Evaluation Method for Nitrogen Oxide Emission Reduction Using Hypothetical Automobile Model: A Case in Guangdong Province

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study evaluates NOₓ emissions mitigation via the lens of automobile fleet transition from ICEVs to NEVs in Guangdong Province. Although the topic is interesting, there are some issues should be clarified and justified by the authors as shown below:

• You claimed "Current research reveals a notable gap in evaluating NOx reduction effects associated NEV-induced transformations in automobile fleet structures[12-17]." This is unproven claim. You should summarize the relevant studies in the literature, provide their main findings and then present your contirbution.
• The model depends on two assumptions: hypothetical vehicle equivalency and constant emission coefficients in adjacent years. I have conerns about the first assumption as it simplifies the model significantly. It is unclear how accurate the hypothetical "average" vehicle (p₁ and p₂) is for both NEVs and ICEVs given varying usage profiles and technologies (e.g., plug-in hybrids vs. BEVs). Moreover, the emission factors for ICEVs are derived from literature, but the method lacks explicit references or error propagation analysis. Including a sensitivity analysis on key input parameters (e.g., travel distance, fuel consumption) would increase credibility. These concerns should justified in the manuscript.
• Table 6 presents p₁ and p₂ without units or clarification of normalization (e.g., per vehicle, per km). This ambiguity makes interpretation difficult. Moreover, the sudden drop of p₂ from -0.241 (2021) to -0.074 (2022) is not explained sufficiently. Is this due to reduced NEV mileage, fewer new NEVs added, or data limitations?
• In Table 5 and figure 6, the NOx emissions in 2019 (peak year) are noticeably higher than in prior and later years. The text attributes this to rebound effects, but further elaboration (e.g., specific policy delays, vehicle surges) is needed. Clarify whether the emissions include all mobile sources or only light-duty vehicles.
• Figure 10 on "reduction ratios" is informative. However, a cumulative view of ICEV vs. NEV total contributions over the study period would add value.
• The claim that NEVs contribute 38.7% of total reductions (Line 282) is strong but needs numerical justification and citation or derivation steps. Moreover, the observed post-2021 decline in NEV impact (p₂ less negative) could imply saturation or rebound effects. Authors should explore and discuss this more critically.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

The authors have developed a model for calculating NOx emission inventory in order to quantify the impact of introducing new energy vehicles on emissions in Guangdong between 2013 and 2022. The research method is based on the statistical analysis of near-surface NO2 concentration data from Guangdong Province from 2013 to 2022, which reveals distinct temporal trends. As a result of the study, the authors have developed policy recommendations aimed at reducing nitrogen oxide emissions, such as: 1. Energy conservation and emissions reduction require strong government-enterprise collaboration. 2. The vigorous promotion of NEVs should be continued, as it is a key path towards promoting green development in the transportation sector. 3. Policy-driven clean energy transition and continuous deepening of demand-side reforms are recommended. Overall, the article receives a positive assessment. The strength of the research is that it draws on real data gathered in the Guangdong province. However, there are some drawbacks to the study, primarily due to the inherent limitations of the statistical methods employed by the authors. In particular, statistical methods differ from simulation-based and optimization-based approaches in that they do not allow for the analysis of the consequences of different scenarios for ecological policies or the search for optimal solutions within the available trade-offs. This limitation can be partially addressed if the authors incorporate forecasts into their research. For example, they could provide forecasts for emission reductions of NEVs and ICEVs in Guangdong Province between 2025 and 2030, for example, in accordance with the policy recommendations outlined in section 4.2.

Therefore, the following recommendations are provided to improve the article:

1. The literature review can be improved. It would be beneficial to broaden the methodological foundation and incorporate studies that utilize, for instance, simulation-based and optimization-based techniques to reduce harmful emission concentrations, such as:

[1] Akopov A.S., Beklaryan L.A., Saghatelyan A.K. Agent-based modelling of interactions between air pollutants and greenery using a case study of Yerevan, Armenia.
Environmental Modelling & Software, Volume 116, 2019, Pages 7-25, https://doi.org/10.1016/j.envsoft.2019.02.003.

[2] Zhou H., Cen K., Fan J., Modeling and optimization of the NOx emission characteristics of a tangentially fired boiler with artificial neural networks, Energy, Volume 29, Issue 1, 2004,
Pages 167-183, https://doi.org/10.1016/j.energy.2003.08.004

2. Section 2.3: The method should be improved. It would be beneficial for the author to include a subsection on predictive analytics for NOx reduction modelling. Predictive analytics can use well-established statistical methods, such as systems of simultaneous equations, to achieve this goal. It is crucial that the variables in these equations are time-dependent, and certain parameters are set within specific scenarios. 

