Long-Term Impacts of COVID-19 Lockdown on the NO₂ Concentrations and Urban Thermal Environment: Evidence from the Five Largest Urban Agglomerations in China

Round 1

Reviewer 1 Report

A thorough methodical paper of great relevance.

Author Response

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Reviewer 2 Report

The topic of the paper: IMPACTS OF COVID-19 LOCKDOWN ON THE NO₂ CONCENTRATIONS AND URBAN THERMAL ENVIRONMENT: EVIDENCE FROM THE FIVE LARGEST URBAN AGGLOMERATIONS IN CHINA is relevant, timely, and of interest to the audience of this journal. In this paper an exploration of the changes in the numerical and spatial distribution of NO₂ concentration and surface temperature during four different COVID-19 lockdown stages in 2019, 2020, and 2021 was presented. The long-term effects of lockdown policies in China on the urban environment were analysed. The results of this findings suggest that improvements to the air quality in most regions are short-term and unsustainable. The NO₂ concentration increased again rapidly with the resumption of production. The variations in the urban thermal environment were much more complex. Compared with air pollutants, there was no similar declining variation pattern in the high-and sub-high-temperature area proportions during the lockdown period of the epidemic.

This research area is great importance not only on for the China but in the many places of the world. The paper is clearly written and is very easy to read. The paper is based on academic standards. The abstract is concise and sufficient. The introduction provides the necessary background information. References are complete and appropriate. In the Discussion section, there should be a relation to other research projects … or other regions … set your results in relation to other works. The manuscript should be published after a minor revision.

The following comments are suggested to be addressed in the revised manuscript:

The Figure 1 is not readable enough.

You should unified the notation of equations in Table 3.

The descriptions of the Figures 4 and 5 are not sufficiently clear.

The description of the charts presented in the figure 6 is too short.

Author Response

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Reviewer 3 Report

GENERAL COMMENTS

In this study, Zhang et al. examine the evolution of NO2 concentrations and surface temperatures retrieved from satellite over five urban agglomerations in China in 2019, 2020 and 2021. The temporal and spatial variations seen in 2020 compared to 2019 and 2021 are attributed to the effect of the COVID-19 lockdown policies. Whilst a significant decline is found in absolute NO2 concentrations, only a geographical redistribution is claimed for the heat sources.

Monitoring the long-term impact of the lockdown policies is an important topic which may be of interest to the Remote Sensing readers, and the paper is written in good English. However, in my opinion, the authors should address some major issues before publication, which are described in the following section.

SPECIFIC COMMENTS

1/ My first concern is of editorial nature. The manuscript deals with two very different topics, i.e. air quality (using NO2 as the only proxy) and the urban heat sources/thermal environment. With the exception of the impact of lockdown policies, these topics have very little in common (measurement techniques, causes, analysis methods, etc.) and the reader gets the feeling of reading two different papers pasted together. While this is not an issue from a scientific point of view, the editorial board should consider if publication is suitable in this form;

2/ The scientific questions are not introduced suitably, which makes the overall outcomes quite trivial. It is stated that "the impact of shorter lockdowns was short-term" and that "NO2 concentrations increased again with the resumption of production" (l. 25-26). Why would the authors expect the opposite? A possible reason is related to the difference between short-term "behaviour" and long-term "habits", which is mentioned too late in the manuscript (e.g., l. 272-273) and not dealt with in detail. Also, the authors found that "the thermal environment was not only related to human activities, but also mainly affected by the composition and configuration of the land cover" (l. 295-296). This sounds quite obvious to me. Is there any reason to believe otherwise? If so, this should be clearly stated in the introduction;

