Hybrid Dielectric Barrier Discharge Reactor: Production of Reactive Oxygen–Nitrogen Species in Humid Air

Abstract

Reactive oxygen–nitrogen species (RONS) production in a Peltier-cooled hybrid dielectric barrier discharge (HDBD) reactor operated with humid air is characterized. Fourier-transform infrared spectroscopy (FTIR) is used to determine the RONS in the HDBD-produced gases. The presence of molecules ${\mathrm{O}}_3$, ${\mathrm{NO}}_2$, ${\mathrm{N}}_2$O, ${\mathrm{N}}_2{\mathrm{O}}_5$, and ${\mathrm{HNO}}_3$ is evaluated. The influence of HDBD reactor operation parameters on the FTIR result is discussed. The strongest influence of Peltier cooling on RONS chemistry is reached at conditions related to a high specific energy input (SEI): high voltage and duty cycle of plasma width modulation (PWM), and low gas flow. Both PWM and Peltier cooling can achieve a change in the chemistry from oxygen-based to nitrogen-based. ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$ are detected at a low humidity of 7% in the reactor input air but not at humidity exceeding 90%. In addition to the FTIR analysis, the plasma-activated water (PAW) is investigated. PAW is produced by bubbling the HDBD plasma gas through 12.5 mL of distilled water in a closed-loop circulation at a high SEI. Despite the absence of ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$ in the gas phase, the acidity of the PAW is increased. The pH value decreases on average by 0.12 per minute.

1. Introduction

The first description of dielectric barrier discharge (DBD) was included in a scientific book published in 1855 by du Moncel [1]. Ozone production for decontamination of potable water is one of the oldest DBD applications [2]. It found wide acceptance as a large-scale water treatment method in the next century [3,4]. Many new applications based on DBD have been developed [5,6], especially in analytical chemistry [7] and for surface sterilization and decontamination [8]. The main driving force behind the success of DBD is its thermal non-equilibrium [9]. The heavy particles of the produced plasma remain cold, but electrons are hot, reaching, for example, 5850 K in air-operated DBD [10], allowing for high-temperature physical (ionization, electronic excitation) and chemical (dissociation) processes in cold atmospheric pressure plasma (CAPP).

The two main types of DBD, volumetric DBD (VDBD) and surface DBD (SDBD), differ essentially in the way the microdischarges develop [11,12]. In VDBD, they spread vertically between the dielectric barrier and the electrode (asymmetric VDBD or DBD with a floating barrier) or the second barrier (symmetric VDBD) [13]. The differences between VDBD and SDBD for microbial decontamination are the subject of current research [14]. An improvement in energy efficiency has been found in hybrid dielectric direct discharge (HDBD) [15,16], allowing the concurrent operation of the surface and volume types of DBD. The term HDBD used in our studies should not be confused with the H-DBD used in the literature on homogeneous DBD, e.g., in articles by Lu et al. [17]. A novel HDBD reactor and its use for the characterization of ozone generation in oxygen, synthetic air, and compressed dry air (CDA) were described in detail in our previous publication [18]. A similar setup (see Section 2.1) is used in this study to examine the production of reactive oxygen and nitrogen species (RONS).

The RONS created in atmospheric pressure plasmas in the air are essential biologically. They play key roles in cell signaling, metabolism, and immunity for plants and animals [19]. The type of discharge has a strong influence on the balance between the oxygen and nitrogen chemistry. Machala et al. [20] compared corona and spark discharges, observing that a corona discharge tends to have an oxygen chemistry and produces ${\mathrm{O}}_3$, while a spark discharge tends to have a nitrogen chemistry and produces ${\mathrm{NO}}_{\mathrm{x}}$. The main method for influencing the chemistry for a given discharge architecture is changing the specific energy input (SEI) [21], called energy density [22], or, alternatively, the beta parameter $\beta$ [23], expressed in ${\mathrm{J/cm}}^3$ or eV/molecule. To enable the desired chemistry, the HDBD reactor allows for SEI tuning via the driver input voltage, the width modulation (PWM) aspect ratio, and the gas flow. A way to influence the chemistry further is by controlling the discharge temperature. The Peltier module is used in the HDBD reactor to cool the dielectric barrier. Unlike in water cooling, no additional media supply is needed. It allows for a much simpler reactor design. Compared to air cooling, it assures much better homogeneity and controllability of the heat flux extracted from the discharge. However, air cooling is still needed to remove the heat from the hot side of the Peltier module. The inferior energy efficiency of Peltier cooling compared to water cooling is a minor problem in cases with a small-scale reactor. The influence of Peltier cooling on the temperature of the gas flowing out of the HDBD reactor is discussed in Section 3.1. One of the main tasks of this study is to examine how Peltier cooling can be used to control the chemistry of the remote HDBD plasma.

