Following the Isotopic Fingerprints of Atmospheric Water Vapor with Balloon-Borne Sampling

Understanding the phase-change history of atmospheric water is essential for constraining the physical parameterizations of the hydrological cycle in general circulation and regional climate models, ultimately improving the accuracy of their predictions. Stable water isotopes are natural tracers of these processes, as they record the integrated effects of phase changes along atmospheric transport pathways and therefore provide constraints for atmospheric models. However, obtaining observations of the stable isotopic composition of water vapor throughout the troposphere remains challenging because of the high costs associated with aircraft-based measurements. In this study, we present the latest results from the Water Vapor Isotopologue Flask sampling for the Validation Of Satellite data (WIFVOS) project, including both recent field observations and the technical developments of the balloon-borne flask sampling system achieved over the past three years, aimed at providing a cost-effective platform for retrieving water vapor mixing ratio (w, ppm) and isotopic composition (δ¹⁸O and δD, ‰ VSMOW). During the 2024 WIFVOS field campaign in Sodankylä (northern Finland), four successful balloon launches were conducted using a newly designed flask sampler. During the descent phase of each flight, four flasks were filled at different altitudes, providing water vapor concentration and isotopic composition at predefined pressure levels up to 3000 m ASL. A Vaisala RS92-SGP radiosonde was attached to the sampler to independently assess the quality of the humidity measurements obtained from the flask samples. Flask analyses were performed offline using a Picarro L2120-i analyzer within a few hours after balloon recovery. The retrieved humidity showed excellent agreement with radiosonde measurements (mean absolute error = 484 ppm), and clear isotopic gradients were observed within the boundary layer and the lower troposphere. To extend the vertical coverage of the profiles, AirCore samples were collected within a few hours of the flask sampling. The flask sample reproducibility was evaluated through two additional low-altitude flights conducted with a hexacopter drone equipped with a modified, lightweight version of the sampler: one flight at a fixed altitude and one sampling the lowest few hundred meters of the atmospheric column. These flights yielded standard deviations fully comparable with uncertainties estimated from dedicated laboratory tests performed prior to field deployment (±0.2 ‰ for δ¹⁸O and ±1.0 ‰ for δD). During the campaign, simulations were performed with the isotope-enabled regional weather prediction model COSMO_iso, providing a highly resolved representation of the vertical distribution of atmospheric water vapor isotopic composition. We demonstrate the applicability of WIFVOS data for satellite validation by comparing the flask-based measurements with observations from the nearby Total Carbon Column Observing Network (TCCON) spectrometer in Sodankylä. Finally, we discuss the potential of the lightweight sampler for measuring additional trace gases, such as CH₄, in the lower atmosphere using conventional drones. The vertically resolved isotopic observations obtained with the complementary techniques presented here provide key constraints for Earth System Models in the Arctic, supporting improved representation of atmospheric moisture processes.