Lange et al. (2024) provide thorough validation of the GEMS NO2 column against ground-based DOAS measurements. The work also provides the comparison of diurnal variation as observed by GEMS and ground-based DOAS measurements.

In my opinion, this work deserves publication. My main concern is the readability of the work. It would be beneficial to condense the figures and main text discussions to help the readers take out the key scientific messages from the work. Most of the comments are related to helping the authors to achieve such a goal.

Recommendation: Minor Revision

Specific Comments

Line 95: These two other works seem relevant for references: Oak et al. 2024 (https://doi.org/10.5194/egusphere-2024-393) and Edwards et al. 2024 (https://doi.org/10.5194/egusphere-2024-570).

Lines 165 – 167: Does GEMS not correct for instrument polarization sensitivity and scene inhomogeneity? I’m curious if this correction is unique to GEMS IUP-UB retrieval.

Section 2.1.2: What’s the rationale behind redoing the DOAS fit for GEMS IUP-UB retrieval? In lines 160 – 162, the most of uncertainties seem to be dominated by AMF calculation. How does it affect the final VCD quality?

Section 2.1.1: More details about L1 to SCD for GEMS official product would be beneficial just like how great details are provided for GEMS IUP-UB retrieval (lines 164 - 168).

Line 217: “The here used tropospheric NO2 VCDs are” might need a grammar fix.

Section 2.3: What’s the quality difference in Pandora data between the direction sun measurements vs. multi-axis mode? Doesn’t multi-axis mode require more assumptions? It might be beneficial to provide the advantages and disadvantages of multi-axis mode.

Lines 253 – 254: Perhaps change “hot spots” to a different language. They have elevated NO2 concentration against the background.  

Line 259: Ulsan is not a remote region. The authors mentioned in line 126 that Ulsan is an important industrial center. Similarly, in lines 383 – 384, I wonder if Ulsan low-polluted sites.

Lines 260 – 265: It might be better to move this to line 249. Otherwise, readers will wonder why GEMS L2 v2.0 has a much coarser resolution than GEMS IUP-UB v1.0 as soon as they see Figure 2. At first, I was wondering how GEMS IUP-UB v1.0 has a finer pixel size than GEMS L2 v2.0 when GEMS IUP-UB v1.0 uses a coarser resolution of chemical transport model for the computation of AMF.

Section 4: It might be better to move Figure 3 to the SI and make the discussion about Figures 3 and 4 more concise. It might be helpful to move 308 – 325 to the Appendix except for lines 321 – 323.

Lines 351 – 353: These lines don’t add value to the scientific discussion in Section 4.1. Correct me if I am wrong, but it seems obvious that GEMS IUP-UB production and TROPOMI product will show good agreement as they use similar retrieval processes.

Figure 5: It might be beneficial to move this figure to the SI as not all sites are discussed in detail in Section 4.1. Showing only the sites that are discussed but moving the rest to the SI might be helpful to improve the readability.

Section 4.1: I wonder if Section 4 and Section 4.1 can be combined into one and become more concise.

Line 361 – 362: A correlation of coefficients of 0.87 and 0.82 are similar. No need to mention that 0.87 is better.

Lines 365 – 358: These lines seem unnecessary.

Section 4.2: It would be helpful to add more scientific discussion to this section. What additional information can we obtain from this section that is different from Sections 4 and 4.1? Can they become one section and be more concise? The conclusion seems to be the same in that GEMS IUP-UB underestimates against car DOAS.

Lines 373 – 374: Perhaps change to “GEMS is the first geostationary instrument providing hourly NO2 data. We compare the diurnal variations observed by GEMS and ground-based instruments.”

Lines 377 – 379: These can go to figure captions.

Lines 379 – 380: Unnecessary line.

Lines 383 – 390: I suggest reducing these lines to one or two sentences. Most sites show no significant bias against Pandora stations while MPIC Seoul and Suwon show more severe bias in the morning. I am noticing that each station has different months and seasons combined. Could it be stemming from averaging different seasons?

