What Makes the Lower Urban Land Coverage City a Deeper Ozone Trap: Implications from a Case Study in the Sichuan Basin, Southwest China

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article analyzes the reasons for the discrepancy between the urban ozone concentration trap and urbanization levels in Chengdu and Chongqing, utilizing atmospheric pollutant concentration products, satellite-based ozone precursor column concentration products, and meteorological observation and reanalysis data. While the topic is compelling, I find the analysis of the results to be insufficiently deep. I suggest the authors address the following two major points during revision:

1. Topographic Influence and Spatial Heterogeneity
Chengdu and Chongqing are the two most representative cities in the Sichuan Basin, yet they possess vastly different topographical features: Chengdu is a plain city, while Chongqing is a mountain city. These distinct terrains lead to significant differences in local thermal circulation. However, the authors did not analyze the potential impact of these topographic differences on ozone concentrations. I believe it is necessary to conduct spatial statistical analyses based on terrain, such as comparing differences in ozone and precursors across different elevation zones based on DEM data, or performing profile analyses along specific topographic gradients.

2. Vertical Meteorological Constraints
The current study considers the influence of near-surface meteorological elements such as visibility, sunshine duration, and surface wind fields. I recommend incorporating a broader range of meteorological factors, specifically focusing on the vertical dimension—such as Planetary Boundary Layer Height 3D temperature fields, and 3D wind fields. The impact of vertical meteorological structures on ozone concentrations should be evaluated in conjunction with the aforementioned topographic differences.

Minor Details and Technical Points:

1. Figure 1: As the map includes China's national boundaries, it is mandatory to provide the standard map review number issued by the Ministry of Natural Resources.

2. Line 186: What is the applicability of ERA5 10m wind speed data in complex terrain regions? It is suggested to cite relevant literature or conduct a validation against local observational data.

3. Figure 12: Why are the winter LST and wind fields for 2018 and 2019 presented separately?

4. Lines 362–364: The argument here lacks rigor. While large-scale circulation systems dictate the overall climatic background of the Sichuan Basin, their influence on the background wind fields of individual cities within the basin is relatively uniform. The multi-fold difference in near-surface wind speeds between Chengdu and Chongqing shown in Figure 12 is essentially a result of micro-scale dynamic processes, such as local topographic relief and urban canopy drag.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

To Authors:

In the manuscript titled " What makes the less urbanized city a deeper ozone trap: implications from a case study in the Sichuan Basin, southwest China" the authors report on a study of the spatial behavior of ozone in urban areas of two megacities (Chengdu and Chongqing) in the Sichuan Basin for the period 2013–2019 based on reanalysis data of near-surface atmospheric pollutants and remote sensing measurements. The results reveal a dipole structure of ozone trapping in these urban areas with varying levels of urbanization. For the purposes of the study, the authors used various datasets, including: terrain elevation data from GMTED2010; land cover data from the Moderate Resolution Imaging Spectroradiometer (MODIS); a high-resolution O₃ concentration dataset for mainland China (ChinaHighO3); data on ozone, nitrogen dioxide, formaldehyde, and the UV-absorbing aerosol index from TROPOMI; data on atmospheric visibility and sunshine duration from automatic weather stations located in the urban areas of Chengdu and Chongqing; near-surface wind data from ERA5-Land, as well as satellite data from MODIS-Aqua for measuring surface temperature. The authors note that relative O₃ concentrations show an upward trend with increasing distance from central urban areas, while NO₂ shows the opposite downward trend. Furthermore, in the seasonal analysis, O₃ levels increase most rapidly with distance in winter, followed by fall and spring, while NO₂ levels decrease most rapidly with distance in summer, followed by spring, fall, and winter. The topic addressed is timely and falls within the scope of remote sensing, but the originality of the contribution is not sufficiently well substantiated.

The presented study is interesting and potentially significant. Ozone gradients between urban and rural areas, as well as ozone suppression/retention in urban areas, are of great importance for air quality management and the development of effective urban climate policies. The manuscript also attempts to distinguish between chemical and meteorological factors, which adds scientific value. The comparison of two megacities within the same basin topography also adds scientific value, and the results obtained can support differentiated strategies for reducing ozone in megacities. However, in its current form, I believe the article requires substantial revision to meet the criteria and high standards of Remote Sensing. Stronger statistical validation is needed, as well as a clearer delineation of the contribution of remote sensing to the study. The causal interpretation is overly exaggerated and is based primarily on correlational evidence. There is also a lack of methodological rigor, clarity, and consistency.

More specifically:

The article concludes that the “ozone trap” is more closely linked to meteorological conditions than to chemical ones. This statement is too categorical, given the evidence presented. You compare the distribution of NO₂, the distribution of HCHO, the duration of sunshine, and wind speed, but this does not prove a causal relationship. Instead of claiming that meteorological conditions are the primary determining factors, it would be more appropriate to note that meteorological conditions appear to be closely related to the observed spatial gradients of ozone.

