The authors compare the NO2 simulated by CAMS and observed by TROPOMI. The comparison shows better agreement in summer than that in winter. The finding about the vertical profile is very informative. The methodology and conclusions are sound. However, the authors seem to favor super long sentences, which makes it difficult for readers sometimes. I recommend rephrasing the long sentences thoroughly to make them more reader-friendly.

General comments:

1. Section 3. The authors discussed a lot of details about the ensemble database. It is not very clear to me what has been used for comparison in this study, what will be upgraded in the near future, and what has been done by previous studies since all information was mixed. I recommend reorganizing this section.
2. section 5.5. It will be useful to compare the differences between ensemble vs tropomi and ensemble vs individual models.
3. I recommend adding a table listing all products used for comparison in the manuscript and adding a brief description of those products.

Specific comments:

1. line 30. The grammar seems incorrect for the 2^nd Please check.
2. line 40. Line 50. Those sentences are too long to read.
3. Line 70. I don’t see the reason to separate items 2 & 3 as two angles. Additionally, it is useful to point out that the vertical profiles are replaced in item 2. Otherwise, it is confusing for the readers why TM5 is mentioned here.
4. Line 145. What is “compo”?
5. Line 149. Is it operational now?
6. Line 204. I suppose the R in S5P-R represents regional? I suggest putting the name after the description directly. It is easier for the reader to link the name with the product.
7. Line 275. I suggest commenting on the potential reason why TROPOMI cannot detect ship lanes here.
8. Line 281. What is 1^st day forecasts?
9. Line 328. What is “process modelling”?
10. Figure 10. What is “spread”? Do you simply mean NO2 column densities here?
11. Line 513. Do the authors claim a new methodology for satellite-model intercomparison here? What is the improvement compared to Eskes et al. (2003)?

John Douros and co-workers report on comparisons between TROPOMI NO2 column observations and results from the 7 air quality models which are currently operational within CAMS, providing forecasts and analyses over Europe at 0.1x0.1 degree resolution. The comparison shows a reasonable agreement during summer, but a substantial (factor of about two) model overestimation in winter. The use of high-resolution a priori profiles from the CAMS model ensemble (instead of the global 1x1 profiles) in the tropospheric NO₂ column retrieval from TROPOMI results in higher retrieved columns over emission hotspots by about 30%. The authors further performed validation of the new satellite TROPOMI NO2 dataset using remote sensing column measurements and found that despite the overall overall bias reduction compared to the operational TROPOMI product, the new dataset is not able to close the large gap between observed and modelled NO₂ columns in wintertime.

The manuscript does not include significant advances in modelling and has quite limited novelty. It uses pre-existing models and their outputs which are routinely available. However, the study proposes an alternative TROPOMI NO2 dataset over Europe based on high resolution model profiles which could be useful for the community. The method used for this derivation has been already applied in previous studies. I find the comparison of the 

data with the output of the 7 models interesting, in spite of the fact that the reasons for the large mismatches are not investigated in the manuscript. The scientific approach and the methods are not new but they are valid and widely used in the literature. The results are discussed in a balanced way, 

although in most instances the discussion is only qualitative. The writing is not always very precise. The language should therefore be improved in the revised version. Some references are incomplete or not defined, and additional references are needed. I could recommend publication after the following points are adequately addressed.

  

Comments: 

 l.8: "7 up to 11 models". Not precise and misleading since the manuscript presents only results from 7 models.   

 l.13: remove "quantitative"

 l.13: provide information (e.g. bias, correlation) about how close this agreement is 

 l.14: 'significant discrepancy', provide numbers

 l.25-28: here again provide figures of the bias reduction and correlation obtained from this validation 

 l.34: 'values above the surface which are otherwise very scarce', replace by 'measurements at the surface which are very scarce'

 l.36: read 'at kilometer scale'

 l.39-44: This information does not seem relevant for this paper.

 l.50: What are the CAMS systems? I would replace by 'CAMS makes'

 l.51: Inness et al. 2019b is not defined

 l.53: 'consistent' appears twice in the same line, replace by "to daily (re)analyses of concentrations and emissions which are consistent with..."

 l.57: changes are not sharp for pollutants other than NO2, see  https://doi.org/10.1029/2020GL091265, https://doi.org/10.3390/atmos12080946, DOI: 10.1126/sciadv.abg7670. I suggest to drop 'sharp' from the sentence and add some more references. 

 l.58-60: poor wording, Replace 'dedicated studies have been launched to study' by 'dedicated studies have been performed to investigate'

 l.62: near daily basis

 l.72: TROPOMI appears twice, replace 'measurement series' by 'measurement period', mention that TROPOMI NO2 is derived using the global TM5-MP profiles

 l.73: mention clearly the horizontal resolution of the CAMS and the TM5 models

 l.71-75: improve the clarity

 l.81-82: remove 'very small', replace 'very large' by 'high'

 l.84: provide references for your statement

 l.85: remove 'the paper by' here and throughout the manuscript

 l.85-93: check your references, for example Eskes et al., 2021a is missing

 l.97-98: 'to force the stratosphere to be consistent with TROPOMI', weird statement

 l.108-114: 'do not have a large impact', 'rather stable', 'considerable change', provide quantification

 l. 121: MAXDOAS or MAX-DOAS, not both

 l.120: mention that the Verhoelst et al. comparisons do not account for averaging kernels

 l.124: reference missing

 l.135: could you mention the impact of the new version v2.2 described in https://doi.org/10.5194/amt-15-2037-2022 ?

 l.146-48: link not accessible (and too long)

 l. 153: correct typo

l.164: 'have', not 'has'

Figure 1: Acronyms are not explained in the caption. 

l.179: not necessary

Section 4.1: I find this section describes well-known methods in a confusing way. 

l.225-26: avoid repetition of the word 'gridded' in the same line

Sections 5.1, 5.2, and 5.3 could be merged, all of them consist in briefly presenting the figures 5-8

l.325: I could not find Huijnen et al. 2010b in the list

Table 3, add additional columns with the ratio S5P and S5Pcams and CAMS-RG-A. Or add another table. This would help your discussion.  

l.455: did you use 8 or 9 MAX-DOAS stations for validation? In the abstract you mention 8

Section 6.1, the discussion is again only qualitative. For example, 'CAMS is higher close to he suface': higher by how much?

Fig.15 inset statistics are too difficult to read

Fig.16: Is S5Pcams and S5P-RG the same thing?

l.500: 'this is not done here', improve the wording

l.564: '10% column enhancement', is this on average?