Scientists will begin collecting the first near-real-time measurements of the isotopes in water vapour on Hawaii's Mauna Loa this week, trialling what could prove to be a new way to study climate and weather systems.

Streams of vapour above Mauna Loa, Hawaii. Credit: F. LANTING/CORBIS

The month-long experiment will deploy a trio of laser-based instruments to measure the isotopic composition of water vapour at least once every few minutes. That information can then be used to create a kind of 'life history' of the vapour, whether the source is a nearby forest, evaporation from the ocean or a cold air mass descending from higher in the atmosphere.

"This could be a new way of detecting fundamental changes in Earth's atmospheric circulation," says Joe Galewsky, an atmospheric scientist at the University of New Mexico in Albuquerque.

He says that Mauna Loa was the obvious place to conduct the experiment because of its sheer elevation and its location in the subtropics, which is influenced by both tropical and northerly atmospheric currents. These same attributes made the volcano an ideal place for the atmospheric carbon-dioxide counts recorded by the late Charles David Keeling, a long-time affilliate of the Scripps Institution of Oceanography in San Diego, California.

At almost 3,400 metres above sea level, the main observatory on Mauna Loa is comfortably above the fray of surface air currents. Galewsky and his team conducted a trial-run on nearby Mauna Kea using flask samples in 2006 and found that the air currents seem to be governed by large-scale atmospheric processes rather than local weather trends (J. Galewsky et al. Geophys. Res. Lett. doi:10.1029/2007GL031330; 2007).

The research ultimately relies on minute differences in the way the isotopologues of water are affected by variables such as evaporation, transpiration and condensation. Heavy water (in which the molecule has an extra neutron) precipitates out of the atmosphere faster than the more common 'light' water isotopologue, and light water evaporates faster.

Scientists commonly study isotopic ratios in things such as ice cores and ocean sediments, but extracting them from water vapour in the air is more difficult. The advent of lasers that can be tuned to the specific absorption frequencies of individual isotopes made this work easier, as did the fact that such devices are now portable.

The team plans to test three such devices simultaneously: one from NASA's Jet Propulsion Laboratory in Pasadena and two commercially available devices that use slightly different technologies. The team will also gather air samples for comparison back in the lab. "We really have to convince ourselves that this works," says David Noone, a climatologist of the University of Colorado at Boulder, who is working on the project.

The next — and for many the most important — step is tying these measurements back to satellite data. A comparison of this sort was first published last year by Noone and colleagues in Nature (J. Worden et al. Nature 445, 528–532; 2007). In that study, the team analysed water-vapour isotopes using measurements from a passive emissions sensor aboard NASA's Aura satellite, one of two that will be collecting data during the current project.

"This is brand new. This is something that really revolutionizes the study of water vapour," says Gavin Schmidt, a climate modeller at the NASA Goddard Institute for Space Studies in New York, who is not involved in the study. Schmidt calls water isotopes "the most super-duper fantastic thing ever", and says that these types of data could fill a enormous gap in the scientific record by allowing scientists to analyse where water vapour comes from, what it is doing and where it is going.

"The data that are going to come out of this experiment really will allow us to go forward with confidence and know what it is we are modelling," Schmidt says.