In Situ Measurement of Water Vapor Isotopic Composition Near the Tropopause: First Results From the Harvard ICOS Isotope Instrument (original) (raw)

AGUFM, 2004

Abstract

ABSTRACT In recent years the atmospheric science community has placed high priority on obtaining measurements of the isotopic composition of water vapor at near-tropopause altitudes. Because water isotopic composition is altered by all processes involving condensation or evaporation, it can serve as a tracer of the processes governing the water content of air ascending to the stratosphere. Obtaining scientifically useful measurements is made difficult, however, by the scarcity of HDO, the most useful water isotopologue; by the precision and accuracy required to differentiate between dehydration scenarios; and by the potential for contamination. We report here on the first flights of a new instrument designed to meet these needs and to provide high-sensitivity in situ measurements of HDO, H218O, and H2O in the near-tropopause region. The Harvard ICOS Isotope Instrument uses the relatively new technique of Integrated Cavity Output Spectroscopy (ICOS) to provide mid-infrared absorption spectra with a pathlength of 4 km, as opposed to the < 100 m of traditional multipass cells. The instrument offers more than an order of magnitude improvement in sensitivity over traditional in situ mid-infrared spectroscopic instruments. Integration times are 1-3 s, short enough to capture the small spatial scales of tropical deep convection plumes and cirrus layers. Instrument design also focuses on minimizing measurement hysteresis due to contamination, using pharmaceutical-grade fittings and welds to eliminate trapped volumes. We discuss the specific measurement needs for studying near-tropopause dehydration and show validation of the performance of this instrument on a series of test flights in autumn 2004 on NASA's WB-57 high-altitude aircraft. The aircraft payload includes for comparison a second instrument measuring HDO and H2O by a fully independent method (the Harvard Hoxotope Instrument, using photofragment laser-induced fluorescence) as well as two well-established and validated instruments measuring water vapor and total water. The Harvard ICOS Isotope Instrument is proposed for use on upcoming science missions studying water vapor in the near-tropopause region.

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