This well-written paper provides a detailed description of the Harwell TCCON site and its operation since its inception in ~2020.  TCCON sites are encouraged to publish such a detailed description, in particular to demonstrate their compliance with the standard operational procedures (both hardware and software) set out by TCCON.  This paper is valuable reading for any TCCON practitioner (I learned a few things) and is well suited to publication in this journal.  I recommend acceptance for publication after addressing several general and specific comments listed below.

General comments:

The technical descriptions are comprehensive with one glaring omission – there is no description of the path from fitted level 1 spectra  to level 2 column averaged mole fractions (Xgas). This should be added, at least in summary, for completeness and readability of the paper, even if it is described elsewhere. The essential steps to describe are:

1. Retrieval of total column amounts of each gas from spectra, including the profile scaling approach (as distinct from profile retrieval). The is already partly covered.
2. Definition and calculation of total column dry air mole fractions as the ratio of total column of gas to O2 times O2 mole fraction, corrected for water vapour
3. Definition of Xluft and why it is useful

This description belongs in section 5.1.

Mole fraction vs VMR:

The quantities Xgas such as XCO2 are mole fractions, not volume mixing ratios – they are not the same thing: 

Mole fraction(A) = (moles(A)/total moles in mixture)

Volume mixing ratio(A) = volume(A)/(total volume – volume(A))

For TCCON purposes and in in situ measurements  we wish to work with mole fractions. VMR introduces two errors:

1. For trace gases at the ppm level vmr and mole fraction may be insignificantly different (eg 0.04% for CO2 in air) but for O2 the difference is large: mole fraction = 0.21, VMR = .21/.79 = 0.26.
2. VMR assumes an ideal gas and that volume is proportional to amount (moles)

Unfortunately, incorrect use of VMR is entrenched in some TCCON  literature and more widely, but its use should be discouraged.  For example using VMR(O2) rather than  mol frac (O2) in calculating Xgas (point 2 above) would lead to a >20% error in Xgas!  It is not used in metrology literature, which also prefers “amount fraction” to mole fraction”.

My recommendation is to change all usage of VMR to mole fraction throughout the paper, and at first use point out that although historical terminology often refers to VMR, it is actually mole fractions that are calculated and used.

Technical corrections

L4: The TCCON geometry is not solar occultation, it is (direct) solar absorption. Occultation refers to something like an eclipse, or the earth’s atmosphere moving directly between the observer (eg satellite or high altitude balloon) and the sun during sunrise/sunset.

L10: First use of VMR – see general comment above

L13: “zenith solar occultation mode” => direct solar absorption mode”.

L15: it would be good to reference here the early TCCON papers led by Debra Wunch (2011, 2010) which provided the first description of the TCCON (2011) and its “calibration” (2010)

Table 1:  If possible please provide a reference (eg wiki page) for these criteria and requirements. Eg from the wiki

https://tccon-wiki.caltech.edu/pub/Main/TCCONCharter/ETL_TCCON_application.pdf

This is not a public page, maybe it could be referenced and copied to an appendix (without the ETL responses)

Table 1: “ME variation over OPD” is not clear – perhaps substitute “ME variation from 1.0”

Table 1: Lineshape analysis “In beam” => “HCl cell in beam”

L57: “input FTS aperture (field stop)”

L57: This input accepts an f/6.5 beam (full angle of view 8.7 deg)

L59: the exit aperture of the interferometer (replace “beamsplitter”)

L61: one side of the dichroic is in nm and the other in cm-1.  For FTIR, better to use cm-1, replace 625-825 nm with 12000-16000 cm-1 (or quote both nm and cm-1) in both places

L66: an InGaAs detector (not “a”)

L76: Much but not all of the “development” was done at NIWA. Perhaps a clearer sentence would be “A high precision alt-azimuth solar tracker was built in house based on the design of Robinson et al (2020) with some adaptions described below to capture…”

L78: Specify the manufacturer (Physik Instrumente) for the rotators. “PI” is not clear

L94 “magnification of 0.25” Is this correct?  Is the image of the 1 mm input aperture just 0.25 mm on the camera sensor? 

L98: remove “occultation”

L102: NI => National Instruments

L111: pure HCl at ~ 5 hPa pressure

L114: Is your HCl cell really 25mm diameter?  Every one I’ve seen is 50 mm.

