Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of “Measurement and Analysis of Brake and Tyre Particle Emissions from Automotive Series Components for High-Load Driving Tests on a Wheel and Suspension Test Bed” by M. Kupper et al. submitted for publication in Atmosphere.

A feasibility study is presented to measure tyre and brake related particles observed at a test bed for wheel suspension and brakes. Particle and size distributions were measured at the air inlet and at the exhaust of the ventilation system from the test bed. As mentioned in the title the brake and tyre particle emissions were measured under high-load conditions – it needs to be discussed if these conditions may occur during real on-road driving? Altogether, the study may be published, after some revision and additional information has been added.

 

Line 137: the authors should add data from test cycle. What are the applied decelerations? Do they occur during on-road driving? Are the applied slip angles representative?

 

Is the combination of braking and slip taking from real drives? Please add a table with the parameters - or range - you have measured.

 

Fig 4 b:      400-450 C Brake disc temperature is very high and not at all representative. Although mentioned later in the paper, it should also be stated here, that the observed temperature is far beyond normal, or even severe braking.

Chapter 3.2: A further analysis could be added: What is the weight loss per driven km?

 

Since the exhaust air flow should be known, and and the airborne particle mass per volume was measured, what would be the airborne PM emission rate (mg/km) for each test sequence. Were are the 5 sets identical? If not, one could look at the variation (after stabilization and omission of the first cycles).

Figure 5: some explanation should be added what can be seen on the pictures? Tyre wear agglomerates? Any hint for metallic abrasion particles? Any individual particle EDX analysis?

Line 264: “Driving the cycle with a vehicle is not possible…” o.k. - however what is the merit of the feasibility study? Did you try to measure PM and size distributions under more moderate conditions? It would be helpful to learn if the method would be feasible under realistic driving conditions?

Typos:

Line 47: Total Particle Number (TPN), additionally to th for 47 engine emissions established Solid Particle Number (SPN),

 

Line 243 … particle sized ….

Line 260 which

Author Response

We would like to thank you very much for your feedback and helpful comments, we have carefully considered all of them and implemented them as follows:

Regarding the comment, that it needs to be discussed if the conditions from the used test cycle may occur during real on-road driving, we now mention in the introduction that our test cycle does not represent real driving behaviour, we furthermore included statements in the method section where we describe the test program and in the first section of the discussion, where we also address what the merit of our study is. Finally we added suggestions for future work, to conduct measurements with the method described for realistic drive cycles.

We added visualisations of important set parameters the test cycle in Appendix B, as suggested we explicitly also added the acceleration/deceleration and the slip angle. The test cycle will be published as a table in the online supplementary.

As suggested, we now mention in the caption of fig. 4 that the brake disc temperatures are very high and not representative for on-road driving, and we added the emission factors for the weight loss per km driven calculated from the gravimetric analysis. Time-resolved PM correlated data is only available from light scattering instruments, using an assumed density. Variation in-between the several sets was not observed.

We added additional explanation to the caption of figure 5 and added the reference to the single-particle EDX analysis results in table 3. Furthermore, we described some EDX results in the results section.

Reviewer 2 Report

Comments and Suggestions for Authors

The work presented in this manuscript focuses on measurements of non-exhaust particles, using a test-bed set up to measure total particles from the entire wheel, including brakes and tyres. Measurements of particle size distribution along with EM of single particles was performed to gather information on the sizes and composition of the particles emitted. The results demonstrate that this method of sampling combined brake and tyre wear is effective, but has some limitations, which are discussed in the manuscript. It is recommended for publication once the following have been addressed.

Section 2.2: Were the tyres and brakes bedded-in in order to stabilize them before the testing began? It would be good to mention this in the text.

Section 3.3: Could you provide an explanation as to why the first two cycles were so different to the remaining cycles? Was it just a matter of the instruments requiring stabilising?

L243-244: It is stated that "The found particle sized do not match the expected sized according to the calculations".  Could you provide further information on these differences and what this means for the results.

L287-288: This sentence does not look complete.

L298: Please state the PM concentration values that you are comparing to within the literature. Also it should be stated whether the comparison is made for brake and tyre wear values in the literature separately or combined.

L308: Please state the UFP PN concentration value you are discussing here.

Comments on the Quality of English Language

A number of typograpical errors were observed in the text.  

Author Response

We would like to thank you very much for your feedback, comments, and questions. We did our best to carefully consider all of them and implemented them as follows:

We added into the description of the test program, that balanced tires and rims were used.

We added additional explanation regarding the stabilisation of the emissions and the connection to the brake disc temperature.

