Numerical analysis of the effect of nitrogen and oxygen admixtures on the chemistry of an argon plasma jet operating at atmospheric pressure

The room temperature, non-equilibrium atmospheric-pressure argon plasma jet considered in this work is a commercial device, the so-called kinpen (neoplas GmbH, Germany) [22, 23]. It is driven by 1 MHz RF electric excitation and can be described as a cold atmospheric pressure plasma jet [24].

Figure 1 depicts the basic geometrical and electrical configuration which consists of a high-voltage (HV) needle electrode centred within a dielectric capillary of radius 1.6 mm. The electrode potential is brought from 2 to 6 kV and dissipates an average power ranging from 0.9 to 2.2 W in the plasma (see also section 2.3).

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A feed gas flow rate ranging from 0.5 to 3.0 slm of dry argon can be blown through the capillary and after excitation produces a visual plasma outside the nozzle between 0.3 and 15 mm length. The plasma length depends on the admixture type and fraction. Typically, ${{{\rm O}}_{2}}$ and ${{{\rm N}}_{2}}$ can be admixed to the feed gas up to 2.0%. Also water vapour can be added to the feed gas in a smaller proportion (about 1400 ppm) to control the production of OH or hydrogen peroxide (${{{\rm H}}_{2}}{{{\rm O}}_{2}}$) in the plasma and its effluent [11, 25].

In order to control the interaction of the plasma and its effluent with the surrounding atmosphere, an external gas flux is implemented by means of a gas curtain device. More information can be found in [4]. This gas curtain can be fed with different gases, however, for the purpose of this work dry air is used, only to keep the ambient humidity out of the active region, as reported in [26]. Additionally, the gas curtain enhances the reproducibility and the stability by excluding any variation of water vapour concentration which strongly depends on the location and time of operation. Furthermore, this gas curtain device is used to couple the plasma jet with the measurement chamber providing a similar atmosphere near the plasma effluent as in open conditions.

Absolute density measurements of ${\rm N}{{{\rm O}}_{2}}$ and ${{{\rm O}}_{3}}$ produced by the kinpen are performed by laser infrared absorption spectroscopy in the mid-infrared. The experimental setup is identical to the one we reported in previous work by Isni et al and which will be described briefly here [27].

The diagnostic apparatus is initially based on the Q-MACS system (neoplas control GmbH, Germany) although some optimizations were performed to allow investigations of plasma jets operating at atmospheric pressure [13, 27].

For the measurement of both species, ${\rm N}{{{\rm O}}_{2}}$ and ${{{\rm O}}_{3}}$, a nanosecond single mode pulsed quantum cascade laser (QCL) is used as a mid-infrared source driven in inter-pulse mode [28, 29]. Unfortunately, pulsed QCLs have typically an emission range only within a few wavenumbers (about 5–15 cm⁻¹), whereas both ${\rm N}{{{\rm O}}_{2}}$ and ${{{\rm O}}_{3}}$ have their absorption bands separated with a gap of about 600 cm⁻¹. Consequently, two different QCLs (Alpes Lasers SA, Switzerland) are used alternatively, emitting from 1607.72 to 1619.43 cm⁻¹ and from 1024.5 to 1029.9 cm⁻¹ to match the absorption band positions of ${\rm N}{{{\rm O}}_{2}}$ and ${{{\rm O}}_{3}}$ respectively.

The intensity stability is enhanced by a Peltier element which permanently regulates the temperature. The tuning of the laser wavelength is performed by temperature variation induced by a fast current ramp. This method allows us to shift the laser wavelength over a range of 0.8 cm⁻¹ in order to produce an absorption spectrum of many ro-vibrational transitions.

As the absorption properties of each molecule in the mid-infrared as well as the expected densities are rather low [30], a 60 cm multipass White cell is used in order to increase the absorption length. The laser beam is focused on the entrance of the multipass cell and is then reflected several times before reaching a very fast mercury cadmium telluride detector (Q-MACS IRDM-600A, neoplas control GmbH, Germany). The number of passes through the cell is tunable. In this work, a number of 32 passes and a total absorption length of 19.2 m is sufficient to observe a good absorption signal. The signal is acquired by a digitizer board controlled via a computer. The latter monitors the complete system by means of QMACSoft Monitor software [31].

Figure 1 illustrates the diagnostic bench and focuses on the coupling of the plasma source to the measurement chamber. In order to collect the reactive species produced by the plasma jet, a measurement chamber is mounted within the multipass cell. The chamber is a cylinder of 9.0 cm inner diameter and 57.5 cm length, yielding a volume of 3660 cm³. It is made of glass in order to prevent reactions with the wall and thus reduces chemical losses.

The coupling of the plasma jet to the measurement chamber is achieved by an opening located at the half-height of the cylinder at the centre of the multipass cell to conserve symmetry between the two exhausts. The gas curtain cap mounted on the plasma device helps to keep the connection with the chamber tightened. It also provides air around the plasma effluent to reproduce the ambient conditions and prevent the chamber to be filled with argon.

As shown in figure 1, the mid-IR beam is reflecting on the multipass cell mirrors and passes through the chamber without interaction with the active visual plasma itself. The homogeneity of the gas mixture within the chamber has been checked in a previous work and confirmed by a numerical CFD model [27].

