Sustainable Urbanization: Unpacking the Link Between Urban Clusters and Environmental Protection

Abstract

Comments 1:

Sentence: “The findings show that the increase in urban clusters contributes to the reduction of environmental pollution...”

Issue: The phrasing is overly general. The exact magnitude of the reduction should be stated to make the abstract impactful.

Suggestion: Rephrase to include quantitative findings, e.g., “The findings show that a 1-unit increase in urban clusters corresponds to a 7.5% reduction in the comprehensive environmental pollution index.”

Response 1: Thank you for pointing this out. The sentence has been revised accordingly. This change can be found on page 1, lines 17-18 of the manuscript.

The findings show that a 1-unit increase in the degree of urban cluster is associated with a reduction in the comprehensive pollution index by approximately 7.5%, and the main mechanisms which urban clusters facilitate environmental protection are congestion alleviation, industry structural optimization, and technological innovation.

Introduction

Comments 1:

Sentence: “The pursuit of economic development by humans while neglecting environmental protection can have serious consequences, such as environmental degradation and resource constraints.”

Issue: The sentence is repetitive and lacks specificity. The next sentence echoes the same idea.

Suggestion: Combine or condense “Unchecked economic development has often led to environmental degradation and resource depletion, with irreversible ecological consequences.”

Response 1: Thanks for your comments. This and the next sentence have been combined to “Unchecked economic development has often led to environmental degradation and resource depletion, with irreversible ecological consequences.” This change can be found on page 3, lines 35-36 of the manuscript.

Comments 2:

Sentence: “As China’s urbanization enters a new phase, the effectiveness of individual city development is declining, and the marginal benefits of reform are diminishing.”

Issue: The term “effectiveness of individual city development” is vague. What metric is being used to measure this?

Suggestion: Specify metrics such as GDP growth, urban livability indices, or pollution levels.

Response 2: Thank you for pointing this out. The sentence has been revised accordingly. This change can be found on page 3, lines 59-60 of the manuscript.

As China’s urbanization enters a new phase, the effectiveness of individual city development is declining, e.g., the GDP growth rate of many developed cities in recent years has been less than 5%.

Comments 3:

Sentence: “To provide more understanding of the linkages between urban clusters and environmental pollution, this paper empirically examines the impact of urban clusters on environmental pollution and investigates the mechanisms through which this effect operates.”

Issue: The repetition of “environmental pollution” within the same sentence is stylistically awkward.

Suggestion: Simplify: “This paper empirically examines the impact of urban clusters on environmental outcomes and the mechanisms driving these effects.”

Response 3: Thank you very much for your careful revision of the language. The sentence has been revised accordingly. This change can be found on page 5, lines 116-117 of the manuscript.

Materials and Methods

Comments 1:

Sentence: “Small amount of missing data is addressed using interpolation.”

Issue: This raises methodological concerns. How was interpolation performed, and how was its reliability verified?

Suggestion: Expand: “Small amounts of missing data were addressed using linear interpolation, validated against available data for accuracy.”

Response 1: Thanks for your comments. The sentence has been revised accordingly. This change can be found on page 6, lines 140-141 of the manuscript.

Comments 2:

Sentence: “Urban clusters are defined as collections of high-density, closely connected urban units within a geographical area.”

Issue: This definition is vague and lacks specificity regarding measurement or criteria.

Suggestion: Add specifics: “Urban clusters are defined as collections of urban units with population densities exceeding [X] and economic interdependencies within a radius of 150 km.”

Response 2: Thanks for your comments. It needs to be explained that the concept of urban clusters may be difficult to define by precise numbers. As can be seen in the subsequent analysis on the same page (page 7), there are various criteria for calculating the degree of urban clusters, and this paper has selected one that is appropriate for the Chinese scenario. Of course, we also discuss the impact of different criteria on the main conclusions in the robustness tests (page 18).

Comments 3:

Sentence: “The comprehensive pollution index is derived using the entropy weighting method.”

Issue: The steps are listed but lack explanation of why the entropy method was chosen over alternatives.

Suggestion: Justify the choice: “The entropy weighting method was selected due to its ability to objectively weight variables based on variability and significance.”

Response 3: Thank you for pointing this out. The reason for using entropy weight method has been added accordingly. Please see page 6, lines 157-159 of the manuscript.

Results and Discussion

Comments 1:

Sentence: “The coefficient for urban clusters remains significantly negative.”

Issue: While statistically clear, the interpretation lacks depth. What does “significantly negative” mean for policymakers?

Suggestion: Expand: “The coefficient for urban clusters remains significantly negative, indicating that higher urban cluster integration correlates with reduced environmental pollution by approximately 7.5%.”

Response 1: Thanks for your comments. The sentence has been revised accordingly. This change can be found on page 11, lines 283-285 of the manuscript.

