Development of an OF-CEAS laser spectrometer for water vapor isotope measurements at low water concentrations

The determination of the isotopic composition of water vapor is an important tool in atmospheric research. The isotopic composition of water in Antarctic or Arctic glacial ice can be used as a paleo-thermometer in the reconstruction of climate changes in the past. The isotope ratios of water vapor in the atmosphere provide information about the water cycle and are an important factor for atmospheric circulation models. This thesis describes the development of an innovative, highly sensitive infrared laser spectrometer, based on the method of Optical Feedback Cavity Enhanced Absorption Spectroscopy, for the quantitative determination of water isotopes in very dry air. Such measurements are not possible with conventional isotope ratio mass spectrometry, nor with commercially available laser instruments. We present a general characterization of the instrument, as well as a detailed discussion of the dependence of the isotopic measurement on the water concentration at which the measurements are performed. We also describe a novel calibration system based on nanoliter syringe pumps, designed to generate water vapor of known isotopic composition in a concentration range from 5 to 15 000 ppmv.This newly developed equipment is used in the context of this thesis for measurements of the atmospheric water isotopic composition at the Norwegian station Troll in Antarctica and as part of the international ISOCLOUD project to study (super) saturation effects during cloud formation in the AIDA simulation chamber of the Karlsruhe Institute of Technology. We conclude with a discussion of the optimal measurement strategy and give an outlook for the future