The diminishing importance of nitrogen dioxide emissions from road vehicle exhaust

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 .