Have vehicle emissions of primary NO\(_2\) peaked?

Abstract

Reducing ambient concentrations of nitrogen dioxide (NO\(_2\)) remains a key challenge across many European urban areas, particularly close to roads. This challenge mostly relates to the lack of reduction in emissions of oxides of nitrogen (NO\(_x\)) from diesel road vehicles relative to the reductions expected through increasingly stringent vehicle emissions legislation. However, a key component of near-road concentrations of NO\(_2\) derives from directly emitted (primary) NO\(_2\) from diesel vehicles. It is well-established that the proportion of NO\(_2\) (i.e. the NO\(_2\)/NO\(_x\) ratio) in vehicle exhaust has increased over the past decade as a result of vehicle after-treatment technologies that oxidise carbon moNO\(_x\)ide and hydrocarbons and generate NO\(_2\) to aid the emissions control of diesel particulate. In this work we bring together an analysis of ambient NO\(_x\) and NO\(_2\) measurements with comprehensive vehicle emission remote sensing data obtained in London to better understand recent trends in the NO\(_2\)/NO\(_x\) ratio from road vehicles. We show that there is evidence that NO\(_2\) concentrations have decreased since around 2010 despite less evidence of a reduction in total NO\(_x\). The decrease is shown to be driven by relatively large reductions in the amount of NO\(_2\) directly emitted by vehicles; from around 25 vol% in 2010 to 15 vol% in 2014 in inner London, for example. The analysis of NO\(_x\) and NO\(_2\) vehicle emission remote sensing data shows that these reductions have been mostly driven by reduced NO\(_2\)/NO\(_x\) emission ratios from heavy duty vehicles and buses rather than light duty vehicles. However, there is also evidence from the analysis of Euro 4 and 5 diesel passenger cars that as vehicles age the NO\(_2\)/NO\(_x\) ratio decreases. For example the NO\(_2\)/NO\(_x\) ratio decreased from 29.5 ± 2.0% in Euro 5 diesel cars up to one year old to 22.7 ± 2.5% for four-year old vehicles. At some roadside locations the reductions in primary NO\(_2\) have had a large effect on reducing both the annual mean and number of hourly exceedances of the European Limit Values of NO\(_2\).

Have vehicle emissions of primary NO\(_2\) peaked?

D.C. Carslaw, T.P. Murrells, J. Andersson, M. Keenan

Faraday Discussions, 2016

Reducing ambient concentrations of nitrogen dioxide (NO\(_2\)) remains a key challenge across many European urban areas, particularly close to roads. This challenge mostly relates to the lack of reduction in emissions of oxides of nitrogen (NO\(_x\)) from diesel road vehicles relative to the reductions expected through increasingly stringent vehicle emissions legislation. However, a key component of near-road concentrations of NO\(_2\) derives from directly emitted (primary) NO\(_2\) from diesel vehicles. It is well-established that the proportion of NO\(_2\) (i.e. the NO\(_2\)/NO\(_x\) ratio) in vehicle exhaust has increased over the past decade as a result of vehicle after-treatment technologies that oxidise carbon moNO\(_x\)ide and hydrocarbons and generate NO\(_2\) to aid the emissions control of diesel particulate. In this work we bring together an analysis of ambient NO\(_x\) and NO\(_2\) measurements with comprehensive vehicle emission remote sensing data obtained in London to better understand recent trends in the NO\(_2\)/NO\(_x\) ratio from road vehicles. We show that there is evidence that NO\(_2\) concentrations have decreased since around 2010 despite less evidence of a reduction in total NO\(_x\). The decrease is shown to be driven by relatively large reductions in the amount of NO\(_2\) directly emitted by vehicles; from around 25 vol% in 2010 to 15 vol% in 2014 in inner London, for example. The analysis of NO\(_x\) and NO\(_2\) vehicle emission remote sensing data shows that these reductions have been mostly driven by reduced NO\(_2\)/NO\(_x\) emission ratios from heavy duty vehicles and buses rather than light duty vehicles. However, there is also evidence from the analysis of Euro 4 and 5 diesel passenger cars that as vehicles age the NO\(_2\)/NO\(_x\) ratio decreases. For example the NO\(_2\)/NO\(_x\) ratio decreased from 29.5 ± 2.0% in Euro 5 diesel cars up to one year old to 22.7 ± 2.5% for four-year old vehicles. At some roadside locations the reductions in primary NO\(_2\) have had a large effect on reducing both the annual mean and number of hourly exceedances of the European Limit Values of NO\(_2\).

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