The absolute density measured within the chamber is determined as follows: the laser is tuned to scan a range of about 0.8 cm⁻¹ yielding the absorption spectrum. The latter is fitted with a simulated spectrum based on the line strength reported in the HITRAN database [30, 32]. The procedure is automatically implemented through the QMACSoft Monitor software and allows a sample rate of 0.5 Hz in our experimental conditions. The absolute wavelength position and wavelength scale distortion during the laser tuning are corrected via a calibration procedure reported in [13]. Herein, a more detailed description of the fitting method is given.

In order to dry the pipes and the measurement chamber, the gas curtain was flushed with 5.0 slm dry air, at least 12 h before to start. Similarly, the argon pipes were flushed in advance with 0.5 slm for over 6 h. Indeed, water is known to have many broad absorption bands in the mid-IR which can lead to a significant disturbance and to an over-estimation of the production rates (or concentrations) of the species but also because even very low amounts of water will influence the plasma chemistry [9, 11, 26]. Moreover, this protocol enhances the repeatability and stability of the diagnostic.

From the measured densities ${{{\rm n}}_{i}}({\rm c}{{{\rm m}}^{-3}})$ of species ${\rm i}$, the net production rates ${{{\rm R}}_{i}}$:

$$\begin{eqnarray*}&&{{{\rm R}}_{i}}={{{\rm n}}_{i}}\times \ 8\times \ {{10}^{3}}/60\;\;{\rm molec}\;{{{\rm s}}^{-1}}\end{eqnarray*}$$

are calculated accounting for the total gas flux of 8 slm, i.e. combined curtain gas and argon feed gas flux.

The power dissipated at the electrode is one of the key parameters to determine how much energy excites the electrons and it is a crucial input parameter for the simulations (see below).

In this work, the power measurement is performed in the same way as reported by Hofmann et al [33]. The current and voltage probes (Tektronix Tek CT-2 and Tek P6139A respectively) are installed before the matching coil directly on the kinpen electronics. Thus, current and voltage are recorded simultaneously with an oscilloscope (Tektronix DPO 4104) and allow the determination of the average power dissipated by the HV electrode. Losses induced by the coil and wires are taken into account following the protocol suggested in [33].

This results in a measured dissipated power ranging from 1.4 to 1.8 W in the case of impurity admixtures up to 1.0%. These values of dissipated power are in agreement with the power reported by other groups using similar argon RF atmospheric pressure plasma jets, i.e. with similar length and gas temperatures, geometry, gas flow rate, etc [24, 34].

The gas temperature value is a crucial parameter for the application as it needs to be close to room temperature (a few millimeters from the nozzle) in order to avoid any thermal damage on the sample. Moreover, it is also important to know the evolution of the temperature throughout the plasma to be able to accurately model the reaction chemistry.

The gas temperature is measured with a non-metallic fiber-optics probe (diameter 1 mm) mounted on a three-axis linear table. The device determines the temperature by spectroscopically measuring the band gap of a GaAs crystal deposited at the tip of the optical fiber (FOTEMP1-OEM and TS3, Optocon AG, Germany). Unlike for metallic probes, there is no visible change in the emission of the plasma when the probe is brought in contact with the visible plasma. However, it is noted that with the probe merely average temperatures can be detected, while in a turbulent flow the local temperature can be expected to exhibit significant statistical fluctuations.

The kinpen in this work is operated with 3.0 slm argon (99.999% purity) gas feed flow rate and an additional gas curtain of 5.0 slm dry air. The gas flow is regulated by mass-flow controllers (MFC, MKS Instruments, USA). To prevent any impurities and ambient humidity from penetrating the pipes and contaminating the argon, stainless-steel and PTFE gas tubes are used [11].

In this work, an admixture variation of ${{{\rm O}}_{2}}$ and/or ${{{\rm N}}_{2}}$ is applied from 0.0 to 1.0% of the total feed gas flow rate (0.0–30.0 sccm in absolute values, respectively). The purity is 99.995% for ${{{\rm O}}_{2}}$ and 99.999% for ${{{\rm N}}_{2}}$ according to the provider specifications (Air Liquide GmbH).

Additional water impurities may result from residual humidity in the tubing as well as diffusion through the tube walls. In the previous work [26] the resulting feed gas humidity was determined to be less than 20 ppm for this setup after flushing the tubes as specified in section 2.2. 1 ppm level water impurity in the feed gas was thus also included in the numerical model used in this work. Furthermore, the molecular admixtures ${{{\rm N}}_{2}}$ and ${{{\rm O}}_{2}}$ are expected to be much larger (0.1–1%) than the residual impurities from the gas bottles and the tubing and hence are expected to dominate the RONS chemistry.

The mixing of either of these molecular gases with argon is performed before the gas is blown through the plasma jet in order to obtain a better homogeneity. The admixture step is 0.1% (3.0 sccm) and controlled by calibrated mass-flow controllers.

Additionally, an artificial mixture of both ${{{\rm N}}_{2}}$ and ${{{\rm O}}_{2}}$ is also used in this work besides admixing both gases separately. Obviously, this should result again in a different gas composition of the plasma and the effluent. In this case, the argon/admixture ratio is fixed (Ar 99.0 + 1.0% admixture) but the content of the admixture fraction itself is varied from 0.0 to 100.0% ${{{\rm O}}_{2}}$ with 100.0–0.0 % ${{{\rm N}}_{2}}$. Hence, a 0.2% ${{{\rm O}}_{2}}$/0.8% ${{{\rm N}}_{2}}$ ratio is equivalent to a 1.0% dry air/99.0% argon admixture.

