Reliable water vapour isotopic composition measurements at low humidity using frequency-stabilised cavity ring-down spectroscopy

Abstract. In situ measurements of water vapour isotopic composition in polar regions has provided needed constrains of post-deposition processes involved in the archiving of the climatic signal in ice core records. During polar winter, the temperatures, and thus the specific humidity, are so low that current commercial techniques are not able to measure the vapour isotopic composition with enough precision. Here, we make use of new developments in infrared spectroscopy and combine an optical-feedback frequency-stabilised laser source (OFFS technique) using a V-shaped cavity optical feedback (VCOF) cavity and a high-finesse cavity ring-down spectroscopy (CRDS) cavity to increase the signal-to-noise ratio while measuring absorption transitions of water isotopes. We present a laboratory infrared spectrometer leveraging all these techniques dedicated to measure water vapour isotopic composition at low humidity levels. At 400 ppmv, the instrument demonstrates a precision of 0.01 ‰ and 0.1 ‰ in δ18O and d-excess, respectively, for an integration time of 2 min. This set-up yields an isotopic composition precision below 1 ‰ at water mixing ratios down to 4 ppmv, which suggests an extrapolated precision in δ18O of 1.5 ‰ at 1 ppmv. Indeed, thanks to the stabilisation of the laser by the VCOF, the instrument exhibits extremely low drift and very high signal-to-noise ratio. The instrument is not hindered by a strong isotope–humidity response which at low humidity can create extensive biases on commercial instruments.