Evaluating calibration strategies for isotope ratio infrared spectroscopy for atmospheric <sup>13</sup>CO<sub>2</sub> / <sup>12</sup>CO<sub>2</sub> measurement

Abstract

Abstract. Isotope ratio infrared spectroscopy (IRIS) provides an in situ technique for measuring δ13C 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 12C and 13C 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.

Non-destructive estimates of soil carbonic anhydrase activity and associated soil water oxygen isotope composition

“…Measured averages for the calibration cylinders were linearly interpolated and correction coefficients calculated from two-point linear regressions between cylinders, allowing correction of sample isotopologue concentrations and calculation of total concentration and δ 18 O of CO 2 (Bowling et al, 2003;Wen et al, 2013;Wingate et al, 2010). This calibration scheme was validated for our isotopic measurements by allowing 40 L of pure CO 2 to equilibrate with 4 L of water in a 25 L barrel at 24 • C. Approximately 3 months after filling, the δ 18 O of CO 2 in the barrel head space, calibrated using the scheme described above, was measured in dilutions with concentrations ranging from 390 to 560 ppm.…”

“…The Wen et al (2013). Measured averages for the calibration cylinders were linearly interpolated and correction coefficients calculated from two-point linear regressions between cylinders, allowing correction of sample isotopologue concentrations and calculation of total concentration and δ 18 O of CO 2 (Bowling et al, 2003;Wen et al, 2013;Wingate et al, 2010).…”

Intercomparison of two cavity ring-down spectroscopy analyzers for atmospheric <sup>13</sup>CO<sub>2</sub> ∕ <sup>12</sup>CO<sub>2</sub> measurement

“…Isotope ratio infrared spectroscopy (IRIS) permits in situ and continuous isotope measurements under ambient conditions and overcomes the shortcoming of traditional isotope ratio mass spectrometers (IRMS), which involve relatively labor-intensive sample collection and preparation (Bowling et al, 2005;Schaeffer et al, 2008;Wingate et al, 2010;Griffith et al, 2012;Werner et al, 2012;Griffis, 2013). To date, various IRIS techniques are commercially available for measuring stable carbon isotopes, including lead-salt tunable diode laser absorption spectrometry (TDLAS, Campbell Scientific Inc.), wavelengthscanned cavity ring-down spectroscopy (WS-CRDS, Picarro Inc.), off-axis cavity output spectroscopy (OA-ICOS, Los Gatos Research), quantum cascade laser absorption spectrometry (QCLAS, Aerodyne Research), and difference fre-quency generation laser spectroscopy (DFG, Thermo Scientific; Griffis, 2013;Wen et al, 2012Wen et al, , 2013. All the data monitored by IRIS analyzers should capture the δ 13 C variations of atmospheric CO 2 with high precision under ambient conditions and should be traceable to the standard Vienna Pee Dee Belemnite (VPDB) scale (Bowling et al, 2005;Schaeffer et al, 2008;Griffis, 2013).…”

“…All the data monitored by IRIS analyzers should capture the δ 13 C variations of atmospheric CO 2 with high precision under ambient conditions and should be traceable to the standard Vienna Pee Dee Belemnite (VPDB) scale (Bowling et al, 2005;Schaeffer et al, 2008;Griffis, 2013). To assess the data comparability of different experiments, it is important to conduct an intercomparison of different IRIS instruments to ensure their compatibility (Flowers et al, 2012;Griffith et al, 2012;Wen et al, 2013).…”

“…Previous studies have shown that temperature dependence, concentration dependence, and spectroscopic interference are among the major sources of errors for IRIS measurements (Griffith et al, 2012;Guillon et al, 2012;Vogel et al, 2013;Wen et al, 2013). Instrument long-term drift is another source of errors affecting IRIS performance (Rella et al, 2013;Vogel et al, 2013).…”