## 1 Introduction  
Nitrogen dioxide (NO2) is a prominent air pollutant emitted from combustion processes, burning fuel, cars or power plants. Exposure to elevated levels of NO2 in the atmosphere causes serious respiratory diseases [1]. Accurate and reliable measurement methods for amount fraction (concentration) levels below 1 ppm are needed for atmospheric NO2 air quality control [2] and to monitor NO2 emitted directly from the exhausts of cars (1-2500 umol/mol, note: umol/mol is often called ppm) as required for the type approval emission limits quality control test (i.e., the EURO 5/6 emission standard states the range of NO2 emission levels, which need to be measured during type approval, to be in the 1-30 umol/mol for cars with a functional exhaust catalyst, and 30-1000 umol/mol for cars with a malfunctioning exhaust catalyst). The standard reference method (SRM) for measuring NO2 is based on chemiluminescence detection (CLD), as documented in the European standard EN 14211:2012 [3], where NO2 is only measured _indirectly_ (i.e., NO2 in a sample is calculated from the difference of NOx and NO after the conversion (catalytic or photolytic) of NO2 to NO. The NO then reacts with the ozone (O3) to produce electronically excited NO*2 which decays to NO2 and emits light (luminescence) that is detected by, e.g., a photomultiplier tube. The NOx concentration is determined using the measured signals, and the NO\({}_{2}\) concentration by subtracting NO (detected in an independent parallel channel) from NOx [4,5]). This indirect measurement of NO\({}_{2}\) can lead to an overestimation of NO\({}_{2}\), as other NOy gas species (HNO\({}_{3}\), HNO\({}_{2}\), N\({}_{2}\)O\({}_{5}\)) in the sample can also be converted into NO as well. It is worth noting that NO\({}_{2}\) is the only air pollutant where the SRM is based on an _indirect_ measurement. Due to the fact that NO\({}_{2}\) is only indirectly measured, very reactive and that there is a lack of accurate static calibration reference materials CRM (gases), e.g., for car exhaust gas applications, in the 1-2500 ppm range with \(\leq\)1% uncertainties, there is the need to implement new accurate and reliable measurement methods (reference methods for ambient air and other applications) for _direct_ NO\({}_{2}\) measurements [2] and alternative calibration approaches (e.g., an instrument standard OGS) to complement CRMs [6]. Laser spectroscopic techniques provide an option to develop opportunities for accurate and reliable measurement methods for direct NO\({}_{2}\) measurements [2]. Laser absorption spectroscopy has been demonstrated for direct measurements of the concentration of a variety of molecules in different applications such as atmospheric monitoring, combustion diagnostics and breath analysis [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21], and therefore can serve as a good candidate to develop a reference method for direct NO\({}_{2}\) measurements. There have been several studies to perform direct NO\({}_{2}\) measurements and also addressing the shortcomings in the SRM-based measurements [2, 22, 23, 24, 25, 26]. For atmospheric NO\({}_{2}\) measurements, spectroscopic techniques such as tunable diode laser absorption spectroscopy (TDLAS), quantum cascade laser absorption spectroscopy (QCLAS), cavity ringdown spectroscopy (CRDS), photoacoustic spectroscopy (PAS) and cavity attenuated phase shift spectroscopy (CAPS) have been reported with sub ppb detection limits [22, 24, 27, 28, 29]. Differential optical absorption spectroscopy (DOAS), TDLAS, PAS and CRDS have been successfully demonstrated for the detection of NO\({}_{2}\) emitted from automobile and aircraft engines, with detection limits in ppm down to the ppb range [23, 30, 31, 32, 33]. To date, for these spectroscopic NO\({}_{2}\) measurements, information on the metrological traceability and uncertainty assessment following, e.g., the guide to the expression of uncertainty in measurement (GUM) [34] is rarely reported.