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

Modelling study quantifying how increasing primary NO₂ emissions from road vehicles raises the risk of exceeding the EU hourly NO₂ limit value at roadside monitoring locations, highlighting the regulatory challenge posed by direct NO₂ emissions.

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

D.C. Carslaw, S.D. Beevers, M.C. Bell

Atmospheric Environment, 2007

In London, analysis of ambient measurements shows that directly emitted (primary) nitrogen dioxide (NO₂) from road transport sources has increased, with these increases appearing to be mostly due to certain after-treatment devices, such as oxidation catalysts and particle filters fitted to diesel vehicles. A constrained simple chemical model is used to predict hourly concentrations of NO₂ at a busy roadside site in central London in order to investigate the dependence of peak hourly concentrations of NO₂ on the ratio of NO₂ emitted by road vehicles to the total emissions of NO_x. It is shown that if the NO₂/NO_x 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.

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