Evaluating calibration strategies for isotope ratio infrared spectroscopy for atmospheric (CO2)-C-13/(CO2)-C-12 measurement

Isotope ratio infrared spectroscopy (IRIS) provides an in situ technique for measuring delta C-13 in atmospheric CO2. A number of methods have been proposed for calibrating the IRIS measurements, but few studies have systematically evaluated their accuracy for atmospheric applications. In this study, we carried out laboratory and ambient measurements with two commercial IRIS analyzers and compared the accuracy of four calibration strategies. We found that calibration based on the C-12 and C-13 mixing ratios (Bowling et al., 2003) and on linear interpolation of the measured delta using the mixing ratio of the major isotopologue (Lee et al., 2005) yielded accuracy better than 0.06 %. Over a 7-day atmospheric measurement in Beijing, the two analyzers agreed to within -0.02 +/- 0.18% after proper calibration. However, even after calibration the difference between the two analyzers showed a slight correlation with concentration, and this concentration dependence propagated through the Keeling analysis, resulting in a much larger difference of 2.44% for the Keeling intercept. The high sensitivity of the Keeling analysis to the concentration dependence underscores the challenge of IRIS for atmospheric research