Evaluation of calibration strategies for accurate <i>δ</i> ¹³ CH ₄ 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 introduce 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 difference (Δδ13CH4) was observed, with which exhibiting a strong correlation with 1/CH4, indicating non-linear spectral effects are most pronounced at lower CH4 concentrations and 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.