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The novel Hygrometer for AtmosphericInvestigation (HAI) realizes a unique concept for simultaneousgas-phase and total (gas-phase + evaporated cloud particles) watermeasurements. It has been developed and successfully deployed for the firsttime on the German HALO research aircraft. This new instrument combinesdirect tunable diode laser absorption spectroscopy (dTDLAS) with afirst-principle evaluation method to allow absolute water vapor measurementswithout any initial or repetitive sensor calibration using a reference gasor a reference humidity generator. HAI contains two completely independentdual-channel (closed-path, open-path) spectrometers, one at 1.4and one at 2.6 µm, which together allow us to cover the entireatmospheric H<sub>2</sub>O range from 1 to 40 000 ppmv with a single instrument.Both spectrometers each comprise a separate, wavelength-individualextractive, closed-path cell for total water (ice and gas-phase)measurements. Additionally, both spectrometers couple light into a commonopen-path cell outside of the aircraft fuselage for a direct, sampling-free,and contactless determination of the gas-phase water content. This noveltwin dual-channel setup allows for the first time multiple self-validationfunctions, in particular a reliable, direct, in-flight validation of theopen-path channels. During the first field campaigns, the in-flightdeviations between the independent and calibration-free channels (i.e.,closed-path to closed-path and open-path to closed-path) were on average inthe 2 % range. Further, the fully autonomous HAI hygrometer allowsmeasurements up to 240 Hz with a minimal integration time of 1.4 ms. Thebest 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 first in-flightperformance of the hygrometer are described and discussed in this work

HAI, a new airborne, absolute, twin dual-channel, multi-phase TDLAS-hygrometer: background, design, setup, and first flight data