The presence of moisture in the gas treated in CAPP is an important factor influencing the plasma’s physics [24] and chemistry [25,26]. Previous studies showed that the inactivation efficacy of microorganisms increases with air humidity [27]. A better understanding of CAPP processes in humid air is achieved using computer simulations. Kinetic models have been developed, e.g., for corona discharge [28] and volumetric DBD [29]. Sakiyama et al. [30] compiled an extensive set of chemistry model reactions in humid air (1% ${\mathrm{H}}_2$O molar) valid for the remote zone of the surface microdischarge (SMD), a sub-type of SDBD, but applicable also for the HDBD remote plasma. Kruszelnicki et al. [31] simulated the chemical interactions of CAPP with water droplets.

The review by Locke [32] summarizes that plasma enriched with water in the form of molecules (vapor) [33,34], clusters [35,36], aerosols [37,38], droplets [39,40,41], and surfaces [42], can be considered as a continuous phase. Different practical methods have been reported for the introduction of water in the discharge zone of the CAPP. A porous wall was used to seep water into the discharge zone in [43]. Kovacevic et al. [44] used a film of water overflowing one of the dielectric barriers. In the experiment of Wang et al. [45], the bulk water played the role of a grounded DBD electrode. Recently, hollow-fiber humidifiers [46,47] have gained importance. Müller et al. [48] used a bubbling flask for air humidification. We use a similar method, as described in Section 2.2. The unique feature of the materials used in the HDBD reactor structure is their resistance against the aggressive conditions of CAPP in humidified air. It enables the investigation of the influence of high humidity on remote HDBD plasma, one of the main goals of this study.

Several physical methods are available for chemical analysis of the plasma products. Optical emission spectroscopy (OES) allows for measurement of reactive species generated in DBD in direct contact with water in several gases [44]. The UV absorption technique is an efficient method for determining the ozone concentration [49,50]. It is used in our study as described in Section 2.5. Many molecular plasma products in the gas phase can be detected using Fourier-transform infrared (FTIR) spectroscopy [51]. Waskow et al. [52] used it for a parameter study of DBD with synthetic air. Al Abduly et al. [53] conducted an in situ and downstream study of non-thermal plasma chemistry in air-fed DBD. They detected ${\mathrm{O}}_3$, ${\mathrm{N}}_2{\mathrm{O}}_5$, ${\mathrm{HNO}}_3$, and ${\mathrm{N}}_2$O molecules in the downstream exhaust FTIR absorbance spectra. The importance of ${\mathrm{N}}_2{\mathrm{O}}_5$ for biomedical applications due to its high reactivity inspired Wang et al. [54] to investigate the energy efficiency of ${\mathrm{N}}_2{\mathrm{O}}_5$ production in DBD. Chen et al. [55] investigated the mechanisms of NO destruction by SDBD using FTIR measurements. The FTIR spectra showed a decrease in the NO as a function of the SEI. At the same time, a strong increase in ${\mathrm{NO}}_2$ and ${\mathrm{N}}_2$O was observed. Ibba et al. [56] investigated RONS in remote gases of SDBD using laser-induced fluorescence (LIF) and FTIR. The characteristic peaks of ${\mathrm{N}}_2$O, ${\mathrm{NO}}_2$, ${\mathrm{N}}_2{\mathrm{O}}_5$, and ${\mathrm{O}}_3$ were distinctly observed for the following input gas: ${\mathrm{N}}_2+6$% ${\mathrm{O}}_2+$NO. A limited appearance of ${\mathrm{N}}_2{\mathrm{O}}_5$ for medium values of SEI was determined. Saturation and a slight decrease in ${\mathrm{NO}}_2$ for a high SEI were observed. Tomeková et al. [57] investigated the influence of the seed’s treatment in SDBD on the FTIR spectra. They observed that, at a very high SEI coupled into an ${\mathrm{O}}_2{\mathrm{/N}}_2$ gas mixture, more than 60% oxygen was needed to initiate the ozone production. The increase in ozone promoted the generation of ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$ for 60 and 80% oxygen. Kimura et al. [58] used FTIR spectroscopy to examine the gaseous products of humid-air plasma. The mechanism of the reaction of ${\mathrm{N}}_2{\mathrm{O}}_5$ from the gaseous phase to the ${\mathrm{NO}}_3^{-}$ ions in the liquid phase was investigated and identified as a crucial for nitration reactions in the water solutions. Our study presents the results of FTIR measurements conducted for characterization of HDBD reactor operation in humid air for both low and high SEI values. The technical details are described in Section 2.4. For quantitative analysis, the calibration gases are used as described in Section 2.3. The measured spectra and species concentrations are presented and discussed in Section 3.4 and Section 3.5 for CDA and humidified air, respectively.

