The study investigates the diurnal variation of the NO2 column-to-surface-concentration relationships using observations and corresponding model simulations from three DISCOVER-AQ campaigns and worldwide Pandora NO2 total column measurements. The authors correct the Pandora measurements by considering biases in effective temperature and local solar timing along the line-of-sight used in Pandora retrieval. Regarding model simulations, the manuscript compares two simulations with different horizontal resolutions (12 vs. 55 km) to evaluate the impact of horizontal resolution on model performance in reproducing observed NO2 surface concentrations and columns. The authors demonstrate that NO2 above 500 m has weak temporal variability but dominates total NO2 columns, dampening the variation of total NO2 columns, although NO2 below 500 m shows apparent diurnal variation. The results are interesting and useful to the community. However, I have two major concerns about the quality of the manuscript.

Major comments:

1 Section 3.5 is entirely based on model results. However, Figures 4 and 5 show that the model simulation results differ from the Pandora measurements in the diurnal variations of NO2 total columns even if the Pandora measurements have been slightly corrected. Are you assuming the model is correct? If so, can you explain why Pandora shows weaker diurnal variations than model results? According to the model results, even if NO2 below 500 m contributes less than 50% to total NO2 columns, we should still observe apparent diurnal variations in NO2 columns (Figures 4 and 5). Why doesn’t it occur in Pandora measurements? On the other hand, if you assume Pandora is correct, how can Section 3.5 convince the community, considering that it is based on model results different from observations?

2 Mostly, I can understand what the authors want to say. However, I suggest further improvement of the language of the manuscript. Please find below for further details.

Minor comments:

Line 22: “campaign” to “campaigns”

Line 24: “Pandora columns” to “Pandora NO2 columns”? In addition, do you refer to vertical columns or slant columns? Please use accurate terms.

Line 25-26: Please rewrite the second part of the sentence. I understand what you want to say, but the sentence needs to be clarified.

Line 26: Again, what are the Pandora observations? NO2 vertical columns? Tropospheric or total?

Line 31: “versus” to “against” and delete the second “versus”.

Line 37: “at the surface” to “NO2 surface concentrations”?

Line 39: What do you mean by the integral of weakly connected layers? The sentence following it? Please rewrite the sentence which is too long.

Line 48: Add “spatial” before “gaps”.

Figure A1 (Line 670): Add the period of the DISCOVER-AQ Colorado campaign.

Line 136-137: Please rewrite the sentence.

Lines 145-147: Doesn’t it depend on the accuracy of the stratospheric NO2 vertical columns?

Line 161: Write down the full name of VOC at its first appearance.

Line 200-202: Please rewrite the sentence and explain Equation (1).

Line 209: Add a period after “photometer”.

Figure 2. The observational lines are messed up. Does different coloring make the figures clearer?

Line 236: How did you calculate normalized biases?

Line 241: What do you mean by left panel?

Lines 254-256: Please explain it.

Equation 2: Why did you multiply 0.2? How did you determine this value?

Lines 278-282: I wonder whether comparing the total NO2 columns in the left panel to partial NO2 columns between 300 m and 4 km in the right panel is correct, although it seems the NO2 columns between 300 m and 4 km dominate the NO2 total columns. In addition, is the left panel of Figure 4 for DISCOVER-AQ Pandora or PGN Pandora?

Line 282-287: I think you have applied the correction to the corrected Pandora NO2 total columns. If so, how much does it affect the NO2 total columns?

Line 348-349: Please rewrite this sentence.

Line 397: Does the PBLH modification increase NBs?

Line 403: Add “total” before “NO2”?

Line 408: “versus” to “against”.