The numerical simulations are performed with a 0D chemical kinetics model which is based on the original GlobalKin source code, developed by Kushner and co-workers [35]. Previously we developed a large Ar/N$_{{\rm 2}}$/O$_{{\rm 2}}$/H$_{{\rm 2}}$O reaction set of 85 species interacting by means of 302 electron impact reactions and 1626 heavy particle reactions. This extensive reaction set was presented previously [36] and for more details about this input data we refer to the supplementary information. This is necessary to calculate the densities of biomedically active species, which often have relatively low concentrations, within a broad parameter range. Some minor adjustments to the initial reaction set were reported by Van Gaens et al [9, 37].

Several modifications to the original code enable us to evaluate the species densities and chemical reaction pathways throughout the plasma jet propagating in an open humid air atmosphere. These modifications are thoroughly discussed in a previous paper [36], but the most important adjustments are briefly summarized below and in the appendix.

We assume that the active plasma species can be found within a cylindrical area along the plasma jet and its effluent, which has the same diameter as the inner diameter of the plasma jet tube. This area corresponds to the visual plasma jet and beyond (see figure 2). During the simulation, we track a cylinder segment that flows along the symmetry axis of the jet. The length of the segment is the distance that the gas travels within one time step of the calculations (order of μs ). Since our model is 0D the plasma properties (such as the species densities) are volumetrically averaged over this cylinder segment.

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The plasma properties for this cylinder segment will change when it moves further along the flow because it is subjected to a varying power deposition, flow velocity, gas temperature and impurities diffusing from the gas curtain, as a function of its position.

It is very important to mention that these plasma parameters are not calculated self-consistently within our model.

The gas flow velocity and the gas curtain entrainment rate follow from an external 2D computational fluid dynamics simulation of the neutral gas at room temperature using Reynolds-averaged Navier–Stokes equations with a standard k– model to account for the turbulence [38]. At a gas flow rate of 3 slm and a corresponding Reynolds number of $\operatorname{Re}\approx 3000$ the flow is expected to be fully turbulent. The computed ambient species densities agree well with mass spectrometric measurements [38–40].

In the far effluent region the gas will eventually become stationary and there we assume that the 3 slm argon feed gas is fully mixed with the 5 slm dry air of the gas curtain. However, it is obvious that there is also an admixture gradient of the gas curtain in the radial direction of the jet. As our model is 0D without a degree of freedom in the radial direction, we simply tested which of the admixture profiles (from different off-axis positions) resulted in the best agreement between the simulations and the measurements. Eventually, the gas curtain admixture profile at r = 0.6 mm (determined by the 2D fluid dynamics simulations) resulted in the best correspondence between the experimental and the calculated species densities. Due to this approximation some effects related to multiple dimensions might be neglected. In reality the situation is more complex as the propagation of the ionization wave is strongly linked to the turbulent flow pattern. It was recently found that the ionization wave preferentially propagates through the channel with the highest noble gas content and thus the lowest concentration of admixtures (in the order of 1% and lower)[39]. Moreover, as the ionization wave follows the vortices occurring in the turbulent flow, it is often positioned off-axis [41].

The power deposition profile within the device and the visual plasma obviously depends on the electric field generated by the electrodes and the plasma itself. It is greatly affected by the electrode setup, as was already demonstrated by this model in a previous work [9]. Because the electrode configuration of the kinpen has a very similar geometry, we use the same shape for the power deposition profile (see figure 2) but the magnitude of the power density is scaled down to match the operating conditions of the kinpen; we set the total power deposition equal to 1.5 W (see section 2.3). The power density determines in every timestep how much energy is transferred to the electrons in the electron energy equation.

Important is also that our power deposition profile shown in figure 2 does not explicitly take the RF excitation waveform into account. We apply this simplification since we are primarily interested in the dynamics of long-lived species and therefore neglect fluctuations on the sub-microsecond timescale. Thus, it should be sufficiently accurate that the total deposited power for the calculations is equivalent to the experimentally measured plasma power deposition, as long as the bulk chemistry does not change by neglecting the RF excitation. Our previous work showed that this assumption is valid for argon plasma jets operating at a higher frequency of 13.56 MHz [9, 42].

The experimentally measured gas temperature profile is plotted in figure 2 as well. We directly used the on-axis probe measurement, i.e., maximum temperature values, as described above and did not account for the effect of a radial gradient in the temperature. The gas temperature value can influence reaction rates since the coefficients are a function of this parameter. Also note that the experimental measurement (see section 2.4) indicates that the gas temperature already starts decreasing at a few millimeters from the nozzle exit. However, within the apparatus the temperature could not be measured accurately (with spatial resolution) and therefore needed to be estimated.

In some of our previous work a temperature profile that starts rising (from room temperature) at the needle tip position was used as an input, assuming that the temperature rise is mainly caused by exothermic reactions of highly reactive radicals or highly energetic species created by electron impact processes. Indeed, when the temperature is calculated by the 0D model itself, these processes cause a maximal temperature value at the same position, shortly after the nozzle exit. Nevertheless, we opted not to adopt the calculated temperature profile because it is not consistent with the experimentally measured values further into the effluent. The deviation is probably related to turbulent heat transfer.

