadside sites in inner London and at several such sites in outer London. PM10 at background sites was largely below the Objective.

The main PM10 incidents were measured during February, March, April and August, with lesser incidents measured during September and November. The first 5 episodes were mainly caused by secondary PM10 from distant sources, with summer episodes also being linked to photochemistry. The November episode was associated with Guy Fawkes Night. Roadside sites measured additional PM10 from local traffic. This additional PM10 increased the roadside concentrations during PM10 incidents and caused additional incidents not measured at background sites. The incidents during 2003 reversed the established trend of improving PM10, with levels returning to those experienced during 1998.

The O3 Objective was exceeded at all sites except Marylebone Road. The O3 season in 2003 was exceptionally long and the hot summer weather caused the highest concentrations measured in the 10 year history of the London Air Quality Network (LAQN).

All roadside and kerbside sites exceeded the NO2 annual mean Objective. The annual mean Objective was also exceeded at background sites in inner and west London. Several roadside and kerbside sites exceeded the hourly mean Objective; some outer London sites exceeded the hourly mean Objective for the first time.

All sites met the CO and SO2 Objectives.

During 2003, 12 new monitoring sites joined the LAQN.