Technical note: A new monitoring approach to measure water vapor isotopes in high altitude regions

Abstract. Water vapor isotopes provide a comprehensive perspective on the moisture source dynamics for tracing the physical processes in hydrological and climatic studies. Continuous real-time water vapor isotopic measurements using Cavity Ring-Down Spectroscopy (CRDS) techniques are mostly based at stations located in high latitudes and the low-lying tropical regions. Such investigations from high-altitude, tropics, subtropical – mid-latitude transition zone – the Himalayas is limited owing to challenging physical conditions and multiple forms of precipitation occurring in the region. In this study, we report the establishment of the first continuous high-altitude isotope-monitoring laboratory in Northwest Himalayas windward side Manali (2,050 above msl) and leeward side Sissu (3,120 above msl) using the Picarro L2140-i Cavity Ring-Down Spectroscopy (CRDS) analyzer. This instrument enables real-time measurements of δ¹⁷O, δ¹⁸O, and δ2H in local atmospheric water vapor. Our laboratory setup integrates installation of Picarro analyzer, a heating air inlet system, meteorological sensor, lightning arrester and calibration protocols suited for optimum performance of the instrument in such challenging high-altitude Himalayan environment. Our laboratory setup protocols integrate the best practice and published guidelines with some additional modifications to mitigate the challenges in water vapor isotopic measurements in high altitude environment. A limitation of the current dataset is that no calibration has been performed since July 2025, due to relocation of JRF recruited to Delhi resulting in the unavailability of the trained personnel to carry out routine calibration cycles. We acknowledge this as a significant shortcoming and highlighted here for transparency. In addition to these continuous water vapor isotope measurements, precipitation events are also recorded, which could be helpful in investigating serious calibration problems should they arise.