We adopted the paragraph regarding the differences between the expected deposited particle sizes on the deposition plates.

We revised the uncomplete sentence in L. 287-288

Regarding the comment on L298, we revised the paragraph to make our statement more clear.

We added the discussed concentration in L 308.

Reviewer 3 Report

Comments and Suggestions for Authors

The introduction is too lengthy, suggests to be shortened.

1.       Is ‘th’ a typo in line 47?

2.       Line 212-222, more description of Figure 3 should be added. Only Figures 3a and 3c have been mentioned in the text. What do Figures 3b and 3d present? I can understand that Filtered supply air in Figure 3a means the background air supply through the inlet into the chamber. What is the difference between exhaust air and filtered exhaust? What does filtered exhaust mean?

3.       Line 213-215, a calculation with the density of soot helps to contextualise the detected amounts. Where does the soot density come from? Other literature? Can the reference be added?

4.       The estimated PM values were between 300 µg/m3 to 400 µg/m3, can you describe the estimated equations for the calculation? Or can you add a reference for this calculation? What effective density have you used for the calculation of each size channel? Is the density constant for every channel?

5.       Line 227, it seems that cycle 1 and 2 are bedding in period. It is not proper to say that since cycle 1 and 2 are different from the cycles after 3, you didn’t consider the first two cycles. Maybe it is better to say that you paid more attention to the steady state/after bedding in period.

6.       Figure 4b, what does brake pressure a.u. means? There is no y axis for brake pressure. Does the a.u. mean normalized? If this is the case, please add full name of it before abbreviation.

7.       is the size distribution shown in Figure 3 the average values of the 200 s measurement?

8.       Line 233, how can you say that the increase in temperature is followed by an increase in the UFP PN concentration? The CPC also counts particles up to 3 µm, not only the UFP range.

9.       Figures 5b-5d give three images of particles collected on different stages. The caption also says that to illustrate the differences of the particles found. Unfortunately, I cannot really tell the differences among these three images. You didn’t give enough description either in the text. How can the readers recognized the difference? I think you’d better to give basic description and help the readers understand the results.

10.   Line 286 287, incomplete sentences.

Comments on the Quality of English Language

Moderate editing is needed. 

Author Response

We would like to thank you very much for your helpful feedback, comments, and questions. We considered all of them and implemented them as follows:

Ad 1) Thank you for the hint.

Ad 2) We added descriptions as suggested.

Ad 3) We revised the mentioned passage, the used density for the presented values was not one taken from soot but from literature – we added citations.

Ad 4) We added the suggested details regarding the calculation

Ad 5) Thank you very much for the suggestion, we revised the passage concerned.

Ad 6) We expanded the caption of fig 4b) to avoid the mentioned confusion

Ad 7) The PSD from the aerosol instruments show mean values from one set, only considering the cycles after the bedding-in period.

Ad 8) Thank you very much for pointing this out, we removed the indication of UFP.

Ad 9) We added a description in the manuscript and expanded the caption of the figure.

Ad 10) Thank you for the hint, we removed the incomplete sentence.

Reviewer 4 Report

Comments and Suggestions for Authors

The tite and the abstract suggest that the manuscript reports on a comprehensive study on particulate matter (PM) emissions from brake and tyre wear. However, the manuscript has serious drawbacks.

1. The size range of the particulate matter in the study is not set, therefore it is difficult to compare the results with the general size fractions for PM mass, i.e. PM₁₀, PM_2.5, PM₁ and PM_0.1.

2. The upper size limit for SMPS is not mentioned, the combination of SMPS and APS data are not clear (electrical mobility diameter vs. aerodynamic diameter).

3. The bar graphs intending to present size distributions are completely misleading, the size distribution function should be dN/d(logd), with a well scaled x axis of d (log scale).  This is also valid for SEM data.

4. Since the three devices measure particle diameters based on different physical processes, the relation between electical mobility diameter, aerodynamic diameter, and geometrical diameter must be clarified.

5. The selection of the sampling substrate is questionable since Cu might be important for brake wear particles.

6. The authors state that the cut-off diameters of the electroscatic precipitator could not be verified using SEM. However, agglomeration of particles is clearly visible on the micrographs presented in Fig. 5, but not discussed at all.

7. The presentation of EDX data is a mess. It is not clear how the elemental concentrations were calculated. It is strange that some elements were identified as originating from the substrate, these elements were not omitted during concentration calculations.

8. The conclusions are very weak.

Author Response

We would like to thank you very much for honest feedback and your comments. Please find our response below, we addressed them as best we could in the given time.