The main advantage of our 0D semi-empirical approach is the possibility to implement the large chemistry set of an Ar/N$_{{\rm 2}}$/O$_{{\rm 2}}$/H$_{{\rm 2}}$O mixture necessary to simulate the kinetics of the biomedical species (which are often not the main plasma components) without excessive calculation times, and while staying close to the experimental conditions.

Evidently, this model needs a good validation by thorough comparison with experimental measurements, as previously demonstrated in Zhang et al [42] and Van Gaens et al [9]. In the latter paper, a good correspondence with the measured NO and O densities was presented for a kHz pulsed RF driven argon plasma jet with air admixtures. In Zhang et al [42] a good quantitative agreement was obtained for the ${{{\rm O}}_{3}}$ density in an RF driven argon plasma jet with ${{{\rm O}}_{2}}$ admixture. Note that in both studies this validation was done with spatial resolution along the symmetry axis of the jet.

Figure 3 (top) demonstrates a very good agreement between the experimentally measured and simulated NO₂ production rate, for the investigated range of ${{{\rm N}}_{2}}$ admixtures between 0 and 1%: the trend as a function of the N₂ admixture and the absolute values are very similar, with at maximum a factor 2 difference for 0% N₂ . Unfortunately, this is not the case for the O₃ production rate where the difference is up to a factor 3 (see figure 3 (bottom)). Furthermore, the model predicts a steeply increasing O₃ production rate between 0 and 0.2% admixed N₂ , whereas the experiments indicate only a very slight increase over the entire operating range. Note that this might be explained by the fact that these ${{{\rm O}}_{3}}$ concentrations are near the detection limit of the diagnostic setup under the present experimental conditions. Moreover, experimentally, very slight differences in the jet properties, such as turbulence or impurities in the tubes, can easily occur. This determines the concentration of molecular gases in the argon and therefore indirectly affects the O₃ concentrations. Indeed, we will show in this paper that the balance between production and loss rates is often very delicate and that a slight inaccuracy can have a large influence on the NO₂ and O₃ net production rate. In this context it needs to be mentioned that we chose to keep the gas curtain entrainment rate consistent for all the simulations, so for the different admixture amounts and for the different admixture gases.

We will now further clarify the production pathways of O₃ and NO₂ by means of a detailed chemical analysis, as obtained from the model, but now focussing mainly on the chemistry that occurs on the time scale of milliseconds. Indeed, this is the time frame where, chemically speaking, the most interesting changes happen. Obviously, this time frame corresponds to the distance that a gas element travels within the kinpen device, the active/visual plasma jet and finally the initial effluent region (thus not the entire measurement cell). The displayed values in figure 4–9 below (i.e. density, the total production and loss rates, the relative contributions of the reactions and electron temperature) are averages for one of these three regions, obtained by performing an integration along the symmetry axis of the jet. Accordingly, the most important chemical phenomena can be identified for each region.

For example, the density evolution of a species, from the vicinity of the needle electrode tip until the early effluent, is thus reduced to only three data points. Indeed, this turned out to be crucial for maintaining a relatively simple overview of the changing chemistry when varying the admixture ratios and at different distances from the nozzle and the needle tip. To make this concept easier to interpret, we added the schematic of the plasma jet above the reaction kinetics data in figure 4.

Also important is that in each figure the production rates are displayed in the upper part of the top graph and the loss rates in the bottom part of the top graph (both with white dots, left axis). Note that the y-axis of the loss rates increases from top to bottom, hence opposite to the y-axis of the production rates, to clearly indicate their opposite effect. The same top graph also displays the contribution of the different reactions to the total production and loss rates by colour areas (right axis). We only show the reactions which contribute more than 10% to the total loss or production of a species. Note that we show all these contributions, for the sake of completeness, but in the text we only discuss the most important production and loss processes. Therefore, the sum of the different contributions often does not reach the full 100%. The most important processes are indicated in bold in the legends at the right of the graphs. The bottom graph of these figures presents the species densities.

The short time-scale information in these plots (e.g. Figure 4) is, in general, sufficient to explain the trend observed on the longer time-scale (see figure 3).

3.1.1. O₃ formation

Figure 4 illustrates that ${{{\rm O}}_{3}}$ is mainly formed in the effluent region (see white dots in the top graph) for all N₂ admixtures investigated. Indeed, the formation rate in the plasma jet is about one order of magnitude lower, and there is no ${{{\rm O}}_{3}}$ formation at all inside the device. The figure shows the following dominant pathway:

$$\begin{eqnarray}&&{\rm O}+{{{\rm O}}_{2}}+{\rm Ar}\to {{{\rm O}}_{3}}+{\rm Ar}.\end{eqnarray} \tag{ 1 }$$

It is clear from figure 4 that the production rate in the effluent region is more than two orders of magnitude higher than the loss rate (see upper and lower part of the top graph). Indeed, the main species responsible for the loss of ${{{\rm O}}_{3}}$ are the electronically excited states ${{{\rm O}}_{2}}$(a) and ${{{\rm O}}_{2}}$(b) (see figure 4 as well), but they do not reach sufficiently high concentrations. The ${{{\rm O}}_{3}}$ density as a function of the N₂ admixture in the early effluent therefore corresponds to the production rate in the far effluent as predicted by the model and depicted in figure 3 (note the logarithmic scale in figure 4). Recall, however, that the agreement with the experimental result was not very good in this case.

