Performance improvement and airborne employment of a highly sensitive tunable-diode laser spectrometer for in-situ measurements of the isotopic composition of water vapor

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Gavin Schmidt calls water isotopologues “the most super-duper fantastic thing ever” [Tollefson, J. (2008), Vapour spies to reveal climate clues , Nature 455, 714-715]. Indeed, the measurement of isotope ratios in water vapor (H2O) can significantly enhance our understanding of many crucial processes in which atmospheric water is involved. The H2O isotope ratios collect and conserve the H2O evaporation and condensation history prior to sampling. They thus contain supplementary information on the hydrological atmospheric cycle which H2O-concentration measurements cannot provide. Furthermore, due to the considerable temperature-dependent isotope fractionation, H2O-isotope ratios of cloud particles are a measure of the supersaturation present during cloud formation. While a number of satellite measurements have recently been realized, they only provide the global picture. Airborne in-situ measurements − providing the often necessary high spatial resolution − have very scarcely been performed, and only about four instruments exist worldwide, one of which is the tunable diode-laser spectrometer ISOWAT. The goal of the present proposal is the performance enhancement, detailed characterization, and airborne employment of the diode-laser spectrometer ISOWAT for in-situ measurements of water-isotope ratios (18O, D). ISOWAT was developed during the foregoing DFG HALO funding period (ZA 550/1-1). The instrument already outperforms the very few other instruments worldwide in terms of its detection limit and size/weight, which is documented by a strong interest of a group of US scientists led by Dr. R. Talbot (University of New Hampshire) to fly a copy of ISOWAT aboard the NASA WB-57. However, we believe that we can improve the performance of ISOWAT and thus even enhance the bandwidth of potential scientific achievements. ISOWAT will be highly useful for a number of future HALO missions, including ML-CIRRUS, TACTS, and POLSTRACC, for which ISOWAT is part of the payload. Isotope-ratio measurements with ISOWAT are directly linked to the main research topics of the present call for proposals by the DFG, namely to Clouds and Precipitation as well as Transport and Dynamics in the Troposphere and Lower Stratosphere. For example, the temperature history prior to sampling determines the H2O-isotope ratios enabling e.g. a direct distinction between (i) slow ascent and cooling with resulting condensation and precipitation being in equilibration with the remaining vapor (named as Rayleigh distillation that leads to very low -values), and (ii) fast convective lofting with potential overshooting in the lower stratosphere which results in much higher -values. Furthermore, our isotope-ratio measurements will provide an indirect link to stratospheric Photochemistry (in combination with remote sensing instruments) as they will provide the isotope ratio of the water vapor entering the stratosphere. Thus, the H2O-isotope ratios are a powerful tracer to study the stratospheric water budget. ISOWAT will hence contribute to at least 50% of the scientific questions to be answered by HALO campaigns.