Some species present in HDBD remote plasma either have good solubility in water or react with it. They can be investigated using plasma-activated water (PAW) or plasma-activated liquid (PAL) [59]. A characteristic feature of PAW distinguishing it from ozonated water [60,61,62,63,64,65] is its nitrogen chemistry, which, in many cases, shows superior microbiological effects [66]. The production of PAW is demonstrated in a large variety of CAPP discharges. Some examples are corona discharge [20,67], gliding arc discharge [40,68,69,70], different types of needle discharges directed on the surface of the water [71,72], which result, depending on polarization conditions, in high-voltage sparks or atmospheric pressure glow [17,73], atmospheric pressure plasma jets (APPJ) [74,75], e.g., RF APPJ [76] and micro-jets [77], DC-pulsed [78] and AC [79] discharge, 2.45 GHz microwave discharge [80], inductively limited discharge [81], piezoelectric direct discharge (PDD) [82,83,84,85,86,87,88,89], and, last but not least, DBD in different configurations [90].

Different methods are used to promote the transfer of chemical species from DBD plasma to PAW. Several PAW studies used a direct plasma discharge geometry, with the water surface working as a VDBD electrode [91,92,93]. Others have described the operation of SDBD in proximity to the water surface [94]. In numerous DBD architectures, the remote plasma is used for PAW production [66,95,96,97,98].

Indirect treatment produces the long-lived species ${\mathrm{H}}_2{\mathrm{O}}_2$, ${\mathrm{HNO}}_2$, and ${\mathrm{HNO}}_3$, and direct treatment provides short-lived, highly reactive radicals, atoms, and molecular species such as O, NO, and OH. Ziuzina et al. [99] investigated the difference in direct and remote DBD treatments of microorganisms in PAW. Both methods were successful in decreasing the PAW pH value and enhancing decontamination. The remotely positioned substrates required a three-times-longer treatment time than the ones positioned directly in the DBD. Kim et al. [100] used the bubbler [101] for dispersing plasma gases into water. This last principle is applied in our study for PAW production, as described in Section 2.6. The plasma gas is circulated in a closed system to improve the energy efficiency and achieve a higher RONS concentration in the PAW.

Several physical methods are used for PAW characterization. The simplest one uses the influence of the plasma treatment on the PAW surface tension. It is smaller for PAW than for distilled water, manifesting a smaller contact angle [68]. The other method explores the electrical conductivity of PAW, which is significantly higher than that of distilled water. Judee et al. [102] measured an increase from 610 to $730\mathsf{\mu }$S/cm after DBD treatment in air. Furthermore, the concentrations of chemical species such as ${\mathrm{H}}_2{\mathrm{O}}_2$, NOx, and ${\mathrm{O}}_3$ in PAW were measured. An established method based on the influence of the CAPP on the acidity of the aqueous solutes is pH value measurement [103]. It is used in our study and described in Section 2.6. The results of the PAW generation are shown in Section 3.6.

The main goal of this study is to show how the balance between nitrogen and oxygen chemistry can be controlled in a novel HDBD reactor. The obtained results are interesting and important for CAPP applications requiring a high level of such control [104]. These exist in fields such as disinfection and sterilization [105,106], medicine and dentistry [107], hygiene and sterilization of medical products [108], textiles [109], nonwoven fabrics [110], polymer foils [111], bacterial inactivation [112,113,114,115], inactivation of fungal spores or conidia [116,117], food processing [118], better germination of seeds and plant growth [119,120,121], and many others.

2. Materials and Methods

2.1. Setup

The experimental setup for the characterization of the plasma gas is shown schematically in Figure 1. Its central part is the HDBD reactor (MediPlas reactor, TDK Electronics, Munich, Germany). It is powered by a high-voltage (HV) DBD driver (MediPlas driver, TDK Electronics, Munich, Germany). Both were described in detail in our previous study [18]. The Peltier module, an important part of the reactor, is supplied with current-stabilized DC power with adjusted current and voltage levels, (BaseTech BT-305, Conrad Electronic SE, Hirschau, Germany). The temperature sensors are mounted in the gas inlet and outlet of the reactor for thermal characterization. The gases produced in the HDBD reactor are guided first through the UV absorption ${\mathrm{O}}_3$ sensor, and then through the FTIR measurement system. After measurement, the plasma gas is extracted and guided out of the system to neutralize the ozone and other oxidizing species.

2.2. Air Humidification

If not specified, CDA with a relative humidity of 7% is used as an input gas for the HDBD reactor. The CDA can be additionally humidified before flowing through the HDBD reactor. A bubbler consisting of a 500 mL borosilicate glass bottle with lid (GL45, Shott Duran, Wertheim, Germany) filled with distilled water, an input pipe ending with a diffusor immersed in water, and an output pipe for guiding the humidified water further is used for this purpose. The bubbler works at the ambient temperature of +25 °C. The humidifying is used for FTIR measurements conducted with a CDA flow of 0.5 SLM and 2.0 SLM (standard liter per minute). The relative humidities of 92% and 96% are measured for these two flows, respectively. The humidity sensor is used to determine the relative humidity of the HDBD input gas. Even though the temperature of the output gas line and the gas cell is below the dew point, the same cannot be said for the reaction products such as ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$. To minimize the sampling loss, the output gas line and the gas cell are preheated by reactor operation before the start of each FTIR measurement.

