Electrical measurements (voltage and current):
The voltage drop between the electrodes was measured with a high voltage probe (Tektronix P6015A, 1000×) connected to a digital oscilloscope (Picoscope 5244B 200 MHz) set at 15 MS s−1 (megasamples per second) and the current was assessed from the voltage drop over a series resistor (100 Ω) towards ground, by connecting the resistor to the 1 MΩ oscilloscope input via an RG58 cable.

Fourier transform infrared (FTIR) spectroscopy:
FTIR measurements were used to determine absolute values of, e.g. O3, NO, NO2, N2O, N2O5, HNO2, HNO3 and H2O2 – if above detection threshold. The IR absorption spectra were measured between 700 and 4000 cm−1 with the Fourier transform infrared spectrometer. The measuring setup consists of a 15 l multipass cell with 32 m absorption length attached to a Bruker Vertex 80 v FT-IR spectrometer. In order to perform this measurement, the plasma device was placed in a 0.5 l chamber at atmospheric pressure connected by a needle valve to the multipass cell. The latter was held at 75 mbar to slow down the reactions. The chamber has also an exhaust pipe for the excess gas. The wavenumber range of the measurements (700–4000 cm−1) ensures the detection of O3, NO, NO2, N2O, N2O5, HNO2, HNO3 and H2O2.
From the absorbance spectra, absolute species densities were calculated with a curve fitting approach used before. It is based on reference absorbance functions obtained from a spectrum from the PNNL quantitative infrared database for HNO2, and calculated from spectral line information collected in the HITRAN database for all other species of interest. In these reference functions calculated from the absorption cross section σi of the species i, the absorption length L and an imitated instrument function, the species densities ni were fitted to the measured absorbance A(ν) using Beer’s law.
The measured absorbance is calculated from the signal intensity measured with the sample I(ν), and the background intensity I0(ν), which is measured in the same configuration, but without plasma and hence without reactive oxygen and nitrogen species.
In absorbance spectra measured using FTIR spectroscopy, baseline anomalies cannot always be ruled out. Therefore, a linear baseline correction was performed in the transmittance domain. This process was also used to assess to estimate a confidence interval for the calculated concentration values. By deliberately shifting the baseline up and down in the transmittance domain within the noise level of the baseline, a concentration range was determined rather than a single value.

Optical emission spectroscopy (OES) side-on and end-on:
The emission spectra of the plasma jet devices were measured between 250 and 900 nm with a spectrometer (1236 OMA, Princeton Applied Research) equipped with a CCD detector and an entrance slit of 100 μm. The plasma plume of the first device was measured both end-on and side-on in order to distinguish the emission produced in the discharge region from the light emitted only by the plasma plume. The second device was only measured end-on, because it does not feature a protruding plasma effluent.

Reaction kinetic modelling:
The main idea is that the species kinetics is initiated and maintained by the energetic plasma electrons, whose action can be quantified from the measured electric current and voltage supplied to the discharge. To model this process we consider a generic reaction that generates molecules of species q by electron impact on molecules of species m with reaction rate k_{qm}: dn_q/dt = k_{qm} * n_e * n_m expressed in terms of the number densities of electrons and species q and m, and which can be conveniently expressed in terms of the electric current i_p in the plasma.
For the given conditions of the gas inside the discharge region: composition, pressure, and temperature, the G-values for all reactions due to electron impact are evaluated as functions of the electric field using the Bolsig+ code [25]. In the case of the arc plasma jet the integration is between both electrodes, considering the spatially differentiated regions of cathode layer and main channel, which are modeled as described in [16]. For the DBD plasma jet the integration in (2) is done along the streamer channel and in the region around the streamer head.
The gas used in both discharges was humid air with a composition of 78% of nitrogen, 21% of oxygen and 1% of water vapor. The water vapor percentage equals to around 30% relative humidity, corresponding well with feed air humidity measurements. The reactions considered in this medium due solely to electron impact are:
e+N_2 →e+N*_2, e+O_2 →e+O∗_2 , e+N_2 →e+N+N, e+O_2 →O^− +O, e+O_2 →e+O+O,, e+O_2 →e+e+O+O^+, e+H_2 O→OH+H^−, e+H_2 O→e+H+OH, e+H_2O→H_2 +O^−, e+N_2 →e+e+N^+_2 ,e+O_2 →e+e+O^+_2 , e+O_2 →e+O^−_2.
The reactive species accounted for in the model were: O, O-, O*, O2*, O2+, O2-, O3, N, N2*,N2+, NO, NO2, NO3, N2O, N2O3, N2O4, N2O5, H, HO2, OH, H2O+, H2O2, HNO, HNO2, HNO3 and a large number of possible reactions among all present species were also considered.