Abstract. Using laser absorption spectrometry for the measurement of stable isotopes of atmospheric CO2 instead of the traditional Isotope Ratio Mass Spectrometry (IRMS) method decreases sample preparation time significantly, and uncertainties in the measurement accuracy due to CO2 extraction and isobaric interferences are avoided. In this study we present the measurement performance of a new dual-laser instrument developed for the simultaneous measurement of the δ13C, δ18O and δ17O of atmospheric CO2 in discrete air samples, referred to as the Stable Isotopes of CO2 Absorption Spectrometer (SICAS). We compare two different calibration methods: the ratio method (RM) based on measured isotope ratio and a CO2 mole fraction dependency correction (CMFD), and the isotopologue method (IM) based on measured isotopologue abundances. Calibration with the RM and IM is based on three different assigned whole air references calibrated on the VPBD scale. An additional quality control tank (QC) is included in both methods to follow long-term instrument performance. Measurements of the QC tank show that best performance is achieved with the RM for both the δ13C and δ18O measurements with mean residuals of 0.007 ‰ and 0.016 ‰ and mean standard errors of 0.009 ‰ and 0.008 ‰ respectively, during periods of optimal measurement conditions. The δ17O standard error in the same measurement period is 0.013 ‰. In addition, intercomparing a total of 14 different flask samples covering a CO2 mole fraction range of 344–439 ppm with the Max Planck Institute for Biogeochemistry shows a mean residual of 0.002 ‰ and a standard deviation of 0.063 ‰ for δ13C, using the RM. The δ18O could not be compared due to depletion of the δ18O signal in our sample flasks because of too long storage times. Finally, we evaluated the potential of our Δ17O measurements as a tracer for Gross Primary Production (GPP) by vegetation through photosynthesis. Here, a measurement precision of
Simultaneous measurement of δ<sup>13</sup>C, δ<sup>18</sup>O and δ<sup>17</sup>O of atmospheric CO<sub>2</sub> – Performance assessment of a dual-laser absorption spectrometer