2.3. Special Gases

Synthetic air is used to flush the FTIR spectrometer (see Section 2.4) It is also used to rinse the gas cuvette and as an ${\mathrm{H}}_2$O-free reference. The synthetic air used in the study is a hydrocarbon-free gas mixture from Linde GmbH, Pullach, Germany with certified conformity according to ISO/IEC 17050-1:2004 [122]. It consists nominally of 20% oxygen and 80% nitrogen. The residual amount of water vapor is less than 2 ppm. The residual CO and ${\mathrm{CO}}_2$ are less than 1 ppm each. The hydrocarbons are less than 0.5 ppm. The contents of the nitrogen oxides NO, ${\mathrm{NO}}_2$, and ${\mathrm{N}}_2$O are below 0.02 ppm each. The calibration gases nitrogen monoxide 500 ppm NO in ${\mathrm{N}}_2$ and nitrogen dioxide 500 ppm ${\mathrm{NO}}_2$ in air are from All-in-Gas GmbH, Starnberg, Germany. All ppm specifications are based on ideal gas volumes (mole/mole).

2.4. FTIR Measurement

A Fourier-transform infrared spectrometer (FTIR) MB3000 from ABB Measurement & Analytics, Quebec City, Quebec, Kanada, is used for chemical analysis of the plasma gases. The IR light beam used for the FTIR analysis is produced by a ceramic Globar and guided through a tubular gas cell filled with the gaseous products from the HDBD reactor. The solid-state laser beam is used for measurement purposes. All optics used are non-hygroscopic (ZnSe). The length of the gas cell is 160 mm and its diameter is 30 mm. The plasma gas is introduced at the middle of its length. Both ends of the gas cell are opened to allow the transmission of the MB3000 light beam through the entire length of the gas cell with no light-absorbing obstacles other than the gas itself. The gas cell and the spectrometer are placed in a transparent cabinet with a gas extraction system mounted on its roof. The collected spectra show the absorbance (extinction) of the beam. The spectral resolution of the spectrometer is $4{\mathrm{cm}}^{-1}$. Each spectrum presented is the result of an accumulation of 16 single spectra. The typical measurement time is 45 s. The FTIR measurement starts 2 min after the plasma in the HDBD reactor is switched on, so thermally and aerodynamically stable conditions can be reached before the measurement.

2.5. UV Absorption Measurement

In addition to the FTIR measurement, ozone concentration is determined using the Ozone Analyser BMT 965 ST of BMT Messtechnik GmbH, Berlin, Germany. It utilizes a dual-beam 253.7 nm UV photometer, a low-pressure mercury lamp for generating UV light, and a built-in sample gas filter. It allows the determination of ozone concentrations ranging from $1{\mathrm{g/Nm}}^3$ to $600{\mathrm{g/Nm}}^3$. The subtraction of the reference spectrum measured in gas with filtered ozone allows for the determination of the ozone concentration of air with relative humidity up to 50% ± 5%.

2.6. PAW Production and Characterization

The chemical properties of the gas from the HDBD reactor are evaluated using the PAW produced by washing the RONS out of the plasma gas. The setup used for this purpose, shown in Figure 2, differs from the one for FTIR measurements. Leaving the HDBD reactor, plasma gas is guided into the bubbler with $12.5{\mathrm{cm}}^3$ of water through a bubble stone. The role of the bubble stone is to distribute the gas in water in the shape of a large number of small bubbles, which extends the area and prolongs the time of interaction between the plasma gas and water [123]. To realize closed circulation, the plasma gas released from the water is collected over the water’s surface, and the membrane pump pumps it back to the HDBD reactor. An additional gas port with a vacuum valve is used to compensate for the gas losses due to the solution in the water. A dosing valve controls the gas flow of 0.3 SLM. A stopper is used to measure the time it takes for the gas to circulate. The pH value of water is measured using a pH meter Apera PC-400S, Apera Instruments GmbH, Wupperal, Germany. The concentration of ${\mathrm{H}}_2{\mathrm{O}}_2$ is determined using test strips Reflectoquant, Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany. Since the properties of the PAW, including the pH value, strongly depend on the kind of water [124], distilled water must be used for the compatibility of the experiments.

3. Results and Discussion

3.1. Thermal Considerations

3.1.1. Temperature Difference

The direct measurement of the gas temperature by sensors placed either in the discharge zone or at the dielectric barrier is difficult and affects the obtained temperature value or the discharge itself. Considering these constraints, the indirect method of thermal characterization is chosen, based on measurement of the gas temperature at the inlet $T_{\mathrm{in}}$ and outlet $T_{\mathrm{out}}$ of the reactor.

