The manuscript presents an approach to comparing CO₂, CH₄, and CO measurements from Total Carbon Column Observing Network (TCCON) instruments by using a travel standard EM27/SUN instrument. TCCON measurements are the reference standard for satellite measurements of total column CO₂ and CH₄. Consequently, having accurate measurements from TCCON is valuable to global GHG observations. Because TCCON instruments are not amenable to transport, it is not possible to move them for co-located instrument intercomparisons that would result in identification and elimination of intra-network variability in observations. This work demonstrates that more compact EM27/SUN instruments can be used as a travel standard to visit TCCON sites and perform co-located measurements, thereby tying the visited TCCON instrument back to a common reference TCCON site. The intercomparison approach presented has clear benefits and resulted in identification of issues to be addressed in instruments visited during this demonstration period. The manuscript does a good job of covering how they implement the transfer standard approach and should be published once improvements are made to the manuscript. Considerable copy editing is needed, both in terms of typos and text-figure mismatches in colors used. Comments/suggestions are below.

General Comments:

The Travel Standard is clearly a step forward in assessing the performance of TCCON instruments, and presumably a similar approach will be used for COCCON instruments. The result of the Travel Standard intercomparison is a series of correction scaling factors that ultimately relate the visited instruments back to the KIT TCCON instrument. Some questions that flow from this:

How often should Travel Standard visits be conducted?

Ultimately we need to tie all the measurements back to the WMO scale. If the KIT TCCON instrument is going to be the reference, should aircraft overflights or AirCore launches be conducted more regularly at the site to keep that instrument tightly related to the WMO scale? If, for logistical reasons, the KIT TCCON site isn’t ideal for overflights/launches, could another site be used for this purpose and the Travel Standard relate that TCCON instrument to the KIT instrument?

Specific Comments:

Consider defining Xgas early in your paper.

Line 23: Give some context to your “pressure analysis” statement here. What pressure is this? Why does it matter?

Line 49, 50: Is there a reference you could provide for the WMO trace gas scale, or the process/importance of tying measurements (in general) to that scale?

Lines 128-133: There isn’t much more info here than in the intro. Perhaps you could trim the corresponding section in the intro for brevity.

Lines 153-155: Please consider providing units for all equation terms.

Lines 165-168: Having the same name for a value and its inverse is unfortunate. Perhaps using a subscript to designate which convention is being used (GGG or PROFFAST) would provide clarity, assuming that the packages won’t settle on a common convention.

Lines 172-178: This text is largely redundant with text earlier in this section and in the intro. Consider removing for brevity.

Lines 212-216: I am not entirely clear on what you are trying to say here. Are you saying that the recorded shocks are at the very start and the very end of the recorded period, and therefore may be from rough handling of the logger when the logger wasn’t attached to the spectrometer?

Lines 218-219: This sentence isn’t necessary—consider removing for brevity.

Lines 288, 295, Fig 2 caption, and Table 1 caption: You’ve got a mismatch between colors mentioned and colors actually used in the figure. Also, since you use different symbol shapes (a good idea), it is worth mentioning them in the text in addition to the color here and in other figures.

Line 309: “It is assumed that the reference in Karlsruhe does not drift in time.” How is the validity of this assumption verified?

Lines 342-349: This correction approach effectively assumes the dependence on SZA is 100% in the Travel Standard instrument. How does the uncertainty in this assumption propagate into the calculation of absolute uncertainty of the CO measurements?

Line 353-354: Not important to this manuscript, but I was surprised that the Karlsruhe TCCON station doesn’t have its own high accuracy pressure measurement considering the importance of this measurement to TCCON data processing. What is the benefit of relying on the DWD data?

Line 434: The ‘yellow “x”-shaped markers’ are actually black triangles in the figure.

Figure 8 caption: For clarity, it is worth mentioning in the caption that the TK-LR, GGG data were only plotted for XCO.

Lines 448, 449, 464, Figure 10 and caption: The colors in the figure do not match the colors in the text.

Line 512: The WG-HR data are red in the figure, not green.

Section 7.1: I did not follow the explanation regarding the Wollongong HR/LR noise level relationship. Why does this explanation only apply to Wallongong and not the other TCCON sites?

