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.