Comments 2:

Section: Congestion Alleviation

Issue: The claim that “urban clusters alleviate congestion effects” is not substantiated with examples or case studies.

Suggestion: Include a real-world example, such as traffic congestion trends in specific urban clusters.

Response 2: Thanks for your comments. We have added corresponding example, please refer to page 13, lines 316-325 of the manuscript.

The alleviating congestion of urban clusters has also been reflected in China. Particularly in the development of the Beijing-Tianjin-Hebei urban cluster, its center city, Beijing, has been facing severe congestion in terms of traffic, population, and so on. In order to solve these problems, in recent years, Beijing has utilized the advantages of urban clusters to transfer “non-capital functions”. That is, relocating some industries and public service functions (including non-core government offices, education and medical resources, etc.) to neighboring cities. There has also been the construction of intercity railroads, satellite cities etc. With the implementation of these measures, Beijing's growing congestion effect has been somewhat alleviated by the development of urban clusters.

Comments 3:

Sentence: “From an environmental standpoint, well-planned urban clusters can reduce resource consumption, mitigate urban sprawl, and promote land use efficiency, all of which are essential for sustainable urban growth.”

Suggestion: please provide relevant references (WOS:000521153200005).

Response 3: Thanks for your comments. The reference has been added accordingly. Please see page 21, lines 497 of the manuscript.

Conclusions

Comments 1:

Sentence: “Urban clusters contribute to enhanced environmental protection.”

Issue: This is overly broad and adds little value.

Suggestion: Rephrase to reflect specificity: “Urban clusters significantly reduce environmental pollution through mechanisms such as congestion alleviation, industrial optimization, and technological innovation.”

Response 1: Thanks for your comments. The sentence has been revised accordingly. This change can be found on page 19, lines 462-463 of the manuscript.

Comments 2:

Sentence: “The planning and construction of urban cluster should actively incorporate environmental considerations.”

Issue: Singular/plural agreement error with “urban cluster.”

Suggestion: Correct: “The planning and construction of urban clusters should actively incorporate environmental considerations.

Response 2: Thank you for pointing this out. This error has been corrected. Please see page 20, line 496 of the manuscript.

Comments 3:

Sentence: “The planning and construction of urban cluster should actively incorporate environmental considerations.”

Issue: Singular/plural agreement error with “urban cluster.”

Suggestion: Correct: “The planning and construction of urban clusters should actively incorporate environmental considerations.”

Response 3: Thanks for your comments. This comment was repeated once and has been revised in the last response.

General Methodological Issues

Comments 1: Use of Dynamic Panel Regression:

Issue: The paper does not justify why dynamic panel regression was chosen over other methods.

Suggestion: Add a sentence explaining its relevance, such as “Dynamic panel regression was used to account for the historical inertia of environmental pollution and to improve causal inference.”

Response 1: Thanks for your comments. The manuscript slightly explains the reason for using dynamic panels regression on page 11, lines 275-277. Of course this is not enough. We added a sentence accordingly based on your suggestion. Please see page 11, lines 289-292 of the manuscript.

Comments 2: Robustness Tests:

Issue: The explanation for robustness tests is incomplete. For example, why were 200 km and 250 km chosen as alternative thresholds?

Suggestion: Provide rationale: “Alternative thresholds of 200 km and 250 km were tested to assess the sensitivity of results to variations in urban cluster definitions, reflecting potential regional differences in connectivity.”

Response 2: Thank you for pointing this out. The explanation has been added accordingly. Please see page 18, lines 439-441 of the manuscript.

Comments 3: Missing Discussion on Endogeneity:

Issue: The paper mentions using lagged variables to address endogeneity but does not elaborate on potential remaining biases.

Suggestion: Expand: “While lagging independent variables addresses simultaneity bias, residual endogeneity due to omitted variable bias or measurement errors may persist.”

Response 3: Thank you for pointing this out. The discussion on endogeneity has been added accordingly. Please see page 19, lines 449-451 of the manuscript.

References (suggestion)

Comments:

· Establishing a robust connection between scientific content and urban environmental pollution is essential to addressing recent societal challenges, as it fosters evidence-based decision-making, enhances public awareness, and drives innovative solutions for mitigating pollution in increasingly complex urban contexts (WOS:000318592900003).

Response: Thanks for your comments. The corresponding statements and the reference have been added. Please see page 22, lines 507-511 of the manuscript.

Comments 1:

In this study, Xu et al. investigated the role of urban clusters on environmental pollution and found that urban clusters contribute to the reduction of environmental pollution, and the main factors underlying this improvement have been identified as congestion alleviation, industry structural optimization, and technology innovation. The paper is well-written, and the findings are interesting. However, I am not an expert on the specific statistical methods used in this study and therefore could not critically review the results section. The work is publishable. I only have the following minor comments.

