Validation of an extractive, airborne, compact TDL spectrometer for atmospheric humidity sensing by blind intercomparison

Abstract

Accuracy, precision, repeatability and long-term stability, are the most important requirements to enable reliable airborne humidity measurements, which are needed for climate models or to validate e.g. remote sensing instrumentation like satellites. However, various hygrometer artifacts which depend on the individual sensor principle and the application profile frequently cause problems and significantly complicate the hygrometer choice. Sensor intercomparisons are one way of providing the information for an optimal choice.In this paper we present the first part of a blind, static, laboratory-based intercomparison of a new, calibration-free, 1.4 µm diode laser-based, optical hygrometer (SEALDH) with the two most important measurement principles for airborne hygrometry (frost-point hygrometers, FPH, and Lyman-alpha fluorescence hygrometers, LAFH). During three days of measurement, the TDL-hygrometer achieved a H 2 O resolution of up to 0.5 ppmv ( t = 2 sec) at tropospheric pressure and H 2 O concentration levels (100- 800 hPa, 10 to 8000 ppmv H 2 O). Its absolute accuracy was investigated via blind intercomparison with two reference FPHs and a LAFH. Without any calibration of SEALDH, i.e. without a comparison to a water vapor standard, we achieve an excellent agreement with the reference sensors, with an average systematic offset (over all three days) of −3.9 % ± 1.5 %, which is fully consistent with the sensor's uncertainty bounds.BFurther we also reevaluated the SEALDH data of day 2 and 3 in a calibrated mode using an independent set of FPH data from the first day and found an 8-fold accuracy improvement, yielding an excellent overall relative deviation of only 0.52 % ± 1.5 % with respect to a LAFH and a D/FH sensor.

Robust, spatially scanning, open-path TDLAS hygrometer using retro-reflective foils for fast tomographic 2-D water vapor concentration field measurements

“…Direct tunable diode laser absorption spectroscopy (dTD-LAS) is a favorable method for field measurements for various reasons: first of all, it can provide absolute gas concentrations without the need for any reference gas measurements (Buchholz et al, 2014). Besides, the technical components are industrially available and easily transportable.…”

“…This version of direct TDLAS has been presented for numerous applications, e.g., biological (Hunsmann et al, 2008;Wunderle et al, 2009), engineering (Ortwein et al, 2010Wagner et al, 2009;Teichert et al, 2003), or atmospheric research (Buchholz et al, 2013).…”

“…Although this bias lies within the measurement uncertainty of the static reference instrument (Buchholz et al, 2013), the systematically negative deviation of the scanning measurement should be kept in mind and evaluated further. We also compared the continuous scanning mode to stepwise measurements, where the motor is sequentially moved but stopped for measuring the absorption signal.…”

“…There are instruments for gas concentration measurements (Babilotte et al, 2010;Crosson, 2008;McManus et al, 2008) or even flux measurements (McDermitt et al, 2011;Hendriks et al, 2008;Tuzson et al, 2010) that are well-suited for field applications but that do not provide a 2-D-concentration distribution. Certain variants of direct TDLAS (tunable diode laser absorption spectroscopy) are powerful techniques for field applications, as no reference gas measurement is needed for calibration (Buchholz et al, 2014) they are based on robust, industrially available components. For receiving spatially resolved concentration fields from line-of-sight data, multiple measurement paths have to be combined into a tomographic setup.…”

“…A fiber-coupled DFB-laser at 1370 nm with a current tuning range of about 2 cm −1 is modulated over the 211 ← 110 H 2 O absorption line at 7299.43 cm −1 (Hunsmann et al, 2008;Buchholz et al, 2014). For this purpose, a function generator produced a triangular current modulation signal.…”