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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.

This article belongs to the Special Issue Diode Laser Spectroscopy – Robust Sensing for Environmental and Industrial Applications