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We report the first realization of a TwOGaSt, a trace water  o ptical  g as  s tandard (OGS) based on direct tunable diode laser absorption spectroscopy (dTDLAS). The TwOGaSt ( T race  w ater  O ptical  Ga s  St andard) hygrometer is equipped with a fiber-coupled DFB-laser emitting at 1.37 µm, a 36 m multi-pass astigmatic Herriott gas cell and a room-temperature InGaAs photo detector. TwOGaSt is capable of directly measuring absolute H 2 O concentration without any previous calibration of the spectrometer with reference gas mixtures. Water vapour amount fraction (= concentration) results delivered by TwOGaSt covering the concentration range from 10 to 350 µmol/mol (µmol/mol = ppm) were demonstrated and showed an optimum precision of 0.023 µmol/mol at a time resolution of 2 s. Further, we demonstrate the first metrological validation of this new OGS for absolute “calibration free” trace H 2 O amount fraction measurements in N 2  and air, by comparing it with the primary coulometric trace water standard (CSFG) of PTB. The TwOGaSt results were in excellent agreement with the CSFG, PTB’s primary trace water vapor standard [ 1 ]. The relative standard uncertainty of the laser-hygrometer was determined to be is 1.2 %. This demonstrates that after SEALDH [ 2 ] and HAI [ 3 ], we now realized with TwOGaSt a laser-based, calibration-free, first-principles, optical gas standard (OGS) for the trace water vapor range, which is highly relevant for the upper troposphere and lower stratosphere (UT/LS) i. e. below 100 µmol/mol down to 4 µmol/mol (the tropopause H 2 O minimum).

First metrological validation of TwOGaSt, a new, absolute dTDLAS-trace-hygrometer, using the primary, coulometric, trace water vapour generator at PTB