The 1997 UK National Air Quality Strategy (NAQS) gives the following provisional objectives for nitrogen dioxide (NO₂) to be achieved by the end of 2005 (DoE et al, 1997):

Annual mean: The annual mean must not exceed 21 ppb.

Hourly mean: The hourly mean must not exceed 150 ppb.

The recently published consultation document, The Air Quality Strategy for England, Scotland, Wales and Northern Ireland (DETR et al, 1999b) proposes the retention of the provisional annual mean objective and the replacement of the hourly mean objective by the following:

Hourly mean: 104.6 ppb, not to be exceeded more than 18 times a year.

The annual mean objective is likely to be the most stringent of the objectives, particularly at the roadside.

Concentrations of both NO₂ and oxides of nitrogen (NO_x, the sum of NO and NO₂) are currently monitored at a wide range of automatic monitoring sites around the UK (Broughton et al, 1998). Annual mean NO₂ concentrations in 1998 were higher than 21 ppb at about half of the UK national monitoring sites. Emissions of NO_x from road traffic sources make an important contribution to urban background NO₂ concentrations and will clearly dominate concentrations at the roadside. National measures are likely to deliver significant reductions in road traffic NO_x emissions between 1996 and 2005 (Murrells, pers comm). It is likely, however, that there will still be exceedances of the annual mean objective in 2005 if national measures are considered in isolation, particularly at the roadside. It was envisaged that this 'policy gap' would be addressed by implementing local air quality management to reduce concentrations in locations that are at risk of exceeding the objective.

Maps of estimated annual mean NO₂ concentrations for 2005 in both urban background and roadside locations have recently been prepared as part of the review of the NAQS (DETR et al, 1999a DETR et al, 1999b , Stedman et al, 1998). These maps indicate that current national policies are likely to result in annual mean urban background NO₂ concentrations in 2005 of below 21 ppb in all areas except inner London. Roadside NO₂ concentrations in urban areas in 2005 are expected to be significantly higher, with about 10% of the total number of UK urban major road links likely to have concentrations higher than the annual mean objective of 21 ppb. The majority of these links are expected to be in the Greater London area. The remainder are generally confined to the most heavily trafficked roads in other big cities. The review concluded that on the evidence available, achieving the annual mean NO₂ objective at the roadside is likely to be very challenging in London and may also be difficult in some other major conurbations. For this reason and because of the uncertainties associated with the NO₂ modelling work carried out for the review, the Government decided that the annual mean nitrogen dioxide objective should remain as a provisional objective at this stage, to be reviewed in two years time.

Site specific projections of annual mean NO₂ concentrations in 2005 for the review of the NAQS were also calculated by Stedman et al (1998) based on 1996 and 1997 monitoring results. These site specific projections have the advantage that they do not have the additional uncertainty associated with the mapping methods. The site specific projections were found to be generally consistent with those derived from mapping methods.

Site specific projections for NO₂ and NO_x for all years between 1990 and 2005 are presented here by projecting both forwards and backwards in time from 1996 and 1997 monitoring data. Projecting backwards as well as forwards provides an indication of the reliability of the prediction methods that have been used in the review of the NAQS. The inter-year changes in measured concentrations are also put into the context of changes in emissions, enabling the identification of years with unusually efficient or poor dispersion of primary pollutants. The projections also clearly illustrate the impact of emissions reductions on ambient NO_x concentrations and the correspondingly smaller changes in annual mean NO₂.

Executive Summary         Chapter 2