A strong decrease in the output–input temperature difference $\Delta T=T_{\mathrm{out}}-T_{\mathrm{in}}$ as a function of the Peltier module voltage is demonstrated in Figure 3. Two main factors influence these results: the power coupled into the discharge and the heat flow extracted from the discharge by the Peltier cooler. The temperature differences obtained for the zero Peltier module voltage are a result of discharge power coupling only. The temperature differences for five gas flows at a driver DC voltage of 12 V are presented in Figure 3a. The gas flow has a strong influence on the measured curves. The temperature difference varies strongly for high gas flows (5 and 10 SLM), but the relative difference between these curves is small. Small variations but large relative differences between the curves are observed for low gas flows (0.5 and 1 SLM).

Figure 3b shows the dependence of the temperature difference as a function of the Peltier module voltage for three driver DC voltages at a gas flow of 10 SLM. The output–input temperature difference strongly increases with driver voltage. It decreases with Peltier module voltage, as shown already in Figure 3a, due to the cooling effect of the Peltier module. However, the temperature difference of ca. 13.6 K reached for the decrease between the smallest (0 V) and the highest (9 V) Peltier module voltage, is almost the same for all three driver DC voltages.

3.1.2. Heat Transfer

The large temperature differences for high gas flows can be explained by the increase in the heat transferred out of the discharge zone with the gas flow. The heat transfer rate $Q_{\mathrm{air}}$ can be expressed as a function of temperature difference $\Delta T$ by the following formula:

$$Q_{\mathrm{air}}(\Delta T)=f_{\mathrm{air}}{c}_{\mathrm{air}}{\displaystyle \frac{p_0{M}_{\mathrm{air}}}{R_0{T}_0}}\Delta T$$

(1)

where $T_0=275.15\mathrm{K}$ is the absolute temperature of 0 °C, $f_{\mathrm{air}}$ is the air flow in ${\mathrm{m}}^3·{\mathrm{s}}^{-1}$, $p_0$ is the pressure of the standard atmosphere (101,325 Pa), $c_{\mathrm{air}}$ is the heat capacity of dry air ($1005\mathrm{J}·{\mathrm{kg}}^{-1}·{\mathrm{K}}^{-1}$), $M_{\mathrm{air}}$ is the molar mass of dry air ($0.0290\mathrm{kg}·{\mathrm{mol}}^{-1}$), $V_{\mathrm{m}}$ is the molar volume under standard conditions ($T_0$ and $p_0$), and $R_0$ is the universal (molar) gas constant ($8.314\mathrm{J}·{\mathrm{K}}^{-1}·{\mathrm{mol}}^{-1}$).

Using this formula and the results in Figure 3b, the heat transfer rate calculated for 10.5 V, 12.0 V, and 15.0 V at a gas flow of 10 SLM with no Peltier cooling is 4.00 W, 5.30 W, and 7.79 W, respectively. Subtracting from these values the ones obtained for the Peltier module working with 9 V (0.86 W, 2.33 W, and 4.59 W, respectively), the same value of about 3.1 W is obtained. This power can be interpreted as the effective heat transfer ratio of the Peltier module for heat transferred from the discharge to the heat sink of the Peltier module. The increase in this heat transfer rate correlates with the difference between the input gas temperature and the temperature of the heat sink, measured directly at the hot side of the Peltier module. This temperature difference is shown as a function of Peltier module voltage in Figure 3a, depicted as a black line with open circles for the driver voltage of 12 V and air flow of 5 SLM. It depends only weakly on the gas flow, and the curves for 0.5 and 1 SLM are not shown in the plot because they overlap with the one for 5 SLM. It is plausible because the heat transferred by the gas flows of 0.5, 1.0, and 5.0 SLM is small (below 500 mW) compared to the heat transfer ratio of the Peltier module.

3.1.3. Influence on Humidity

The relative humidity has a strong influence on the chemistry in the discharge itself but also in the remote plasma zone. On the other hand, the relative humidity at the output of the HDBD reactor depends not only on the known relative humidity at the HDBD reactor input, but also on the actual output gas temperature. If the input and output temperatures, $T_{\mathrm{in}}$ and $T_{\mathrm{out}}$, respectively, and the input relative humidity $h_{\mathrm{in}}$ are known, the output relative humidity $h_{\mathrm{out}}$ can be calculated. Assuming that the water content ratio (kg/kg) does not change when passing through the reactor, the relative humidity is as follows:

$$h_{\mathrm{out}}=h_{\mathrm{in}}{\displaystyle \frac{p_{\mathrm{S}}\left(T_{\mathrm{in}}\right)}{p_{\mathrm{S}}\left(T_{\mathrm{out}}\right)}}{\displaystyle \frac{T_{\mathrm{in}}}{T_{\mathrm{out}}}}$$

