Effect of Lockdown Measures on Atmospheric Nitrogen Dioxide during SARS-CoV-2 in Spain

Round 1

Reviewer 1 Report

The paper presents variations in the NO₂ column over Spain during the spring of 2020. Positive changes in air quality became one of indirect results of the COVID-19 pandemic. The changes appeared due to numerous quarantine restrictions with a sharp traffic decrease. The paper includes a useful information to understand variations in the Europe air quality in the spring 2020. Some recommendations are noted below.

L.2. The paper title is too broad. The manuscript is really devoted to the nitrogen dioxide column but air quality is more general term including particulate matter, ozone, sulfur dioxide and so on.

L. 45-47. It is desirable to rewrite the text more clearly.

L. 69. The singular form should be ("ability.... has improved")

L. 71. Last word in the name of the instrument is usually spelled as "CHartography", see, for example https://www.iup.uni-bremen.de/sciamachy/.

L. 74. "Sentinel-5 Precursor" should be (instead of "Precurser").

L. 81. "Tropospheric" should be instead of "Thropospheric".

L. 83-84. Definition of Vertical Column Density is presented but it is worth to explain briefly how to separate tropospheric VCD from the stratospheric one.

L. 89. I suppose that at least United Kingdom should be mentioned among the most suffered European countries.

L. 115. "these eleven cities" should be

L. 117. The label "Alicante" should be displaced to the left.

L. 210, 233. I think that it is necessary to mention NO₂ in the captions of Figs. 5 and 6.

L. 229. Monthly values are plotted therefore it is possible to indicate about 30 days of limitations (in average) instead of 45.

L. 251. The colour scale is sharply non-linear, it is necessary to describe in text the correspondence between NO₂ variation and colour.

L. 253-259. The text should be rewritten to become more comprehensible for readers. Firstly, negative relationship with both reduction and variation is mentioned that is not clear. Secondly, burning of fossil fuels is noted as one of the main sources of NO₂, but the next sentence supposes that motor vehicles are the predominant source.

L. 259. An incorrect form "is also increases" is used.

L. 261. The ordinate axis in Fig. 8 exhibits change in vertical column that should be presented in the label.

L. 332. The singular form should be: "The comparison... shows"

L. 336, 338. Commas between initials of the authors are necessary.

References. Several positions from the available list should be corrected. In particular, this relates to the references 7 (Publisher should be: John Wiley and Sons), 16, 24 (first names are interpreted as last names). It is desirable to check the whole list of references.

Author Response

The paper presents variations in the NO₂ column over Spain during the spring of 2020. Positive changes in air quality became one of indirect results of the COVID-19 pandemic. The changes appeared due to numerous quarantine restrictions with a sharp traffic decrease. The paper includes a useful information to understand variations in the Europe air quality in the spring 2020. Some recommendations are noted below.

Thank you for the opportunity to resubmit our manuscript. Our manuscript has benefited greatly from your review and we appreciate the favorable comments from reviewer. We have revised it to clarify your suggestions. We believe our revised manuscript aligns with your comments.

L.2. The paper title is too broad. The manuscript is really devoted to the nitrogen dioxide column but air quality is more general term including particulate matter, ozone, sulfur dioxide and so on.

Your suggestion has been taken into account. See Line 2

“Effect of Lockdown Measures on Atmospheric Nitrogen Dioxide During SARS-CoV-2 in Spain”

45-47. It is desirable to rewrite the text more clearly.

Your suggestion has been taken into account. See Line 44-47.

NO₂, specifically, has been correlated with mortality in studies in different parts of the world [15,16]. It is true, however, that there is no clear evidence to establish that NO₂ acts as an independent agent causing said mortality [17].

69. The singular form should be ("ability.... has improved")

Your suggestion has been taken into account. Sentence has been rewritten, see Line 69.

“The ability to observe and monitor air pollutants from sensors onboard satellite platforms has improved in the last two decades.”

71. Last word in the name of the instrument is usually spelled as "CHartography", see, for example https://www.iup.uni-bremen.de/sciamachy/.

Your suggestion has been taken into account. See line 71.

“…followed by the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartography (SCIAMACHY)…”

74. "Sentinel-5 Precursor" should be (instead of "Precurser").

Your suggestion has been taken into account. See line 74.

“…the European Space Agency launched the Sentinel-5 Precursor (S5P) to provide data on air quality,…”

81. "Tropospheric" should be instead of "Thropospheric".

Your suggestion has been taken into account. See line 81.

