Following more than ten years of collaboration between LSCE and LIPHY, a new infrared spectrometer dedicated to measuring water isotopic composition at very low humidity has been developed specifically for Antarctica in situ monitoring of isotopic exchange between the snow and the atmosphere.
The new instrument takes advantage of two techniques (Figure 1): Cavity Ring Down Spectroscopy (CRDS), an extremely sensitive infrared spectroscopy technique enabling to measure at very low humidity; and Optical Feedback Frequency Stabilisation (OFFS), which limits the drift of the instrument and thus the need for calibration. Combining both, this infrared spectrometer is able to measure with satisfactory precision for humidity levels close to 1 ppmv, corresponding to more than 95% of the conditions found at Dome C, a station on the top of the East Antarctic plateau where some of the coldest conditions on earth are found with temperature going down to -90°C.
To prepare for this instrument to be deployed in the field where the laser frequency cannot be monitored as easily as in a fully equipped spectroscopy laboratory, we implemented a frequency auto-referencing scheme. We make use of the very fine absorption pattern of a Lamb dip, a saturated effect visible at the very top of absorption lines, as a frequency etalon. By measuring it every hour, we limit the drift of the frequency of the laser source (Fig. 1a) below 10 kHz, ensuring the the associated drift of the isotopic measurement of the instrument remains below 0.01‰.
Authors: M. Casado, A. Landais, T. Stoltmann, J. Chaillot, M. Daëron, F. Prié, B. Bordet, S. Kassi