Why were only three pollution indicators used? Could you explain why these three are the most important factors and the other factors, particularly the air quality factors such as ozone, NOx, PM2.5, and GHGs, are not important and, if considered, will not reverse the results?

Response 1: Thanks for your comments. There are two main reasons for using these three pollution indicators. One is that the indicators we tried to select can comprehensively respond to the overall environmental level of a city, including the status of air pollution, water pollution, etc., rather than focusing only on air pollutants as in previous studies. On page 6, lines 141 to 142 of the manuscript, we also mention another reason, the availability of data. Historical data for these three indicators for the past two decades are relatively complete. The lack of data on other pollutants such as ozone is very serious in some cities. This is a reflection of the fact that the government and the public attach different importance to different pollutant indicators. Of course, the problem you mentioned is unavoidable, in order to solve this problem, we newly included PM2.5 in the dependent variables, the comprehensive index of urban pollution, to conduct a robustness analysis. The regression results, as shown in Table 5 on page 20, are consistent with the baseline regression results, which illustrates the robustness of the main conclusions of this study.

Comments 2:

In conclusion, the authors offered policy recommendations. However, many uncertainties remain regarding the effectiveness of urban clusters, and therefore, any policy recommendations could be avoided at this time.

Response 2: Thank you for pointing this out. We agree with you that it is not appropriate to give policy recommendations. But several other reviewers had some other comments on the policy recommendations. We removed the original policy recommendations from the manuscript and rewrote a small number of new policy recommendations by synthesizing other reviewers' comments. Please see lines 497-506 on page 21 of the manuscript.

Comments 3: Comments on the Quality of English Language

The English language used in this manuscript is overall good. You could modify/correct grammars in some places.

Response 3: Thanks for your comments. We have revised the grammar throughout the manuscript.

Comments 1:

The article addresses the topic of urban districts (referred to as "clusters") and their potential contribution to or impact on the ecological footprint of Chinese cities. While this subject is inherently well-researched and explored—dating back to studies on "eco-neighborhoods" and even the concept of the garden city—the authors aim to innovate by presenting additional data and focusing on the Chinese context. Framed in this way, the topic could serve as a valuable starting point, and I agree that extending discussions, which are still overly Western-centric, to Eastern regions and cities is essential. However, I find the research question excessively vague, and the results lacking specificity, thus contributing only marginally to the ongoing debate. The attempt to address a poorly defined and somewhat trivial research question using sophisticated (and numerous) indicators results in a study that appears superficial, methodologically weak, and limited in scope. I therefore recommend that the journal does not publish the article in its current form and request a thorough revision of both the research premises and outcomes. Attached, you will find additional comments and notes on the text for the authors' consideration.

Response 1: Thanks for your comments. First of all, thank you very much for recognizing the discussion of this paper's extension of eco-neighborhoods to the eastern regions. Regarding the issues you mentioned about the research questions being too vague and the lack of specificity in the results, we have made the following main revisions. Firstly, we have added research on eco-neighborhoods and provided some description and definition of urban clusters in introduction. This is because we found that perhaps the fact that the definition of urban clusters was not provided earlier was the main reason for the lack of readability. Secondly, we provided more precise descriptions of the independent variables as well as using more precise statements at some important beginnings. Finally, we have made major revisions to the conclusion of the manuscript to avoid self-evident sentences and unspecific conclusions. And we provide a new policy recommendation based on the empirical results of the study. Thank you very much, your comments are very helpful to improve the quality of the manuscript.

Comments 2:

Line 2: C

Response 2: Thank you for pointing this out. We have revised “clusters” to “Clusters”.

Comments 3:

Line 13: what does this mean?

Response 3: Thanks for your comments. Inappropriate words could be a major cause of confusion. We have revised the sentence accordingly.

City clusters are the main trend of current and future urbanization worldwide, but its impact on environmental pollution has been controversial.

Comments 4:

Line 35: please consider including also more recent reference from IPCC, this would provide a broader insight to your text.

Response 4: Thank you for pointing this out. We have added the IPCC's latest views on environmental issues such as climate change in recent years. Please see lines 37-40 on page 3 of the manuscript.

Comments 5 & 6:

Lines 35 – 38 & 38-42: This information is repetitive and does not add meaningful insights to the research in this field, as it merely highlights well-known or self-evident points

Response 5 & 6: Thank you for pointing this out. We have almost entirely deleted this paragraph, replacing it with literature on recent IPCC views and eco-neighborhoods, and adjusting the logic of this paragraph. Please see lines 35-46 on page 3 of the manuscript.

Comments 7:

Line 42: you should immediately explain what you mean by city cluster

Response 7: Thank you for pointing this out. We have added an explanation about urban clusters here. Please see lines 44-48 on page 3 of the manuscript.

