The diminishing importance of nitrogen dioxide emissions from road vehicle exhaust

David C Carslaw,William S Drysdale,Stuart Young,Naomi J Farren,Adam R Vaughan,James D Lee

doi:10.1016/j.aeaoa.2018.100002

David C Carslaw, William S Drysdale + Show 4 more

Open Access

https://doi.org/10.1016/j.aeaoa.2018.100002

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Abstract

The direct emission of nitrogen dioxide (NO2) from road vehicle exhaust has been an important contributor to near-road ambient concentrations of NO2 in many European cities. Diesel vehicles and their use of emission control technologies such as Diesel Oxidation Catalysts, have dominated the emission of NO2 from road vehicles. In this work, we summarise findings from recent vehicle emission remote sensing measurements in the UK that provide detailed information on the emissions of NO2 and total NOx(NO2 + NO). We show that while new diesel cars and light commercial vehicles are associated with high (typically 30%) proportions of NO2/NOx, the amount of absolute NOx and NO2 emitted by most Euro 6 vehicles has decreased substantially and that absolute emissions of NO2 have been reducing since around 2007. Additionally, we find that the amount of NO2 decreases as the vehicle mileage increases. Taken together, these factors have led to substantial reductions in emissions of NO2 in recent years from light duty diesel vehicles, which has contributed to reduced roadside NO2 concentrations. There is a need however for commonly used emission factor models to account for these changes in emissions of NO2.

Highlights

IntroductionIn Europe there remains considerable concern over the ambient concentrations of NO2, which in many locations exceed European Directive limit values
This paper aims to better understand the recent evidence from comprehensive vehicle emission remote sensing measurements relating to the emission of NOx and NO2
It is apparent for example, that vehicles fitted with Selective Catalytic Reduction (SCR) technology have much lower emissions of NOx compared with those using Lean NOx Traps (LNT) i.e. by a factor of about three

Summary

Introduction

In Europe there remains considerable concern over the ambient concentrations of NO2, which in many locations exceed European Directive limit values. Exceedances of the annual mean NO2 limit value of 40 μg m-3 is the principal concern. Locations where the 40 μg m-3 limit are exceeded tend to be restricted to the near-road environment. It is these locations where directly emitted (primary) NO2 emissions from vehicles has greatest impact. The extent to which primary NO2 emissions affect roadside concentrations can vary considerably depending on the nature of the vehicle fleet and the absolute concentrations of NOx and NO2

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