1 Introduction

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

• Annual mean:The annual mean must not exceed 21 ppb.
  
• Hourly mean: The hourly mean must not exceed 150 ppb.
  
• 
  The European Union (EU) 'Daughter Directive' gives the following limit values for NO2 to be achieved by 1 January 2010:
  
  
  • Annual mean limit value of 40 µgm-3 (21 ppb).
    
  • 1-hour limit value of 200 µgm-3 (104.6 ppb), not to be exceeded more than 18 times a year.
    
  
  This report focuses on the annual mean objective and annual mean limit value, since these are likely to be the most stringent of the targets, particularly at the roadside. Projections of annual mean concentrations of NO2 have therefore been calculated for both 2005 and 2009 for comparison with the NAQS objective and EU limit value.
  
  Concentrations of both NO2 and oxides of nitrogen (NOx, 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 are currently higher than 21 ppb at the majority of urban background sites and almost all of the roadside or kerbside monitoring sites. Emissions of NOx 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 NOx emissions between now and 2005 (Murrells, pers comm). It is likely, however, that there will still be significant exeedances of the annual mean objective in 2005 if national measures are considered in isolation, particularly at the roadside. Concentrations of NO2 in some locations may also still be higher than 21 ppb in 2009. It is envisaged that this 'policy gap' will be addressed by implementing local air quality management to reduce concentrations in locations that are at risk of exceeding the objective. One of the first stages in this process is to identify those areas that are most at risk of exceeding the objective annual mean NO2 concentration in 2005.
  
  Automatic monitoring sites within the UK national monitoring networks are classified according to site type and this classification distinguishes between kerbside (within 1 m of the road) and roadside (1 - 5 m). In this report we have considered all locations within 5 m of a road as roadside. This enables a larger number of monitoring sites to be considered as a single group and our analysis does not show any systematic difference between these two site types.
  
  An empirical model for estimating annual mean roadside NO2 concentrations has been developed and used to estimate concentrations for 1996, 2005 and 2009. The expected changes in urban road traffic emissions of NOx are discussed in Section 2 and the methods that are used to predict NO2 concentrations from NOx concentrations are discussed in Section 3. Concentrations within roughly 5 m of a major road have been considered as the sum of two components:
  
  roadside concentration = background concentration + 'roadside enhancement'
  
  The model builds on previously published maps of annual mean background NO2 and NOx concentrations, to which the roadside enhancement is added. Section 4 describes the derivation of the maps of background concentrations. Section 5 shows how the relationship between the roadside enhancement of NOx concentrations and individual road link NOx emissions has been derived from an analysis of automatic monitoring data for roadside sites. The roadside enhancement is then added to background concentrations to produce estimates of roadside concentrations for individual road links for 1996, 2005 and 2009.
  
  The roadside NO2 concentrations that are likely to result from a range of alternative traffic emissions reduction scenarios are listed in Section 6. A preliminary investigation of the likely presence of housing at the roadside of the roads with the highest predicted concentrations is described in Section 7.