Intercomparison of two cavity ring-down spectroscopy analyzers for atmospheric ¹³CO₂  ∕ ¹²CO₂ measurement

Abstract. Isotope ratio infrared spectroscopy (IRIS) permits continuous in situ measurement of CO₂ isotopic composition under ambient conditions. Previous studies have mainly focused on single IRIS instrument performance; few studies have considered the comparability among different IRIS instruments. In this study, we carried out laboratory and ambient measurements using two Picarro CO₂δ¹³C analyzers (G1101-i and G2201-i (newer version)) and evaluated their performance and comparability. The best precision was 0.08–0.15 ‰ for G1101-i and 0.01–0.04 ‰ for G2201-i. The dependence of δ¹³C on CO₂ concentration was 0.46 ‰ per 100 ppm and 0.09 ‰ per 100 ppm, the instrument drift ranged from 0.92–1.09 ‰ and 0.19–0.37 ‰, and the sensitivity of δ¹³C to the water vapor mixing ratio was 1.01 ‰ ∕ % H₂O and 0.09 ‰ ∕ % H₂O for G1101-i and G2201-i, respectively. The accuracy after correction by the two-point mixing ratio gain and offset calibration method ranged from −0.04–0.09 ‰ for G1101-i and −0.13–0.03 ‰ for G2201-i. The sensitivity of δ¹³C to the water vapor mixing ratio improved from 1.01 ‰ ∕ % H₂O before the upgrade of G1101-i (G1101-i-original) to 0.15 ‰ ∕ % H₂O after the upgrade of G1101-i (G1101-i-upgraded). Atmospheric δ¹³C measured by G1101-i and G2201-i captured the rapid changes in atmospheric δ¹³C signals on hourly to diurnal cycle scales, with a difference of 0.07 ± 0.24 ‰ between G1101-i-original and G2201-i and 0.05 ± 0.30 ‰ between G1101-i-upgraded and G2201-i. A significant linear correlation was observed between the δ¹³C difference of G1101-i-original and G2201-i and the water vapor concentration, but there was no significant correlation between the δ¹³C difference of G1101-i-upgraded and G2201-i and the water vapor concentration. The difference in the Keeling intercept values decreased from 1.24 ‰ between G1101-i-original and G2201-i to 0.36 ‰ between G1101-i-upgraded and G2201-i, which indicates the importance of consistency among different IRIS instruments.