As the O atoms are fully responsible for the ${{{\rm O}}_{3}}$ production, we will now analyze the chemistry of this species.

Figure 5 illustrates that the variation of the O density as a function of the ${{{\rm N}}_{2}}$ admixture is very similar to that of O$_{{\rm 3}}$. Furthermore, it can be seen that the O atom production occurs mainly in the plasma jet region. Indeed, in this region ${{{\rm O}}_{2}}$ from the gas curtain starts to diffuse into the argon jet, as shown in [39, 40, 43].

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Yet for 0% N₂ admixture, the O atom production inside the device is quite significant. Indeed, there is only some N₂ present in the form of impurities, because, as initial conditions of our simulations, we always impose a slight amount of N₂ , O$_{{\rm 2}}$ and H$_{{\rm 2}}$O in the argon feed gas (1 ppm) to mimic the impurities in the gas bottles and desorbed molecules from the piping. This N₂ impurity level is insufficient to reduce the electron temperature as drastically as for cases with significant N₂ fractions (see figure 6), where a lot of energy is used for vibrational excitation of N₂ . Fast electron impact dissociation of O$_{{\rm 2}}$ and electronically excited OH radicals, i.e. OH(A) created from H$_{{\rm 2}}$O, are therefore possible within the kinpen device at 0% N₂

$$\begin{eqnarray}&&{{e}^{-}}+{{{\rm O}}_{2}}\to {{e}^{-}}+{\rm O}+{\rm O},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{{e}^{-}}+{\rm OH}({\rm A})\to {{e}^{-}}+{\rm O}+{\rm H}.\end{eqnarray} \tag{ 3 }$$

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As a result, the O atom density is already significant within the kinpen device when no ${{{\rm N}}_{2}}$ is admixed to the argon.

This also explains the maximum in the O atom density in the plasma jet region, at 0% N₂ , and the clear drop upon addition of small N₂ admixtures. However, at larger N₂ admixtures the O atom density increases again. Here, the O atoms are mainly created from collisions of ${{{\rm O}}_{2}}$ species with the nitrogen metastable (N₂ (A)):

$$\begin{eqnarray}&&{{{\rm N}}_{2}}({\rm A})+{{{\rm O}}_{2}}\to 2\;{\rm O}+{{{\rm N}}_{2}}.\end{eqnarray} \tag{ 4 }$$

Moreover, in the plasma jet region the rates of the reactions (2) and (3) become insignificant for 0% ${{{\rm N}}_{2}}$. This is because the electron density drastically drops as a result of electron attachment to oxygen species (see figure 6; note that the electron chemistry itself is, however, not explicitly shown in this paper) since the ${{{\rm O}}_{2}}$ density quickly rises due to the mixing of curtain gas with the argon.

Note that figure 7 illustrates that ${{{\rm N}}_{2}}({\rm A})$ is also significantly quenched in this region but the quenching of this state is much slower than that of the electrons. Additionally, the rising ${{{\rm O}}_{2}}$ density compensates for this ${{{\rm N}}_{2}}({\rm A})$ quenching and still causes the rate of reaction (4) to be high in the plasma jet region.

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Besides, figure 7 also illustrates that N₂ (A) is mainly formed inside the kinpen device, i.e. by electron impact excitation of ground state N₂ :

$$\begin{eqnarray}&&{{e}^{-}}+{{{\rm N}}_{2}}\to {{e}^{-}}+{{{\rm N}}_{2}}({\rm A}).\end{eqnarray} \tag{ 5 }$$

Thus one might expect a rising N₂ (A) density upon increasing N₂ admixture, simply because the density of one of the reactants becomes higher.

However, above 0.4% N₂ the electron density and electron temperature, plotted in figure 6, become quite low (due to the vibrational kinetics, as mentioned above) and this compensates for the rising N₂ density. This even causes a slight drop in the rate of reaction (5) above 0.4% N₂ and the same behavior is therefore seen for the N₂ (A) density in the bottom graph of figure 7. As a result the rate of reaction (4) (causing the dissociation of O$_{{\rm 2}}$ into O atoms in the plasma jet) will also first increase upon increasing N₂ fraction (after the initial drop, as explained above), but after 0.2% N₂ it will remain more or less constant (see figure 5). This explains the behaviour of the O atom density upon rising N₂ fraction, and because the O atoms are mainly responsible for the O₃ production, this also clarifies why the O₃ production and O₃ density first increase upon rising N₂ fraction, then remains more or less constant and eventually slightly decreases for N₂ fractions above 0.5% (see figure 4 and also figure 3 above).

The above explains the production rates predicted by the model. We believe that we at least identified all the dominant reactions correctly, although the agreement with the experiments is not perfect. Note that deviations can easily occur since the balance between the production and loss processes of all the species involved here is delicate. A more complete model approach might be better in this case.

