The diminishing importance of nitrogen dioxide emissions from road vehicle exhaust
The diminishing importance of nitrogen dioxide emissions from road vehicle exhaust
Atmospheric Environment: X, 2019
The direct emission of nitrogen dioxide (NO\(_2\)) from road vehicle exhaust has been an important contributor to near-road ambient concentrations of NO\(_2\) in many European cities. Diesel vehicles and their use of emission control technologies such as Diesel Oxidation Catalysts, have dominated the emission of NO\(_2\) 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 NO\(_2\) and total NO\(_x\) (NO\(_2\) + NO). We show that while new diesel cars and light commercial vehicles are associated with high (typically 30%) proportions of NO\(_2\)/NO\(_x\), the amount of absolute NO\(_x\) and NO\(_2\) emitted by most Euro 6 vehicles has decreased substantially and that absolute emissions of NO\(_2\) have been reducing since around 2007. Additionally, we find that the amount of NO\(_2\) decreases as the vehicle mileage increases. Taken together, these factors have led to substantial reductions in emissions of NO\(_2\) in recent years from light duty diesel vehicles, which has contributed to reduced roadside NO\(_2\) concentrations. There is a need however for commonly used emission factor models to account for these changes in emissions of NO\(_2\).