The authors focus on the declines and peaks in NO2 pollution during the multiple lockdown phases in the New York metropolitan area and disentangle the contribution of anthropogenic emissions sources and role of meteorology. In general, I find this manuscript to be of interest for publication and appropriate for Atmospheric Chemistry and Physics. I have one main concern and several minor suggestions for improvement listed below that should be considered by the authors before publication.

The authors use total column NO2 in this study and a few references are made to tropospheric column NO2 throughout the manuscript. The authors can be specific about whether the “column NO2” refers to “total column NO2” or “tropospheric column NO2”. I have pointed out a few instances below. Typically, studies use tropospheric column NO2 to relate to changes in NOx emissions. How do the values of total column NO2 compare to tropospheric column NO2 for New York metropolitan area? The authors should consider discussing why total column NO2 is used and the possible impacts on the results.

Minor comments

Line 68 Population and area of New York metropolitan area?

Line 81-86 Do both these studies use total column NO2? Or tropospheric column NO2?

Line 85 “0.4° radius”. Also mention in kms to compare to 100km radius in previous sentence.

Line 101 “4% yr^-1 decrease …” is in tropospheric column NO2. How would the trend values be for total column NO2?

Line 106 “…highest national NO2 levels.” The authors can give value of annual mean NO2 and compare to the recently updated WHO guidelines.

Line 117 Here and everywhere else, the authors mention high-frequency observations from Pandora but do not provide any value.

Line 131 Section heading can be changed to “Materials and Methods” as the subheadings also focus on the various datasets.

Line 135 Last assessed date for the URL.

Line 169 Tropospheric columns of?

Line 173 Filtering criteria such as TCNO2>0 can lead to a positive bias in the mean TCNO2. Is it possible to quantify how much data is removed because of this filtering criteria and if the positive bias is large?

Line 187 The authors use total column NO2, but the retrieval steps are also given for tropospheric vertical column.

Line 189-190 Would the version change have a significant impact on the results?

Line 193 Here again, the authors mention about validation of “tropospheric columns”.

Line 253 “(v) in March-April 2021” should be “March-May 2021”, right?

Line 266 URL or DOI for MTA data?

Line 297 “Variability in TCNO2 also decreased…” except New Brunswick?

Line 320 “Fig.5, right panel.” “Middle and right panels” perhaps?

Line 324-326 This statement starts suddenly and the value for NO2 changes need to be given before explaining why they are lower compared to NOx changes.

Line 339-344 The authors can consider giving a brief description of how these changes in NOx emissions were obtained (either here or in section 2).

Lines 484-492 Interesting result!

Table 1 The table caption can include details of the time. “Present” may be replaced by “05/2021” Additional footnotes can be used on the column of “Temporal range of data” to mention data unavailability. For example, data from PSI#135 not available for Jan-Mar 2020 is mentioned in Figure 4 and should also be mentioned in table 1. The value of stdev for PSI#56,#69 for Sept-Nov reads as “0.24x”. Check for typo. Lastly, are both the PSIs at Queens, New Brunswick and New Haven in operation for the time period stated and is the data from both of them used?

Figure 1 Is there a way the different major pollutant emitters can be identified on the map? For example, by use of numbers with the circles and the numbers can be stated in the figure caption. This would aid the interpretation of the results on lines 419-424.

Figure 2 What is the “ratio difference”? It is not clear in the figure caption also.

Figure 7 The colors for TROPOMI and Pandora bars in (a) are different from the ones in the key. In panel (b), please add the year next to the months and consider adding error bars.

Figure 8 The caption can be slightly modified to reflect that TROPOMI observations are only over Manhattan.

Figure 9 The authors can consider labeling the panels and refer to the individual panel in the text.

Figure 12 The time and location are missing in the figure caption.

Declines and peaks in NO2 pollution during the multiple waves of the COVID-19 pandemic in the New York metropolitan area

Maria Tzortziou, Charlotte F. Kwong, Daniel Goldberg, Luke Schiferl, Róisín Commane, Nader Abuhassan, James J. Szykman, Lukas C. Valin

This paper presents an extensive study of declines and peaks in NO₂ levels durng different phases in the COVID-19 pandemic in New York city. Columns of NO₂ from TROPOMI observations and PGN for four stations in the New York metropolitan area are presented for periods pre- and post-pandemic. These observations shown changes in NO₂ columns up 36%. Additionally,  model simulations of air masses and meteorological information are combined with PGN and TROPOMI observations in order to evaluate impact of NO₂ pollution in the New York metropolitan area during the multiple waves of the COVI-19 pandemic.

The topic of this work fits well within the scope of ACP. The manuscript is well structured and well written. However, the main concern is about the lack of evidence of decrease solely to lockdown periods during pandemic but not to as a general reduction NO2 emissions in New York. Would be possible to evaluate this decrease during lockdown as consequence of only mobility reduction in New York city and not as result of decreasing tendency of general reduction of NOx emission as suggest other studies (e.g. Zangari et al., 2020) by comparing with earliest years?

I recommend acceptance to ACP after addressing the comments above and few minor comments below.

Additional comments:

L179-180: Please add reference for OMI and SCIAMACHY instruments.

L247: Similar to the comment above, have you checked the NO₂ amounts for earliest years thatn 2018 for PGN or any satellite observation?

L273: Would possible to generate a similar figure to SI-2 but comparing with 2018?

L276: Are you comparing tropospheric NO2 columns with total columns from ground based measurements? Or are both tropospheric columns?

L284: Would be possible to repeat the same figure for OMI or GOME-2 NO2 observations but including earliest years to confirm real reduction to COVID-19 lockdown and not a combination of other factors?

L288: Already Manhathan and Queens present a significan reuction for 2018 in comparison 2019 before pacdemic lockdown, e.g. June-September and October 2018 and January 2019?

L290: Figure 3, TROPOMI columns are very low and difficult to observe the reduction in NO2 columns. What is the detection limit of NO2 trospospheric columns from TROPOMI?

L331: Can be possible to quantify the increase or decrease of Nox emission due to only energy sector? It is clear that contribution from transportation sector is reduced, but energy sector, specially dosmestic consumption probably has an significant increase due to the COVID-19 lockdown.

L340: Decrease of power demand/usage in New York city is associated to?

Figure 6: Please check the title display of subfigures.

Line 409: It would be worth to add a figure showing this data in the supplementary material.

L427: Would be possible to add the location of the power plants in the maps similar to Figure 1?

L448-448: Please add a figure if it is possible in the supplementary material for the matched of TCNO2 in 23 and 25 April.

References:

Shelby Zangari, Dustin T. Hill, Amanda T. Charette, Jaime E. Mirowsky, Air quality changes in New York City during the COVID-19 pandemic,

Science of The Total Environment, Volume 742, 2020, https://doi.org/10.1016/j.scitotenv.2020.140496.