“The Tropospheric Vertical Column Density (VCD) data of …”

83-84. Definition of Vertical Column Density is presented but it is worth to explain briefly how to separate tropospheric VCD from the stratospheric one.

Your suggestion has been taking into account and a new reference has been added. See lines 85-88.

Tropospheric and stratospheric column densities are separated using a data assimilation system based on the 3-dimensional global Tracer chemical Transport Model (TM5-MP), after which they are converted to VCD by a look-up table of altitude dependent air-mass factors and information on the vertical distribution of NO₂ [41].

Reference:

Van Geffen, J.H.G.M.; Eskes, H.J.; Boersma, K.F.; Maasakkers, J.D.; Veefkind, J.P. TROPOMI ATBD of the total and tropospheric NO2 data products; 2019.

89. I suppose that at least United Kingdom should be mentioned among the most suffered European countries.

Your suggestion has been taken into account. See line 104.

“Unfortunately, the spread of the coronavirus SARS-Cov-2 has been unstoppable, having become a pandemic [46], with dramatic results in countries like Spain, Italy and the United Kingdom [59] in Europe’s case.”

115. "these eleven cities" should be

Your suggestion has been taken into account. See line 130.

The relationship between the number of inhabitants in these eleven cities and the variation of NO₂ VCD under the lockdown measures is the focus of this analysis.

117. The label "Alicante" should be displaced to the left.

Your suggestion has been taken into account. See line 132.

I think that it is necessary to mention NO₂in the captions of Figs. 5 and 6.

Your suggestion has been taken into account. See lines 226 and 249.

Figure 5. Monthly maps of average values of Tropospheric vertical column of NO₂ in Spain for the years (I) 2019 and (II) 2020 during the months of (a) January, (b) February, (c) March and (d)April.

Figure 6. Monthly maps of average values of Tropospheric vertical column of NO₂ in (a) Madrid, (b) Barcelona and (c) Valencia for 2020 during the months of (I) January, (II) February, (III) March and (IV) April

229. Monthly values are plotted therefore it is possible to indicate about 30 days of limitations (in average) instead of 45.

Your suggestion has been taken into account. See line 245.

“Thus, after 30 days of limitations and restrictions in mobility and activity, the values of NO₂ CDV in these urban areas were similar to those of non-urban areas.”

251. The colour scale is sharply non-linear, it is necessary to describe in text the correspondence between NO₂variation and colour.

Your suggestion has been taken into account. See line 253-255.

“The highest NO₂ VCD reductions are represented in red, while a severe reduction is represented in yellow. On the other hand, those areas with a low reduction are represented in green and those areas with no discernible variation are in cyan.”

253-259. The text should be rewritten to become more comprehensible for readers. Firstly, negative relationship with both reduction and variation is mentioned that is not clear. Secondly, burning of fossil fuels is noted as one of the main sources of NO₂, but the next sentence supposes that motor vehicles are the predominant source.

Your suggestion has been taken into account. See lines 285-289..

A negative relationship was expected between population activity and NO₂ levels, where greater activity leads to a higher level. This component is one of the most important in urban air pollution, with the burning of fossil fuels such as coal, oil and gas being one of the main sources of NO₂. It is estimated that about 86% of nitrogen dioxide in European cities are caused by fossil fuels emitted from motor vehicles [52].

259. An incorrect form "is also increases" is used.

Your suggestion has been taken into account. See line 290.

That means that as populations increase, NO₂ also increases

261. The ordinate axis in Fig. 8 exhibits change in vertical column that should be presented in the label.

Your suggestion has been taken into account. See line 294.

332. The singular form should be: "The comparison... shows"

Your suggestion has been taken into account. See lines 352.

The comparison between the NO₂ concentration values before and after the lockdown measures shows a strong relationship with the number of inhabitants.

336, 338. Commas between initials of the authors are necessary.

Your suggestion has been taken into account. See lines 356-359.

F.-J.M.-C., A.G.-F. and J.E.M.-L. conceived and designed the experiment, F.-J. M.-C., F.P.P. and P.T.-T performed the experiment; F.-J.M.-C., F.P.P. and P.T.-T analyzed the data and F.-J.M.-C. wrote the paper and A.G.-F. and J.E.M.-L. collaborated in the discussion of the results and revised the manuscript. All the authors have read and approved the manuscript.

References. Several positions from the available list should be corrected. In particular, this relates to the references 7 (Publisher should be: John Wiley and Sons), 16, 24 (first names are interpreted as last names). It is desirable to check the whole list of references.

We apologize, there were in citation files. Your suggestion has been taken into account.