Today, however, urban development places greater emphasis on the synergistic development and comparative advantage of urban units within a certain region [5]. That is, as the level of urbanization increases, multiple cities form urban clusters through well-developed transportation networks, industrial division and economic and technological links in order to promote the better development of the member cities.

Comments 8:

Line 44: recognized for what purposes?

Response 8: Thank you for pointing this out. We have added here the reasons why governments have introduced policies that emphasize urban clusters. Please see lines 48-50 on page 3 of the manuscript.

Urban clusters contain a large number of closely linked cities at different development stages and are strategically important in terms of socio-economic impact [6].

Comments 9:

Line 45: are these city clusters? you should rearrange all the definitions around the paper

Response 9: Thank you for pointing this out. This was an error and we have revised it to urban clusters. Also, although we have slightly explained the difference between urban clusters and similar concepts such as urban agglomerations in lines 168 - 170 on page 7 of the manuscript, the presence of urban agglomerations in the preceding text tends to confuse readers. We have replaced all “urban agglomerations” with “urban clusters” in the introduction of the manuscript.

Comments 10:

Line 46: what do you mean by these? please provide explanation to those who don't know about reform or else

Response 10: Thank you for pointing this out. Reform and opening up is a major strategic decision that China has implemented since 1978 to promote economic development and social progress through “internal reform” and “opening up to the world”. Internal reform includes economic and political reform, while opening up to the world involves a wide range of economic cooperation and international exchanges.

The explanation of the term may require many words and is less relevant to the topic of this study. We have therefore removed the term and revised the sentence, which does not quite change the original meaning.

In China, the main development has also been in the form of rapid urbanization over the past several decades.

Comments 11:

Line 50: I believe this expression is very strong and would require a reference

Response 11: Thanks for your comments. We have added a reference on urban disease in China here.

The added reference:

Yan, H.; Liu, Z., A New Perspective on the Evaluation of Urbanization Sustainability: Urban Health Examination. Sustainability 2023, 15, (12).

Comments 12:

Line 53: what is this 5 years plan? you should introduce it first and then refer to it as the primary form of urbanization if necessary

Response 12: Thanks for your comments. We have briefly described the Five-Year Plan and revised the sentence. Please see lines 60-62 on page 3 of the manuscript.

Consequently, one of China’s most important national development strategies, the National Economic and Social Development Plan, advocated for urban clusters as the primary form of urbanization in 11th Five-Year Plan.

Comments 13:

Line 55: explain “group warming”

Response 13: Thank you for pointing this out. It is an inappropriate expression. We used this phrase originally to express the meaning that cities within a city cluster can collaborate to promote development. Since we have already expressed a similar meaning in the revision above (lines 44-50), we have deleted this sentence.

Comments 14:

Line 57: the lack of an explanation for what you mean by "urban cluster" makes it difficult to understand and appears overly vague in the context of the research

Response 14: Thanks for your comments. In addition to some descriptions of urban clusters added in the responses above (lines 44-48 of manuscript), we have added a definition of urban clusters, as shown on page 3, lines 48-50.

The added reference:

Portnov, B. A.; Schwartz, M., Urban Clusters as Growth Foci*. Journal of Regional Science 2009, 49, (2), 287-310.

Comments 15:

Line 76: maybe you meant CO₂?

Response 15: Thanks for your comments. We reread the literature and determined that SO₂ is being meant here.

Comments 15:

Line 81: innovation of which kind?

Response 15: Thanks for your comments. This reference does not explicitly indicate what kind of innovation is being referred to, but we have found in the variable descriptions in this reference that the measure of innovation is based on invention patents, so we have revised innovation here to technological innovation.

Comments 15:

Line 87: “making them become public issues within urban clusters” this sentence should be revised

Response 15: Thanks for your comments. We have revised the sentence accordingly. Please see lines 94-97 on page 5 of the manuscript.

This results from the “adjacency effect” between cities, whereby increasing interdependence and collaboration have caused internal problems in individual cities, such as environmental pollution, to spill over into public issues within urban clusters.

Comments 16:

Line 88: Fan

Response 16: Thank you for pointing this out. We've revised “fan” to “Fan”.

Comments 17:

Lines 101-102: can you be more precise? this is another self-evident point that is not even necessary to insert in a scientific paper

Response 17: Thank you for pointing this out. We have rewritten this reference to more accurately express its scientific contribution. Please see lines 109-113 on page 5 of the manuscript.

Wang et al. verified that the impacts of China’s urban economic development and population size on haze pollution conformed to the environmental Kuznets curve, but that the impact of rapid land urbanization on haze pollution was mainly monotonic and increasing, and that the incompatibility of population and land in the process of urbanization exacerbated haze pollution [11].