(2)

where $p_{\mathrm{S}}\left(T_{\mathrm{in}}\right)$ and $p_{\mathrm{S}}\left(T_{\mathrm{out}}\right)$ are the saturated water vapor pressures for these two temperatures. They can be calculated using the Magnus formula, determining the maximum (saturation) vapor pressure for a given absolute temperature T as follows:

$$p_{\mathrm{S}}\left(T\right)=a_{\mathrm{M}}exp{\displaystyle \frac{b_{\mathrm{M}}(T-T_0)}{c_{\mathrm{M}}+(T-T_0)}}$$

(3)

where $a_{\mathrm{M}}=611.2\mathrm{Pa}$, $b_{\mathrm{M}}=17.62$, and $c_{\mathrm{M}}=243.12\mathrm{K}$ are the empirical parameters valid in the temperature range from $-20$ °C to 60 °C for water vapor.

Using this formula for a sublimation temperature of dinitrogen pentoxide of $33$ °C, the output relative humidity reaches 3.78% and 48.6% for input relative humidities of 7% and 90%, respectively. Equation (2) is used for the calculation of relative humidities for the selected spectra presented in Section 3.4.

3.2. Expected Molecular Species

The driving force of the RONS production in the CAPP comes from the atomic and molecular radicals. The atomic oxygen is generated in the DBD from oxygen molecules by electron impact dissociation [125]. The same physical mechanism is responsible for the production of nitrogen atoms [126]. The electron impact dissociation of gas-phase water molecules is the source of OH radicals [127], having the highest oxidation potential (2.8) of all radicals available in the plasma gas [128]. The byproduct of this reaction is the hydrogen radical. However, O, N, H, and OH radicals belong to the group of short-living species [129]. The length of the decay zone depends on the gas flow and the energy coupled in the discharge. In our case, they decay within centimeters of travel from the microdischarge zone where they are created. Consequently, they are not expected in the remote location where the FTIR measurement is conducted. Only the long-living species are relevant for the FTIR measurements in the plasma gas. Earlier investigations of DBD in humid air showed the presence of the molecules ${\mathrm{O}}_3$, ${\mathrm{N}}_2{\mathrm{O}}_5$, ${\mathrm{HNO}}_3$, and ${\mathrm{N}}_2$O in the downstream exhaust [53]. In addition to these species, the kinetic models for DBD in humid air predicted the presence of ${\mathrm{H}}_2{\mathrm{O}}_2$, NO, and ${\mathrm{HNO}}_2$ [129].

The ozone production and destruction mechanisms in air plasma due to oxygen, nitrogen, and humidity are summarized in Sections 3.5.2, 3.6.1, and 3.7.1 of our previous work [18], respectively. A brief overview of the essential generation mechanisms of the other RONS in plasma gas is given in the following. The presence of atomic nitrogen and oxygen allows for the generation of nitric oxide (NO) according to the following reactions [130]:

$$\mathrm{O}+{\mathrm{N}}_2⟶\mathrm{NO}+\mathrm{N}$$

(4)

and

$$\mathrm{N}+{\mathrm{O}}_2⟶\mathrm{NO}+\mathrm{O}$$

(5)

In the presence of humidity, an additional reaction path is possible:

$$\mathrm{N}+\mathrm{OH}⟶\mathrm{NO}+\mathrm{H}$$

(6)

Atomic oxygen allows for reactions converting the nitrogen oxides to higher oxidation states. It oxidizes the nitric oxide [131] according to the reaction

$$\mathrm{O}+\mathrm{NO}+\mathrm{M}⟶{\mathrm{NO}}_2+\mathrm{M}$$

(7)

and the resulting nitrogen dioxide according to the highly unstable nitrogen trioxide (nitrate radical) as follows:

$$\mathrm{O}+{\mathrm{NO}}_2+\mathrm{M}⟶{\mathrm{NO}}_3+\mathrm{M}$$

(8)

where M represents any other species.

The inverse reactions are caused by atomic nitrogen. They produce oxides with a lower oxidation state. An example is the generation of the oxide with the lowest oxidation state, nitrous oxide (${\mathrm{N}}_2$O), from nitrogen dioxide (${\mathrm{NO}}_2$) according to the following reaction [132]:

$${\mathrm{NO}}_2+\mathrm{N}⟶{\mathrm{N}}_2\mathrm{O}+\mathrm{O}$$

(9)

The equal amounts of NO and ${\mathrm{NO}}_2$ react together at a temperature of $-21$ °C to create dinitrogen trioxide (${\mathrm{N}}_2{\mathrm{O}}_3$). No such product is expected at a much higher HDBD reactor temperature. This reaction path is also not expected because the nitric oxide concentration is very low due to the following reaction [132]:

$${\mathrm{O}}_3+\mathrm{NO}⟶{\mathrm{NO}}_2+{\mathrm{O}}_2$$

(10)

Reaction (10) and the reaction [133,134]

$${\mathrm{O}}_3+{\mathrm{NO}}_2⟶{\mathrm{NO}}_3+{\mathrm{O}}_2$$

(11)

are important loss mechanisms of ozone. Their contribution to the global balance of the species increases with temperature.