3. Section 3: Results and Discussion should be improved. It is essential to provide forecasts for the reduction of emissions from NEVs and ICEVs in Guangdong Province for the period between 2025 and 2030, for example, in line with the policy recommendations set out in Section 4.2. It would also be advantageous to establish a clear link between these scenario forecasts and the policy recommendations outlined in section 4.2.

Therefore, the paper requires major revision.

 

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

1. The content of the abstract is already quite detailed, but there may be some ambiguity in some words. For example, negative emission contributions and so on.
2. The abstract section of the article can be supplemented with data evidence to describe the contribution of the research.
3. Some of the sentences in the article are too lengthy. It is recommended to reorganize the sentences and clarify the viewpoint through more concise and clear statements.
4. The coherence of each paragraph lacks logic. It is suggested to add some sentences at the beginning of the paragraph to enhance the connection with the previous text and improve the logical coherence of the article.
5. In the introduction section of the article, the second paragraph contains repetitive sentences, such as significantly reducing greenhouse gas emissions and decreasing greenhouse gas display.
6. The introduction section of the article is brief and repetitive in meaning. It is recommended to add content or references to enrich this section.
7. Some statements in the article tend to lean towards a national perspective, and it is suggested to discuss the contribution of the article from an international perspective for greater persuasiveness.
8. In the table of the data exposition section of the second part, it is recommended to add units of various scales to avoid ambiguity.
9. The image expression in the article is too simplistic, which can add some complexity and aesthetic appeal.
10. The citation format of some references is not consistent. It is recommended to standardize the citation format.
11. The model proposed by the author is overly simplistic and contains numerous assumptions, such as those in line 144 and line 170. What is the basis for these assumptions? What kind of impact will they have on the results?
12. In the process of compiling the emission list, uncertainty analysis is usually conducted. However, no such content is found in the manuscript.

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

The authors have conducted a comprehensive evaluation of NOₓ emissions from Internal Combustion Engine Vehicles (ICEVs) and New Energy Vehicles (NEVs) in Guangdong Province, China, over the period 2013–2022. I suggest the following major modifications and revisions before the manuscript can be considered for publication.

1. The methodology heavily relies on "hypothetical automobile equivalents" for ICEVs and NEVs. However, the basis of assigning a single emission factor to diverse vehicle types oversimplifies real-world variations. 
2. The NEV-related NOₓ emissions are represented as negative values (i.e., emission reduction). However, the paper does not account for indirect NOₓ emissions from power generation (especially if electricity is generated from fossil fuels). This omission could lead to an overestimation of NEV benefits.
3. The model outputs (values of p₁ and p₂, NOₓ reductions) are presented as fixed values without any uncertainty bounds or confidence intervals. 
4. The fuel consumption values (kg/km) listed for medium and large cars appear identical (0.163 kg/km), which is unrealistic and needs verification. Additionally, the table does not clarify whether these are gasoline or diesel vehicles.
5. The model does not include the effect of vehicle age, driving patterns (urban vs. rural), congestion, or cold starts, all of which significantly affect NOₓ emissions. 
6. While the paper provides NO₂ concentration trends from satellite or near-surface data, it does not explicitly compare modeled NOₓ values with measured NO₂ trends. 
7. The equations assume linear scaling of national NOₓ emissions to regional levels using vehicle stock ratios. This oversimplifies spatial variability in fuel quality, traffic density, and enforcement of emission norms. 
8. Although the authors mention improvements in exhaust systems and combustion technology, there is no quantitative discussion or data supporting this claim. 
9. The policy section provides generalized suggestions (e.g., promote NEVs, phase out ICEVs), but lacks quantitative policy scenarios or impact projections. 
10. Figures 6 to 10 are discussed in detail, but graphical quality, axis labels, units, and legends are not clear from the text. 

Author Response

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Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed all the comments. 

Author Response

Thank you again for your review.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have improved the article based on the reviewers' recommendations. This study makes a significant contribution to the development of the method for evaluating nitrogen oxide emission reduction using two hypothetical automobile models. The authors have developed a novel framework for NOx emissions accounting that allows for the quantification of the effectiveness of NOx mitigation resulting from the adoption of NEVs using the Guangdong Province as a case study. The paper is recommended for publication.

Author Response

Thank you again for your review.

Reviewer 3 Report

Comments and Suggestions for Authors

I noticed that the author has made certain revisions. Although the method involves quite a few assumptions, there is still a significant lack of uncertainty analysis for the emission inventory. No necessary analysis has been conducted on the uncertainty analysis during the compilation of the emission inventory. It is suggested that the author make necessary supplementary analyses.

Author Response

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