Articles | Volume 9, issue 8
Research article
22 Aug 2016
Research article |  | 22 Aug 2016

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

Jiaping Pang, Xuefa Wen, Xiaomin Sun, and Kuan Huang

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.