Meta-Analysis as Early Evidence on the Particulate Emissions Impact of EURO VI on Battery Electric Bus Fleet Transitions

Round 1

Reviewer 1 Report

Notes are included in the appendix

Comments for author File: Comments.pdf

Author Response

Firstly, thank you for your time and input. Your contributions and those of the other reviewer, the editor and staff at Sustainability are greatly appreciated, and I hope you find my responses (below), which I believe address your comments appropriately, acceptable.

In the meantime, I have updated the acknowledgements by adding the following additional sentence:

“All authors also gratefully acknowledge the time and input of the editors and staff at Sustainability and the anonymous referees whose value comments improved the quality of this paper.”

1. Agreed, full both gas and dust emissions as the result of the full life-cycle of any vehicle is an important consideration but regrettably, it is outside the scope of both of the project and paper. The work it describes, which was funded by NERC as part of its Clean Air programme, focuses on Non-Exhaust Bus Emissions. But, by way of compremise, added sentence to end of paper to acknowledge this (line 447 onwards in pdf):

“Likewise, manufacturers need to be actively working to address the full life-cycle costs, both financial and environmental, of these incoming vehicle and battery technologies if we want to re-position ourselves as a truly circular economy, and tools like the methods presented here can help fleet managers and policy makers facing a marketplace full of choices.”

2. Thank you for spotting that. Table 2 now reported consistently, in line with assumed accuracy. (Also, updated Tables 5 and 7, which had similar inconsistency.)
3. Happy to discuss this further, and will defer to your and editor if you feel this is needed, I would like to keep the appendices as is because several of the figures are referred to in the main text as part of the explanation of trends, and others, most notably figures and tables relating to later models do provide additional information that is distracting as part of main text but I would wanted by many readers, so suitable for inclusion.

[If the editor would rather and Suitability allows the inclusion of a separate Supporting Information document, I can re-format them as such.]

4. I (KR) think this relates to duplication of information in some tables and figures, also commented on by other reviewer.

[If not, could you let me know?]

As with comment 3, happy to discuss this further, and will defer to your and editor if you feel this is needed, but I would like to highlight that repeated information is in the form of both a summary figure (like figure 1) and a table of specific values (like in Table 6) for a specific bus comparison model. The figure is helpful for those after a quick measure of the relative importance of different emission contributions but impractical for anyone that wants specific values, e.g. a fleet manager wanting to compare predictions with regulatory standards. Likewise, few wanting an overview of trends, find several large tables of numbers helpful. So, feel we should include both. It is maybe also worth noting that it is not abnormal to include both, see e.g. Beddows and Harrison’s paper on car NNEs: https://doi.org/10.1016/j.atmosenv.2020.117886  

 

Reviewer 2 Report

This study investigated the vehicle exhaust emissions and Non-Exhaust Emissions impact of E6DV-to-BEV transitions by meta analysis. The work is interesting, abundant and meaningful. However, there is some issues that can be improved.

Specific comments:

1.      For abstract part, there are lack of the finding descriptions of this study. It is recommended to broaden this part.

2.      For section 2, it is better to add the descriptions of meta analysis?

3.      The contents of figure 1 and Table 6 are duplicate. There are many same issues in other part.

4.      For section 4, it is lack of comparative analysis with other published research results. It is better to add discussions.

Author Response

Firstly, thank you for your time and input. Your contributions, like those of the other reviewer, the editor and staff at Sustainability are all greatly appreciated. I hope you find my responses, which I believe address your comments fully, acceptable.

In the meantime, I have updated the acknowledgements by adding the following additional sentence:

“All authors also gratefully acknowledge the time and input of the editors and staff at Sustainability and the anonymous referees whose value comments improved the quality of this paper.”