3.1.2. NO₂ formation

Figure 8 illustrates the NO₂ production and loss rates, as well as the relative contributions of the different processes and the NO₂ density, as a function of N₂ admixture. It is clear that NO₂ is mainly produced from NO, especially by reaction (6), and to a lower extent also by reaction (7):

$$\begin{eqnarray}&&{\rm O}+{\rm NO}+{\rm Ar}\to {\rm N}{{{\rm O}}_{2}}+{\rm Ar},\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{\rm H}{{{\rm O}}_{2}}+{\rm NO}\to {\rm OH}+{\rm N}{{{\rm O}}_{2}}.\end{eqnarray} \tag{ 7 }$$

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These processes are especially important in the plasma jet. Indeed, the total production rate of NO₂ is almost an order of magnitude larger than the total loss rate here. In the effluent region this production rate has already decreased by at least a factor 2, but additionally the loss rate (i.e. mainly the reaction ${\rm O}+{\rm N}{{{\rm O}}_{2}}\to \ {\rm NO}+{{{\rm O}}_{2}}$) is now of about the same magnitude. The net production rate is therefore much smaller in the effluent than in the plasma jet region.

Still, NO₂ is a rather long-lived species because there are no highly reactive species present in the effluent that are sufficiently abundant to cause considerable NO₂ loss within these time scales. Note that the O atoms are involved both in the main production and loss reactions of NO₂ and this species will eventually get depleted in the effluent region (see figure 5 above. The same is true for N, OH and HO$_{{\rm 2}}$, which are involved in several other loss processes).

Important to mention is that NO₂ reaches only its maximum concentration towards the end of the plasma jet region, whereas the density practically does not change any more in the effluent region and stays continuously high here. The averaging we performed thus results in a relatively low NO₂ density in the plasma jet region, although it is mainly produced here.

Because NO is the main NO₂ precursor, we show the NO chemistry in figure 9. NO is produced in large amounts early in the plasma jet region, by a reaction between N₂ (A) and O atoms:

$$\begin{eqnarray}&&{{{\rm N}}_{2}}({\rm A})+{\rm O}\to {\rm NO}+{\rm N}{{(}^{2}}{\rm D}).\end{eqnarray} \tag{ 8 }$$

The reason for this is twofold: first, the maximum N₂ (A) density is located close to the needle electrode where the power density is at maximum; further in the plasma jet it is rapidly quenched by ${{{\rm O}}_{2}}$, thereby creating O atoms or excited ${{{\rm O}}_{2}}$ molecules (see figure 7). Second, the O atoms only reach a maximum density in the plasma jet region, as illustrated in figure 5, because of the oxygen entrainment from the ambient into the argon, which obviously only starts after the nozzle exit. The combination of these two effects explains why the maximum rate of reaction (8) is located in the early plasma jet, close to the nozzle exit. Nevertheless, the production of NO by this reaction is also non-negligible inside the device, as is clear from figure 9.

As the excited N₂ (A) molecules mainly determine the NO formation, and NO is the dominant NO₂ precursor, the NO₂ production as a function of the N₂ admixture therefore largely follows the trend of the N₂ (A) state, which depends both on the N₂ fraction and on the electron temperature inside the kinpen device, as the latter determines the rate coefficient. Because the electron temperature (and hence the rate coefficient) drastically decreases upon increasing N₂ fraction, at least within the device (see figure 6 above), these two effects are opposite to each other.

Thus, the NO₂ production (and density) steeply rises immediately when small amounts of N₂ are added, but beyond 0.15% N₂ the NO₂ production (and density) drops again, because the effect of the electron temperature starts to play a more dominant role. Indeed, this explains the trend seen for the NO₂ production rate in figure 3 above.

This subsection is structured in exactly the same way as the previous one for nitrogen admixtures. We first look at the longer timescale, comparing experiments with simulations and consequently we explain the observed trends on the basis of a chemical reaction analysis of the short timescale.

The trends of both the NO₂ and the O₃ production rate as a function of the O$_{{\rm 2}}$ fraction in the argon feed gas (from 0 and 1%) are reproduced well by the model, as demonstrated in figure 10. The NO₂ production rate decays exponentially as a function of the O$_{{\rm 2}}$ fraction, while the O₃ trend is practically the inverse with a sharp increase between 0 and 0.2% ${{{\rm O}}_{2}}$. The absolute values are, however, not fully comparable, although the difference is at most one order of magnitude within the investigated range of the ${{{\rm O}}_{2}}$ fraction. As previously stated, this might be related to the inaccuracy on the amounts of curtain gas that diffuses into the jet (i.e. the absence of a radial gradient in the model or consequences of turbulence).

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3.2.1. O₃ formation

As described in section 3.1.1, ${{{\rm O}}_{3}}$ is almost exclusively produced by reaction 1. However, in the case of adding ${{{\rm O}}_{2}}$, this occurs already in the plasma jet region and even inside the device, at least for ${{{\rm O}}_{2}}$ fractions above 0.2%, as demonstrated in figure 11.

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For lower ${{{\rm O}}_{2}}$ levels there is clearly not yet enough O$_{{\rm 2}}$ present inside the device and in the (early) plasma jet to yield a large rate for reaction (1). Therefore, the production will occur in such a case mainly in the effluent region when there is enough diffusion of O$_{{\rm 2}}$ from the gas curtain.

