Risks of exceeding the hourly EU limit value for nitrogen dioxide resulting from increased road transport emissions of primary nitrogen dioxide

In London, the recent analysis of ambient measurements has shown that directly emitted (primary) nitrogen dioxide (NO2)(NO2) has increased as a fraction of total nitrogen oxides (NOX)(NOX) from road transport sources. These increases appear to be mostly due to certain after-treatment devices, such as oxidation catalysts and particle filters fitted to diesel vehicles. This study uses a constrained simple chemical model to predict hourly concentrations of NO2NO2 at a busy roadside site in central London to investigate the dependence of peak hourly concentrations of NO2NO2 on the level of assumed primary NO2NO2 emitted by road vehicles. The model performance is shown to be good across the full range of hourly NOXNOX and NO2NO2 concentrations over seven years. Monte Carlo simulations are used to predict future hourly NO2NO2 concentrations and the probability of exceeding the EU hourly limit value for NO2NO2 by considering the model errors, uncertainties in future NOXNOX trends and the inter-annual variability of meteorology. It is shown that if the NO2/NOXNO2/NOX emission ratio of 22.0% by volume, as calculated at the end of 2004, is sustained into the future, it is likely that the hourly EU limit value will not be met. However, the probability of not meeting the limit value in 2010 depends strongly on the meteorological year and varies from 16% to 88% depending on the year considered. This work shows that further increases in the NO2/NOXNO2/NOX ratio beyond those observed at the end of 2004 would considerably increase the probability of the EU hourly limit for NO2NO2 being exceeded. Additionally, the analysis of road network emissions in London shows that many other roads are likely to be at risk from exceeding the hourly limit value. Further work is required to improve the quantification of NO2NO2 in vehicle exhausts to determine the likely future risks of exceeding the hourly limit value in other European cities.