Accurate humidity probe for persistent aviation-contrail conditions

Abstract. We present a state-of-the-art humidity probe for in situ airborne water vapor measurements tailored to the humidity range relevant for the formation of persistent aviation contrail cirrus clouds. Our probe is based on tunable infrared laser direct absorption spectroscopy (TILDAS). We scan a laser in wavelength across an isolated water (H₂O) absorption line near 7205 cm^-1 (1.39 µm) to obtain the H₂O volume mixing ratio in sample air at 1 Hz in real time. Our novel optical design features a combination of single-mode optical fibers and a short-path absorption cell with integrated focusing optics and detector to avoid any absorption path outside of the sample volume. A parallel fiber path and detector capture the laser power profile during the wavelength scan for spectral-baseline normalization. We have built and tested two prototypes and tested them side-by-side in the laboratory against a humidity standard as well as against each other. Without any calibration, we found agreement against the humidity standard of better than 98 % for the relevant H₂O range below 200 ppm. The agreement between both instruments when operated in series measuring the same sample air was 99.7 %. The stability of both instruments was quantified to be ±1 % (1 σ) during a 5-day long period where both prototypes operated without any temperature control. We show that our probes can measure H₂O with linear response over 4 orders of magnitude.