Recent evidence concerning higher NO$_x$ emissions from passenger cars and light duty vehicles


Ambient trends in nitrogen oxides (NO$_x$) and nitrogen dioxide (NO$_2$) for many air pollution monitoring sites in European cities have stabilised in recent years. The lack of a decrease in the concentration of NO$_x$ and in particular NO$_2$ is of concern given European air quality standards are set in law. The lack of decrease in the concentration of NO$_x$ and NO$_2$ is also in clear disagreement with emission inventory estimates and projections. This work undertakes a comprehensive analysis of recent vehicle emissions remote sensing data from seven urban locations across the UK. The large sample size of 84,269 vehicles was carefully cross-referenced to a detailed and comprehensive database of vehicle information. We find that there are significant discrepancies between current UK/European estimates of NO$_x$ emissions and those derived from the remote sensing data for several important classes of vehicle. In the case of light duty diesel vehicles it is found that NO$_x$ emissions have changed little over 20 years or so over a period when the proportion of directly emitted NO$_2$ has increased substantially. For diesel cars it is found that absolute emissions of NO$_x$ are higher across all legislative classes than suggested by UK and other European emission inventories. Moreover, the analysis shows that more recent technology diesel cars (Euro 3e5) have clear increasing NO$_x$ emissions as a function of Vehicle Specific Power, which is absent for older technology vehicles. Under higher engine loads, these newer model diesel cars have a NO$_x$/CO2 ratio twice that of older model cars, which may be related to the increased use of turbo-charging. Current emissions of NO$_x$ from early technology catalyst-equipped petrol cars (Euro 12) were also found to be higher than emission inventory estimates e and comparable with NO$_x$ emissions from diesel cars. For heavy duty vehicles, it is found that NO$_x$ emissions were relatively stable until the introduction of Euro IV technology when emissions decreased by about 30%. The more limited data available for urban buses shows that there has been little change in NO$_x$ emissions from Euro I to Euro IV. There is general much better consistency across the different estimates of heavy duty vehicle NO$_x$ emissions than for light duty vehicles.

Atmospheric Environment