Night-Time Skyglow Dynamics during the COVID-19 Epidemic in Guangbutun Region of Wuhan City
Round 1
Reviewer 1 Report
In the article " Artificial Night-time Skyglow Dynamics during the COVID-19 Epidemic in Guangbutun Region of Wuhan, China", the authors observed COVID-19 epidemic lockdown has a direct influence on urban socioeconomic activity, including night-time light (NTL) changes in Wuhan. This manuscript utilized Night-time light images to map urban skyglow. The writing of the manuscript looks good overall. I see the novelty in both scientific findings and methodological approaches. The authors clearly state the scientific significance of Night-time light changes in Wuhan city during and before lockdown. The manuscript shows a clear picture of the scientific foundation, structure, focus and clarity of argumentation. The objectives are very clearly outlined in the introduction, the used datasets are well described with sufficient detail.
However, I have a few concerns/suggestions, particularly centering on the method section for improvement.
The author mentioned that they have started monitoring and collecting the night-time sky data from November 1, 2019, to April 12, 2020, before lockdown and during the lockdown. The key concern is about the Chinese New Year vacations. I have personally observed a drastic change in commercial activities near the Jiedaokou area during the Chinese New Year break that was right before the lockdown in Wuhan. Did the author observe any change during that particular time period and mentioned it in the manuscript?
Furthermore, in Figure 2 (a, b and c) the date is not mentioned at the time of the pictures taken. Either these are taken before lockdown, during a lockdown or after lockdown?
Detailed comments by line numbers (as templated by authors):
1. Line 73 to 75: The statement is incorrect, as there are few studies/research published about the Wuhan night-time related.
https://doi.org/10.1007/s10311-021-01222-x
https://doi.org/10.14358/PERS.87.3.197
2. Line 284: The start time was November 1, 2019, but the end date is January 19, 2020. Why? It should be January 22, 2020, before the lockdown was implemented, not January 19, 2020.
3. Line 286: The lockdown was implemented on January 23, 2020, but the author put data from January 28 to January 31, 2020, only. Why!
I would suggest a minor revision for this manuscript and accept the paper after revisions are done. My sincere congratulations to the authors, although there are works with similar objectives in the bibliography, the proposed methodology is adequately detailed and facilitates the interpretation of the results obtained, showing an arduous scientific and technical work for its processing. I encourage you to continue on the path you have begun.
Author Response
Thank you for providing constructive and helpful comments, and we have revised the manuscript according to these comments. The following pdf file is our point-to-point answer to your concerns.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript describes long-term measurements of the brightness of the night sky in Wuhan (China) before, during and after the Covid-19 lockdown. The instrument used is a standard SQM-DL. The main findings are: the the advertising/facade switch-off time started earlier during lockdown and did not recover to the pre-lockdown value; the fluctuations of the skyglow (authors associate it with the changing illumination of the advertising banners and with other human activities) was significantly lower during the lockdown at 20:30-22:00; time between 20:30-21:00 seems to be most sensitive to the human activities; the sky brightness after midnight was stable but did not recover to the pre-lockdown values probably due to less polluted atmosphere (associated with the lockdown).
The findings can be interesting for the wider community, currently the analysis of the influence of the lockdown on the skyglow is quite popular topic.
SQM is a commonly used instrument, but requires regular calibration due to its long-term instability. Observed long-term changes in SQM sensibility are smaller than changes observed in Wuhan, so the measured values are valid. However, I recommend to add the information about the long-term stability of the SQM and corresponding reference(s).
I have concerns about the conclusions in the section 4.1, where the time-dependence of the skyglow is analysed, particularly the dramatic increase in sky brightness on 21.2.2020 (end of the lockdown). As authors stated properly in sec. 4.2, the sky brightness strongly depends on the aerosol content in the atmosphere. However, the aerosol content wasn't determined, so the changes in the measured sky brightness are caused by the combination of both amount of light emitted by the Wuhan-city and the atmospheric conditions. This means that only long-term averages are relevant. The point at 21.2.2020 (in fig. 7) looks like a single outlier - the values before (first three weeks in February) and after (middle of March) looks similar in average. Unfortunately, the Moon was in the first quarter in the days after 21.2., which did not allow measurements before midnight. In my opinion, an alternative explanation of the decrease is the change in atmospheric condition. Aeronet data from the cities in north, ost, south and west directions from Wuchan shows a dramatic increase of the aerosol optical depth (AOD) in days close to 21.2.2020. The same effect can be seen in air quality data from Wuchan, where PM2.5 concentration changed from ca 15-20 to ca 40-75 (main change on 21.2.). I recommend to add the discussion about the influence of atmospheric conditions on the sky brightness in sec. 4.1 and to formulate the conclusion more carefully.
The manuscript needs the check for mistakes, e.g. date 29.1.2020 is shown in figure 4, but 20.1.2020 is written in the figure caption.
I recommend also to check the English, for example in line 377: is: „near to the central of a city“, should be „close to the city centre“.
Author Response
Thank you for summarizing our work and providing constructive comments, and we have revised the manuscript according to your comments. The following pdf file is our point-to-point answer to your concerns.
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper deals with Artificial night-time skyglow during 2 COVID-19 lockdown in Wuhan, China. The paper is well-written and uses actual data for light pollution. Although the paper is of interest, there are several comments on the paper as follows;
The paper on night-time skyglow COVID19 lockdown I reviewed was conducted in Germany. All the variables should be considered were recorded in this paper better than the draft of the paper at the time. However, there are no results for specific ripple effects due to the research results. It merely suggests the amount of star brightness reduction in the night sky caused by human activities. What is the author's opinion on this point?
Since there is no multi-dimensional analysis of the necessity and differentiated insight, The research paper looks like a measurement report.
Is it correct that the measurement results of just one place are presented? I don't think the results of the measurement at one point can represent the artificial lighting characteristics of a large city. Otherwise, the characteristics of each measurement region should be presented. This is because the measurement results of one place are bound to show fixed characteristics depending on the influence of surrounding buildings, making it difficult to reach a conclusion at the urban level.
Author Response
Thank you for providing helpful and constructive comments, and we have revised the manuscript according to your comments. The following pdf file is our point-to-point answer to your concerns.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors edited the revised manuscript according to all my comments.
Reviewer 3 Report
The author clearly presented the limitations of the study and evaluated the value of this paper appropriately.
