Summary:

This is a more impactful paper than the current title implies (see specific comments below). The study describes sources of systematic error affecting methane flux estimation using the tracer dispersion method, with acetylene as the tracer. It considers water vapour and other sources of spectroscopic/retrieval error, but the true novelty in the paper is most captured by the potential flux error that arises from non-calibration of acetylene analysers when used as a proxy for CH4.  While the study is specific to the Picarro C2H2 analyser, the general need remains the same. The paper arrives at one flux bias number (7.6% - I think there’s more to offer than this though), which is not insignificant in CH4 emissions quantification methods. Calibration such as that described here is rarely used by those using tracer methods for survey work, but clearly should be.

The paper is an excellent fit to AMT and its readers. The paper has value to scientists and industry using Picarro (and other IR) analysers and those following methane emissions quantification methods and datasets. It is a well-presented, rigorous, and detailed study. While it is refreshing to see such a detailed, lengthy, and complete technical study such as this, some of the important conclusions of the paper get lost at present (see specific comments).

I wholeheartedly recommend publication, and the comments below are constructive in the hope that the potential of the study can be better realised. I also have some technical questions which may need to be addressed satisfactorily.  

Specific comments

1. Title: When I first saw the title, I thought this paper was about the simple calibration of an analyser. It is a fair bit more than this. It is about how calibration of an instrument significantly affects flux calculations. I would recommend a better title to attract readers to this important fact. It would be good to be explicit in the title that this is about impact on accuracy of fluxes.
2. Introduction: I’m really not sure the historical discussion of the discovery of the greenhouse effect is needed (with very old references in places). It is not explicitly linked to the content of the paper. Also, CH4 is the second-most important GHG according to the IPCC, after CO2 (not the third) – please correct, or otherwise clarify. If you are referring to GWP rather than RF (or something else), please be clear.  Line 42 cites Dlugokencky et al., 2011 – please use the most recent reference for this regular review from Dlugokencky).
3. Line 100 – N2O has a lifetime >100 years (greater than CH4). The sentence here implies that use of N2O may be less problematic than CH4 (calling N2O “finite lifetime”). All gases have a finite lifetime in any case. Please clarify and correct this misleading sentence.
4. Acetylene calibration: Ok…. So C2H2 standards aren’t readily available. I understand that. Instead, an empirical calibration of mass flow controllers was used to produce a diluted air matrix. To do this, certified CH4 standards were used to define the dilution characteristics of specific mass flow controllers. This sounds ok, but I do have some questions. 1/ Was the same MFC tested repeatedly (e.g. on different days, after switching on and off etc), and were the results always consistent? If not, can you be confident that there was no drift in the MFC rates when switching between CH4 and C2H2? 2/ Can you be certain that there are no biases between these 2 gases in terms of the experimental setup, e.g. C2H2 may be more sticky than CH4 in sample lines – was flushing and equilibrium time used, or can you be sure there are no residual effects? 3/ When you take into account the precision of the CH4 standard (i.e. how well known the concentration of the CH4 reference cylinder is), how does that precision manifest as a percentage of the concentration of acetylene after dilution? And is that percentage similar to the gain factor calculated for C2H2? It would be worth including a discussion of this in the paper for completeness and the avoidance of doubt.
5. Flux error: A single number is given from the field trials of 7.6%. But a percentage isn’t really that useful to those reading the paper, as a percentage of the flux will depend on all the factors you list that affect the quality of tracer release experiments and fluxes derived from them. A reader taking away the 7.6% number and using it as a guide to the effect of calibration would not be remembering something useful. However, I think you have all that you need here to derive a range of biases for different CH4 fluxes and acetylene release rates. They’re all linearly scalable. So, rather than 7.6%, would it not be better to include a table in the paper that describes CH4 mass flux (g/s) error (and percent error), for a range of different assumed CH4 emission fluxes, or even simply C2H2/CH4 concentration ratios? This would be far more meaningful, and a better guide to others assessing how important calibration may be to their surveys. If you don’t take up this recommendation, the discussion needs to be very explicit that the 7.6% number is not indicative of mass flux error due to instrument calibration and is unique to the conditions of this study (reducing the paper’s impact).
6. There are a lot of important points to take from this paper, but they get lost in the extremely thorough discussion (and the conclusions). I would recommend completely changing the conclusions section to avoid unnecessary repetition of methods and instead focus on the salient conclusions about water vapour, need for acetylene calibration, and potential for flux error. It may also be useful to contextualise the error potential in terms of the magnitude of error from other flux methods (i.e. is it a comparable error if this calibration is ignored?). The important aspects of this useful paper will otherwise get missed.

Technical comments:

1/ Line 152 – Full stop after bracket.

The manuscript entitled “Measuring acetylene with a cavity ring-down spectroscopy gas

analyser and its use as a tracer to quantify methane emissions” by Shah et al. highlights the necessity of careful calibration when apply a commercial instrument measuring acetylene in quantifying methane emission flux. It provides valuable insight on the uncertainty of methane emission flux estimation which could be easily overlooked. The manuscript is well organized and presents instructive information on the experimental setup. I think the it well fits the scope of AMT journal and recommend the publication after minor revision. The following comments shall be considered during the revision.

1. Compared to the major message (i.e., systematic bias can originate from uncareful calibration of the PICARRO instrument) the authors want to deliver, some detailed description on the experiment seems redundant in the main text. For example, discussion on the Allen variance obtained from zero air measurement, description on safety control of C2H2 release, etc.. I suggest the authors move those parts which are not directly relevant on the major message to Supplementary Materials, and revise the manuscript in a more concise way.
2. The PICARRO G2303 instrument used in this study showed 6% bias of measured C2H2 concentration. This results in on average 7.6% bias in estimating CH4 emission. It is well known that the sensitivity of optical instrument could drift with time. Therefore, the 6% bias could be largely instrument dependent. I suggest the authors specify this point. Moreover, the authors applied a complicated dilution system for their calibration. Detailed correction for the bias introduced by dilution is thus needed. The authors have described the reason of using high concentration C2H2. Does it mean that the calibration of PICARRO G2303 shall all following the same way?
3. Line 169: Why applying such a measurement cycle?
4. Line 170: “… are defined as wet measurement”. This is confusing since the field measurements were conducted in dry conditions.
5. Line 245: There are biased between the red dots and the gray lines under dry conditions in panel (a). Is this caused by the error introduced during dilution? Better provide a short statement in the following paragraph.
6. Line 273: “6%”. Should it be “6 ppb”?
7. Line 276: “[H2O]”. Should it be “[C2H2]r”?
8. Line 291: What dose [C2H2]t stand for?
9. Line 432: “ppm” should be “ppb”.
10. Line 439: “shorter” should be “longer”.
11. Line 508: What is the placement height of the anemometer?
12. Line 524: What is the sampling height? Would the height influence the large fluctuation showing in Figure 13?