The air pollution incidents experienced in London during 2003 were not repeated during 2004. As a consequence, there was a substantial improvement in air quality in London during the year.

During 2004, the annual mean AQS NO2 Objective was exceeded alongside the vast majority of major roads and also at background locations in inner and west London. Four kerbside sites also exceeded the incident-based Objective for NO2. New research has pointed to the additional influence of increases in the emission of primary NO2 that may be masking underlying reductions in the concentration of NO2 brought about by NOX reduction (Carslaw, 2005).

The PM10 pollution incidents which affected London during 2003 were not repeated during 2004. The number of days per year with mean PM10 greater than 50 µgm-3 therefore decreased substantially at background sites during 2004, returning to levels that prevailed before the pollution incidents in 2003. However, at kerb and roadside sites in inner London, the annual number of daily means greater than 50 µgm-3 did not return to pre-2003 levels. The PM10 Objective was exceeded alongside major roads during 2004 and at the two sites adjacent to waste facilities. The annual mean concentration of PM10 decreased by 30% between 1996 and 2004. However, this reduction was achieved before the turn of the century and there is evidence of an increase in the mean concentration of PM10 since 2000.

During August 2003, O3 concentrations in south east England reached their highest levels measured since 1990. During 2004, such pollution incidents were not repeated. However, the AQS Objective for O3 was exceeded throughout suburban areas and in the Home Counties. The annual mean concentration of O3 in London has risen by 33% since 1996.

All monitoring sites met the AQS Objectives for CO and SO2. Since 1996 annual mean concentrations of CO and SO2 decreased by over 50%.