| 19 Nov 2025
Evaluation of Calibration Strategies for Accurate δ13CH4 Measurements in Dry and Humid Air
Abstract. Accurate determination of the methane isotopic composition (δ13CH4) is essential for attributing emission sources of methane (CH4). However, for measurements with optical instruments, spectral interference from water vapor and instrumental drift often introduces substantial biases in δ13CH4 measurements, particularly for humid air measurements. Although multiple calibration strategies exist, a systematic evaluation of their performance under diverse field conditions remains lacking. Here, we evaluate two calibration strategies for a cavity ring-down spectrometer: a delta-based calibration for δ13CH4 and an isotopologue-specific calibration for 12CH4 and 13CH4. We performed laboratory experiments over a water vapor range of 0.15–4.0 % to establish empirical correction functions, quadratic for 12CH4 and 13CH4, and linear for δ13CH4, to remove humidity-induced biases. These correction functions were then applied to field measurements in both dried air at the SORPES stie and humid air at the Jurong site. At the SORPES site where air samples were dried using a Nafion™ dryer, the mean difference in δ13CH4 between the two strategies was ~0.29 ‰. In contrast, for humid air at the Jurong site, significant inter-method biases were observed, with Δδ13CH4 exhibiting a strong correlation with 1/CH4, indicating non-linear spectral effects at high concentrations that compromise the performance of delta-based calibration. Notably, only the isotopologue-specific calibration, coupled with an explicit water vapor correction, delivered stable and accurate δ13CH4 measurements across all conditions. This work underscores the need for robust calibration strategies to minimize bias in CH4 isotopic composition measurements.
Competing interests: Some authors are members of the editorial board of journal Atmospheric Measurement Techniques.
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This paper provides a thorough analysis of water vapour correction and calibration strategies for measurements of delta-13C in CH4 using a Picarro G2201-i CRDS analyser in humid- and dried-air field measurements. The paper is very clearly written and well structured, and comprehensive in its treatment. There are few typos or cases of unclear language. The biases between the two calibration methods (isotopologue- vs delta-based) are clearly documented and assessed. It is clearly suitable for publication in AMT and I recommend publication with only minor technical corrections and clarifications listed below.
General comments:
L118 et seq. It isn’t clear at this point if the Picarro water corrections on d13CH4 are applied automatically, or replaced by new ones from this work. Are the water-corrected and un-corrected raw data available from the Picarro logs, and if so which have you used to define your own water vapour correction coefficients?
Sections 3.2-3.4 compare calibration strategies and identify systematic CH4-dependent inter-method bias, and discuss the linear and inverse CH4 dependence. As a comment here, perhaps useful for future work, if the analyser can be calibrated for the delta method with a set of at least 3 reference gases covering a reasonable CH4 range, and the simple linear delta-based calibration equation (6) is replaced with a 3-parameter equation of the form of Eq 15 in Griffith 2018,
Del13CH4(cal) = alpha*del13CCH4(meas) + (alpha-1) + beta/CH4 + gamma*CH4
(alpha ~ 1, gamma ~ 0, see also section 3.4) the concentration dependence is automatically taken into account in the delta calibration. No spread in del13C values across the reference gases is required.
Technical comments:
L 21 introduce not introduces
L28 Site not stie
L31 In the abstract, better to avoid terms that you have not yet defined, here Dd13CH4. Better here to say “difference in d13CH4 between the calibration methods” and define Dd13CH4 at first use in the main text.
L33 if there is a strong correlation with 1/CH4, do you mean non-linear effects at LOW concentrations, where the 1/CH4 term is largest?
L 150 et seq – This section is unclear - there has been no reference to Ref1 and Ref 3 yet in the text and it is not clear if the calibrations referred to are for CH4 or for isotopic quantities. Firstly, please cross reference Table 1. Further, Ref1 and Ref 3 cover a wide CH4 span but only a narrow del13CH4 span. I assume that this “correction” refers to CH4 not del13CH4. At line 152-153 specify that that ref4 and ref5 have similar CH4 composition
Fig 1 The multiport valve is labelled solenoid valve in one diagram and Valco in the other – is this correct? A solenoid would not normally be an 8-port valve.
L 167 Perhaps, in context, move the sentence “A detailed description of all reference gases is given in Table 1.” up before first reference to ref gases at line 150
Eq 1 and 2 – subscript on LHS should be true not ture
Eq 1-7 I believe the IUPAC preferred, safest and least ambiguous way to include delta values in equations is to NOT include the factor 1000‰ explicitly. Thus Eq 3 for example would be
del13CH4 = (13r/13rref-1).
If the delta value is say 0.001 it can be referred to in text as d = 0.001 or more commonly d = 1‰. This is the same way % is normally treated. 1000‰ simply says “multiply by 1000 then divide by 1000” .
L218 and/or caption to Table 1, the quantity Rsum (introduced in Griffith 2018) may not de widely recognised. Although Griffith 2018 is widely cited elsewhere (perhaps even repetitively so), it would be useful to cite it directly here for practitioners following the calculations.
L220 It would be helpful to calculate and quote the error in del13C caused by an error in the assumed value of delD and hence Rsum for atmospheric air, to confirm that this is a minor error over reasonable range of values of delD in air.
L240 Just to clarify, please confirm that all measurements were made with the same Picarro G2201 analyser, but several years apart? The correction factors may vary slightly from one instrument to another. This could also be done in section 2.2.