Analysis of Passenger Car Tailpipe Emissions in Different World Regions through 2050

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study presents the projection of tailpipe emission (CO2, NOx and PM2.5) in nine world regions in the future until year 2050 using a bottom-up calculation method. The authors claimed that emissions of passenger car would drop by 45%, 63% and 54% for CO2, NOx and PM2.5, respectively in 2050 as compared to that in 2015. Electric vehicles will lead in stock share after 2040. The rising transport demand in some regions would offset the emission reductions.

The structure of the study was logically formulated. The aim of the study was also stated clearly from the beginning to the end. I also appreciate the work they integrated information from different sources for different world regions to this comprehensive analysis. I would recommend publication after minor revision.

11. I understand this analysis is about tailpipe emission of passenger cars. However, when we consider electric cars as a future trend, non-exhaust emission has been discussed also quite a lot. Some literatures also pointed out that electric cars would generate more non-exhaust PM emission due to their weights. Have the authors considered that as well?

22. Similar to the previous question, have the authors also quantified indirect CO2 emissions? As we have more and more electric cars in urban areas, on-road emission would be reduced, but indirect emissions from power plants remain. As CO2 is a greenhouse gas, it have strong climatic effects no matter where it is emitted.   

Author Response

Dear Reviewer,

Thank you for your time and the valuable feedback you provided, which has greatly helped us improve our manuscript. Attached, please find our detailed responses to your comments.

Best regards

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

thank you very much for submitting this paper for review. I found the article interesting and a good starting point, but I unfortunately think that in its current status can not be published. I find particularly problematic the lack of any sensitivity analysis or effort to verify the robustness of the overall results (for example through comparison with other similar works, except for what is done in Figure 5). This for me in a major flaw that need to be addressed before any discussion on the details can be done. I also find not serious to propose in the paper that "future emission inventory compilers to cover this.". In my humble view since you are presenting a model/modelling exercise is your duty to ensure that results are robust and also to show how they compare with existing literature (for example, in Europe, there are thorough modelling exercises done by the European Commission in multiple impact assessments for different regulations on the topic, just to mention an example).

Beside this major issue, I have some general comments that you may want to consider

·        I think the novelty of the work and the motivation can be made clearer. There are several similar exercises in the literature, and it should be made more evident what make this one relevant. In addition, in the introduction, an effort to review existing work of this type should be done. In Europe there are state of the art model to do the same exercise done in this manuscript.

·        To improve readability, also for the non expert reader, I would suggest that units are used in the equations when describing the model (e.g. rho_f(kg/m3)). Also note that it is formally wrong to use square parenthesis for units of measure (like it is done in the figure). Square parentheses are normally reserved for dimensional analysis.

·        I think the title of the paper is confusing since the analysis is not really done for world region but rather for specific countries. I would suggest, to improve transparency, to modify the title and the manuscript accordingly. Especially being not clear what is the impact/uncertainty associated with such hypothesis.

·        I think the clarity of the paper would increase if some of the commentary done at the end is moved at the beginning. For example the fact that non-exhaust PM is not included or that only CO2 from fossil fuel origin is considered (and the CO2 reported is not tailpipe CO2 as one could imagine). Moreover, is not clear to me if other GHGs are considered or not.

·        Is not clear to me how the air pollution EF for PHEV have been calculated. It has been demonstrated that PHEV can have very high air pollutant emissions due to repeated cold start event during the driving cycle.

·        Is not clear to me what is the projection for transport activity in Europe. It looks like it is stable up to 2050. I just want to point out that, if this is the case, this is at variance with last modelling done for Europe in European Commission Climate 2040 targets.

 

Author Response

Dear Reviewer,

Thank you for your time and the valuable feedback you provided, which has greatly helped us improve our manuscript. Attached, please find our detailed responses to your comments.

Best regards

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Analysis of Passenger Car Tailpipe Emissions in Different World Regions through 2050

(futuretransp-2906165)

 

 

General Comments:

 

This paper presented a very interesting study that was well aligned with the concerns of global GHG emissions. It provided insights into the next decades about passenger car CO₂, NOx and PM emissions considering the impact of various changes in vehicle composition as well as EV adoptions and market shares. It represents some important contributions to this area of research. However, the review is of the view that the manuscript could further be improved by considering the following comments.

 

1.    Methodology:  The current study is related to a very large scale of modelling work in which the calculation methodology has divided into a number of sub-models such as market share model and emission calculation models etc., and the application of these models varies across different regions / countries investigated. Overall, it is complicated. It would be better if the overall approach could be outlined in a flow chart showing the structure, the relationships between key elements of the model, as well as the major inputs and outputs to/from each key element within the modelling framework. Model descriptions could then base on this outline to supplement various details and variations when applying to different regions (e.g. gap filling in the China region’s calculation as described in the manuscript). This helps the reader to more easily follow the logic and details of the modelling works.

 

2.    Model Results:  The mode results are very interesting and insightful. It is suggested that more interpretation could be done for some distinct patterns for a certain region(s) that might be more different from others. For example, the emission patterns projected for the China region appeared to be quite different from other regions.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English of the manuscript is generally well written.

Author Response

Dear Reviewer,

Thank you for your helpful feedback. We have made improvements based on your comments. Attached are our responses.

Best regards

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The authors point out that this study uses a diffusion model to predict transport carbon emissions and pollutant emissions in nine countries and regions worldwide from 2015 to 2050. In general, there was a lack of explanation of the profile of the study area. In addition, contributions to this research should be clearly provided. Currently, the results are not well displayed and it is difficult to correlate the figures with the text.

 

The main problem with the manuscript is that it is written, which makes the evaluation and interpretation of the main contributions of the paper obscure. It is noted that the manuscript needs careful editing by someone with expertise in technical English editing, paying particular attention to English grammar, spelling, and sentence structure, to make the manuscript more readable. To further improve this manuscript, I have the following suggestions:

 

Specific comments:

 

1. Abstract: The abstract section should reflect the purpose of the research. And the abstract needs to indicate that the nine countries and regions are those

 

2. Introduction: Provide a brief overview of why your work is important, including any background context, state the objectives of your research, and elaborate on its unique contributions to the literature, avoiding stacking useless literature a detailed literature review. It lacks the description of the study area and the significance of selecting these 9 countries and regions, and how to judge it as representative.

 

3. Line 39, L41 repeat reference.

 

4. Line 296 “and”

 

5. Theoretical Framework and Methodology: It has to be described why the sample has been selected and whether the sample selection is scientific. A comparative analysis of the advantages and disadvantages of this method should be added.

 

6. Results: The result graph needs to be clearer and marked with the graph number to match the conclusion text. In addition, some result graphs need to pay attention to the units, as shown in Figure 7 (b).

 

7. Conclusions: Key results, associated policy implications, limitations, and/or recommendations for future research should be presented in a short Conclusions section.

 

 

Comments on the Quality of English Language

It is noted that the manuscript needs careful editing by someone with expertise in technical English editing, paying particular attention to English grammar, spelling, and sentence structure, to make the manuscript more readable.

Author Response

Dear Reviewer,

Thank you for your time and valuable input. We have updated our manuscript accordingly. Please see the attached document for our detailed responses.

Best regards,

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

 

thanks for addressing the comments. 

Reviewer 4 Report

Comments and Suggestions for Authors

The article has been further improved, but in terms of formatting and layout, please check the manuscript again.

Comments on the Quality of English Language

There are still some grammatical flaws in the article. Please check the manuscript again.