Comments 18:

Lines 130: wasn’t it 2006?

Response 18: Thank you for pointing this out. We do apologize that this was a slip of the pen. We have revised it to 2006.

Comments 19:

Lines 131: please be specific.

Response 19: Thanks for your comments. The cities with significant missing data are mainly some autonomous regions for ethnic minorities and are numerous. We have revised this sentence accordingly to your comments and have provided Fig. 1 to reflect more accurately which cities were excluded. Please see line 141 and lines 150-151 on page 6 of the manuscript.

Many cities in Inner Mongolia, Hainan, Sichuan, Guizhou, Yunnan, Tibet, Qinghai and Xinjiang with significant missing data are excluded.

Comments 20:

Lines 132: what do you mean by abolished?

Response 20: Thanks for your comments. During the study period, a small number of cities were downgraded to a lower administrative level because of population loss, resource depletion, and other reasons. That is, they ceased to be a prefecture-level city and became a county or other low-level administrative unit. The current statement is inappropriate word, so we have revised the sentence accordingly. Please see lines 141-142 on page 6 of the manuscript.

Additionally, both canceled and newly established cities during the study period were excluded.

Comments 21:

Lines 156: this is the first time a definition of urban cluster appears and it is not even referenced. I think you should rearrange the text to have this stated at the beginning a with much core consistency to have a strong base for your hypothesis which in my opinion is missing now.

Response 21: Thank you very much for pointing this out. We have added a description and definition of urban clusters in the introduction. We have also added the corresponding reference for the definition here. Your comments are very important to improve the readability of the manuscript.

Comments 22:

Lines 265: “does increased urban cluster affect urban environmental pollutions”. what do you mean by this? more quare meters, more people? wider? more buildings? more green areas? the question to me is absolutely vague and, even though you provide specific indicators it does not provide an answer to a vague question.

Response 22: Thanks for your comments. The expression here is not precise enough and we have revised the sentence (lines 277-278). Urban clusters are a well-defined indicator with a number of measurements, but the vagueness of definitions in the previous section may be the main cause of your confusion. Urban clusters and the degree of urban clusters in a city are two different concepts. The former can simply be understood as collections of cities. The latter could be popularly interpreted as a measure of how closely an individual city is connected to an urban cluster. So, the implication of the sentence in the manuscript is whether a city having a higher degree of urban clusters affects its environmental pollution? Or does a city's deeper linkage into the urban network affect its environmental pollution? We have added some corresponding statements in the previous section to minimize any confusion among readers. Please see lines 177-178 on page 7 of the manuscript.

Comments 23:

Lines 461-463: To be able to have this as a recommendation you should provide proof that this is not already something policymakers are encouraging. At least in China.

Response 23: Thanks for your comments. It is to be explained that this sentence is not a policy recommendation, but merely a brief summary of current urbanization trends. Specific policy documents are mentioned in the introduction to the manuscript (lines 61-64).

Comments 24:

Lines 461-463: To be able to have this as a recommendation you should provide proof that this is not already something policymakers are encouraging. At least in China.

Response 24: Thanks for your comments. It is to be explained that this sentence is not a policy recommendation, but merely a brief summary of current urbanization trends. Specific policy documents are mentioned in the introduction to the manuscript (lines 61-64). But this sentence is in an inappropriate place in the whole paragraph. In addition, other reviewer felt that the topics of this study are still relatively controversial and that it would be inappropriate to provide too many policy recommendations. As a result, we made major revision policy recommendations.

Comments 25:

Lines 463-465: same as above.

Response 25: Thanks for your comments. We have added some evidence about the lack of planning in urban clusters in the past, especially in terms of environmental benefit. Please see lines 497-501 on page 21 of the manuscript.

The first mention of urban clusters at the national strategic of China appeared in the 11th Five-Year Plan in 2005, but urban clusters in practice had been developing for many years by then. It is therefore reasonable to believe that previous development of urban clusters was inadequately planned, but proactive urban cluster planning is essential for sustainable urbanization growth [12].

Comments 25:

Lines 469-470: this is another self-evident sentence. Scholars are addressing this issue since decades and for this reason this should not be "sold" as a recommendation. Recommendations should be based on empirical findings and be thoroughly specific. The problem here is that with such large and open research question you risk banalyzing the answers

Response 25: Thanks for your comments. We have revised this sentence to better provide more specific recommendations based on empirical results. Please see lines 501-508 on page 21 of the manuscript.

Comments 26:

Lines 476: again, this is an overy simplified statement, not worthy of a scientific journal

Response 26: Thanks for your comments. We have deleted this sentence and rewritten the policy recommendations. Please see lines 496-513 on page 21 of the manuscript.

