The novel Hygrometer for Atmospheric Investigations (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurement. It has been developed and suc-cessfully employed for the first time on the German HALO research aircraft. This new instrument combines direct Tunable Diode Laser Absorption Spectroscopy (dTDLAS) with a first-principle evaluation method to allow absolute water vapor measurements without any initial or repetitive

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... calibration using a refer-ence gas or a reference humidity generator. HAI contains two completely independent dual-channel (closed-path, open-path) spectrometers, one at 1.4 µm and one at 2.6 µm, which allow together to cover the entire at-mospheric H₂O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers comprise each a separate, wavelength-individual extractive, closed-path cell for total water (ice and gas-phase) measurements. Additionally, both spectrometers couple light into a common, open-path-cell outside of the aircraft fuselage for a direct, sampling-free and contactless determination of the gas-phase water content. This novel twin du-al-channel setup allows for the first time multiple self-validation functions i.e. in particular a reliable, direct, in-flight validation of the open-path channels. During the first field campaigns, the in-flight deviations be-tween the independent and calibration-free channels (i.e. closed-path to closed-path and open-path to closed-path) were on average in the 2 % range. Further, the fully autonomous HAI hygrometer allows measurements up to 240 Hz with a minimal integration time of 1.4 ms. The best precision is achieved by the 1.4 µm closed-path cell at 3.8 Hz (0.18 ppmv) and by the 2.6 µm closed-path cell at 13 Hz (0.055 ppmv). The requirements, design, operation principle and in-flight performance of the hygrometer are described in this work.