TRUE publication

Evaluation of real-world vehicle emissions in Brussels

The Real Urban Emissions (TRUE) Initiative conducted an extensive vehicle emissions testing study in Brussels in the fall of 2020, performing more than 260,000 in-use emissions measurements of 130,588 unique vehicles using remote sensing technology. During the study, researchers also used a portable instrument to measure the tailpipe particle number (PN) emissions of close to 600 light-duty vehicles. The data collected was analyzed with the dual goals of identifying the real-world effectiveness of key policies and regulations impacting the Brussels fleet and providing recommendations for their future development. These include the low emission zone (LEZ) implemented in the Brussels-Capital Region, the introduction of new stages of European emission standards for new vehicles, and more stringent periodic technical inspection requirements that Belgium has recently introduced.

Key findings from the analysis include:

  • The real-world nitrogen oxide (NOx) emissions from diesel passenger cars operating in Brussels greatly exceed regulatory limits for vehicle groups not subject to Real Driving Emissions (RDE) type-approval requirements. These groups include vehicles certified to Euro 4, 5, and 6 (pre-RDE) standards, which were found to have real-world NOx emissions three, four, and five times the respective laboratory type-approval limits. These results are consistent with TRUE Initiative studies in other European cities.
  • The NOx emissions of Euro 6d-TEMP and 6d diesel cars, which are subject to RDE testing, are 63% and 74% lower, respectively, than those from vehicles certified to previous stages of the Euro 6 standard. However, average emissions from Euro 6d-TEMP diesel cars remain 60% greater than those of petrol vehicles certified to the same standard.
  • Euro 4 diesel cars accounted for only 12% of the passenger cars measured during this study. However, the study estimates that they contribute 26% of total passenger car NOx emissions and 47% of total particulate matter (PM) emissions. Similarly, Euro 4 diesel light commercial vehicles (LCV) constituted 15% of the measured fleet but account for more than half of total PM emissions from this vehicle type. Beginning in January 2022, Euro 4 diesel light-duty vehicles will be restricted from accessing the Brussels LEZ.
  • The two vehicle groups estimated to account for the greatest shares of NOx emissions—Euro 5 diesel cars and LCVs—will be allowed to circulate within Brussels until 2025 under the current LEZ implementation schedule. These vehicles are estimated to contribute approximately 40% of total NOx emissions from passenger cars and nearly 50% of emissions from LCVs. An earlier phase-out would accelerate the NOx emissions reduction benefits achievable from removing these high-emitting vehicles from the streets of the city.
  • Tailpipe PN testing showed that approximately 5% of the measured light-duty diesel vehicles that were equipped with a diesel particulate filter (DPF) for which a type-approval limit for PN applies (Euro 5b and higher) were found to have PN emissions indicative of some level of failure of the emission control system. We estimated this small group of very high-emitting vehicles to be responsible for more than 90% of total particles emitted from the test group.


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