Particles: The main PM10 episodes 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, which 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. As a consequence of these PM10 episodes, 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.

Ozone: The O3 season during 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). The O3 Objective was exceeded at all sites except Marylebone Road.

Nitrogen Dioxide: All roadside and kerbside sites exceeded the NO2 annual mean Objective. The annual mean Objective was also exceeded at background sites in inner, north-east 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.

The annual mean concentration index increased for all pollutants during 2003. The largest increases were exhibited by NO2 (8%), PM10 (11%) and O3 (16%). These changes differ markedly from the decreasing trends in air pollution seen in London over the previous 7 years when annual mean concentrations of all pollutants, except O3, decreased during the period November 1996 to the end of 2003. The greatest reductions in annual mean concentration were exhibited by SO2 (63%) and CO (50%). A lesser reduction was achieved for PM10 (25%) and NOX (29%). Despite the 29% reduction in NOX concentration, the annual mean concentration of NO2 at the end of 2003 was only 2% below its value during November 1996