Dear authors,

I congratulate you to the fact that both referee reports indicate that your paper is well suited for publication after some technical corrections. Please take their comments into account and update the document accordingly. I would also like to point out two issues that might or might not relate to the observed discrepancy between the NDIR instrument and the airplane profiles.

First, the abstract is misleading since it compares laboratory calibrations between 1010 and 250 hPa and aircraft data up to altitudes up to 7 km (500 hPa). It emphasises on the agreement in that range even though data from higher altitudes exist, albeit with discrepancies on the claimed accuracy level. This should be clearly stated in the abstract and in the main text, as pointed out by one of the referees.

Second, eq. (1) is strictly valid only for high resolution spectra at individual wavelengths. From this, the NDIR signal can be obtained by spectral integration, but this certainly will lead to a deviation from the law, taking into consideration that the strongest transitions will be saturated. Also, if one applies the series expansion in eq (2), one expects a relative error on the order of about 1 to 2% (4 to 8 ppm at 400 ppm). This is quite large compared to the intended measurement accuracy. It would be important to point out these inherent methodological drawbacks and explain why they don't matter in the current application.

As pointed out by one of the referees, the English could be improved at instances. Don't hesitate to ask a native speaker for checking the manuscript once again.

Technical comments

Has the acronym NDIR been defined ?

Abstract/ please follow the recommendation of the second referee concerning the abstract and include the instrument performance in the document.

L 27 - 29. The phrase is difficult to understand. Maybe you should regroup : Vertical profiles of atmospheric CO2 can be measured with a 240-400 m altitude resolution through regular onboard calibrations using two different CO2 standard gases.

L 31 - 32. This sentence is not clear. Please revise.

L 180 proportional constant -> proportionality constant

L 530 frequently -> frequent

L 531 any parts of the world. This is probably too optimistic considering safety regulations. You should replace "any" by "many"

L 534 will help improve -> could help improve. Depending on the satellite program, a bias on the order of 1 ppm or larger might be too large for validation.

Dear Authors

Thank you for having responded to all open questions. There is still one minor request linked to the functioning of the optical analyzer.

As you state correctly in the corrected version of your manuscript,

"Although the NDIR analyzer potentially exhibits non-liner absorption due to the saturation of strong absorption lines, the NDIR analyzer is known to have a good linearity within a certain concentration range [Galais et al. 1985]. In our analyses of the balloon data, eq. (1) was used only for the interpolation between the low and high mole fractions of the in-flight calibration gases to obtain the ambient CO2 mole fractions.”

the linearity of the instrument is restricted to a certain range. I have modelled the conditions of 0 - 500 ppm CO2 in a dry O2/N2 mixture in a 12 cm cell at 1 atm and 296 K (see top of attached figure that can be downloaded as comments-to-author file) using actual spectroscopic data from the HITRAN data base. The bottom figure shows the integrated absorption signal when a rectangular filter function is used (such that 400 ppm gives about 3 % of absorption). It is apparent that the peak absorbance is about 0.7 at 400 ppm (thus it is not small) and that the broadband signal does not follow the equation in line 177 of your revised manuscript.

Please revise this paragraph so that theory and your application become consistent. I propose to clearly state that eq 1 and 2 hold for monochromatic light only and that eq 2 only holds for small absorptions. Then you can continue with the above phrase.

I also strongly suggest removing the word potentially in your above phrase.

Best regards

Dear authors,

I congratulate to the final article which can be published with the following technical correction :

Please revise lines 180 - 183 in the following way:

Eqs. (1) and (2) hold for monochromatic light only and eq. (2) only holds for small absorptions. Although the NDIR analyzer exhibits non-liner absorption due to the saturation of strong absorption lines, it is known to have a good linearity within a certain concentration range (Galais et al. 1985) and eq (2) may be used correspondingly.