At higher O$_{{\rm 2}}$ fractions, the production of O₃ is smaller in the effluent than in the plasma jet region or even inside the device. Nevertheless, since the O₃ loss is clearly negligible in the effluent region, the O₃ density is still at maximum here (see figure 11). This explains why O₃ is a relative long-lived species. As described in section 3.1.1, the O₃ production is mainly determined by the O atoms; therefore, the chemistry of the O atoms is displayed in figure 12. The O chemistry, however, is now considerably different from the case when adding ${{{\rm N}}_{2}}$ (section 3.1.1). Indeed, the O atoms are now mainly produced within the kinpen device, so mainly from the oxygen admixture itself and not from the gas curtain.

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Secondly, in the absence of significant ${{{\rm N}}_{2}}$ admixtures, the O atom production is now mainly due to the dissociation of ${{{\rm O}}_{2}}$ by direct electron impact and by collisions with energetic argon species (i.e. Ar$_{{\rm 2}}*$excimers, Ar($^{{\rm 4}}$S($^{{\rm 3}}$P$_{{\rm 2}}$)) metastables, as well as higher excited states, grouped in the model as Ar($^{{\rm 4}}$P), but not by collisions with N₂ (A). Note that the heavy particle reactions with energetic argon species are in fact also an (indirect) result of electron impact reactions, because these species are created by electron impact excitation of Ar ground state atoms, possibly followed by an association with another Ar atom to form the Ar$_{{\rm 2}}*$excimers (data not shown).

A third difference is that the contribution of ${{{\rm O}}_{2}}$ dissociation upon collisions with the argon species increases with rising ${{{\rm O}}_{2}}$ fraction, whereas the contribution of direct electron impact dissociation drops (at least between 0 and 0.2% ${{{\rm O}}_{2}}$). Indeed, the contribution of the latter process is more than 60% at very small ${{{\rm O}}_{2}}$ concentrations, compared to about 30% above 0.2% ${{{\rm O}}_{2}}$. This can be explained by the electron temperature evolution as a function of the ${{{\rm O}}_{2}}$ admixture (see figure 13). Clearly, the average electron temperature rises for higher ${{{\rm O}}_{2}}$ admixtures and argon excitation therefore becomes relatively more important than ${{{\rm O}}_{2}}$ dissociation because of the higher threshold energy and since the reaction coefficient thus drastically increases with the electron temperature.

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Finally, note that after the initial rise in net O atom production (and thus O atom density) until 0.2% O$_{{\rm 2}}$, the O atom density stays relatively constant at higher O$_{{\rm 2}}$ admixtures (i.e. between 0.2 and 1.0% O$_{{\rm 2}}$). This is not only because the average electron temperature is rather constant in this range but also because the O atoms seem to be 'self-quenching' as the main loss pathway of the O atoms creates ${{{\rm O}}_{3}}$ (see reaction (1)) and the latter is also quite important for the loss of O atoms (see figure 12 again):

$$\begin{eqnarray}&&{{{\rm O}}_{3}}+{\rm O}\to 2\;{{{\rm O}}_{2}}.\end{eqnarray} \tag{ 9 }$$

The ${{{\rm O}}_{3}}$ production is therefore almost linearly dependent on the O$_{{\rm 2}}$ fraction between 0.2 and 1.0% (or between 0.3 and 1.0% for the experimental values), as was depicted in figure 10. Indeed, the rate of reaction (1) is practically first order because the O atom concentration does not change much in this range.

3.2.2. NO₂ formation

The admixture of ${{{\rm O}}_{2}}$ does not change the dominant production pathway for NO₂ compared to the admixture of ${{{\rm N}}_{2}}$ (see section 3.1.2). NO₂ is still mainly formed by the reaction between O and N₂ (A) producing NO (by reaction (8) above, data not shown again here), which then oxidises further by the three-body reaction with O atom and Ar as the third collision partner (see reaction (6) above), as depicted in figure 14.

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The net production rate is the highest towards the end of the plasma jet region (the loss is significantly lower than the production although this is difficult to see due to the log scale). Note that some NO₂ production also occurs in the effluent region but the loss rate is now more comparable to the production rate here. However, the NO₂ density shown in figure 14 is not as high in the plasma jet region as in effluent region, again simply due to the averaging (as explained above).

The NO₂ formation once more depends indirectly on the N₂ (A) formation and this species is produced less upon increasing ${{{\rm O}}_{2}}$ admixture (see figure 15). This is because N₂ (A) is mainly formed from electron impact excitation, as can be seen from this figure, and the density of the electrons, which are involved in this reaction, rapidly drops when increasing the ${{{\rm O}}_{2}}$ content (see figure 13). The latter is caused by efficient electron attachment processes for ${{{\rm O}}_{2}}$ species. Obviously, the reaction rate of electron impact excitation (reaction (5)) also depends on the electron temperature which determines the rate coefficient, but this is less important in this range of electron temperatures (3–3.5 eV). Therefore, the production of N₂ (A), as well as the N₂ (A) density, clearly drop upon increasing O$_{{\rm 2}}$ fraction, as shown in figure 15.

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Secondly, the NO₂ formation also depends on the concentration of the other reactant, i.e. the O atoms. The density of this species drastically rises when very small amounts of ${{{\rm O}}_{2}}$ are added to the feed gas, but afterwards (beyond 0.1%) the density remains more or less constant, as discussed above (see figure 12).

