Air quality in London in 1996 follows the pattern of previous years and is characterised by air pollution episodes interspersed with lower level background conditions.

New health effects based standards for the comparison of air quality are now available to us from the work of the Government's Expert Panel on Air Quality Standards. These have formed the basis of the objectives within the Government's New Air Quality Strategy. During 1996 National Air Quality Strategy Objectives were exceeded at:

• The majority of sites measuring nitrogen dioxide.
• Sulphur dioxide monitoring sites in Greenwich, Bexley, Wandsworth and Sevenoaks.
• Kerbside and Roadside carbon monoxide monitoring sites in Bromley, Ealing and Hounslow.
• The majority of sites measuring ozone. These are chiefly located away from highly trafficked areas.

• All PM10 monitoring sites.

Changes in air quality year on year are due to a wide variety of factors that may mask underlying trends in air quality and emissions. However, a comparison between 1995 to 1996 in the context of the National Air Quality Strategy Objectives indicates that:

Lower annual averages of nitrogen dioxide were measured during 1996 but no significant overall change was observed for the number of exceedences of the objective.

Reductions in carbon monoxide, measured against the objective, were seen at all sites except London Bloomsbury.

No significant changes were observed in sulphur dioxide exceedences of the objective.

A reduction in ozone, measured against the objective, was observed at the majority of sites

Increases were observed in PM10 exceedences, measured against the objective, at all sites except Bexley.

Incidents from each of the quarters of 1996 are discussed in detail in this report.

When reviewing results from air quality monitoring it is always necessary to consider its accuracy, since this will allow confidence in its use. Uncertainties in both precision and accuracy can arise and these can be estimated by calculation or by testing. An analysis of these supports working uncertainties in the region of ±10%. Uncertainty does not translate to an equivalent uncertainty in statistics which count the number of exceedences. Although this does not justify precautionary reductions in these Standards and Guidelines it does highlight an issue which should be addressed as part of guidance for the NAQS.

The high density of particulate monitoring sites in London and the South-East has permitted a statistical analysis to help inform knowledge, as to the origin and behaviour of this pollutant. This has shown good correlations between different sites, which further confirms the importance of transboundary influnces. An additional analysis, using correlations with carbon monoxide, provides estimates of traffic related particulate for Inner and Outer London Sites.