This report summarizes the results of the air pollution monitoring carried out by the London Air Quality Network (LAQN) during 2001.

During 2001 the LAQN continued to expand with data available from an additional eleven monitoring sites.

Network annual mean concentrations have been analysed for the period November 1996 to December 2002.

• Network annual mean NO2 has declined by around 10 % overall. NO2 concentrations have been largely stable since the start of 2001 and provisional measurements for 2002 suggest a slight increase. Overall NOX concentrations have fallen by 29%.
• Network annual mean O3 shows an overall rise of around 25 %.
• Network annual mean CO, SO2 and PM10 show marked overall decreases. However, the rate of decline has slowed substantially since the start of 2000.

The report also analyses the number of PM10 exceedence days at three sites since 1994, clearly showing the effect of past episodes and the difficulty of achieving the PM10 daily mean objective at the Marylebone Road kerbside site.

Each of the pollutants monitored by the LAQN during 2001 has been compared to the Air Quality Strategy (AQS) Objectives (DETR 2000a, DETR 2000b).

• The CO objective was exceeded at the kerbside site Redbridge 2.
• The annual mean objective for NO2 was exceeded at the majority of kerbside and roadside sites. The objective was also exceeded at many inner London background sites and those in west London. Around one quarter of the area of Greater London exceeded the objective.
• The incident based objective for NO2 was exceeded at kerbside sites at Marylebone Road, Redbridge 2 and Redbridge 3. The objective was also exceeded at the roadside sites Kensington & Chelsea 3 and Brent 2.
• The O3 objective was exceeded at sixteen outer London sites.
• The PM10 incident based objective was exceeded at ten sites.
• The annual mean objective for PM10 was exceeded at the kerbside site in Marylebone Road and the roadside site Bexley 4.
• All LAQN sites met the objectives for SO2.

For the first time, the LAQN Annual Report presents measurements as mass per unit volume in addition to volume mixing ratios. To further assist air pollution modellers the report also includes annual mean measurements for NOX. Hydrocarbon measurements undertaken by the LAQN and national networks have also been included in more detail.

A measurement cannot be used with confidence unless its uncertainty can be estimated allowing it to be seen in context. Estimates of uncertainty produced by calculation and audit suggest an uncertainty of ±10 % as a good working figure to be considered when comparing measurements to AQS Objectives and EU Limit Values. This, however, does not imply a ±10 % uncertainty for those objectives that are determined by exceeding a threshold on a given number of occasions.