Fieldwork in Antarctica is approaching, and besides the packing, another important task that had to be carried out was the calibration of the instruments that we will use to measure stable water isotopologues in water vapor in Antarctica. These instruments are cavity ring down laser spectrometers, measuring the water vapor mixing ratio, which provides a measure of humidity, and the concentration of the stable water isotopologues 1H218O and HDO (deuterium).
Two of our instruments will travel to Antarctica. One is operated by the Cloud Dynamics group at the Institute of Geography at the University of Bern. It is the most experienced instrument for field campaigns, as it has already been deployed at several places from Barbados in the Tropics to the poles during ship expeditions in the Arctic and the Antarctic. During the most recent field campaign, it was deployed in Brittany, where it was operated on an aircraft. The other analyzer belongs to the Institute for Snow and Avalanche Research SLF in Davos. It is not used to travel this far, as it has mostly been measuring in the laboratory and during a campaign at Weissfluhjoch, the field measurement site of the SLF located just above Davos. A third cavity ring down spectrometer was also used during the calibration as an additional reference for comparison and to support its analyzer colleagues before their adventure.
Now that we have introduced our main characters, let’s turn to the one-month calibration experiment they have been part of.
First, the raw water vapor mixing ratio and isotopic compositions given by the instruments do not perfectly correspond to their actual values. They must therefore be calibrated against well defined reference values. Second, previous studies have shown that the isotopic composition measured depends on the water vapor mixing ratio and deviates considerably from the actual composition at low humidities. Therefore, a correction for this dependency must be applied before analyzing the measurements. The low humidity correction is particularly relevant for our fieldwork in Antarctica, as it is the largest desert on Earth and therefore experiences extremely dry conditions. We are looking forward to putting all the skills developed during this calibration procedure in the lab to use in the field at Neumayer III, starting from this austral summer and continuing until early 2029, hopefully without any major instrument failures.