3/ For both NO2 and temperature, the discussion of the weather effects is too quick and simplistic. All variations found in NO2 are attributed to changes in emissions/implementation of control measures (e.g., l. 248-249, l. 271, etc.), and the impact of the meteorology is not even mentioned. The authors should either explicitely include it in their calculations, as done in the majority of the published literature dealing with the effect of the lockdown policies on air quality (e.g., using meteorological normalization techniques, or chemical transport models), or at least state an upper limit of that effect based on weather observations and their changes in 2019, 2020 and 2021. At lines 275-282, even the seasonal changes in the observed concentrations are only attributed to the emissions without any reference to the weather conditions (mixing layer height, circulation regimes, etc.). This is likely due to lack of experience by the authors on the topic of air quality and I suggest them to catch up with the current scientific literature in the field. The same shortcomings apply for surface temperature: sometimes the observed variations are attributed to "climate influences" (l. 398-399; incidentally - this should be "weather", not "climate"!) and other times to the lockdown policies (l. 385-387, l. 434). However, I cannot find in the paper a serious analysis discriminating the two effects, nor a discussion of the possible interfering factors in satellite imagery.

TECHNICAL REMARKS

If the paper undergoes further review, I suggest the following changes.

- Title: please, specify "_Long-term_ impacts of COVID-19...", as thousands of other papers have already explored the short-term impacts of the lockdown policies;

- In the manuscript, the distribution of temperatures in an urban environment is defined a "problem" or an "issue" rather than a "feature". Please, provide an explanation why heat sources in an urban environment are dangerous for the human health, especially in the examined period (January-February);

- Sect. 1: an overview of the published studies related to the geographical area of interest is missing;

- l. 27 and anywhere else: the results from NO2 analysis cannot be generalized to all air pollutants without proper discussion. Please, do not refer to "air pollutant/pollution" in these cases and just stick to "NO2";

- l. 43-47: just before and after this paragraph, the topic is the pandemic. Air quality and urban heat island should be discussed in more depth elsewhere;

- l. 47 and anywhere else: I think it is more appropriate to use the word "pandemic" instead of "epidemic";

- l. 49: "January, 23" add "2020";

- l. 50-51: "restricting... restricting";

- l. 62-65: the authors start from too specific examples, and perhaps of minor importance. A number of study have already addressed changes in NO2 concentrations over much larger areas and of bigger magnitude (e.g., 60% or more). No reference to studies in the European continent is made here;

- l. 82-83: please, specify "heat release" from where. What are the major influencing factors of this phenomenon?

- l. 93-96: explain why long-term changes should be seen;

- l. 97-100: I cannot see any link between the two topics;

- Sect. 2.2.1: can you provide more details about the troposphere/stratosphere partitioning algorithm?

- l. 178, "via mathematical operations": this is obvious, please remove;

- Sect. 2.3.1: as far as I understand, this is not a novel method. Hence, this section can be included in Sect. 2.2.2;

- l. 185, "at 250 m": specify "at 250 m spatial resolution";

- l. 193, "Elimination", and l. 200, "eliminate": I cannot understand the use of this verb. The effect was calculated, not eliminated;

- l. 208-209: not clear;

- Table 2: please, explain to what "T", "p", and "std" refer;

- Table 3: please, add bibliographic references for each method;

- l. 250-252: not clear. Also, why should there be a reduction in 2020, in the pre-lockdown period?

- l. 254: not clear;

- Sect. 4: I wonder why the analysis of temporal variations should be in the "Results" section and the analysis of spatial patterns in the "Discussion". Please, use "Results" for results and "Discussion" for discussion;

- l. 314, "conspicuously aggregated": not clear;

- l. 327-328: not clear;

- Figure 5: the color scales are not centered, which makes the interpretation of the plots very complex (where is the zero in the maps?). Also, common color scales should be used in all figures for consistency and better interpretability. I additionally suggest to plot "20 minus 21" instead of "21 minus 20" for an easier comparison with column a. Caption: add "during the lockdown period";

- l. 342, "a decline in the point aggregation": not clear;

- l. 377-397: not clear;

- Figure 6, caption: specify "during the lockdown period";

- l. 417: what "thermal environment problems" were alleviated in January-February, specifically?

Author Response

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