Specific responses:

1. Upon re-reading, we agree. Extended end of abstract as follows (line 5 onwards in pdf):

“Here, as part of early work on the scoping of the First Bus EURO VI Diesel Vehicle (E6DV) to BEV fleet upgrades, we discuss the complex interaction between E6DV/BEV weight difference, regenerative braking and journey type, and propose metrics that would allow fleet operators more insight into a wider range of emission outcomes at the scoping stage of a fleet upgrade.” TO “Here, as part of early work on the scoping of the First Bus EURO VI Diesel Vehicle (E6DV) to BEV fleet upgrades, we estimate E6DV total particulate emissions to ca. 62 - 85 and 164 - 213 mg.veh−1.km−1 for PM2.5 an PM10, respectively, and that the majority, typically 93-97%, are NEEs. We also discuss the complex interaction between E6DV/BEV properties, and estimate potential changes resulting from the transition to BEV to ranging from a decrease of ca. 2 - 12% to an increase of ca. 12 - 50% depending on combination of weight difference, regenerative brake performance and journey type. Finally, we propose metrics that would allow fleet operators more insight into a wider range of emission outcomes at the scoping stage of a fleet upgrade.”

2. Again, upon re-reading, we agree. Modified methods as follows (line 102 onwards in pdf):

Expanded first paragraph from “The approach adopted here to estimate bus emissions is based on that previously applied to BEV, gasoline and diesel passenger cars by Beddows and Harrison [14]. Summarising, briefly:” TO “As part of the first round of data gathering and literature review for the meta-analysis, Beddows and Harrison [14] methods previously applied to BEV, gasoline and diesel passenger cars were selected as the starting-point for the estimation bus emissions. We also report exhaust PM EFs as a point-of-reference for discussion of trade-offs between Conventional ICE and BEV vehicles, and discuss proposed modifications to Beddows and Harrison [14] for use in E6DV/BEV bus PM emissions scoping exercises to extend weight-based corrections to weight-and-route-based corrections.

Summarising briefly, the main methods were:”

We then replaced the text after the summarised methods with the following: “Our results and a critique of our findings, based on the external evidence identified during the literature review, are presented in Section 3 (Results) and 4 (Discussion and Model Refinements) of this paper.”

3. A similar point was raised by the other reviewer, as stated there we are happy to discuss this further, and will defer to your and editor if you feel this is needed, but would like to keep the appendices as is if possible? We respectfully suggestion that including both figure and table is helpful to readers. The figure is helpful for those after a quick measure of the relative importance of different emission contributions but impractical for anyone that wants specific values, e.g. a fleet manager wanting to compare predictions with regulatory standards. Likewise, few wanting an overview of trends, find several large tables of numbers helpful. So, feel we should include both. It is maybe also worth noting that it is not abnormal to include both, see e.g. Beddows and Harrison’s paper on car NNEs: https://doi.org/10.1016/j.atmosenv.2020.117886

[I am going to confess, rather shamefully, that word-for-word this is same case I made to reviewer one.]

4. We already do some of this but though out sections 3 and 4 alongside specific findings. For example:

In section 3.1: “on-road surveillance indicates that these PM EFs are similar to those observed for real-world bus fleets (see e.g. [ 30], [31 ], Euro VI Bus EF(PM) ca. 5 mg.veh−1.km−1).”

In section 3.2: “…broadly consistent with those reported in the literature for heavy duty vehicles (c f . 20 - 80 mg.veh−1.km−1 for HDV EF(PM10)brake in [ 33 ] and references therein).”

There is less evidence for other NEE source types, which we acknowledge in the paper by generally we are good agreement e.g. NAEI estimates: In section 3.3. “…consistent with those estimated for tyre emissions in UK NAEI guidance ([26])”

We suggest that such source-type-related information is best placed presented alongside the associated calculation to provide direct context. We, however, acknowledge that it would also be useful to include an overview statement regarding the comparison of our outputs and those of other similar studies. We therefore propose the following addition to the conclusions:

Added extra sentence after bullets conclusions (line 421 onwards in pdf): “Although there is still relatively little source apportionment evidence regarding the NEE impact of the transition to BEV bus fleets, these findings are broadly consistently with other public evidence. For example, the much amounts of NEE PM by comparison to exhaust PM for modern (post-EURO V) vehicle fleets (see e.g. [ 34], [23]), Bus NEEs of the order of 50-100 and 150-350 mg.veh−1.km−1 for PM2.5 and PM10, respectively (see, e.g. [ 9 ], [13]), and the impact of heavier vehicles more generally ([22]).”