Talk by Mathieu Casado from LSCE

New developments in infrared spectroscopy for monitoring of water isotopes at low humidities

ABSTRACT Cavity Enhanced Absorption spectroscopy techniques are widely used to monitor isotopic composition of very dilute gaseous samples. As the absorption spectrum of water is not perfectly known, the infrared spectroscopy water isotopes ratio retrievals rely on reference samples. In particular important discrepancies are found in the relative peak intensities of the different isotopes. In practice, to precisely infer the isotopic composition of an unknown sample, several interlacing measurements of the sample and of references with different (known) isotopic composition have to be performed (calibrations). This is both time-consuming and limits the algorithm precision. One way, to circumvent these problems is to provide an adequate model for the water spectra so the retrieval from single sample measurement would be possible. In order to provide such model a detailed study (with high sensitivity and precision) of water vapour absorption spectrum is necessary.

Here, we detail the different the new possibilities open with cavity ring down spectroscopy and the performances obtained down to 10 ppmv of water vapour. This study combine laboratory and field tests of different instruments and also the results of a new spectrometer being developed in LIPHY.