Mid-IR Laser Spectrometer for Balloon-borne Lower Stratospheric Water Vapor Measurements

Water vapor is the dominant greenhouse gas, and its abundance in the upper tropospheric/lower stratospheric region (UTLS, 8-25 km altitude) is of great importance to the Earth's radiative balance. Reliable predictions of the climate evolution as well as the understanding of cloud-microphysical processes require the accurate and frequent measurement of water vapor concentrations at these altitudes. The only established method for high-accuracy UTLS water vapor measurements aboard of meteorological balloons is cryogenic frost-point hygrometry (CFH). However, the cooling agent required for its operation (CHF₃) is to be phased out due to its strong global warming potential. It is, therefore, a major, worldwide challenge to ensure the continuation of the observation of this key Environmental Climate Variable (ECV) of the World Meteorological Organization (WMO). As an alternative method, we present a compact and lightweight instrument based on quantum cascade laser absorption spectroscopy (QCLAS) that reduces systematic errors by contactless and contamination-minimized measurements. Its construction addresses the stringent constraints posed by the harsh environmental conditions found in the UTLS. This is achieved by a fundamental reconsideration of main components of the spectrometer. We developed a highly versatile segmented circular multipass cell (SC-MPC) which supports compact and well-controlled beam folding [1]. The SC-MPC consists of a monolithic aluminum ring with 10.8 cm inner radius, containing 57 quadratic, spherically curved segments, seamlessly shaped into the internal ring surface. The collimated mid-IR beam (λ = 6 µm) from the distributed feedback quantum cascade laser (DFB-QCL) is directly coupled to the MPC without the need for additional beam-shaping optics. This leads to a resilient optical setup suitable for mobile applications and rough environmental conditions. Water vapor amount fractions of <10 ppmv can be measured with a precision better than 1% at 1 Hz. Measuring in open-path mode ensures quick response and minimal interference by water desorbing from surfaces. The instrument weighs less than 4 kg (including battery) and has an average power consumption of 15 W. An elaborate thermal management system that comprises phase change materials and thermoelectric cooling ensures excellent internal temperature stability despite an outside temperature difference of up to 80 K. Specifically developed hard- and software guarantee autonomous operation for the duration of flight [2]. Extensive stability assessments in climate chambers as well as validation experiments using dynamically generated, SI-traceable water vapor mixtures were performed in collaboration with the Swiss Federal Institute of Metrology (METAS). In cooperation with the German Weather Service (DWD) in Lindenberg, the instrument was successfully tested and compared to CFH in two consecutive balloon-ascents in December 2019 up to 28 km altitude, experiencing temperatures and pressures as low as –65°C and 16 hPa, respectively. The drastic reduction in mass and size of a laser absorption-spectrometer and its successful deployment under harshest conditions represents a paradigm change in portable laser spectroscopy and opens the door to previously inaccessible applications.

[1] Graf, M.; Emmenegger, L.; Tuzson, B. Opt. Lett. 2018, 43, 2434-2437

[2] Liu, C. et al., L. Rev. Sci. Instrum. 2018, 89 (6), 065107 (9 pp.)