L183: O2 A-band (760 nm, 13000 cm-1)

L185: remove “occultation”

L200: suggest rephrase as …the requirement of ME = 1.0 +/- 0.05 or 5% over the range of OPD

L265: Debra Wunch has vry recently prepared a detailed paper on GGG2020 vs GGG2014 that has just been published on the Caltech server, it would be useful to reference that alongside Laughner 2023b

The Total Carbon Column Observing Network's GGG2020 Data Version: Data Quality, Comparison with GGG2014, and Future Outlook (GGG2020.R0). CaltechDATA. https://doi.org/10.14291/TCCON.GGG2020.DOCUMENTATION.R0

L271 and following: VMR => mole fraction

L279: No definition of Xgas etc, see general comments.

L281: best to avoid “calibration” as it does not have the same meaning as commonly used in the in situ community (and has raised ire…)  Suggest replace with “ in the absence of independent site scale factor determination.”

L294: This ZPD location error has now been corrected using a different, simpler  approach and will be implemented in I2S in the next GGG2020 version. 

L296 it is not “incompatible with I2S”, it is “incorrect”.  The GGG2020.next fix identifies and corrects it in I2S. (see GGGBUGS)

L314, table 3: Please provide the Lindqvist equation that is L-M-fitted, it would be much easier to follow the meanings of the fitted parameters in Table 3.

L329: Explain Xluft, it is not obvious to the reader. It is important that the 0.2095 factor for O2 is described as a mole fraction not a VMR (see general comments).

L373: In acknowledging the TCCON community, it would also be appropriate to mention the QAQC teams that especially provide a lot of time and feedback to all sites.

“The Harwell TCCON Observatory” by Weidmann, Brownsword and Doniki, seeks to describe the implementation of a TCCON site at the Rutherford Appleton Laboratory in the UK and demonstrate that the data is of high quality and compliant with TCCON (Total Carbon Column Observing Network) protocols and then goes on to describe a few of the interesting and notable features of the dataset.

Site and dataset descriptions such as this provide an invaluable resource to the user community as well as those seeking to start their own measurement system or even to diagnose issues with existing systems, and their creation and publication should be encouraged.

The manuscript achieves nearly all its objectives in a thorough, technically competent and well written way, with a couple of minor exceptions described in the general comments below. Once these have been addressed, publication of this manuscript would constitute a valuable contribution to the literature.

General comments:

The introduction of the column averaged dry mole fractions of trace gases (denoted X_gas), which are the major outputs of the TCCON processing system, is vague to the point of being non-existent. In particular the fact that these quantities are the result of scaling by the co-retrieved oxygen column means that various instrumental/systematic errors are removed. The introduction of the scaling to produce the X_gas quantities would also lead naturally to the definition of the X_luft diagnostic, which is mentioned in the manuscript but not described. The correct units for X_gas should be mole fractions, e.g. nano or micro moles per mole, although ppb and ppm respectively seem to be used interchangeably in the literature.

Throughout the manuscript the measurement principle is described a solar occultation. This is not correct; occultation refers to the process of the source becoming hidden or eclipsed over time, whereas these measurements are made nearly instantaneously whilst the source (the sun) is fully visible.

Specific comments:

Abstract: It would be nice to have more of the meta-data included in the abstract, such as the latitude and longitude of the site. As the data set is fundamentally associated with this paper it would be justified to include the full DOI either here or prominently in the introduction.

L9. At the first mention of the TCCON would be a logical place to cite Wunch et. al. 2011.

L10. (and subsequent mentions) column averaged dry mole fraction (or ratio). This is also a good opportunity to introduce the X_gas notation, even if it is defined later.

L18. References for CO2M and MicroCarb.

L25. TCCON has already been introduced and so the acronym could be used here.

L31. It is mentioned later in the manuscript, but the dates of the change of location would be useful here.

L39. The description of the wind rose implies that this is only selected data. If this is so, it should be made explicit here, and in the caption of Fig. 1.

Fig. 1. The reproduction of this (and several other figures) is small and difficult to read, a larger font would be helpful.

L50. Although reproduced fully here, the TCCON requirements are also documented on the TCCON wiki, a reference and last accessed date here would be informative.

L61. Inconsistent use of wavelength and wave number.

L73. Are solar measurements DC?

L73. (or Table 1) define HCl at first use.

L78 and 81. It can be assumed that the parenthesised codes are part numbers, but this should be made more explicit.

L110. The acronym OAPM has previously been defined, one acronym should be used consistently.

L113. Define CaF₂

L124. Enumerate the second item.

L154. It would be more normal to specify rainfall as a rate, mm hr^-1, or simply state “no rainfall”.

Fig. 5. Sub figure labels are missing.

L231. Remove second “to” or consider modifying the sentence to make it more understandable.

L313. “the associated modelled seasonal variation.” Might be clearer.

L318. Including a description of the model as an equation would help the reader.