Towards an Optical Gas Standard for Traceable Calibration-Free and Direct NO₂ Concentration Measurements

We report a direct tunable diode laser absorption spectroscopy (dTDLAS) instrument developed for NO₂ concentration measurements without chemical pre-conversion, operated as an Optical Gas Standard (OGS). An OGS is a dTDLAS instrument that can deliver gas species amount fractions (concentrations), without any previous or routine calibration, which are directly traceable to the international system of units (SI). Here, we report NO₂ amount fraction quantification in the range of 100–1000 µmol/mol to demonstrate the current capability of the instrument as an OGS for car exhaust gas application. Nitrogen dioxide amount fraction results delivered by the instrument are in good agreement with certified values of reference gas mixtures, validating the capability of the dTDLAS-OGS for calibration-free NO₂ measurements. As opposed to the standard reference method (SRM) based on chemiluminescence detection (CLD) where NO₂ is indirectly measured after conversion to NO, titration with O₃ and the detection of the resulting fluorescence, a dTDLAS-OGS instrument has the benefit of directly measuring NO₂ without distorting or delaying conversion processes. Therefore, it complements the SRM and can perform fast and traceable measurements, and side-by-side calibrations of other NO₂ gas analyzers operating in the field. The relative standard uncertainty of the NO₂ results reported in this paper is 5.1% (k = 1, which is dominated (98%) by the NO₂ line strength), the repeatability of the results at 982.6 µmol/mol is 0.1%, the response time of the instrument is 0.5 s, and the detection limit is 825 nmol/mol at a time resolution of 86 s

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)