With interest I read your manuscript where three different types of measurement are combined.

I have the following comments and questions:

Line 20. Define which correlation statistic is used here. Is it Pearson-R?

Line 21. NMB abbreviation is used in the abstract but is not defined. Does it mean 'normalized mean bias' or 'normalized median bias'

Line 114-115. "Pan #59 at Aldine, #61 at La Porte, and #25 and University of Houston were chosen..."

59 is not in Table 1. 61 is not at La Porte but at Aldine, according to Table 1.

Table 1. I would include here also the altitude, as it is a reason for not using P188.

Line 172. "We download the publicly available data (https://data-portal.s5p-pal.com/products/no2.html) "

In fact, only data processed with processor version 2.3.1, with data up to November 2021, can be found here. The latest reprocessed version 2.4.0 (and 2.5.0 for measurements since March 2023) are currently available from the Copernicus Data Space Ecosystem (https://dataspace.copernicus.eu/)

Line 186. "A comparison between TROPOMI version 2.4.0 and a MAX-DOAS network found that in moderately polluted locations, TROPOMI had a median bias of -35% (Lambert et al., 2023). "

Not only MAX-DOAS, but also Pandora measurements from PGN are included in this quarterly updated validation report. Given that Pandora measurements are used in the current paper, it is logical to mention these results as well.

Line 189-190. What kind of regridding approach is applied?

Line 196-198.  More detail is needed here. Which TROPOMI version is used? How exactly are the total and tropospheric values calculated? By calculating new total and tropospheric AMF and in this way recalculating from the available tropospheric and total VCD available in the TROPOMI file?

Note also that in the operational product, there are two total column variables. Normally only the 'summed column' is recommended to be used.

Also, there is already a stratospheric NO2 available in the TROPOMI data files. Why was this not used to correct the GCAS and WRF-CAMx data?

Line 309. r^2 and NMB are used but not introduced. Is Pearson-R used? What is the exact mathematical definition of NMB?

I see later in the text 'normalized mean bias' appearing, but that does not yet explain how exactly it is normalized (using only reference data or e.g., the mean of reference and satellite data), and if the normalization happens before or after the averaging.

When using relative measures, I recommend using median rather than the mean, to prevent small numbers in the denominator to dominate.

I recommend also to add a statistic on the dispersion. E.g., the standard deviation of the differences.

Fig. 2. MBE, NMB are used in the figures but are not defined.

Also there is a typo in the unit: this should be molecules cm-2 10^16, not molecules cm-2 10^(-16). I recommend to add a dispersion measure like the standard deviation of the differences.

Line 312. "but there was a negative bias (NMB=-22.8%) in v2.4.0. "

Here, it is relevant to compare with the TROPOMI vs Pandora results on the validation server (https://mpc-vdaf-server.tropomi.eu). 

Results using Pandora #25 (university of Houston; PGN location name HoustonTX) are at

https://mpc-vdaf-server.tropomi.eu/no2/no2-offl-pandora/houston-tx 

Results using Pandora #58 are at https://mpc-vdaf-server.tropomi.eu/no2/no2-offl-pandora/la-porte-tx-gsfc058

While it is not fully clear to me what is the meaning of MBE and NMB, the numbers seem broadly consistent with the bias statistics obtained from the validation server, despite the fact that the sampling is not the same (as on the validation server no subsampling to the airborne measurements is done). This indicates that the sample taken in the current paper is a representative set.

Note that on the server, the filter qa_value>75% is not applied yet (only qa_value>50%), so you'll first have to download the data and apply this more strict filter yourself to obtain exactly the numbers below. This setting was taken to have more pixels also for the sites where the stratospheric contribution is dominant.

 

La Porte TX P58:

mean difference=-2.5 10^15 molec/cm2

median difference =-2.2 10^15 molec/cm2

mean relative difference (i.e., mean of (SAT-REF)/REF)=-21%

median relative difference (i.e., median of (SAT-REF)/REF)=-24%

Houston TX P25:

mean difference=-2.3 10^15 molec/cm2

median difference =-1.9 10^15 molec/cm2

mean relative difference =-21%

median relative difference=-23%

Finally, also the comparison with La Porte TX P63 (mentioned in Table 1 but not used in the paper) is available on the validation server. However, in this case the bias is quite different, due to Pandora values being lower (about 2 Pmolec cm-2  lower when calculating on an overlapping time range 2021-10-07 to 2022-01-22). 

In general, it would be interesting if the authors added a consistency check of the Pandora instruments that are at the same location, perhaps in section 2.1.

Line 607. 'both outperform version 2.4.0'. The bias of NASA MINDS and v2.3.1 is indeed slightly better, but the correlation of NASA MINDS is lower than that of 2.4.1. Also, it would be interesting to look at a dispersion measure (e.g., standard deviation of the differences).

Line 643.  "It is common to consider TROPOMI measurements as accurate representation of NO 2  column concentrations; however, if we had done so in this study, we would have failed to identify the substantial negative bias"

Here, it is useful to remind that a negative bias has been mentioned in earlier literature (e.g., Verhoelst et al, 2021) and in the quarterly issued operational validation reports (available at https://mpc-vdaf.tropomi.eu/). This information is therefore already available to the user.

Line 648. In the acknowledgments, there should as a minimum appear: 'contains modified Sentinel data'. 

An example sentence could be 'This work contains modified Copernicus Sentinel-5 Precursor

data processed by KNMI and post-processed by ... (ending with the name of your research institute).