Abstract

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$_2$.