Another two ozone loss mechanisms that increase with humidity are the reaction with hydrogen [132]

$$\mathrm{H}+{\mathrm{O}}_3⟶\mathrm{OH}+{\mathrm{O}}_2$$

(12)

and the reaction with OH groups

$$\mathrm{OH}+{\mathrm{O}}_3⟶{\mathrm{HO}}_2+{\mathrm{O}}_2$$

(13)

At room temperature, the nitrogen dioxide remains in balance with dinitrogen tetroxide (${\mathrm{N}}_2{\mathrm{O}}_4$):

$$2{\mathrm{NO}}_2⟷{\mathrm{N}}_2{\mathrm{O}}_4$$

(14)

At $0$ °C, the dimer ${\mathrm{N}}_2{\mathrm{O}}_4$ mainly occurs. In contrast, at +$140$ °C, only the monomer ${\mathrm{NO}}_2$ is observed.

A crucial role for the generation of ${\mathrm{HNO}}_3$ in the gas phase and for the acidity in PAW is played by dinitrogen pentoxide ${\mathrm{N}}_2{\mathrm{O}}_5$. Under room-temperature conditions (+$23$ °C), ${\mathrm{N}}_2{\mathrm{O}}_5$ is a solid. It sublimates, yielding a colorless gas. Its sublimation and boiling points are $+32.4$ °C and $+41$ °C, respectively [135]. Consequently, its gaseous phase can be expected only under experimental conditions with a temperature higher than the sublimation point (see the threshold line shown in Figure 3b). For low gas flows, the amount of heat transferred from the discharge into the gas cell is not sufficient to heat it up. The parts of the setup in contact with the plasma gas remain cold. Even though the water relative humidity is far from the dew point, ${\mathrm{N}}_2{\mathrm{O}}_5$ can condensate at such cold surfaces with a temperature below $33$ °C and cannot be detected by FTIR spectroscopy.

The dinitrogen pentoxide (${\mathrm{N}}_2{\mathrm{O}}_5$) is produced from the ${\mathrm{NO}}_2$ dimer and ozone accordingly to the reaction

$${\mathrm{N}}_2{\mathrm{O}}_4+{\mathrm{O}}_3⟶{\mathrm{N}}_2{\mathrm{O}}_5+{\mathrm{O}}_2$$

(15)

or from nitrogen trioxide and nitrogen dioxide by the reaction [132]

$${\mathrm{NO}}_3+{\mathrm{NO}}_2+\mathrm{M}⟶{\mathrm{N}}_2{\mathrm{O}}_5+\mathrm{M}.$$

(16)

It is unstable at higher temperatures and decomposes spontaneously into monomer ${\mathrm{NO}}_2$ and oxygen ${\mathrm{O}}_2$:

$$2{\mathrm{N}}_2{\mathrm{O}}_5⟶4{\mathrm{NO}}_2+{\mathrm{O}}_2$$

(17)

Consequently, in a hot reactor, ${\mathrm{NO}}_2$ is expected because it cannot be more oxidized in dry air.

${\mathrm{N}}_2{\mathrm{O}}_5$ is easily dissolved into water to form ${\mathrm{HNO}}_3$ [135], so it can only be stable under dry conditions. In humid-air discharge, an important role is played by the fast reaction of the dinitrogen pentoxide with water:

$${\mathrm{N}}_2{\mathrm{O}}_5+{\mathrm{H}}_2\mathrm{O}+\mathrm{M}⟶2{\mathrm{HNO}}_3+\mathrm{M}$$

(18)

The presence of OH radicals allows for the production of nitric and nitrous acids according to the reactions

$$\mathrm{OH}+{\mathrm{NO}}_2+\mathrm{M}⟶{\mathrm{HNO}}_3+\mathrm{M}$$

(19)

and

$$\mathrm{OH}+\mathrm{NO}+\mathrm{M}⟶{\mathrm{HNO}}_2+\mathrm{M}$$

(20)

respectively, and their production increases with the amount of water and decreasing temperature.

The production of hydrogen peroxide can be expected under low-SEI conditions according to the following recombination reaction [136]:

$$\mathrm{OH}+\mathrm{OH}+\mathrm{M}⟶{\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{M}.$$

(21)

Due to the high reactivity in reactions (19), (20), and (21), OH radicals are short-lived and present only at a small distance from the microdischarge zones. This property explains the absence of OH in the remote plasma.

3.3. FTIR Spectra for Test Gases

The reference spectra are collected. First, the reference spectra for CDA with gas flows of 0.5 and 2.0 SLM without plasma ignition are measured. In this case, the gas flow is controlled by the pressure reducer valve of the test gas bottle.

