An empirical model for predicting urban roadside nitrogen dioxide concentrations in the UK

An annual mean concentration of 40 mug m(-3) has been proposed as a limit v alue within the European Union Air Quality Directives and as a provisional objective within the UK National Air Quality Strategy for 2010 and 2005, re spectively. Emissions reduction measures resulting from current national an d international policies are likely to deliver significant reductions in em issions of oxides of nitrogen from road traffic in the near future. It is l ikely that there will still be exceedances of this target value in 2005 and in 2009 if national measures are considered in isolation, particularly at the roadside. It is envisaged that this 'policy gap' will be addressed by i mplementing local air quality management to reduce concentrations in locati ons that are at risk of exceeding the objective. Maps of estimated annual m ean NO2 concentrations in both urban background and roadside locations are a valuable resource for the development of UK air quality policy and for th e identification of locations at which local air quality management measure s may be required. Maps of annual mean NO2 concentrations at both backgroun d and roadside locations for 1998 have been calculated using modelling meth ods, which make use of four mathematically straightforward, empirically der ived linear relationships. Maps of projected concentrations in 2005 and 200 9 have also been calculated using an illustrative emissions scenario. For t his emissions scenario, annual mean urban background NO2 concentrations in 2005 are likely to be below 40 mug m(-3), in all areas except for inner Lon don, where current national and international policies are expected to lead to concentrations in the range 40-41 mug m(-3). Reductions in NO, emission s between 2005 and 2009 are expected to reduce background concentrations to the extent that our modelling results indicate that 40 mug m(-3) is unlike ly to be exceeded in background locations by 2009. Roadside NO2 concentrati ons in urban areas in 2005 and 2009 are expected to be significantly higher than in background locations. 21% of urban major road links are expected t o have roadside NO2 greater than or equal to 40 mug m(-3) in 2005 for our i llustrative emissions scenario. The continuing downward trend in traffic em issions is likely to further reduce the number of links exceeding this valu e by 2009, with about 6% of urban major road links predicted to have concen trations higher than 40 mug m(-3). The majority of these links are in the L ondon area. The remaining links are generally confined to the most heavily trafficked roads in other big cities. (C) 2001 AEA Technology Plc. Publishe d by Elsevier Science Ltd. All rights reserved.