The potential of the stable isotope analysis of water vapor as a dynamic tracer of water transport in the atmosphere

The stable isotopes of hydrogen and oxygen in water vapor and precipitation in the atmosphere contain an integrated history of the physical processes of evaporation, condensation and isotopic exchange between liquid water and water vapor. Fractionation of the isotope ratios (²H¹H¹⁶O/¹H₂O and ¹H₂¹⁸O/¹H₂¹⁶O) occurs during these processes. The signal to noise ratio encountered in samples using mass spectrometer analysis of these ratios is very large making these analyses uniquely powerful tools for studying water transport and thereby energy transport in the atmosphere. In addition only 100 micro-liters of sample are needed for isotopic analysis of both isotope ratios combined.

Numerous studies have been completed and published on samples gather from land bases, mobile ground units and research aircraft. Most of the analyses have been on samples gathered from within tropical cyclones. Additional samples have been gathered in storms of lesser severity and during calm atmospheric conditions in the tropics and subtropics. Very large decreases in the isotope ratios of water vapor have been found in hurricanes, more moderate decreases in tropical storm systems such as mesoscale convective systems and tropical waves and somewhat elevated values in air parcels exposed to sea spray relative to isotope ratios of water vapor evaporated from the sea under more normal non stormy conditions. A grid of isotope values in time and space from a storm reflect the history of the meteorological processes affecting any given parcel of air that was sampled.

Finally, new infrared laser technology may allow for isotopic analysis aboard mobile units such as ground driven vehicles and research aircraft. These analyses would be made on water vapor in air samples drawn directly from the atmosphere yielding near real time results. Such units promise to be more mobile and less costly than the conventional mass spectrometer methods used presently.