Marsh, W.M.; Grossa Jr, J. Environmental geography: science, land use, and earth systems.; John Wiley and Sons, Ed.; 2nd editio.; John Wiley and Sons: New York, 2002; ISBN 0471503967.

MacIntyre, E.A.; Gehring, U.; Mölter, A.; Fuertes, E.; Klümper, C.; Krämer, U.; Quass, U.; Hoffmann, B.; Gascon, M.; Brunekreef, B.; et al. Air Pollution and Respiratory Infections during Early Childhood: An Analysis of 10 European Birth Cohorts within the ESCAPE Project. Environ. Health Perspect. 2014, 122, 107–113.

Beelen, R.; Hoek, G.; Brandt, P.A. van den; Goldbohm, R.A.; Fischer, P.; Schouten, L.J.; Jerrett, M.; Hughes, E.; Armstrong, B.; Brunekreef, B. Long-Term Effects of Traffic-Related Air Pollution on Mortality in a Dutch Cohort (NLCS-AIR Study). Environ. Health Perspect. 2008, 116, 196–202.

Author Response File: Author Response.pdf

Reviewer 2 Report

line 108 You used Standard Deviation, this is not clear. since standard deviation is used for error while you did not discuss any error or variance in this part.

all your display are in mol/m^2, while the SI unit is µg/m^3 . it would have been much better to present your data in the SI unit in order to compare with regulatory limits. Or if it's possible to show a relationship between the two units.

Your discussion part can be improved. for example the first 2 paragraph of your discussion can easily be moved to your introduction, since they are general information.

Discussing the drop in NO2 concentration in relation to population density seems vert interesting, i hoped that the author focused more on this aspect, maybe extracting a pattern or threshold for definitions of different types of population density that can be later used in future studies. Also, these threshold can be used to compare different location/countries drop in NO2

Author Response

Thank you for the opportunity to resubmit our manuscript. Our manuscript has benefited greatly from your review and we appreciate the favorable comments from reviewer. We have revised it to clarify your suggestions. We believe our revised manuscript aligns with your comments.

line 108 You used Standard Deviation, this is not clear. since standard deviation is used for error while you did not discuss any error or variance in this part.

We have clarified it and sentence has been rewritten. See line 123.

As a result, the average population density in Spain is 416 habitants per square kilometer,

all your display are in mol/m^2, while the SI unit is µg/m^3 . it would have been much better to present your data in the SI unit in order to compare with regulatory limits. Or if it's possible to show a relationship between the two units.

Remote sensors measure VCD [mol/m²] while parameters to represent air quality, as concentrations, are express in mg/m³. Both are necessary to model this relation. One options it is to use ground-based measurements or to use models of chemistry transport model like MOCAGE. For Spain, there is an adaptation called MOCAGE-AEMET. In addition, this model is not accessible.

However, this is not a problem because mol/m² is generally used by scientific community.

Your discussion part can be improved. for example the first 2 paragraph of your discussion can easily be moved to your introduction, since they are general information.

Thank you for helping us improve the readability of our paper. We have re-structured the introduction and the discussion.

Discussing the drop in NO2 concentration in relation to population density seems very interesting, i hoped that the author focused more on this aspect, maybe extracting a pattern or threshold for definitions of different types of population density that can be later used in future studies. Also, these threshold can be used to compare different location/countries drop in NO2

We have taken into account your suggestion. A new explanation has been added and a new figure has been included, see lines 272-282.

Figure 8 shows the average value of tropospheric VCD of NO₂ reduction in relation to the number of inhabitants, resulting in nine categories. In general, as the number of inhabitants increases, NO₂ decreases. Cities with less than 50000 inhabitants had least significant reduction, with an average value of -2.99×10^-5 mol×m^-2. On the other hand, those cities with more than 600000 inhabitants were those that presented the greatest reduction, with average values of less than -1.05×10^-4 mol×m^-2. Considering the first three categories with the lowest number of inhabitants, the factor of increase in the reduction of tropospheric VCD of NO₂ was equal to 1.28 per 50000 inhabitants. On the other hand, it decreased slightly among the categories of 150000 to 600000 inhabitants, being equal to 1.03 per 50000 inhabitants.

Figure 8. Number of inhabitants versus variation of tropospheric vertical column of NO₂.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper applies Sentinel-5P images to analyze spatial distribution of NO2 and its evolution under the lockdown measures in Spain in March and April 2020. The authors show the variation of NO2 before and after the lockdown using the Google Earth Engine as the tool for image processing. They conclude a significant correlation between the population’s activity level and the reduction of NO2 values.

