Executive Summary

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 mgm-3 (21 ppb).
    
  • 1-hour limit value of 200 mgm-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. National measures are likely to deliver significant reductions in the emissions of oxides of nitrogen (NO_x, the sum of NO and NO₂) from road traffic sources between now and 2005. 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. This 'policy gap' can 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.
  
  Method
  
  This report describes an empirical model for annual mean roadside NO₂ concentrations, which has been used to estimate concentrations for 1996, 2005 and 2009. 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 NO_x concentrations, to which the roadside enhancement is added. 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.
  
  Current policies are likely to deliver a reduction of urban road traffic NOx emissions to about 50% of 1996 levels by 2005 and annual mean roadside concentrations of NO_x are therefore likely to fall to approximately 50% of 1996 levels. Annual mean roadside NO₂ concentrations are likely to see a less dramatic reduction, because current NO₂ concentrations are limited by the availability of oxidant. We have assumed that annual mean NO₂ concentrations will fall to about 70% of 1996 levels by 2005.
  
  Results
  
  Annual mean urban background NO₂ concentrations in 2005 are likely to be below 21 ppb in all areas except for inner London, where current national policies are expected to lead to concentrations in the range 21 - 29 ppb.
  
  Roadside NO₂ concentrations in urban areas in 2005 are expected to be significantly higher, with a total of 761 road links likely to have concentrations higher than the annual mean objective of 21 ppb. This represents about 10% of the total number of urban major road links with a total length 670 km. The majority of these links (495, representing 387 km) are in the Greater London area. The remainder are generally confined to the most heavily trafficked roads in other big cities.
  
  Annual mean roadside NO₂ concentrations have also been predicted for 2009, for comparison with the European Union 'Daughter' Directive limit value, which is also 21 ppb. The continuing downward trend in traffic emissions is likely to further reduce the number of links exceeding this value by 2009, only about 2% of urban major road links are predicted to have concentrations higher than 21 ppb.
  
  Roadside NO₂ concentrations in 2005 and 2009 have also been estimated for several alternative emission reduction scenarios. Many fewer road links are expected to have concentrations greater than 21 ppb if additional NOx emission reductions of the order of 30% compared with business as usual can be achieved.
  
  The objectives for 2005 within the NAQS (DoE, 1997) apply to non-occupational near-ground level outdoor locations where a person might reasonably be expected to be exposed over the relevant averaging period. The annual mean NO₂ objective of 21 ppb therefore only applies at the facade of buildings where people actually live. Many of the road links in cities other than London with estimated concentrations greater than 21 ppb are unlikely to have people living close to the road, although there are a number of roads with houses present.