Intercomparison of two cavity ring-down spectroscopy analyzers for atmospheric (CO2)-C-13/(CO2)-C-12 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 CO2 delta C-13 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 delta C-13 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 parts per thousand, and the sensitivity of delta C-13 to the water vapor mixing ratio was 1.01 parts per thousand/% H2O and 0.09 parts per thousand/% 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 parts per thousand for G1101-i and 0.13-0.03 parts per thousand for G2201-i. The sensitivity of delta C-13 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 delta C-13 measured by G1101-i and G2201-i captured the rapid changes in atmospheric delta C-13 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 delta C-13 difference of G1101i-original and G2201-i and the water vapor concentration, but there was no significant correlation between the delta C-13 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