the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Feb 2025
On-road vehicle emission measurements show a significant reduction of black carbon and nitrogen oxides emissions in Euro6c and 6d diesel-powered cars
Abstract. This study compares the results of three on-road chasing campaigns conducted in 2011, 2017, and 2023, making it the first to report how real-world emission factors (EF) for diesel vehicles have changed over a decade. By directly measuring emissions during real-world driving, this research provides critical insights into the effectiveness of emission control technologies and regulatory interventions. The findings highlight the transformative impact of Diesel Particulate Filters (DPFs), which have consistently reduced black carbon (BC) EF from diesel vehicles to levels comparable to gasoline-powered cars. This underscores the success of DPFs in controlling particulate emissions, a major contributor to air quality issues. Real Driving Emissions (RDE) regulations have also proven effective in significantly lowering nitrogen oxide (NOx) emissions. By incentivizing the use of previously underutilized technologies, these regulations ensure better compliance with standards during typical driving conditions. Real-world EF measurements are crucial for bridging the gap between ambient pollution data and traffic emission models, enabling more accurate assessments of vehicle fleet contributions to air quality. This study employs an independent chasing method, demonstrating its value as a practical tool for monitoring fleet emissions, identifying super-emitters, and detecting vehicles with defeat devices. Overall, this decade-long analysis highlights the significant advancements in reducing diesel emissions, while emphasizing the importance of sustained efforts in monitoring and regulation to further improve air quality and refine emission modeling frameworks.
Competing interests: The authors IJB, AG, MI, BA, GM, and MR were employed by Aerosol d.o.o., the manufacturer of Aethalometers, at the time or part of the time while the study was conducted.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2024-3553', Anonymous Referee #1, 28 Mar 2025
The study compares the results of three on-road chasing campaigns (2011, 2017, and 2023) for Diesel vehicles. It is robust, well written and rigorous despite for NOx EF computation for which I strongly suggest to improve the paper as detailed below in the comments.
Comments:
- Line 89: The results of the 2011 campaign were published in (Ježek et al., 2015a)... Change with “The results of the 2011 campaign were published in Ježek et al. (2015a)
- Line 92: the words “at the Ljubljana measurement station” are reported twice in the same sentence. Correct the sentence
- Line 98: improve the writing of (Ježek et al., 2015b) citation as reported at point 1. This comment is valid all along the manuscript. Please take care of citations.
- Line 99: put in brackets the year of reference cited
- Line 102: please discuss in a quantitative way the maximum error that can be done “Assuming equal dilution of all released pollutants and complete combustion of fuel, in which practically all the carbon in the fuel is converted to CO2 (Hansen and Rosen, 1990)”
- Line 111: “NOx was treated as NO2 equivalent with a molar mass of 46 g mol-1 (Wang et al., 2012)” This is wrong. Compare for example Carslaw (2013; https://doi.org/10.1016/j.atmosenv.2013.09.026) where NO2/NOx ratio is well below 50% for Diesel passenger cars and other fleet vehicles. I suggest to review the literature (and the aforementioned paper) to find a proper average NO2/NOx ratio for each group of vehicles and apply a proper new molar mass on its base. Please redo all the computations connected with NOx EF estimation.
- Line 112: “eBC”, referring to Savadkoohi et al. (2024; https://doi.org/10.1016/j.envint.2024.108553) ad a proper nomenclature
- Line 115: “The inlet was positioned on the right-hand side window of the mobile platform”. Please describe the inlet and its capability to work as isokinetic or not before the PM2.5 cyclone. And in the last case discuss the possible influence on the car speed on the capability of AE33 inlet to effectively collect the BC in a proper way.
- Improve Figure 3 by also adding on the x-axis the label of each Euro standard referred to each numerical limit.
Citation: https://doi.org/10.5194/egusphere-2024-3553-RC1 -
RC2: 'Comment on egusphere-2024-3553', Anonymous Referee #2, 28 Mar 2025
This paper summarises the findings of field campaigns that aim to measure / calculate the emission factors for NOx and BC over the period 2011 to 2023. The paper mostly focuses on how emissions have changed over this period, which is of potential interest to the scientific community. The results broadly reflect those already established in numerous emission measurements across Europe, with little new insight.
The paper is lacking in the literature cited. There is a considerable literature available on this topic, which already captures the main findings of this paper. I feel that if the authors had considered more literature they would have focused their paper differently i.e. not summarising what we already know. As it stands, this paper provides an overall summary of emissions but does not present compelling new evidence of wider interest to the community. I have suggested major revision for this (and other) reasons.
The Abstract is rather general and could be made stronger by being more quantitative e.g. how much have NOx emissions reduced through RDE testing, or the extend to which BC has reduced due to DPFs.
There aspects of the data and methods that require further attention.
The field campaigns were conducted in very different conditions - especially the 2011 data; summarised in Table 1. the 2011 data was collected in very low ambient temperatures (-1.3 to -9.3) compared with 2023 (11.9 to 18.1). There are therefore very likely strong differences in emissions between these two years simply because of the conditions. Emissions of NOx (and BC) were likely much higher in 2011 for two reasons: older fleet compared with 2017 and 2023 and much lower temperatures. Such low temperatures are known to affect vehicle NOx emissions e.g. see
Grange, S. K., Farren, N. J., Vaughan, A. R., Rose, R. A., & Carslaw, D. C. (2019). Strong Temperature Dependence for Light-Duty Diesel Vehicle NOx Emissions. Environmental Science and Technology, 53(11), 6587–6596. https://doi.org/10.1021/acs.est.9b01024Suarez-Bertoa, R., & Astorga, C. (2018). Impact of cold temperature on Euro 6 passenger car emissions. Environmental Pollution, 234, 318–329. https://doi.org/10.1016/j.envpol.2017.10.096Wærsted, E. G., Sundvor, I., Denby, B. R., & Mu, Q. (2022). Quantification of temperature dependence of NOx emissions from road traffic in Norway using air quality modelling and monitoring data. Atmospheric Environment: X, 13. https://doi.org/10.1016/j.aeaoa.2022.100160The emissions for NOx and BC are therefore very likely higher in 2011 than they would have been if the temperatures were similar to the later campaigns - but the effect is unknown. However, there is an opportunity to investigate this important aspect of emissions.Figure 2 needs an x-axis label. I also think it could be plotted differently with the x-axis as Euro standard (not year of manufacture). This would make it much easier to contrast the emissions by Euro standard. It would also be worth considering plotting the mean and the 95% confidence interval in the mean to help demonstrate whether there are significant differences between the Euro classes. I don't think considering the interquartile range adds much and it makes it difficult to see the main changes.Figure 3 looks somewhat questionable if I understand it correctly. Why consider median and not mean emissions? It would help if a different symbol was used for each Euro class. It might also be useful to split Euro 6 into pre RDE (b/c) and RDE (d-temp/d) given that there were large changes in emissions through Euro 6 from lab-based factors to on-road measurements and limits.In the Conclusions it is stated: "...Despite these technological achievements, challenges such as improper vehicle maintenance and tampering with exhaust systems remain significant obstacles to achieving further reductions in traffic-related emissions." This may be true but it is a conclusion that is not based on the work that was done e.g. tampered vehicles were not identified, nor any impacts of improper maintenance. In general the Conclusions need to be strengthened e.g. by providing a more quantitative understanding of the changes and whether the result presented challenge wider understanding of these issues.Citation: https://doi.org/10.5194/egusphere-2024-3553-RC2
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