Nitrogen pollution from diesel-fuelled cars even higher than suspected

Euro 6 class diesel vehicles' nitrous oxide emissions are six times higher than allowed

smog of a carClick to enlarge
High nitrogen dioxide emissions occur most notably on cold days.
Source: Stefan Redel / Fotolia

Diesel passenger cars exceed European limits for nitrogen dioxide (NOx) on roads at higher levels than previously thought. In 2016 emissions were assumed to be 575 mg NOx/km, but the diesel-fuelled passenger car fleet in Germany actually had average emissions of 767 mg NOx/km. These findings are based on new calculations done on behalf of the German Environment Agency (UBA). The reassessment considered for the first time the emissions of vehicle engines at operating temperature, measured in all the outdoor temperature conditions typical in Germany. High NOx emissions tend to occur on cold weather days in particular. UBA's President Maria Krautzberger said: "Our new data reflect a much more realistic but unfortunately even more disagreeable situation with regard to nitrogen oxide pollution by diesel cars in Germany. More than ever before we need quick relief for the many hundred thousands of people in inner cities who are suffering from the effects of excessive diesel fumes." Nitrogen dioxide irritates the respiratory tract and impairs lung function in the long term, leading to chronic cardiovascular disease and premature death. This is particularly dangerous for sensitive segments of the population such as children.

To gain the most realistic reflection of emissions possible, the reassessment took into account not only measurements of engines at operating temperature in outdoor temperatures of more than 20°C, but also of emissions from diesel vehicles in all seasons and at all temperatures which typically occur in Germany. At temperatures below which normally occur in the laboratory (20°C - 30°C), NOx emissions increase sharply as outdoor temperature sinks. The biggest polluters in these temperature conditions are Euro 5 diesel passenger cars, which then have average emissions of 906 mg NOx/km (403% above the limit of 180 mg NOx/km). In Euro 4 vehicles, average emissions are 674 mg NOx/km (170% above the limit of 250). In modern, newly approved Euro 6 diesel cars which have not undergone the authoritative RDE (Real Driving Emissions) road test, emissions upon approval are an average of 507 mg NOx/km (534% above the limit value of 80).   

Half of all passenger car driving mileage in Germany is performed at temperatures below 10°C. The fact that exhaust purification of nitrous oxides in on-road diesel passenger cars is impaired in cold weather only came to light in all its details during the diesel scandal. The UBA has updated its publication "Handbook Emission Factors for Road Traffic (HBEFA)“ which features a systematic calculation of the impact of this bad state of affairs. It illustrates the extent to which ambient temperature influences the NOx emissions of an engine which is already at operating temperature. The influence of temperature was previously considered only in cold engines. 

The new data have no effect on current air quality but they do enable making inferences about the impact of countermeasures. Because of these considerably higher baseline figures, the reduction of emissions in new Euro 6 diesel passenger cars, which must meet additional RDE emissions standards, will decrease air pollution more sharply than previously calculated in UBA analyses. UBA's President Maria Krautzberger said: "We must have clean air in cities. It is very clear that the automotive industry must assume responsibility and deliver a solution which consumers can live with." 

Further information: 

The Handbook Emission Factors for Road Traffic (HBEFA) was first published in 1995 and has been updated on a regular basis with financial support from public authorities in Germany, France, Norway, Austria, Sweden and Switzerland. The HBEFA is used inter alia in Germany's Länder and municipalities to determine emissions from traffic. 

Measurements were taken on the test block and in real driving emissions tests to determine the level of NOx emissions in diesel passenger cars. Computer modelling can determine the emissions in all driving conditions. The updated Version 3.3 of HBEFA, which is also available to the public, includes many more measurements of vehicle emissions than the previous version, namely 27 Euro 5 diesel passenger cars and 25 Euro 6 diesel passenger cars ranging in size from small to SUVs. The new data are representative of diesel passenger car emissions in Germany. 

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