Reviewer -AMT

Evaluation of MOPITT and TROPOMI carbon monoxide retrievals using AirCore in situ vertical profiles. Martínez-Alonso et al., 2022

General:

The authors use the advanced AirCore profiles operated by NOAA during the last years to validate the MOPITT and TROPOMI CO products. With the high measurement altitude of the AirCore measurements, the authors quantify the error introduced in MOPITT CO validation by the use of shorter aircraft vertical CO profiles extended upwards. The results are promising, and the error is estimated to be small. The AirCore profiles are also applied to validate the TROPOMI CO under both clear and cloudy conditions. The results are also consistent with previous studies. In general, the paper is well-written and easy to understand. I would like to recommend it to publish on AMT after addressing some minor comments/suggestions below:

lines 5-6 the unit is in mole cm-2 for MOPITT-AirCore, while in line 19 the unit is in % for TROPOMI-AirCore. Please be consistent, better to use %.

Line 88 – 91: it is not so clear for me to understand the uncertainty of the CO. The authors said that “the total uncertainty is typically <5 ppb (Karion et al., 2013)”, but they also pointed out that “stratospheric CO profiles have shown differences up to ~15 ppb”. Besides the uncertainty of the CO at each altitude, it is more important to highlight the uncertainty of the CO total column (or within the AirCore measurement vertical range)

Figure 6, shows that the relative bias at one latitude (or at the same location) can vary from -18% to 28%. Have you ever investigated the cause for this, e.g. AirCore-satellite distance? cloud? surface? meteorology?

This is a well written paper with an important additional validation of MOPITT and TROPOMI CO retrievals. The main addition to previous validation studies is that the validation is more robust at higher altitudes due to the better vertical range covered by AirCore. I can recommend the paper for publication in AMT after a few minor changes. These are mainly related to the description of the errors, as detailed below.

Abstract. l. 12, 19 and 20: I find these percentage values difficult to understand as they do not make clear to what they refer (percentage of what?). Further e_null (l.20) is not explained and will not mean much to the non-expert reader, certainly in the abstract.

l. 21: please include atmospheric before oxidation

l 26. include reference for the statement on radiative forcing

l. 37. please include reference.

l. 29/38: at first mention of MOPITT and TROPOMII I suggest to include a reference to an instrumental description paper.

l. 48: this is the first mentioning of AirCore. I suggest including a ref. here.

l. 62: this dimension is specific for the NOAA AirCore and not a general AirCore feature. Other AirCores have different dimensions.

l. 70: the altitude uncertainty is not only determined by diffusion. The flow into the tube and the time to establish a pressure equilibrium in the tube during fast descent is an important factor (see e.g. Wagenhaeuser et al., 2021).

l. 92: again, assumptions on pressure equilibrium during sampling or modelling of the flow into AirCore play a role.

l. 100: latter (not later)

l. 149: what is meant by the true value? Also AirCore has an uncertainty. I suggest to be more careful with the use of the term "true". This applies also to other places in the manuscript where "true" is used.

l. 175: please explain what  null-space error is in a way that is understandable for a non-expert reader.

l. 192: The value is certainly not 0.0 x 10^7. It is below a certain value. But not 0.

l. 227-234: I'm not usre that this part is really needed. This section should focus on the difference between using truncated and non-truncated AirCore profiles (i.e. what is discussed in lines 235-242).

l. 251: 2.16% of what?