Recent evidence concerning higher NOx emissions from passenger cars and light duty vehicles

Ambient trends in nitrogen oxides (NOx) and nitrogen dioxide (NO2) for many air pollution monitoring sites in European cities have stabilised in recent years. The lack of a decrease in the concentration of NOx and in particular NO2 is of concern given European air quality standards are set in law. The lack of decrease in the concentration of NOx and NO2 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 NOx 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 NOx emissions have changed little over 20 years or so over a period when the proportion of directly emitted NO2 has increased substantially. For diesel cars it is found that absolute emissions of NOx 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 3–5) have clear increasing NOx 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 NOx/CO2 ratio twice that of older model cars, which may be related to the increased use of turbo-charging. Current emissions of NOx from early technology catalyst-equipped petrol cars (Euro 1/2) were also found to be higher than emission inventory estimates – and comparable with NOx emissions from diesel cars. For heavy duty vehicles, it is found that NOx 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 NOx emissions from Euro I to Euro IV. There is general much better consistency across the different estimates of heavy duty vehicle NOx emissions than for light duty vehicles.

► Trends in ambient NOx concentrations have tended to stabilise in recent years.
► Little change in light duty vehicle NOx emissions over 20 years or so.
► Increased diesel car power demand leads to increased NOx for newer vehicles.
► Older catalyst-equipped vehicles emit more NOx than previously thought.
► Important implications at a European level for meeting NO2 limits.