At first, we would like to state that we understand that our study does not meet the requirements of an defined aerosol measurement campaign, in this case, it is also not our claim. The presented was done as a feasibility study to measure particle emissions at a wheel and suspension testbed, which was originally designed for component testing. Also, the study does not aim for PM measurements, so does the title does state “particle emissions”. We regret this misunderstanding and have added a passage in the introduction.

Ad 1) As stated above, our study did not aim for PM measurements. The most reliable PM value  can be extracted from the gravimetric approach in this study. As stated in our manuscript, real-time PM data is based on APS measurements and can in our point of view only be considered as a relative estimator as the density of the detected particles is not known.

Ad 2) We added a paragraph in the section of the size distribution measurements to describe that the presented data are only qualitatively comparable. We also added a note to the caption of figure 3.

Ad 3) The suggestion of the usage of normalised concentrations for the PSD is reasonable. For the data presented in the manuscript we have made a conscious decision not to use dN/(dlogDp) to preserve the order of magnitude of the measurements. In our opinion difference in the concentrations from the differences in the widths of the size bins would not increase the comparability of the data for this study, as only a qualitative comparison is possible. We added a statement in the results section.

Ad 4) Thank you for highlighting this point, we have added a statement in the results section and in the caption of figure 3.

Ad 5) We are aware that Cu is not the ideal sampling substrate for NEP. We added a statement in the suggestions for future studies, to use another material for future studies.

Ad 6) We revised the section on the EM results and added a paragraph to the discussion. In this regard we have also paid attention to clear terminology regarding the description of fractals and agglomerates and.

Ad 7) We sincerely apologize if the representation of the EDX data in table 3 is not clear at first glance. We also here made a conscious decision to show all data in one table, because it facilitates the comparison in-between the different samples. We added a description of how the mass fraction data was obtained. The handling of substrate elements is described in the last paragraph of the results section.

Ad 8) We regret that you are not satisfied with the conclusions. As this is data from a feasibility study, we would like to be very cautious about what goes beyond qualitative statements.

Reviewer 5 Report

Comments and Suggestions for Authors

This work has a very high level of originality and innovation. It is a unique experimental setup to quantify brake and tire wear particles. The enclosing chamber between the brake and rim would be useful in separating the brake wear particles from the exhaust particles, as the author pointed out in the paper. 

Whether or if this experimental configuration can accurately represent the tyre and brake wear scenario in the actual world is the main focus. The characteristics and morphologies of the tire wear particles generated by this test machine may differ significantly from the tire-road wear particles found in the real world because it employs asphat pavement as a surrogate material.  

Furthermore, the emission factors of tire and brake wear particles should be similar to those from earlier studies that assessed the EFs of these particles under practical circumstances if the test equipment is run under typical driving settings. 

In any case, excellent work was done in building this new kind of wear machine that could generate brake and tire wear particles. Soon, I hope to see additional test results.   

Author Response

We would like to thank you very much for your feedback, it encourages us to continue our work!

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for addressing the comments.

However it is still not clear when comparing to the literature values, whether you are comparing brake and tyre wear separately. With the experimental set up in this paper, the brake and tyre wear cannot be distinguished easily. So when comparing to literature values it is important to state whether the literature values are also for brake and tyre wear combined.  

Suggest updating the text in Line 343-349 in the revised manuscript to address this. 

 

 

Comments on the Quality of English Language

Typographical and grammatical errors observed in the manuscript. Please review and correct these.

Author Response

Thank you for this clarification, we changed the passage accordingly.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have made substantial improvement of the manuscript and I think it is qualified to be published.

Author Response

We would like to thank you again for your review.

Reviewer 4 Report

Comments and Suggestions for Authors

The comments were not addressed properly, the manuscript has not been undergone a satisfactory improvement.

Author Response

Dear reviewer, we are sorry that we have not yet been able to make a satisfactory improvement in your interest. As it is not entirely clear to us where we still need to make improvements, we have tried to orient ourselves on the comments from the first round. To address this, we have made the following changes for this revision:

• We changed the y-axis in the representations of the particle size distributions to dN/dlog(Dp) and adopted the description in the manuscript accordingly. We shifted the other figures to the Appendix.
• We added further descriptions regarding how the mass fraction in the EM data was obtained, since a commercial instrument was used from which the data processing manufacturer side is not disclosed, our options here are unfortunately very limited.
• We have revised the conclusion slightly to improve readability and to incorporate further results.

We sincerely hope that we have now adressed your comments and annotations better. If it is still not the case, we would like to ask you to describe in more detail your suggested further improvements.