The AquaVIT-4 intercomparison of atmospheric hygrometers

Abstract. The AquaVIT-4 intercomparison of atmospheric hygrometers was conducted at the AIDA climate simulation chamber of the Karlsruhe Institute of Technology (KIT), Germany, in March–April 2022, within the framework of the HEMERA H2020 EU project. The objectives were to document the performance of existing hygrometers and to support the development of novel methods for water vapor (H₂O) measurements in the upper atmosphere. The AquaVIT-4 intercomparison involved seven hygrometers, based on either infrared laser absorption spectroscopy or frostpoint hygrometry techniques: four deployed on aircraft or stratospheric balloon platforms, and three reference instruments. The simulated conditions in the AIDA chamber reproduced the characteristic atmospheric conditions of the upper troposphere-lower stratosphere (UTLS, altitude range ~5–28 km) in the tropics and mid-latitudes, spanning between 20–600 hPa pressure, 190–245 K temperature, and 0.5–530 ppm H₂O mixing ratio. The campaign was divided into two phases, each consisting of four measurement days: an “open intercomparison”, where the simulated conditions were known to the participants, and a “blind intercomparison”, where the conditions were coordinated by independent referees and unknown to the participating teams. Here we present a statistical analysis of the entire dataset, which allows to assess the accuracy and limitations of each instrument. For the accuracy evaluation, two sets of reference measurements were defined: one for in situ instruments, located inside the AIDA vessel, and one for extractive instruments, sampling the chamber gas through a heated inlet. This distinction accounts for H₂O desorption effects, which are most prominent at low pressures and low H₂O concentrations. All instruments showed a good agreement with the reference values in the range of H₂O > 2 ppm, with mean deviations within ±7 % for H₂O > 10 ppm, and ±8 % between 2–10 ppm H₂O. The largest differences were found for H₂O < 2 ppm, a rarely observed range in the atmosphere, though most of the instruments still achieved average deviations within ±10 %. Overall, the results of AquaVIT-4 demonstrate the high accuracy and reliability of the four involved sensors for upper atmospheric monitoring and research applications.