This report summarizes the results of the air pollution monitoring carried out by the London Air Quality Network (LAQN) during 1999.

During 1999 the LAQN expanded rapidly with the addition of twenty new monitoring sites.

No single major episode occurred during 1999. The year was, however, characterized by a series of 'moderate' ozone (O3) episodes.

Rolling network annual means have been analysed for the period November 1996 to September 2000.

• The rolling annual LAQN mean for nitrogen dioxide (NO2) shows little overall change during the period November 1996 to November 1999 despite the significant reduction in the concentration of nitrogen oxides (NOx). Unratified data for the summer of 2000 suggests a recent modest overall decrease of around 7% for NO2, despite a 23% overall fall in NOx. This rate of change is less than the target rate for inner London kerbside sites but may be sufficient for the annual mean objective to be met at many background sites.
• Following the summer of 1999 the rolling annual LAQN mean for O3 peaked at over 30% above November 1996 levels. Provisional data for summer 2000 indicates a 17% overall increase.
• The rolling LAQN annual means for CO, PM10 and SO2 show significant overall decrease.

Each of the pollutants monitored by the LAQN in 1999 has been analysed in terms of their spatial distribution, and compared to the Air Quality Strategy (AQS) objectives (DETR, 2000a, DETR 2000b).

• The annual mean objective for NO2 was exceeded at an overwhelming majority of kerbside and roadside sites. The objective was exceeded at all inner London background sites and those in West London. Around one third of the area of Greater London exceeded the objective.
• The 'incident' based objective for NO2 was exceeded at Marylebone Road only.
• The CO objective was exceeded at the Redbridge 2 kerbside site only.
• All London sites met the objective for SO2.
• The O3 objective was exceeded at all Outer London sites and several in inner London.
• The PM10 'incident' based objective was exceeded at the kerbside in Marylebone Road, roadside sites in Bromley and Enfield and the suburban site in Havering.
• The annual mean objective for PM10 was exceeded at the kerbside site in Marylebone and at the roadside site in Enfield.