Comments 1:

line 44= rewrite please, I think you missed some words "but are also is fully"

Response 1: Thank you for pointing this out. We have rewritten the sentence accordingly. Please see lines 52-54 on page 3 of the manuscript.

Therefore, urban clusters are fully given adequate attention by governments of various countries, and a series of policies supporting the development of urban clusters have been introduced in many countries.

Comments 2:

line 46-47= some comas are missing to understand the meaning of the sentence

Response 2: Thanks for your comments. Other reviewers also provided comment to this sentence, and thought that non-Chinese scholars would have difficulty understanding the reform and opening up. Therefore, combining all the comments, we have revised the sentence accordingly. Please see lines 54-55 on page 3 of the manuscript.

In China, the main development has also been in the form of rapid urbanization over the past several decades.

Comments 3:

line 52-54= explain better please

Response 3: Thanks for your comments. We have added an example to explain “the effectiveness of individual city development is declining” and deleted “the marginal benefits of reform are diminishing”, which is not relevant to the topic. In the next sentence, we explained the 11th Five-Year Plan. Please see lines 59-64 on page 3 of the manuscript.

As China’s urbanization enters a new phase, the effectiveness of individual city development is declining, e.g., the GDP growth rate of many developed cities in recent years has been less than 5%. Consequently, one of China’s most important national development strategies, the National Economic and Social Development Plan, advocated for urban clusters as the primary form of urbanization in 11th Five-Year Plan.

Comments 4:

line 77-79= explain better, please

Response 4: Thanks for your comments. We have rewritten this reference. Please see lines 87-90 on page 4 of the manuscript.

Liu and Guo argued that foreign direct investment in China reduces haze pollution, and the total effect can be decomposed into scale effect, technology effect and structural effect. Foreign direct investment increases urban haze pollution through scale effects and structural effect, but decreases urban haze pollution through technology effects [10].

Comments 5:

line 88= everytime you write a reference outside the brachet a coma is missing eg.: capital letter and coma

Response 5: Thank you for pointing this out. We have revised all references to ensure that the capital letter and comma are no longer missing. In addition, due to the editor's request, we have revised all references in sequential numerical order instead of authors plus year.

Comments 6:

line 93-96= explain better, please

Response 6: Thanks for your comments. We have revised this sentence to more clearly convey the main points of these references. Please see lines 104-106 on page 5 of the manuscript.

Due to close connections and good cooperation within urban clusters, it is easier to realize the supply of public goods and the integration of environmental policies, which is conducive to solving cross-city pollution governance challenges [12, 13].

Comments 7:

line 93= "environmental quality" what do you mean when you speak about environmental quality? Do you refer to air, water, soil or what else?

Response 7: Thanks for your comments. This sentence summarizes the views of the following references. Environmental quality here therefore refers to the environment quality in total, including air, water, land and so on. However, the later references refer more to “reducing pollution”, so we have revised this sentence and other similar expressions in this paragraph.

Please see lines 103-104 and line 95 on page 5 of the manuscript.

An alternative perspective suggests development of urban clusters can reduce environmental pollution such as haze and acid rain.

Comments 8:

line 196= could you explain the meaning of each parameter, please?

Response 8: Thanks for your comments. The meanings of all the parameters have been added. Please see lines 209-211 on page 9 of the manuscript.

where  refers to the emissions of the pollutant j of city i in year t;  is the Gross Domestic Product of city i in year t; and m and n refer to the number of pollutant types and the number of cities, respectively.

Comments 9:

line 277 =" the paper next adopt a dynamic" what do you mean with next? Maybe you want to write later. Having written consequently you don't need next or later.

Response 9: Thank you for pointing this out. We have deleted “next” from this sentence.

Comments 10:

line 278= GMM the first time you write this acronym, you have to write the complete words, in bracket the acronym and so the second time you can use the acronym.

I suggest you to write a brief explication for this method

Response 10: Thank you for pointing this out. We have added the complete words and an introduction of GMM. Please see lines 292-295 on page 12 of the manuscript.

Comments 11:

line 293= "coefficient of the interaction " explain why it is significant, please.

Response 11: Thanks for your comments. The word “significant” here means that the estimates are significant, as shown in the columns (1) of Table 2. The current expression is ambiguous, so we have revised it. Please see lines 310-311 on page 13 of the manuscript.

The coefficient of the interaction term is significantly negative.

Comments 12:

line 314= "reaching a certain scale" please, define the scale

Response 12: Thank you for pointing this out. “reaching a certain scale” comes from a reference that supports our findings. We reviewed this literature and revised the sentence using more precise numbers. Please see lines 321-324 on page 14 of the manuscript.

Additionally, some studies based in China have highlighted that the effectiveness of urban clusters in alleviating environmental pollution requires reaching a certain scale, for example, Chen et al. suggest that the inflection point of ecological efficiency improvement by agglomeration is 3.4 [15].

