A Review of “The Antarctic stratospheric Nitrogen Hole: Southern Hemisphere and Antarctic springtime total nitrogen dioxide and total ozone variability as observed in Sentinel-5p TROPOMI data” by A. de Laat et al.

< General Comments >

This paper describes a new analysis of stratospheric “Nitrogen Hole” using TROPOMI nitrogen dioxide (NO₂) data and assimilated ozone (O₃) data. The analysis idea is somewhat new and found some new aspects on springtime cross-vortex chemistry/dynamics on NO₂ and O₃. However, since the stratospheric photochemical lifetime of NO₂ (10-100 s) is much shorter than that of HNO₃ (10⁵-10⁶ s), special care is needed to treat the stratospheric NO₂ data. The authors need to more carefully treat this point in the paper, as is pointed out in the following comments. When the modification of these points is completed in the paper, I think that the paper is worth published in Atmospheric Chemistry and Physics.

< Major Comments >

1) P.6, L.160: The authors first tried to validate the TROPOMI SNO2 data with ground-based SAOZ data. However, the TROPOMI SNO2 data are acquired at 13:30 local time, while SAOZ data are acquired at local sunrise. The authors claim that “a diurnal cycle correction is applied based on model calculations”. Since this is a critical point for comparison, more detailed description is needed for this “diurnal cycle correction”.

2) If ascending node crossing local time of Sentinel-5P is 13:30, the descending node crossing local time is 01:30. However, there is no description on whether the authors are using only ascending part of the orbit, or using both ascending and descending parts (full parts) of the orbit. Since the measurement local time is important for NO₂ analysis, please clarify this point.

3) The authors use the term “Noxon cliff” for both the cliff for NO₂ and that of O₃. However, as far as I understand, the “Noxon cliff” can be used only for the cliff of nitrogen species (NOx, HNO₃, N₂O₅, etc.), but not for O₃. Therefore, all the description after Section 3.4, where the authors use the terminology “Noxon cliff for TCO3” should be re-worded.

< Minor Comments >

4) P.6, Figure 1A: Please explain why there are differences in darkness both in S5p total NO₂ data points and reference total NO₂ data points in this plot.

5) P.6, Figure 1B: There is no explanation on different three regression lines. Please explain them either in caption or in the legend. Also, no color bar is shown in the figure. Please add a color bar.

6) P.6, Table 1: Why there are relatively large biases (< -10 %) in Rio Gallegos data? Please add some explanation.

7) P.6, Table 1: The order of stations in the table should be not in alphabetical order, but from lower latitude to higher latitude.

8) P.7, L.195: “MSR-2” first appeared in the text which is not explained elsewhere, nor any reference is shown. Please explain MSR-2 and add some references.

9) P.7, L.197: “TEMIS” first appeared in the text which is not explained elsewhere, nor any reference is shown. Please explain TEMIS and add some references.

10) P.7, L.203: The authors claimed “longitude-latitude grid of 1.5°x1.0° and is re-gridded to 0.8°x0.4° to match …”. How they re-grid the data into finer resolution grid? Please explain.

11) P.7, L.205: “GOME-2 has a 4 DU bias”. Is this a positive bias or a negative bias? Please explain.

12) P.8, Figure 4: The panel for 2020 is wrong (the one shown here is for 2019). Please add a panel for 2020.

13) P.8, Figure 5: Please add panel numbers [A]-[F] in Figure 5. In the figure caption, use [A]-[F] for the corresponding explanation.

< Grammar/Typos >

14) P.1, L.28: This process depletes the Antarctic … --> This process depletes nitrogen oxides (denitrification/denoxification) in the Antarctic stratospheric vortex (Farman …

15) P.2, L.29: Farman et al., 1995 --> Farman et al., 1985

16) P.2, L.31: during Antarctic spring to the then denitrified … --> during Antarctic spring to the denitrified …

17) P.2, L.45: Struthers et al 2004 --> Struthers et al., 2004

18) P.6, L.170: regression coefficients equal 0.94 --> regression coefficients equal to 0.94

19) P.11, L.336: And complex relationships between (long-lived) … --> And complex relationships among (long-lived) …

20) P.14, L.416: J d.L. --> A.d.L.

21) P.15, L.417: P.V. --> J.P.V.

“The Antarctic stratospheric Nitrogen Hole: Southern Hemisphere and Antarctic springtime total nitrogen dioxide and total ozone variability in Sentinel-5p TROPOMI data” provides a scientifically useful analysis of a TROPOMI measurements of nitrogen dioxide during the Antarctic ozone hole. The study demonstrates that co-located TROPOMI NO₂ and O­₃ observations can be used to clearly identify the evolution of chemical differences between inner and outer polar vortex air masses throughout the springtime. While demonstrating the viability of a new dataset for the analysis of the Antarctic ozone hole is scientifically important, improvements to the presentation of the data and details about the design of the analysis are needed.

Please refer to the attached PDF for specific comments.