Questions and Suggestions

1. What type of Sentinel-5P data products (Level 1B or Level 2) being used in study?
2. Please include detailed methods for handling the remote sensing images, such as the pixel filtering method to eliminate errors, and the method to address pixel loss in some areas. (More technical details can be found in below reference:S5P Mission Performance Centre Nitrogen Dioxide [L2 NO2] Readme)
3. To better characterize and validate the selected remote sensing data used in Spain area, please conduct coherence analysis of NO2 ground-based monitoring concentration and tropospheric column concentration.
4. Please verify the content of reference 50 (see line 462 & 463).
5. Relevant references should be included.
1. Zheng, Z.; Yang, Z.; Wu, Z.; Marinello, F. Spatial Variation of NO₂ and Its Impact Factors in China: An Application of Sentinel-5P Products. Remote Sens. 2019, 11, 1939.
2. S5P Mission Performance Centre Nitrogen Dioxide [L2 NO2] Readme. Available online: https://sentinels.copernicus.eu/documents/247904/3541451/Sentinel-5P-Nitrogen-Dioxide-Level-2-Product-Readme-File

 

Author Response

This paper applies Sentinel-5P images to analyze spatial distribution of NO2 and its evolution under the lockdown measures in Spain in March and April 2020. The authors show the variation of NO2 before and after the lockdown using the Google Earth Engine as the tool for image processing. They conclude a significant correlation between the population’s activity level and the reduction of NO2 values.

Questions and Suggestions

Thank you for the opportunity to resubmit our manuscript. Our manuscript has benefited greatly from your review and we appreciate the favorable comments from reviewer. We have revised it to clarify your suggestions. We believe our revised manuscript aligns with your comments.

What type of Sentinel-5P data products (Level 1B or Level 2) being used in study?

We have used Level 3 data, which is the level information stored by Google Earth Engine. The reason is Sentinel 5P Level 2 data are binned by time, not by latitude / longitude. Therefore, it is necessary to convert them to a single grid por orbit. In this stage, moreover, data are filtered to remove pixels taking into account QA values.

We have taking into account your suggestion and we have clarified level processing. See lines 146-147.

“Thus, Sentinel 5P Level 2 data [64] are processed to obtain a single grid per orbit, which allows the Google Earth Engine to process the data”.

Please include detailed methods for handling the remote sensing images, such as the pixel filtering method to eliminate errors, and the method to address pixel loss in some areas. (More technical details can be found in below reference:S5P Mission Performance Centre Nitrogen Dioxide [L2 NO2] Readme)

We have provided information about method to eliminate errors. On the other hand, in this study we did not have problem about pixel loss because we used a synthetic image to represent monthly time series.

Your suggestion has been taken into account, see line 147-149.

In addition, the data are previously filtered, resulting in pixels with quality assurance values less than 75% being removed, such as cloud or partially snow covered pixels, errors and / or problematics retrievals.

To better characterize and validate the selected remote sensing data used in Spain area, please conduct coherence analysis of NO2 ground-based monitoring concentration and tropospheric column concentration.

We were very interested in this as well. Unfortunately, it is not possible at this moment.

While it is true, in Spain there are more than 600 stations all over the country registering air quality parameters, there is no open data policy with updated information. Currently only data until 2018 are published, you can check it in https://www.miteco.gob.es/es/calidad-y-evaluacion-ambiental/temas/atmosfera-y-calidad-del-aire/calidad-del-aire/evaluacion-datos/datos/ .

Please verify the content of reference 50 (see line 462 & 463).

Your suggestion has been taken into account. See line 505-506.

Peto, J.; Alwan, N.A.; Godfrey, K.M.; Burgess, R.A.; Hunter, D.J.; Riboli, E.; Romer, P. Universal weekly testing as the UK COVID-19 lockdown exit strategy. Lancet 2020, 395, 1420–1421.

Relevant references should be included.

Zheng, Z.; Yang, Z.; Wu, Z.; Marinello, F. Spatial Variation of NO₂ and Its Impact Factors in China: An Application of Sentinel-5P Products. Remote Sens. 2019, 11, 1939.

S5P Mission Performance Centre Nitrogen Dioxide [L2 NO2] Readme. Available online: https://sentinels.copernicus.eu/documents/247904/3541451/Sentinel-5P-Nitrogen-Dioxide-Level-2-Product-Readme-File

Your suggestion has been taken into account. References have been added, see references 32 and 64.

Author Response File: Author Response.pdf