Comments 13:

line 335= "industrial structure optimization" I suggest you to explain the industrial structure optimization, eg. increase of technology to reduce the pollutant emissions and so on.

Response 13: Thank you for pointing this out. We have added explanation of the industrial structure optimization. Please see lines 369-373 on page 16 of the manuscript.

More specifically, industrial structure optimization refers to the adjustment and upgrading of the weight and layout of various industries within a city, usually involving the development of high-technology, green and service sectors, and thus often resulting in benefits such as economic efficiency and sustainable development.

Comments 1:

This is a promising article about urban clusters and environmental protection in China. While the article is mostly clearly structured, the use of referencing is lacking in places, the methods used to carry out analysis is not entirely clear and discussion of the findings in relation to previous research is mostly absent.

Response 1: Thank you for pointing this out. We have reinforced the use of references, with a particular focus on linking to previous research in the discussion. We have also clarified your mention to a formula and the software used. Thank you for your valuable comments, which are helpful in improving the quality of the article.

Comments 2:

- Lines 57-58: Please review the verb used in the aim of the paper. It is questionable whether you are addressing or actually assessing the issue.

Response 2: Thank you for pointing this out. We have revised address to assess. Please see line 67 on page 4 of the manuscript.

Hence，from an environmental protection perspective, this paper aims to assess whether urban clusters can reduce environmental pollution and, if so, through which mechanisms this effect occurs.

Comments 3:

Line 74: Please check spelling

Response 3: Thank you for pointing this out. We double-checked line 74, and there should be no spelling errors. However, there are two words in this sentence that are inappropriate. Therefore, we revised “technique” to “technological”, and “researches” to “the existing literature”. Please see lines 84-85 on page 4 of the manuscript.

Regarding technological effects, the existing literature have focused on emission reduction.

Comments 4:

Line 131: what does "so on" refer to here? Unclear expression of areas that are excluded, please revise.

Response 4: Thank you for pointing this out. The cities with significant missing data are mainly some autonomous regions for ethnic minorities and are numerous. We have revised this sentence accordingly to your comments. Please see lines 141-142 on page 6 of the manuscript.

Many cities in Inner Mongolia, Hainan, Sichuan, Guizhou, Yunnan, Tibet, Qinghai and Xinjiang with significant missing data are excluded.

Comments 5:

Data: It would be helpful to see a map of included/ excluded areas as international readers may not know where the named cities are located and otherwise it would be interesting to know the spread of included Vs excluded areas geographically

Response 5: Thank you for pointing this out. We have added a map of included/ excluded areas accordingly. Please see lines 150-151 on page 7 of the manuscript.

Comments 6:

- 2.1 Data: There is no mention of use of software to analyse the data. Please explain how the analysis was completed as it is not sufficient to state the calculations because based on this only the research is not replicable.

Response 6: Thank you for pointing this out. We have stated what statistical software was used. Please see lines 149 on page 6 of the manuscript.

Comments 7:

Lines 155-156: Unclear if this is the definition of the authors or one from previous literature. If the latter please add a reference, if the former, please state this.

Response 7: Thank you for pointing this out. Both this definition and the next sentence are references to the same literature, so we have only added the literature to the next sentence. This is not appropriate. We have added references accordingly. Please see lines 174 on page 8 of the manuscript.

Comments 8:

Lines 298-299 Are there any references to back this claim or are you able to elaborate this further by relating this to previous literature? 

Response 8: Thank you for pointing this out. There is a reference to support this conclusion on pages 313 to 315 of the original manuscript. We had thought it would be inappropriate to have too much discussion when reporting the results, but it seemed that this would lack the logic of the statement. We have therefore repositioned the reference. Please see lines 321-324 on page 14 of the manuscript.

Comments 9:

Lines 317-319 What are these suggestions based on, please provide some references. 

Response 9: Thank you for pointing this out. These suggestions are largely based on the empirical results of the previous section. Of course, these empirical findings are not sufficient for making such a policy recommendation, and we have added a reference. Please see line 355 on page 15 of the manuscript.

Comments 10:

Lines 321-322 Same issue as above. 

Response 10: Thank you for pointing this out. We have deleted this policy recommendation because we suggest it is repetitive and lacks specificity.

Comments 11:

Lines 371-378 Same issue as above: any references to back these claims about technology? 

Response 11: Thanks for your comments. We have revised the sentence accordingly and have added some references. Please see lines 407-412 on page 3 of the manuscript.

Comments 12:

lines 466-468 This sentence is somewhat unclear; please could you revise.

Response 12: Thanks for your comments. We have revised the sentence accordingly. Please see lines 503-509 on page 21 of the manuscript.