Figure 16: Delete the last sentence in the caption—this is a given. What do the whiskers in the plots represent? Consider adding text in the caption to describe them.

General comments:

The manuscript “Using a portable FTIR spectrometer to evaluate the consistency of TCCON measurements on a global scale: The COCCON Travel Standard” by Herkommer et. al. presents the initial results of work to use a portable Bruker EM27/SUN Fourier transform spectrometer (FTS) of the COllaborative Carbon Column Observing Network (COCCON) as a travelling standard to check the consistency of the much larger, more complex high-resolution Bruker IFS125HR FTSs used for the Total Carbon Column Observing Network (TCCON).

This work fulfils both an existing need to provide a cross check between TCCON sites and an emerging requirement to demonstrate the compatibility of the TCCON and the COCCON.

The manuscript is scientifically robust but would benefit from a thorough copy editing to address many typos and grammatical errors as well as technical discrepancies. For example, many of the figures are described with the wrong colours and marker styles in the main text.

This work and its continuation to other sites across the TCCON (and COCCON) will prove to be an excellent resource to both the practitioners within the two networks and the many and varied users of their data and its publication should be supported once the manuscript has been modified.

Specific comments:

Sha et. al. (2020) noted a seasonally varying biases when comparing low and high resolution FTSs which were of a magnitude which could be significant relative to the comparisons made in this work, particularly as the comparisons were only made for short periods and at various times of the year. Was this effect considered by the authors?

In the abstract at lines 20-23 it is not clear if the comparisons described refer to TCCON HR-LR or TCCON-TS.

Do the transport loggers referred to in section 2.2.1 record a time series? If so, it should be easy to identify when during transit events occur. The comparison to Saturn-V rocket launches may not be relevant as the 3.8 g mentioned is likely to be a steady state acceleration not a transient as would be recorded by the loggers. The sixteen plus g experienced by the TS would be consistent with a drop from a small height onto a hard surface if the instrument did not have sufficient protection by deformable material. By resolving the vector of the acceleration, it should be possible to identify where more padding would be beneficial. Fortunately, the rigorous checks carried out by the authors demonstrates that no significant disruption to the TS resulted from these events.

In section 7.2 correction factors between the visited TCCON site and the KA instrument are calculated using a factor derived for the TS to the reference EM27 at Karlsruhe and then onto the KA TCCON site. It would seem more practical to directly derive this for the TS to the KA instrument since the TS will be measuring near KA at this time. This would remove a stage in the derivation of the correction factor and hence a source of error. If there is a good reason not to proceed in this way, it should be made explicit in the main text.

Technical corrections:

The following is not a comprehensive list, and the manuscript should undergo thorough copy editing to pick up on multiple typos, grammatical errors, technical inconsistencies  and improve readability before publication.

L43 has been evaluated

L51 omit "profile observations by"

L208 even -> event

L273 remove first limits

L295 and Fig 2 caption Red crosses should be blue triangles. (plot symbols and colours are consistently incorrect throughout the text)

Figure 4. Delta XCH4 is presented, relative to what? (Ref instrument is assumed)

Figure 6. Although not necessary in this case it is generally more pleasing to have the same y axis scale on adjacent plots.

L407 Extra the

Figure 10 caption. Repeated use of “normed” throughout. It is more normal to use normalised. However, I appreciate that as the scientific vernacular evolves, normed may become the new normal.

Fig 16 caption. Last sentence redundant, every figure should be dicussed in the man text.

L662 l-minute or one-minute?

Equations B7-B9 check the subscripts for consistency

Reference:

Sha,M. K., De Mazière, M., Notholt, J., Blumenstock, T., Chen, H., Dehn, A., Griffith, D.W. T., Hase, F., Heikkinen, P., Hermans, C., Hoffmann, A., Huebner, M., Jones, N., Kivi, R., Langerock, B., Petri, C., Scolas, F., Tu, Q., and Weidmann, D.: Intercomparison of low- and high-resolution infrared spectrometers for groundbased solar remote sensing measurements of total column concentrations of CO2, CH4, and CO, Atmos. Meas. Tech., 13, 4791–4839, https://doi.org/10.5194/amt-13-4791-2020, 2020.