Evaluation of a cavity ring-down spectrometer for in situ observations of 13CO2

With the emergence of wide-spread application of new optical techniques to monitor delta C-13 in atmospheric CO2 there is a growing need to ensure well-calibrated measurements. We characterized one commonly available instrument, a cavity ring-down spectrometer (CRDS) system used for continuous in situ monitoring of atmospheric (CO2)-C-13. We found no dependency of delta C-13 on the CO2 concentration in the range of 303-437 ppm. We designed a calibration scheme according to the diagnosed instrumental drifts and established a quality assurance protocol. We find that the repeatability (1-sigma) of measurements is 0.25% for 10 min and 0.15% for 20 min integrated averages, respectively. Due to a spectral overlap, our instrument displays a cross-sensitivity to CH4 of 0.42 +/- 0.024% ppm(-1). Our ongoing target measurements yield standard deviations of delta C-13 from 0.22% to 0.28% for 10 min averages. We furthermore estimate the reproducibility of our system for ambient air samples from weekly measurements of a long-term target gas to be 0.18 %. We find only a minuscule offset of 0.002 +/- 0.025% between the CRDS and Environment Canada's isotope ratio mass spectrometer (IRMS) results for four target gases used over the course of one year.