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

摘要

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 NO2 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 3-5) 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/CO_h 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 1/2) were also found to be higher than emission inventory estimates - 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.