The first mention of urban clusters at the national strategic of China appeared in the 11th Five-Year Plan in 2005, but urban clusters in practice had been developing for many years by then. It is therefore reasonable to believe that previous development of urban clusters was inadequately planned, but proactive urban cluster planning is essential for sustainable urbanization growth. The empirical findings of this study also reflect more the objective environmental benefits of urban clusters than the active environmental considerations in the design of urban clusters in the past.

Comments 13:

Lines 470-472 This is lacking references, assuming this has been identified by previous research? 

Response 13: Thanks for your comments. We have revised the sentence accordingly and have added a reference. Please see lines 507-508 on page 21 of the manuscript.

There should be a higher level of coordinated action within city clusters, such as industrial planning and the provision of public goods.

Comments 14:

Lines 475-477 Please provide some references. 

Response 14: Thank you for pointing this out. We have revised the sentence accordingly and have added a reference. Please see lines 505-507 on page 21 of the manuscript.

Comments 15:

The discussion section should be more clearly signposted to the reader as it is listed together with the results. Both the discussion and conclusion lack references which makes it unclear which statements relate to the results of this research and what evidence has been provided by previous literature. As sustainability is an international journal, it would be recommendable for the authors to consider how the findings of their research relate to a broader international context outside of China.

Response 15: Thanks for your comments. Firstly, we have subdivided the results and discussion as much as possible and have preceded the conclusions with similar signifiers such as “the above results indicate”. Secondly, in addition to the references added in the response above, we have added a number of references to enhance the credibility and readability of the manuscript. Finally, combining the comments of other reviewers, we have made major revisions to the policy recommendations of the manuscript in order to make it possible to relate the findings to the international context. Please see lines 499-517 on page 21 of the manuscript.

Comments 16:

References 37-39 seem to have an error in the listing of one reference.

Response 16: Thank you for pointing this out. We have revised the reference accordingly.

Comments 1:

The article was reviewed according to my comment. I would suggest to stress more the relevance of this main topic for a global audience. Why should researchers from USA, Europe, Africa, etc. benefit from this research. Sustainability is an international journal with a broad audience and I believe it would be beneficial to highlight the key messages from this work. Furthermore, I would mention clearly the discipline this paper tackles and the target (policymakers, other researchers, designers?) of the recommendations it provides. In the conclusions please provide both limitations and future research perspective clearly.

Response 1: Thank you for your valuable feedback and suggestions. We agree with these three comments and have added a paragraph at the end of the manuscript to respond to all of the issues you mentioned above. Regarding the relevance of this main topic of this study to a global audience, we respond in two ways. (1) The findings and policy recommendations have relevance for regions undergoing rapid urbanization (e.g., Africa, Southeast Asia), and we acknowledge that the policy implications of this study for regions that have largely completed urbanization may be limited. (2) Our study provides empirical evidence and methodological references to address the academic controversy over the environmental impacts of urban clusters. Please see lines 509-517 on page 22 of the manuscript. Second, we state on page 22, lines 510-511 of the manuscript that the disciplines this paper tackles are environmental policy, urban studies and sustainability, and on lines 514-515 that the target population is scholars and policymakers. Finally, we suggest limitations of this paper and corresponding future directions: making cross-regional comparisons, focusing on more specific industries or sectors, and improving inference methods. Please see lines 522-526 on page 22 of the manuscript. Thank you again for your thoughtful suggestions.

In conclusion, this study provides empirical evidence addressing the academic debate in the field of environmental policy, urban studies and sustainability regarding whether urban clusters can mitigate environmental pollution, and highlights the potential of urban clusters as a governance tool for environmental protection and urban sustainability. Consequently, the study’s design and measurement of key variables offer references for scholars interested in these issues, while the findings provide policy insights for policymakers in rapidly urbanizing regions such as Southeast Asia. However, the policy implications for regions that have largely completed urbanization are limited. This study has several limitations. Firstly, it relies on data from China, which may limit the generalizability of the findings, especially for regions at different urbanization stages. Future research should include cross-regional comparisons, which could also capture the environmental impacts of urban clusters at various stages of urbanization. Additionally, while the environmental impacts of urban clusters could be interpreted as a combined effect of all industries, the environmental impacts of agglomeration may vary across different industries. Therefore, future studies could focus on specific industries or sectors. Finally, although a series of robustness tests were conducted, this study has limitations in addressing unobserved confounders, which is a common challenge when using observational data for causal inference. Future research could advance inference methods to facilitate more accurate causal analysis.

Comments 1:

Thank you for addressing the comments in your revised paper. I find the paper is now much clearer to read and the use of references has been substantially improved. I have no further recommendations to make.

Response 1: Thank you very much for your thoughtful feedback. We are glad to hear that the revisions have improved the clarity of the paper and the use of references. We appreciate your positive comments and am grateful for your time and effort in reviewing our work. Thank you once again for your support!