This paper concerns the description and characterisation of a new instrument designed to accurately measure surface spectral emissivity across the far- and mid-infrared wavelength domain. The instrument, called FINESSE (Far INfrarEd Spectrometer for Surface Emissivity), comprises a commercially available EM27 spectrometer from Bruker coupled with an innovative front end viewing and calibration rig, developed in-house at Imperial College London. The rig enables the system to perform views over the full 360° angular range from zenith to nadir on either side, and includes two well-characterised blackbody targets, one at ambient temperature and one held at a temperature above ambient, to provide a built-in radiometric calibration capability. The paper reviewed here is the first part of a two-part study, and focuses on a full and detailed characterisation of the system, uncertainty budget, measurement strategy, and comparison with a line-by-line radiative transfer model, whilst the second part will address how FINESSE measurements are used to obtain surface spectral emissivity in the far- and mid-infrared.

Overall, the paper provides a comprehensive overview of an important new tool for measuring infrared spectral emissivity. The extension of the measurements into the far-infrared spectral region makes this work particularly novel, and timely given the impending launch in the coming years of new satellite missions that will observe these wavelengths at high spectral resolution. I am happy to recommend that this paper is suitable for publication in Atmospheric Measurement Techniques, subject to the following minor and technical suggestions and corrections:

Line 20: suggest “various” in place of “varying”;

Line 23: suggest replacing “profiles” with “column or profile concentrations”;

Line 24: should read “15 µm” instead of “15 mm” (typo);

Line 27: suggest making clear that these are absorption micro-windows;

Line 28: suggest replacing “in some cases” with “under certain conditions”

Line 35: suggest including a reference to the airborne Far-InfraRed Radiometer (Libois et al, 2016) in addition to the TICFIRE reference when citing the planned NASA mission;

Line 43: remove the “0” between “as” and “required” (typo);

Line 200: in Figure 3 and all Figures containing data plots thereafter, it would be clearer to include a legend on the plot themselves so that the reader does not have to refer to the caption to understand what is being shown;

Line 278: the expression for the PRT100 sensor uncertainty is a bit unclear, I assume T is in °C in the expression given? I think it should read something like “±0.15 (1 + 0.002|T|) where T is in °C”;

Line 489: suggest referring to the specific Section when referring to “… self-compensation effects discussed earlier” to help the reader understand the explanation for the lower sensitivity to offsets in the hot target;

Line 497: is the observation altitude 30 m above sea level, or above the ground?

Line 500: did the authors consider using a representative CO₂ profile in the LBLRTM simulation? It’s possible that it doesn’t make a significant difference given the strength of the absorption around 15 µm, but it would be useful to clarify this;

Lines 509-10: replace "mm" with "µm" (typos);

Line 522: suggest also commenting on the potential contribution of the uncertainty in the water vapour continuum absorption model assumed in the LBLRTM simulation here.

Reference:

Libois, Q., Ivanescu, L., Blanchet, J.-P., Schulz, H., Bozem, H., Leaitch, W. R., Burkart, J., Abbatt, J. P. D., Herber, A. B., Aliabadi, A. A., and Girard, É.: Airborne observations of far-infrared upwelling radiance in the Arctic, Atmos. Chem. Phys., 16, 15689–15707, https://doi.org/10.5194/acp-16-15689-2016, 2016.

This paper describes a new instrument that integrates a commercial Bruker EM27 spectrometer with a viewing and calibration apparatus developed by the authors. This spectrometer (FINESSE) is designed to measure radiance in the 400-1600 cm^-1 spectral range.

The paper is the first part of a two-part study and offers a thorough characterization of the FINESSE system, including its uncertainty budget and measurement strategy. Additionally, the authors compare FINESSE's observations to simulations made using reanalysis data and a well-established radiative transfer model.

The authors present a valuable tool for infrared emissivity measurements, particularly significant due to its ability to reach the far-infrared range. This capability is especially opportune with the upcoming launch of satellites designed for observations in these wavelengths.

The paper is well and clearly written and it fits the scope of AMT, I have however some minor and technical suggestions and corrections:

Title: "Part 1" here the Arabic numeral is used, while throughout the paper Part is always followed by Roman numerals "I", "II", please make a consistent choice.

line 24: "15 mm" -> "15 µm"

line 30: add a comma between "that do" and "tend"

line 43: "as0 required" -> "as required"

line 59 add a comma between "Far-IR" and "FINESSE"

line 66: "repeat" -> "repeated"

line 120 delete "of the order"

line 141: can you quote the uncertainty of CO2 probe?

line 163: I suggest to add the resolution used

Fig. 3A: in the Y-axis label: 'Codes' is this a typo?(see also Figs. 4A, 6A)

line 220: add a comma after cavity

Eq. 5: this is not consistent with Eq.4 (and Eq. 8), e_eff -> ε_eff ;  L(σ,T)  -> B(σ,T)

Fig. 4(E) the curve and the legend color for emissivity @150 C don't match

line 450: clarify how L^ext_hot (L^{ext}_{hot}) is determined/modelled when calculating L_scene

line 468: a "signal-to-noise" of NESR sounds odd perhaps -> "assessment"?

line 470: "we take the square root of the spectrally resolved NESR described above as the resultant single scan NESR", please clarify this part. I have two issues here: 

1 - the square root of the rms of the radiance differences would have a measurement units of sqrt(RU)

2 - if I followed your line of reasoning, the NESR on a single scan should be the NESR estimate divided by the square root of 2

line 506: I suggest to change "surface emission temperature" to "blackbody surface emission temperature", since at the end of the next line surface temperature, humidity ... are mentioned the latter being ground surface

line 509 and 510: "15 mm" -> "15 µm"