## 3 Results and Discussion
There is a 15% difference between the NO measurement methods for H\({}_{2}\) and FO combustion and a 10% one for the plasma heating experiments, which we consider as rather good considering the uncertainties in the NO UV absorption and probe measurements. In the evaluation of the NO mole fraction using the NO UV absorption method made for FO combustion, the temperature measured using the TDL was used. At the conditions studied, an increase in the input temperature by 100 \({}^{\circ}\)C results in a decrease in the NO mole fraction by approximately 10%. For H\({}_{2}\) and FO combustion, the NO\({}_{2}\)/NOx ratio is below 10%, which is in accordance with a typical NO\({}_{2}\)/NOx ratio for combustion, see [16]. In plasma heating experiments, the significant concentrations of NO\({}_{2}\) were measured using the probe up to 1000 ppm, the NO\({}_{2}\)/NOx ratio is above 25%. The UV spectra recorded in the ECF do not contain any clear indication of the NO\({}_{2}\) spectral features, similar to our earlier observation reported in [9]. Figure 7a-c show the examples of the experimental and fitted UV NO spectra for the cases when the process air was heated by H\({}_{2}\)-air combustion, N\({}_{2}\) plasma, and Air plasma gases, respectively. The fitted NO and T values are reported in the figures. The agreement between the fitting and experimental spectra is rather good, though the experimental spectra display some disturbances occurring at about the same wavelengths for all the measured spectra. Figure 8 shows the transmission spectra near 225 nm measured when the process air was heated by Air Plasma gases (\(I\)) and when only the process air was in the ECF, \(I_{0}\). The dark current is included in the figure. The \(I_{0}\) signal decreases quickly with a decreasing the wavelength and displays wavelength-dependent features. In the evaluation, the \"real\" background signal \(I_{0}\), measured during the interruption/stoppage of the fuel feeding, is assumed to represent the \"true\" background absorbance (difference in the net absorption of the NO molecules and the measured gross absorbance) during the recording of the transmission signal, \(I\). However, the somewhat different experimental conditions inside the ECF during \(I\) and \(I_{0}\) measurements lead to the deviation between \"real\" and \"true\" \(I_{0}\) signals, which causes the observed disturbances in the experimental spectra. The disturbances are more pronounced at low absorptance. The mismatch described above between \"real\" and \"true\" \(I_{0}\) signals prevented us from evaluating the temperature for FO combustion, characterized by relatively low NO concentration, below 200 ppm. Since the temperature depends on the shape of the NO UV spectra, the spectral disturbances at small signals were too high to allow an accurate evaluation. The NO concentration depends on the area under the spectral curve and was determined for all the measured cases. In our future work, we plan to modify the current NO UV absorption set up for improving the diagnostics of a low NO concentration in hot mixtures. First, the signal-to-noise ratio of the set-up and steepness of the \(I_{0}\) spectral dependence in the region of the NO absorption feature will be improved by selecting a more powerful D2 lamp and using the grating with better transmittance properties for NO diagnostics. Second, we plan to reduce the diameter of the UV beam by using extra optical lenses in a UV beam collimating system and/or by coupling the radiation from the UV source to a fiber with a diameter lower than 600 \(\upmu\)m. Thirdly, the effects of the thickness and material of optical windows on matching \"real\" and \"true\" \(I_{0}\) signals will be attended. Figure 7: Measured and fitted NO spectra (L = 78 cm) in the ECF heated by H2-air combustion (**a**), N\({}_{2}\) Plasma (**b**) and Air Plasma (**c**) gases. Figure 9 shows an overview of the NO measurements made during the campaign; the NO mole fraction obtained using the NO UV absorption is plotted as a function of the probe NO.