Evaluation of the carbon isotopic effects of NDIR and CRDS analyzers on atmospheric CO2 measurements

Non-dispersive infrared (NDIR) and cavity ring-down spectroscopy (CRDS) CO2 analyzers use 12CO2 isotopologue absorption lines and are insensitive to all or part of other CO2-related isotopologues. This may produce biases in CO2 mole fraction measurements of a sample if its carbon isotopic composition deviates from that of the standard gases being used. To evaluate and compare the effects of carbon isotopic composition on NDIR and CRDS CO2 analyzers, we prepared three test sample air cylinders with varying carbon isotopic abundances and calibrated them against five standard cylinders with ambient carbon isotopic composition using CRDS and NDIR systems. We found that the CO2 mole fractions of the sample cylinders measured by G1301 (CRDS) were in good agreement with those measured by LoFlo (NDIR). The CO2 values measured by both instruments were higher than that of a CO2 isotope measured by G2201i (CRDS) analyzer for a test cylinder with depleted carbon isotopic composition δ13C =-36.828‰, whereas no obvious difference was found for other two test cylinders with δ13C=-8.630‰ and δ13C=-15.380‰, respectively. According to the theoretical and experimental results, we concluded that the total CO2 mole fractions of samples with depleted isotopic compositions can be corrected on the basis of their 12CO2 values calibrated by standard gases using LoFlo and G1301 if the δ13C and δ18O values are known.