SEALDH-II-An Autonomous, Holistically Controlled, First Principles TDLAS Hygrometer for Field and Airborne Applications: Design-Setup-Accuracy/Stability Stress Test

Evaluation of Spatial Gas Temperature and Water Vapor Inhomogeneities in TDLAS in Circular Multipass Absorption Cells Used for the Analysis of Dynamic Tube Flows

The use of optical circular multipass absorption cells (CMPAC) in an open-path configuration enables the sampling free analysis of cylindrical gas flows with high temporal resolution and only minimal disturbances to the sample gas in the pipe. Combined with their robust unibody design, CMPACs are a good option for many applications in atmospheric research and industrial process monitoring. When deployed in an open-path configuration, the effects of inhomogeneities in the gas temperature and composition have to be evaluated to ensure that the resulting measurement error is acceptable for a given application. Such an evaluation needs to consider the deviations caused by spectroscopic effects, e.g., nonlinear effects of temperature variations on the intensity of the spectral line, as well as the interaction of the temperature and concentration field with the characteristic laser beam pattern of the CMPAC. In this work we demonstrate this novel combined evaluation approach for the CMPAC used as part of the tunable diode laser absorption spectroscopy (TDLAS) reference hygrometer in PTB's dynH2O setup for the characterization of the dynamic response behavior of hygrometers. For this, we measured spatially resolved, 2D temperature and H2O concentration distributions, and combined them with spatially resolved simulated spectra to evaluate the inhomogeneity effects on the line area of the used H2O spectral line at 7299.43 cm-1. Our results indicate that for dynH2O, the deviations caused by the interaction between large concentration heterogeneities and the characteristic sampling of the beam pattern of the CMPAC are three orders of magnitude larger than deviations caused by small temperature heterogeneity induced spectroscopic effects. We also deduce that the assumption that the "path-integrated" H2O concentration derived with the open-path CMPAC setup represents an accurate H2O area average in the flow section covered by the CMPAC in fact shows significant differences of up to 16% and hence does not hold true when large H2O concentration gradients are present.

H2O Collisional Broadening Coefficients at 1.37 µm and Their Temperature Dependence: A Metrology Approach

We report self- and air collisional broadening coefficients for the H2O line at 7299.43 cm−1 and corresponding temperature coefficients for a temperature range spanning 293–573 K. New laser spectroscopic setups specifically designed for this purpose have been developed and are described. The line parameters reported here are in good agreement with those values reported in the HITRAN 2020 database, but the uncertainties have been reduced by factors of about 4, 1.3 and 4.4 for the self-broadening coefficient, air broadening coefficient and the temperature exponent of air broadening, respectively. Further, we combined our measurement approach with metrological data quality objectives, addressing the traceability of the results to the international system of units (SI) and evaluated the uncertainties following the guide to the expression of uncertainty in measurement (GUM).

Towards a Fast, Open-Path Laser Hygrometer for Airborne Eddy Covariance Measurements

Water vapor fluxes play a key role in the energy budget of the atmosphere, and better flux measurements are needed to improve our understanding of the formation of clouds and storms. Large-scale measurements of these fluxes are possible by employing the eddy correlation (EC) method from an aircraft. A hygrometer used for such measurements needs to deliver a temporal resolution of at least 10 Hz while reliably operating in the harsh conditions on the exterior of an aircraft. Here, we present a design concept for a calibration-free, first-principles, open-path dTDLAS hygrometer with a planar, circular and rotationally symmetric multipass cell with new, angled coupling optics. From our measurements, the uncertainty of the instrument is estimated to be below 4.5% (coverage factor k = 1). A static intercomparison between a dTDLAS prototype of the new optics setup and a traceable dew point mirror hygrometer was conducted and showed a systematic relative deviation of 2.6% with a maximal relative error of 2.2%. Combined with a precision of around 1 ppm H2O at tropospheric conditions, the newly designed setup fulfills the static precision and accuracy requirements of the proposed airborne EC hygrometer.