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

An annual mean concentration of 40 μg m−3 has been proposed as a limit value within the European Union Air Quality Directives and as a provisional objective within the UK National Air Quality Strategy for 2010 and 2005, respectively. Emissions reduction measures resulting from current national and international policies are likely to deliver significant reductions in emissions of oxides of nitrogen from road traffic in the near future. It is likely 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 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 in both urban background and roadside locations are a valuable resource for the development of UK air quality policy and for the identification of locations at which local air quality management measures may be required. Maps of annual mean NO2 concentrations at both background and roadside locations for 1998 have been calculated using modelling methods, which make use of four mathematically straightforward, empirically derived linear relationships. Maps of projected concentrations in 2005 and 2009 have also been calculated using an illustrative emissions scenario. For this emissions scenario, annual mean urban background NO2 concentrations in 2005 are likely to be below 40 μg m−3, in all areas except for inner London, where current national and international policies are expected to lead to concentrations in the range 40–41 μg m−3. Reductions in NOx emissions between 2005 and 2009 are expected to reduce background concentrations to the extent that our modelling results indicate that 40 μg m−3 is unlikely to be exceeded in background locations by 2009. Roadside NO2 concentrations 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 to have roadside NO2 greater than or equal to 40 μg m−3 in 2005 for our illustrative emissions scenario. The continuing downward trend in traffic emissions is likely to further reduce the number of links exceeding this value by 2009, with about 6% of urban major road links predicted to have concentrations higher than 40 μg m−3. The majority of these links are in the London area. The remaining links are generally confined to the most heavily trafficked roads in other big cities.