Lines 389 – 390: Explanation as to why would be helpful.

Line 391: I recommend the line to be rephrased to not contain “interesting”. This and other parts of the manuscript.

Line 393 – 399: It would be helpful to include Yang et al. (2023b) and Edwards et al. (2024) in the discussion instead of studies that used the LEO instruments to investigate the diurnal variation. Both studies found similar results as in lines 391 – 393. But I wonder how averaging across different seasons would play a role in interpreting Figure 8.

Lines 399 – 401: Crawford et al. 2021 (https://doi.org/10.1525/elementa.2020.00163) and Chong et al. 2019 (10.4209/aaqr.2017.09.0341) might be helpful to interpret this diurnal variation.

Section 5: As mentioned in lines 403 – 405, the diurnal variation can vary significantly depending on the season. I would like to suggest moving Figures 8 and 9 to SI and removing Section 5. If there is beneficial information that Section 5 can provide that Section 5.1 cannot, keep both sections.

Line 410 – 411, lines 440 - 441: Edwards et al. (2024) and Yang et al. (2023b) found consistent results.

Line 414 – 415: Yang et al. (2023b) found a more pronounced increase in NO2 VCD over the course of the day in the winter when the wind speed was segregated. Figure 5a showed a similar diurnal pattern in the YSU site as this work where the NO2 decreases after noon. When the wind speed is not segregated, the transport term offsets the emission term leading to weaker diurnal variability.

Lines 415 – 420: It seems like the diurnal variation of NO2 in spring is more similar to the summer following what is being described. The autumn seems to show diurnal variation more similar to that of the wintertime based on SNU and Yonsei sites.  

Lines 421 – 432: It may be better to focus on interpreting the diurnal variation in this section.

Figure 11: This figure is too busy. It might be better to move it to the SI.

Line 445: Not sure what “but already when considering all data” means.

Figure 13. I wonder if the result for one site can be shown and move the other sites that are not thoroughly discussed into the SI section. This might be helpful for readability.

Lines 450 – 451: No need for a separate paragraph for two sentences.

Figures 14 and 15. Can it be condensed to one hour in the morning, noon, and afternoon? It might have to make the figure more concise. The main difference seems to be in 7:45 KST, 11:45 KST, and 16:45 KST. Perhaps one of Figures 14 and 15 can be shown in the main text and the other one in the SI as the general conclusion is the same for both sites.

Figure 16: Can only a few months be shown that shows a prominent difference?

Figure 17. It might be helpful to just show a few sites on which Section 5.3 discusses and move other figures to the SI.

The manuscript by Lange et al. presents a very thorough evaluation of the performance of the operational GEMS tropospheric NO2 product and the scientific NO2 product from the University of Bremen over Seoul. The relevant aspects of the retrieval are evaluated in an exhaustive manner: absolute magnitude, seasonality, weekend effect, and diurnal cycle. Also some plausible interpretation of the measurements is provided in terms of emissions, transport, and atmospheric chemistry, which strengthens the study. Overall, I fully support publication of this work, and only make a few remarks and suggestion for corrections below. I agree with the other reviewer that the amount of material presented is quite overwhelming and some condensing would benefit the readability of the paper.

General remarks

* It may be useful to discuss the differences between stratospheric NO2 in the operational, IUP-UB and TROPOMI products in more detail. Especially since stratospheric NO2 is argued to be one of the reasons for the overestimation in the operational GEMS product. Is there a clear reason why the method from Bucsela et al. (2013) would result in too low stratospheric columns? Have the stratospheric NO2 columns been validated?

* How does stratospheric NO2 change throughout the day in the two GEMS products studied?

* A clear message what the authors think is the main reason for better validation results around noon than in the morning or late afternoon would be appropriate. 

Minor issues

L179-181: does the GEMS IUP-UB product have a similar quality assurance flagging system as TROPOMI?

L291: typo 'sight' --> slight

L445-446: this sentence was a bit difficult to follow. Please consider rephrasing.

L512: stratospheric NO2 columns are usually on the order of 10^15 molec. cm-2