# Towards a Fast, Open-Path Laser Hygrometer for Airborne Eddy Covariance Measurements
## 1 Introduction  
Water vapor is the most important greenhouse gas, and plays a key role in the energy budget of the atmosphere [1]. Water vapor fluxes transport latent heat vertically from the Earth's surface into the atmosphere, and they are a major contributor to many meteorological processes, including the formation of clouds, storms and huricanes [2]. The eddy correlation (EC) method can be used to measure these fluxes by correlating the vertical component of the wind vector with fast and accurate H\({}_{2}\)O concentration values [3]. This method is commonly used in stationary setups, such as meteorological EC flux towers, to determine the water vapor flux from the ground into the atmosphere at one specific location [4, 5]. To study fluxes on larger scales, EC measurements from airplanes are possible but are significantly more challenging from a technical and metrological standpoint [6, 7]. The requirements for a hygrometer used for airborne EC measurements are (A) a high temporal resolution of more than 100 milliseconds (i.e., a 10 Hz rep rate), (B) a large dynamic H\({}_{2}\)O range from below 100 to 30,000 \(\upmu\)mol/mol (ppm, parts per million), (C) resistance to vibrations and large temperature variations, (D) small aerodynamic drag for the components mounted outside the aircraft and (E) compatibility with aviation licensing requirements [3, 8].  
A large number of airborne hygrometers have been demonstrated in the past. Besides the sensor type, the use of a closed- or open-path optical cell is one of the key characteristics of an airborne hygrometer. Notable instruments that utilize an extractive samplingsystem and a closed-path cell on aircrafts are described in [6; 7; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Although closed-path instruments, such as SEALDH-II, can reach a spectrometric temporal resolution of up to 140 Hz, the response time of the instrument is significantly limited by the transport of the gas sample to the optical cell, particularly by H\({}_{2}\)O adsorption/desorption processes in the sampling system [11]. The SPARC Report and AquaVIT-1 intercomparison both found sampling problems to be one of the major contributors to the observed deviation between hygrometers for atmospheric studies of 10% and more [18; 19]. Whereas instruments with open-path optical cells avoid sampling and adsorption limitations, open-path instruments that are used on aircrafts face significant design and approval challenges due to the added complexity of conditions on the exterior of the aircraft [9; 20; 21; 22; 23]. The multi-channel, multi-phase hygrometer for Atmospheric Investigation (HAI), developed for the German research aircraft HALO [24], demonstrated the simultaneous use of an open- and closed-path cell and showed the benefits of an open-path system, especially when fast concentration fluctuations are of interest (e.g., during EC measurements) [9; 25].  
Here, we present a design concept for a new calibration-free direct tunable diode laser absorption spectroscopy (dTDLAS) hygrometer, particularly optimized for airborne open-path EC measurements using a rotationally symmetric optical multipass cell. The multipass cell forms a particularly thin optical plane that is normal to the direction of the flow, thereby minimizing the transition time through the light sheet. This new design is integrated in a setup with fast pressure and temperature probes close to the optical plane. The spectroscopic performance of the newly designed, compact optics setup is validated by static intercomparison with an SI-traceable dew point mirror (DPM) hygrometer.