Furthermore, the description of the chemical processes related to ozone is overly simplified. In the study, the authors place strong emphasis on NO titration and proxy indicators for NO₂ and HCHO. However, ozone formation mechanisms are nonlinear and depend on a number of parameters such as radical chemistry, mixing in the surface boundary layer, photolysis rates, and transport mechanisms. Satellite data on HCHO alone are insufficient to draw a conclusion about the limiting role of VOCs. Recommendation to the authors: if possible, include validation with ground-based stations in Chengdu and Chongqing.

Regarding urbanization, the authors use the ISA indicator (impervious surface area). It is useful but too limited, as urbanization is a multidimensional phenomenon. Parameters such as population density, traffic intensity, industrial emissions, building morphology, and others are missing. It would be more accurate from a scientific standpoint to specify that ISA is an indicator of land cover, not an exhaustive representation of urbanization.

The abstract is well structured and clearly presents the context, objectives, results, and contributions of the study. However, it lacks a clear description of the methodology used, which limits the reader’s ability to fully understand how the reported results were obtained. The Introduction clearly states the objectives and research questions. However, the literature review is not sufficiently up to date and needs to be more detailed. Only 6 of the 55 cited sources are from the past four years. I also recommend, in order to ensure greater clarity and scientific rigor, that a formal definition be provided for the term “ozone trap.”

Regarding the data and methods used in the study, I recommend specifying the versions of the Sentinel-5P products, the methods used to create time series and for cloud removal, how missing data were filtered, why a core radius of 5 km was chosen, and what the sensitivity is for a core radius of 10 km or 15 km. The presented results are interesting but seem too descriptive. There is no assessment of their statistical significance, no confidence intervals are provided, and more tables for quantitative comparison are needed. The Discussion section also needs to be revised and expanded. It is important to point out the discrepancies between the data for the columns and the surface, to indicate the uncertainties in the reanalysis, and to note and discuss missing observations of volatile organic compounds (VOCs). The Conclusions section is well-structured, but it is recommended that the statements be toned down.

Comments on the Quality of English Language

The manuscript contains numerous grammatical and stylistic issues such as subject-verb agreement, the use of definite articles, plural formation, and expressions that are atypical for the language, which make it difficult to read. I recommend a thorough professional edit in English.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors The authors have made revisions based on my suggestions. However, there are still a couple of small points to address before publication:   1. The review number in Figure 1 should be presented in English rather than Chinese.   2. Please add a legend to the wind vectors in Figure 15 to specify what the arrow sizes represent in terms of wind speed.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

I commend the authors for the substantial revisions made to the manuscript. I note that the authors have carefully addressed the recommendations of the reviewers, including the comments raised in my previous review. These recommendations have been incorporated into the revised version through appropriate corrections, clarifications, and additions presented in a rigorous, coherent, and scientifically sound manner. I find the revisions introduced across the individual sections satisfactory, particularly those related to the refinement of terminology, the statistical validations undertaken, and the clarifications provided regarding the methodological framework and analytical procedures applied. The inclusion of the study by Ren et al. (2024) to support the validation of O3 production sensitivity using in situ ground-based measurements is appropriate and strengthens the scientific basis of the manuscript. Overall, the revised manuscript demonstrates a clear improvement in scientific quality, methodological transparency, and clarity of presentation.

The additional investigations and analyses of meteorological constraints, as reflected in the new Figures 14 and 15, significantly enhance the scientific rigor and methodological depth of the study. The findings derived from this component of the work also demonstrate strong potential for future citation. I encourage the authors to further develop this line of inquiry in future studies by examining PBLH variability over longer temporal scales, including its seasonal cycle, as well as under varying environmental conditions such as solar radiation, cloud cover, humidity, and related atmospheric factors.

The revisions concerning the terminology used throughout the methodology and text with respect to ISA—as an indicator of the physical extent of urban land cover and the urbanization footprint—are appropriate. The addition of a dedicated paragraph in the Discussion section addressing the limitations of the present study, particularly the lack of high-resolution spatiotemporal data (e.g., traffic intensity and related variables), represents an important and scientifically transparent acknowledgment.

The inclusion of methodological details in the Abstract, as well as the expanded literature review, is also appropriate. The proposed definition of the “ozone trap” is clear and suitable, and will likely be beneficial for a broader readership.

The descriptions of the filtering procedures and quality assurance/quality control (QA/QC), based on threshold values of QA < 0.75 and cloud cover > 0.3, are consistent with established methodological standards and have been rigorously applied in the revised manuscript. Equally important is the clarification of the spatial–temporal averaging procedures used in constructing the time series.

The sensitivity analysis of the kernel radius, together with the additions to the Discussion addressing inherent uncertainties—such as column-versus-surface discrepancies and limitations associated with reanalysis products—are appropriately presented and further strengthen the overall scientific contribution of the study.

Overall, the English language has been substantially improved, and in its current form I consider it to be of a sufficiently high academic standard.

Author Response

Please see the attached file.

Author Response File: Author Response.docx