How Does COVID-19 Lockdown Impact Air Quality in India?

Round 1

Reviewer 1 Report

This paper needs  improvement: 

• author has to clarify what kind of  tropospheric NO2 it is .... column ?
• also not clear when they refer to  tropospheric NO2, O3  if it is measured from satellite or ground.
• line 104-105 - what is the link to the paper?  may be removed
• line 173 - describe units and measurement method
• line 200 - it is not clear for what ground measurements where considered in this paper. What is the purpose
• line 339 - missing Fig S4 ?
• line 427 - for this values, you can not claim that this is a correlation, similar for line 432
• line 453 - this part is not necessary to be included and needs to be coherent. Authors may consider the high O3 values due to large radiation .... 
• line 478 - add data source 
• line 481- rephrase using % for  the reduction values
• line 492 - a value of 10-20% is not a dominant value, To make a clear statement you have to consider also emission inventory for your study. Since the methods for ground data are not clearly presented, you can not asses the right aspects, precipitation, water vapor in relation with AOD.

Author Response

We would like to thank the reviewer for providing constructive suggestions and comments. We have revised the manuscript based on the reviewer’s comments by providing the following point-by-point responses.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript with the title “How does COVID-19 lockdown impact air quality in India?” is a promising manuscript that studies the of the covid 19 lockdowns over the Indian subcontinent during the lockdown period. Their results are promising, and their study introduces a scenario for future climate actions. This manuscript can be published in “remote sensing” provided the below comments are addressed.

 

Figure 1. 

• Authors should define the "anomaly" mentioned in the figure caption of figure 1 beforehand, even though they have mentioned it in later figures.

 

Figure 2. 

• The last sentence in the figure caption of figure 2 is not clear. Please rephrase it.
• Explicitly state, what does gray shaded area show? 
• Authors should indicate the mean value of NO₂ and AOD for January 01-March 23rd (for example, a black dot) in comparison with the light red dot.
• It seems that for the period of March 24-June 30 the NO₂ and AOD are almost out of phase in all presented regions. The authors should explain the possible reasons for this in the manuscript. 

 

Figure3.

• In figures 3 d, e, and f, an increasing trend in all in AOD after March 24 can be observed, despite decreasing trend of NO₂. Authors should include a possible explanation for this observation in the manuscript.

 

Figure4. 

• One observation of figure 4, is comparing figures 4b and 4c 

That shows a discernable reduction of AOD in 2019 compared to 2020 for the pre-lockdown, which could show the relative success of “NCAP”. The authors should elaborate on this observation in the manuscript. 

 

Figure 8. 

• Page 13. Regarding the description of figure 8, the authors should mention the type of regression performed on the scatter plots. If it is a linear regression it could result in less accurate fit coefficients since the data are scattered. I would suggest, trying orthogonal regression as well.  

Comments for author File: Comments.pdf

Author Response

We would like to thank the reviewer for providing constructive suggestions and comments. We have revised the manuscript based on the reviewer’s comments by providing the following point-by-point responses.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Author improved the document. 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This manuscripts presents an analysis of satellite derived columnar NO2 and AOD over India for the COVID-19 lockdown periods, along with meteorological parameters. While a number of manuscripts have been published on this topic, the layout of the manuscript and the presentation was refreshing.

The manuscript title must change. The paper has nothing to do with NCAP, except for the conclusion that there is a lesson for NCAP from the amount of reductions noticed during the COVID-19 lockdowns.

Line 183 -- what are these NCAP-induced changes? 
Line 256, 283 - reduction due to NCAP?

Line 51-58 talked about objectives of NCAP, but it has nothing on the changes or measures enforced due to the program. As it is explained here and what is available in the literature and media - NCAP is currently a capacity building activity with a goal to reduce 20-30% pollution, but has nothing in the form of control measures to induce changes. It feels like authors are forcefully using the acronym and a program to fit into the manuscript. As is with COVID-19 related data, the manuscript is rich with information and should stick to that story in the paper.

Not clear why TROPOMI datasets were not used in the analysis, which has a higher resolution and other gaseous species as well.

Line 95-111 needs some editing. Some connecting words are missing.
Line 252 - notice(d)

Figure 7 - There is a numbering error in the title. Increase in precipitation (nearly 50%) and increase in wind speed must have a direct impact on aerosol loadings and AOD. 

Reviewer 2 Report

Title may be changed instated of referring to covid to lock down

 recommendation for  authors:

• check paper for spelling and contradictory affirmations example line 182 and line 311
• explain methodology used or developed to analyze data
• units for concentration ..... column or others
• it is not always clear to what you refer ... ground or satellite data
• conclusions and results - present clearly the cause of reduction, it is not due to COVID, it is due to lock down 

Reconsider the paper after major revision!

Reviewer 3 Report

Reference number: remotesensing-1513635

Manuscript title: The impacts of the National Clean Air Programme and COVID-19 on air quality in India

 

1. Introduction: Evaluating the impact of COVID-19 lockdown has been a very hot topic since the start of this pandemic. As a result, there are many papers on this topic from different cities worldwide, including India. Different methods such as air quality monitoring stations, satellite images and air quality modelling were also used. In addition, the conclusions also varied among different studies. Authors need to conduct a more comprehensive review on this topic, to justify the necessity of having another paper on this already intensively covered topic.
2. Method: lockdown was not enforced in one day, and was usually introduced and lifted gradually. Thus it is suggested to use a table to list the dates of key events of this pandemic in India, such as the first case, delta outbreak, various restrictions and their lifts.
3. The analysis covered 2005-2020. It should be noted due to continued air pollution control efforts, air pollutant concentrations in some cities may be on a decrease trend by itself. Therefore, authors need to compare if 2019-2020 changes are more significant to 2018-2019 changes and previous years, so as to conclude if lockdown has obvious effects on air quality in India.

Reviewer 4 Report

Interesting and extensive paper on the use of various satellite instruments/programs/systems for monitoring pollution and human activities which cause pollution as well as NCAP. 

• Introduction - lines 117-119 -  remove sentences in which you describe what is written in each section
• Data and Method - lines 135-137, 152-154, - maybe just mention main web site and not all setting like in lines 168-169, and 172
• Results - line 203 - Figure 1. - can you please label/mark d) part same as a) part?  Also c), e) and f) parts of figure 1 same as b) part?
• Results - line 224 - Figure 2 -  can you please put legend in b), c), d), e) and f) parts of figure same as a) part?  
• Results - line 260 - Figure 3 -  can you please put legend in b), c), d), e) and f) parts of figure same as a) part? 
• Results - line 300 - Figure 4 -  can you please mark major cities in a) and ) part of figure as in Figure 1? And can you mark/label  c), e) and f) parts of figure same as b) part? 
• Supplement - Figure S1 - nothing is drawn - can you make it right? 
• Supplement - Figure S4 - can you please x-axe label months in words - it's little confusing for people from different speaking areas