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

Abstract

Abstract. The novel Hygrometer for Atmospheric Investigation (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurements. It has been developed and successfully deployed for the first time on the German HALO research aircraft. This new instrument combines direct tunable diode laser absorption spectroscopy (dTDLAS) with a firstprinciple evaluation method to allow absolute water vapor measurements without any initial or repetitive sensor calibration using a reference gas or a reference humidity generator. HAI contains two completely independent dual-channel (closed-path, open-path) spectrometers, one at 1.4 and one at 2.6 µm, which together allow us to cover the entire atmospheric H 2 O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers each comprise 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 dual-channel setup allows for the first time multiple self-validation functions, in particular a reliable, direct, in-flight validation of the open-path channels. During the first field campaigns, the in-flight deviations between 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 first in-flight performance of the hygrometer are described and discussed in this work.

Advances in reference materials and measurement techniques for greenhouse gas atmospheric observations

“…One of them could be shown to fulfil WMO requirements in the OGS-mode, which should be interesting for very remote field applications. The most mature TDLAS-OGS-realisations are the two laser-hygrometers, SEALDH [233] and HAI [250], which were successfully validated in [251,252] side by side comparisons with primary metrological humidity generators, but were also successfully used in numerous harsh airborne field-applications [253,254]. For example SEALDH-II is a fully validated transfer standard which offers an extraordinary relative temperature stability of the humidity signal of 0.026% K −1 , and an optimum precision of 56 nmol mol −1 (0.125 µmol mol −1 m −1 Hz −1/2 ).…”

Evaluation of the IAGOS-Core GHG package H<sub>2</sub>O measurements during the DENCHAR airborne inter-comparison campaign in 2011

“…The measurements were conducted by a G2401-m wavelength scanned cavity ring-down spectroscopy analyzer from Picarro Inc. (US) (CFKB2004), which simultaneously measures CO 2 , CH 4 , CO and water vapor (Crosson, 2008;Chen et al, 2010).…”

Comparison of the fast Lyman-Alpha and LICOR hygrometers for measuring airborne turbulent fluctuations

“…On the DLR HALO (High Altitude Long-Range Aircraft), 20 an innovative spectroscopic sensor developed by the Physikalisch-Technische Bundesanstalt (PTB) Braunschweig is deployed (Buchholz et al, 2014). Compared to the LICOR sensor, this tunable diode laser hygrometer can be operated much faster (up to several kHz) and with a known accuracy, providing the most precise humidity values available to date (Buchholz et al, 2013(Buchholz et al, , 2014(Buchholz et al, , 2016. However, this hygrometer requires extensive post processing, and at least so far it is not possible to obtain real-time humidity data.…”