In London, the recent analysis of ambient measurements has shown that directly emitted (primary) nitrogen dioxide
(NO2) has increased as a fraction of total nitrogen oxides
(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
NO2 at a busy roadside site in central London to investigate the dependence of peak hourly concentrations of
NO2 on the level of assumed primary
NO2 emitted by road vehicles. The model performance is shown to be good across the full range of hourly
NOX and
NO2 concentrations over seven years. Monte Carlo simulations are used to predict future hourly
NO2 concentrations and the probability of exceeding the EU hourly limit value for
NO2 by considering the model errors, uncertainties in future
NOX trends and the inter-annual variability of meteorology. It is shown that if the
NO2/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/NOX ratio beyond those observed at the end of 2004 would considerably increase the probability of the EU hourly limit for
NO2 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
NO2 in vehicle
exhausts to determine the likely future risks of exceeding the hourly limit value in other European cities.