Combining the N₂ (A) and the O density trends upon increasing ${{{\rm O}}_{2}}$ admixture indeed leads to the observed behaviour of the NO₂ density as a function of the O$_{{\rm 2}}$ fraction, depicted in figure 14: a steep rise between 0 and 0.1% O$_{{\rm 2}}$ to a maximum of about 10$^{{\rm 1}3}$cm$^{-3}$ followed by a clear drop for higher O$_{{\rm 2}}$ fractions.

Note, however, that the calculated and measured net NO₂ production rates as a function of the O$_{{\rm 2}}$ admixture, illustrated in figure 10 above, do not exhibit this initial rise between 0 and 0.1% O$_{{\rm 2}}$, that is observed in figure 14 (not only for the NO₂ density, but also for the NO₂ production rate). The reason is that the calculated and measured NO₂ net production rates, shown in figure 10 above, apply to a much longer time scale (i.e. in the order of seconds; see the total residence time in the measurement cell, as described at the beginning of section 3), whereas the calculation results of the effluent region depicted in figure 14, apply to a time scale in the order of milliseconds.

Within this shorter time frame, the loss of NO₂ occurs predominantly by collision with O atoms, and these species will eventually disappear from the discharge (for example by forming O₃ as described above). Therefore, in a later stage of the effluent, O₃ will be the only available molecule able to react with NO₂ as it is the only reactive species that is at least as abundant as NO₂ . Note that the reaction NO₂ + O₃ is not displayed in figure 14 because only the dominant reactions contributing more than 10% are presented, but at longer residence times, this reaction eventually becomes important. As the O₃ density is rather small for O$_{{\rm 2}}$ admixtures between 0 and 0.1%, the NO₂ loss at these longer time scales will also be negligible at these low O$_{{\rm 2}}$ admixtures. Therefore the net production of NO₂ will be higher between 0 and 0.1% at these longer time scales, and indeed the NO₂ net production rate will continuously drop from 0 to 1% O$_{{\rm 2}}$ as illustrated in figure 10 above.

An interesting combination of the two chemistries clarified in the previous sections is obtained when ${{{\rm O}}_{2}}$ and ${{{\rm N}}_{2}}$ are simultaneously added to the argon feed gas. Recall that in this case, a fixed 1% O$_{{\rm 2}}$ + N₂ admixture is used, but with the O$_{{\rm 2}}$/(O$_{{\rm 2}}$ + N₂ ) ratio varied between 0 and 100%. Therefore, 0% O$_{{\rm 2}}$/(O $_{{\rm 2}}$ + N₂ ) in figures 17–21 correspond to the conditions of 1% N₂ admixture in figures 4–9, whereas 100% O$_{{\rm 2}}$/(O $_{{\rm 2}}$ + N₂ ) corresponds to the case of 1% O$_{{\rm 2}}$ in figures 11–15.

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Figure 16 illustrates that the simulated evolution of the NO₂ and O₃ net production rates is also well in accordance with the measurements. For both species, the shape of the curve is quite complex. The NO₂ production rate initially drops (except for a first rise between 0 and 5% O$_{{\rm 2}}$ for the model results), but increases again at about 20% O$_{{\rm 2}}$/(N₂ + O$_{{\rm 2}}$). A second maximum is formed at about 50% O$_{{\rm 2}}$/(N₂ + O$_{{\rm 2}}$) before the production rate steeply drops close to 100% O$_{{\rm 2}}$ (thus for low N₂ levels).

The shape of the O₃ production rate as a function of the admixture composition is clearly x $^{{\rm 3}}$-shaped, although more pronounced in the experimental measurements. Indeed, the difference between the simulated and measured O₃ production rate is in general about a factor 2, but in the middle of the investigated range, at an O$_{{\rm 2}}$/N₂ ratio close to 1, this difference has increased to a factor 3.

3.3.1. O₃ formation

3.3.2. NO₂ formation

For the NO₂ production rate as a function of the O$_{{\rm 2}}$/(O$_{{\rm 2}}$ + N₂ ) admixture ratio one might expect the highest value for equivalent O$_{{\rm 2}}$ and N₂ fractions, but this does not seem to be the case (see figure 16 above). From the above sections, we know that NO₂ is formed from NO and this is also valid here (therefore, the NO₂ chemistry is not explicitly shown, as it does not give new information). Thus, by studying the chemistry of NO in figure 20 it is possible to explain the observed trend for the net NO₂ production rate displayed in figure 16.

In the plasma jet region the NO density is, as expected, the highest between 10 and 70% O$_{{\rm 2}}$/(O$_{{\rm 2}}$ + N₂ ) ratio. However, in the effluent region a small dip at 20% starts to develop. This is due to significant NO destruction upon collision with N atoms:

$$\begin{eqnarray}&&{\rm N}+{\rm NO}\to {{{\rm N}}_{2}}+{\rm O}.\end{eqnarray} \tag{ 10 }$$

Indeed, the concentration of the N atoms is at maximum at this O$_{{\rm 2}}$/(O$_{{\rm 2}}$ + N₂ ) ratio, as demonstrated by figure 21.

As a result, the measured and modelled net NO₂ production rate further in the measurement cell (shown in figure 16 above) exhibits a similar profile as the NO density in the effluent region of figure 20, because NO₂ is created from NO.