Strong Temperature Dependence for Light-Duty Diesel Vehicle NO _x Emissions

Diesel-powered road vehicles are important sources for nitrogen oxide (NO _x) emissions, and the European passenger fleet is highly dieselised, which has resulted in many European roadside environments being noncompliant with legal air quality standards for nitrogen dioxide (NO₂). On the basis of vehicle emission remote sensing data for 300000 light-duty vehicles across the United Kingdom, light-duty diesel NO _x emissions were found to be highly dependent on ambient temperature with low temperatures resulting in higher NO _x emissions, i.e., a "low temperature NO _x emission penalty" was identified. This feature was not observed for gasoline-powered vehicles. Older Euro 3 to 5 diesel vehicles emitted NO _x similarly, but vehicles compliant with the latest Euro 6 emission standard emitted less NO _x than older vehicles and demonstrated less of an ambient temperature dependence. This ambient temperature dependence is overlooked in current emission inventories but is of importance from an air quality perspective. Owing to Europe's climate, a predicted average of 38% more NO _x emissions have burdened Europe when compared to temperatures encountered in laboratory test cycles. However, owing to the progressive elimination of vehicles demonstrating the most severe low temperature NO _x penalty, light-duty diesel NO _x emissions are likely to decrease more rapidly throughout Europe than currently thought.