New spectrometer measures water vapor isotopic evolution in cirrus clouds

Cirrus clouds — the wispy, thin ice clouds that form high in the atmosphere — cover about 30% of the Earth’s area and play a pivotal role in shaping climate. However, their high altitude makes them difficult to study, and their nucleation and growth are only poorly understood. New measurements of water vapor isotopic composition in a cloud chamber shed light on these processes.

Sarkozy et al. present a tunable diode laser absorption instrument designed to measure the evolution of vapor-phase water isotopologues in a cloud chamber. The Chicago Water Isotope Spectrometer (ChiWIS) simultaneously measures vapor-phase H₂O and the heavier isotopologue HDO. ChiWIS may allow for a deeper understanding of cirrus clouds, their formation and their effect on Earth’s climate.

The instrument was built for use in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber at the Karlsruhe Institute of Technology in Germany. The AIDA chamber is an experimental facility that can form cirrus clouds over a wide range of atmospheric conditions, including those that mimic important but difficult-to-access altitudes of 60,000 feet (18 km), where temperatures fall to -90 degrees Celsius.

ChiWIS consists of three modular units: a laser head, sample cell and detector module. The laser head and detector module are mounted on the wall of the AIDA chamber. The laser has a simple main beam path, along with a secondary beam that provides power and frequency referencing.

ChiWIS meets its science-required goals of 1% precision in measuring evolving HDO/H₂O ratios in isotopically enhanced cold cirrus clouds. Measurements were used to demonstrate the absense of anomalous supersaturation in cold cirrus and to quantify the HDO/H₂O fractionation factor at temperatures between 190 and 235 K.

Source: “The Chicago Water Isotope Spectrometer (ChiWIS-lab): A tunable diode laser spectrometer for chamber-based measurements of water vapor isotopic evolution during cirrus formation,” by Laszlo C. Sarkozy, Benjamin W. Clouser, Kara D. Lamb, Eric J. Stutz, Harald Saathoff, Ottmar Möhler, Volker Ebert, and Elisabeth J. Moyer, Review of Scientific Instruments (2020). The article can be accessed at http://doi.org/10.1063/1.5139244 .