Development and airborne operation of a compact water isotope ratio infrared spectrometer†

^†Originally presented at the JESIUM 2008 Conference (31 August–5 September 2008, Presqu'île de Giens, France).

Abstract

A sensitive laser spectrometer, named IRIS (water isotope ratio infrared spectrometer), was developed for the in situ detection of the isotopic composition of water vapour in the upper troposphere and the lower stratosphere. Isotope ratio measurements can be used to quantify troposphere–stratosphere exchange, and to study the water chemistry in the stratosphere. IRIS is based on the technique of optical feedback cavity-enhanced absorption spectroscopy. It uses a room temperature near-infrared laser, and does not require cryogenic cooling of laser or detectors. The instrument weighs 51 kg including its support structure. Airborne operation was demonstrated during three flights aboard the European M55-Geophysica stratospheric research aircraft, as part of the AMMA/SCOUT-03 (African Monsoon Multidisciplinary Analysis/Stratospheric Climate links with emphasis on the Upper Troposphere and lower stratosphere) campaign in Burkina Faso in August 2006. One-second averaged, vertical profiles of δ²H, δ¹⁷O and δ¹⁸O in the upper troposphere are shown, as are the δ¹⁷O–δ¹⁸O and δ²H–δ¹⁸O relations. The data are discussed with reference to a Rayleigh distillation model. As expected, there is no indication of non-mass-dependent fractionation (also known as mass-independent fractionation) in the troposphere. Furthermore, improvements to the thermal management system and a move to a (cryogen-free) longer-wavelength laser source are discussed, which together should result in approximately two orders of magnitude improvement of the sensitivity.

Keywords:

• airborne
• hydrogen-2
• in situ infrared laser spectroscopy
• isotope measurements
• methods and equipment
• mass-independent fractionation
• optical feedback cavity-enhanced absorption spectroscopy
• oxygen-17
• oxygen-18
• troposphere–stratosphere exchange

Acknowledgements

This work was supported by the Dutch Foundation for Fundamental Research on Matter (FOM) (Project 99MAP10), the EC EUFAR Transnational Access program and NASA. It used the infrastructure of the AMMA/SCOUT-03 campaign. The authors are very grateful to Henk Been, Jeff Grose and Bruce Borchers for their excellent technical assistance. Thanks are owed as well to the COLD team: Silvia Viciani, Francesco Castagnoli, Alessio Montori and Piero Mazzinghi for their support with the integration of IRIS on the CVI-rack, and for sharing their in-the-field infra-structure and know-how with the authors during the Verona and Ouagadougou campaigns. The authors also thank the WAS-team in particular for their collaboration, and all AMMA teams present in Ouagadougou for their help, support, flexibility and, without exception, very pleasant cooperation. Finally, they thank Francesco Cairo, Cornelius Schiller and two anonymous reviewers for their valuable comments on the manuscript.

Notes

^†Originally presented at the JESIUM 2008 Conference (31 August–5 September 2008, Presqu'île de Giens, France).

The AMMA/SCOUT-03 campaign was funded by the European Union under grant 4089. On the basis of a French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, especially from France, the United Kingdom, the United States and Africa. It has been the beneficiary of a major financial contribution from the European Community's Sixth Framework Research Programme. Detailed information on scientific coordination and funding is available at the AMMA International web site: http://www.ammainternational.org.