## 2 Materials and Methods
### Analytical Determinations  
#### 2.5.1 Setup for Optical Absorption Spectroscopy (OAS) and Data Processing  
The setup for OAS is the same as that used by [PERSON] et al. [13] and is schematically represented in Figure 2. A deuterium-halogen lamp, characterized by a broad band spectrum from UV to NIR radiation, was used as a light source. With the aim of investigating the plasma afterglow, the light beam was focused by means of optical fibers and fused silica lenses (50 mm of focus length) to achieve a parallel beam which passed under the mesh and was collected by a 500 mm spectrometer (Acton SP2500i, Princeton Instruments) to spectrally resolve the light beam in the UV, VIS, and near infrared (NIR) regions. The width of the inlet slit of the spectrometer was fixed at 50 um for OAS acquisitions, and a grating with a resolution of 150 mm\({}^{-1}\) was used. A photomultiplier tube (PMT, Princeton Instruments PD439) connected to a fast oscilloscope (Tektronix DPO40034) was used as detector, allowing fast acquisitions with a time resolution of 40 ms. The PMT amplification factor was kept constant for all acquisitions. In order to ensure identical initial conditions, the discharge chamber was opened and flushed for 30 s with fresh air prior to every measurement.  
In order to quantitatively evaluate the species' concentrations from absorption measurements, the Lambert-Beer law was taken into account:  
\[\frac{I}{I_{0}}=e^{(-L\sigma n)} \tag{2}\]  
where \(I/I_{0}\) is the ratio between the initial light intensity \(I_{0}\) and the light intensity \(I\) after an optical path length \(L\), and \(n\) is the concentration of absorbers. The absorption cross-section \(\sigma\) is a function of the wavelength (\(\sigma=\sigma(\lambda)\)).  
In general, when \(N\) species absorb at the same \(\lambda j\) wavelength, the Lambert-Beer equation can be re-written as:  
\[\frac{I}{I_{0}}\bigg{|}_{j}=e^{(-L\sum\limits_{i=1}^{N}\sigma_{ij}n_{i})} \tag{3}\]  
where \(\sigma_{ij}\) represents the absorption cross-section of the \(i\)-species at \(\lambda_{j}\) and the ratio between the light intensities \(I\) and \(I_{0}\) is referred to the \(j\)-wavelength. The concentration \(n\) of each absorbing species can thus be obtained from the following expression, where the suffix \(k\) indicates a generic absorber:  
\[n_{k}=-\frac{1}{L\sigma_{kj}}\ln\left(\left.\frac{I}{I_{0}}\right|_{j}\right) -\sum\limits_{i\
eq k}^{N}\frac{\sigma_{ij}}{\sigma_{kj}}n_{i} \tag{4}\]  
The wavelengths selected to perform the study and the corresponding absorption cross-sections for O\({}_{3}\) and NO\({}_{2}\) as absorbers are reported in Table 2. These wavelengths were  
Figure 2: Schematic of the experimental setup for optical absorption spectroscopy.  
defined, in accordance with [PERSON] [14], to maximize the absorption of the molecules relevant to our study while minimizing the contribution, and thus the disturbance, of other absorbing molecules.  
For all experiments, the optical path length \(L\) was 21 cm and the contributions of background radiation and spontaneous plasma emission were duly taken into account in the data processing, subtracting them from the acquired values of \(I\) and \(I_{0}\).