The Imperial College London team examined nitrogen dioxide (NO₂ ) and nitric oxide (NO) emissions, collectively known as NO_x, because of their direct link to health problems such as childhood asthma. The UK’s air pollution is thought to contribute to 40,000 premature deaths each year, and NO_x (particularly NO₂ ) plays a large part in this. Air pollution in general contributes to strokes, asthma, heart disease, chronic obstructive pulmonary disease, lung cancer, and lung infections. Across the EU, it is thought to reduce life expectancy by nine months.

Unfortunately the EURO-6 diesel cars seem to be following the pattern of EURO-4 and 5 by far exceeding type approval in real world driving. These results are concerning, particularly for people living in urban areas, as NO₂ is linked to childhood asthma among other health issues.

Higher levels of NO₂ are particularly problematic in urban areas, and we found that many of the diesel cars emitted large proportions of NO₂, directly enhancing concentrations of this toxic component at road-side locations.

—Lead author Rosalind O’Driscoll, Centre for Environmental Policy, ICL

The UK and several other European member states consistently breach the EU air quality limit value for ambient concentrations of NO₂ . This has resulted in the current court battle between the Department of Environment Food and Rural Affairs and the environmental law NGO ClientEarth.

Using portable emissions measurement systems (PEMS) attached to vehicle exhausts, the researchers collaborated with Emissions Analytics to measure the amount of NO_x produced by the 39 Euro 6 diesel cars.

The researchers compared their results to estimates of vehicle emissions using COPERT (Computer Program to Calculate Emissions from Road Transport), an air quality emissions model developed by the European Environment Agency. The average NO₂ emission in the new study was 2.5 times the COPERT model estimate, rising to 2.8 for urban driving.

Comparison of urban and motorway trip average NOx emissions (note: y axis varies). Vehicle ID at top of each plot. O’Driscoll et al. Click to enlarge.

The researchers also found no clear best between the after-treatment technologies. Both SCR and LNT were able to meet the Euro 6 standard and NTE limit while EGR alone was not.

Since Euro 6 vehicles are already in use, the researchers argue that it is important to identify the worst performing cars and ban them, rather than just relying on generic EURO-6 standards for regulation. This would enhance the effectiveness of schemes like Clean Air Zones and allow for consumers to make informed decisions when purchasing a new car. The researchers found that removing the five most polluting vehicles reduced average emissions considerably.

At present, manufacturers must adhere to emissions limits tested only in a lab setting, but next year, type approval testing will include real-world driving environments to tackle real-world emissions levels. The authors also suggest that rather than using generic guidelines for all cars, each car model should be tested on its own merit with regard to emissions.

Resources

• Rosalind O’Driscoll, Helen M. ApSimon, Tim Oxley, Nick Molden, Marc E.J. Stettler, Aravinth Thiyagarajah (2016) “A Portable Emissions Measurement System (PEMS) study of NO_x and primary NO₂ emissions from Euro 6 diesel passenger cars and comparison with COPERT emission factors,” Atmospheric Environment, Volume 145, Pages 81-91 doi: 10.1016/j.atmosenv.2016.09.021