H2O Isotope Ratios
Water vapor (H2O) is one of the most important gases in Earth's atmosphere as it is responsible for two third of the natural greenhouse effect, it carries large amounts of latent heat, and has a strong impact on the radiative budget via cloud formation. These aspects make H2O a key species, and the understanding of the global hydrological cycle a vital task.
The measurement of the H2O concentration alone does not give enough information about the processes involved in the transport of atmospheric H2O or its origin. Here, the stable isotope ratios (δ18O, δD) can be used to study these processes, as they provide important information about the physical processes involved in the transport of H2O and its condensation history.
Despite their importance, in-situ measurements of H2O isotope ratios in the upper troposphere and lowermost stratosphere (UT/LMS) have very rarely been performed due to the high technical difficulty of this task. The humidity range spans almost three orders of magnitude from ground to the LMS, and especially in the UT/LMS the concentration of the less abundant isotopes is extremely low.
We have developed the sensitive tunable-diode laser spectrometer ISOWAT for regular airborne operation aboard the CARIBIC passenger aircraft. The instrument is based on absorption spectroscopy at around 3765 cm-1 in combination with wavelength modulation for noise reduction, and contains a multipass-absorption cell for pathlength enhancement.
Besides the demands towards a high sensitivity, we have realized a very compact (19", 35 cm high) and lightweight spectrometer design. Furthermore, a compact calibration source for in-situ calibration with a known isotope standard was developed and is flown together with the spectrometer. The ISOWAT spectrometer and calibration unit has a mass of only around 40 kg and a total power consumption of less than 350 W.
Based on measurements during various flights in the UT/LMS we have been able to obtain the first in-situ meridional profile of δD at the flight level of a commercial airliner (~12 km altitude). The measurements were obtained between around 40°N and 20°S (Fig. 1). The data are depicted here w.r.t. the latitudinal distance to the inner tropical convergence zone (ITCZ), i.e., the geographic location of strongest convection. We clearly resolve an increase in dD (less depletion) by more than 100 ‰ in the tropics compared to higher latitudes. This is due to increased injection of convectively lofted H2O into the UT, whereby the H2O has higher dD than that of a slowly ascending airmass in mid latitudes.
Figure 1: In-situ meridional profile of δD measured at around 12 km cruising altitude of the CARIBIC passenger aircraft. The in-situ measurements are in good agreement with satellite observations (ACE-FTS).