The reference spectra of test gases, nitrogen with 500 ppm of NO and synthetic air with 500 ppm of ${\mathrm{NO}}_2$, are measured for quantitative evaluation of the plasma gas spectra. The spectra for NO for test gas flows 0.5 and 2 SLM are shown in Figure 4a. The double band with sub-maxima 1900 and $1845{\mathrm{cm}}^{-1}$ confirms the presence of NO. The intensity of these peaks increases with the gas flow because the effective optical path in the gas cell increases.

The spectra for ${\mathrm{NO}}_2$ for gas flows of 0.5 and 2 SLM are shown in Figure 4b. The double bands at 1627 and $1600{\mathrm{cm}}^{-1}$ and 2920 and $2880{\mathrm{cm}}^{-1}$ confirm the presence of ${\mathrm{NO}}_2$. Mixing the test gases in the gas cell with the residual ambient air results in the presence of ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2$O spectral features. Analog to NO, the spectrum peak responsible for ${\mathrm{NO}}_2$ is much higher for the 2 SLM flow than for 0.5 SLM due to the longer effective measurement path in the gas cell for higher gas flow.

Since the concentrations of NO and ${\mathrm{NO}}_2$ in the test gas mixtures are known (see Section 2.3), both spectra are used to calibrate the plasma gas FTIR results. This is important for conditions resulting in a very low ozone concentration, avoiding its use for calibration.

3.4. FTIR Characterization for CDA

CDA with a relative humidity (RH) of 7% without additional humidification is used as an ionization gas of the HDBD reactor. The CDA flow is 0.5 SLM or 2.0 SLM, controlled by a mass flow controller (MFC). The DBD driver voltage is 12.5 V or 15 V. The duty cycle of the 70 Hz PWM signal is 10%, 30%, 50%, and 100%. The DC power supply of the Peltier module is 0, 4, and 6 V.

3.4.1. Specific Energy Input

SEI values expressed in ${\mathrm{J/cm}}^3$ can be calculated for the duty cycle of 100% with the following formula:

$$\mathrm{SEI}={\displaystyle \frac{P}{f_{\mathrm{air}}}}$$

(22)

Here, P is the electric power coupled to the discharge and $f_{\mathrm{air}}$ is the air flow. A linear dependence on the percentage can be assumed for lower duty cycles. The SEI values for the power with the 100% duty cycle and the gas flows used in this study are displayed in

Table 1. The SEI values calculated for the applied powers and air flows.

SEI Level		Low	Moderate	High	Very High
DBD driver voltage	[V]	12.5	15.0	12.5	15.0
Discharge power	[W]	11.0	15.7	11.0	15.7
Air flow	[SLM]	2.0	2.0	0.5	0.5
SEI	[${\mathrm{J/cm}}^3$]	0.33	0.47	1.32	1.88

.

3.4.2. FTIR Spectra for CDA

Figure 5 shows the spectrum collected for plasma gas produced with the HDBD reactor operated with a CDA flow of 2.0 SLM, the Peltier module switched off, the full duty cycle of the PWM, and a driver input voltage of 15 V. The same spectra are taken three times for the same HDBD reactor conditions and averaged. The variation coefficients for the molecular spectral lines are given in

Table 2. Spectral main bands for quantitative evaluation and their coefficient of variation.

	Wave	Lower	Upper	Coefficient
Molecule	Number	Limit	Limit	of Variation	Overlapping	Source
	[${\mathrm{cm}}^{-\mathbf{1}}$]	[${\mathrm{cm}}^{-\mathbf{1}}$]	[${\mathrm{cm}}^{-\mathbf{1}}$]	[%]
${\mathrm{O}}_3$	1054.98	964.34	1074.27	3.0	no	[137]
${\mathrm{NO}}_2$	1627.80	1558.37	1654.80	129.4	with ${\mathrm{H}}_2$O	[138]
${\mathrm{N}}_2$O	2237.26	2160.12	2260.41	14.7	no	[138]
${\mathrm{N}}_2{\mathrm{O}}_5$	742.54	702.04	779.18	9.2	no	[53]
${\mathrm{HNO}}_3$	879.48	833.19	933.48	12.3	no	[139]

. A relative humidity at the output of the HDBD reactor of 2.7% is calculated using Formula (2). These conditions correspond to a moderate SEI according to Table 1. Consequently, the ozone concentration in the plasma output gas reaches a moderate 1520 ppm. The spectrum is an overlap of the spectra of the single molecules. Figure 5a shows the relevant single-molecule spectra from the FTIR database for ${\mathrm{O}}_3$, ${\mathrm{NO}}_2$, ${\mathrm{N}}_2$O, ${\mathrm{N}}_2{\mathrm{O}}_5$, and ${\mathrm{HNO}}_3$. Their wave number characteristics are summarized in Table 2. The residual carbon dioxide present in the ambient air can also be recognized. It confirms the previous findings [48].

For the low CDA flow of 0.5 SLM and driver voltage of 15.0 V, corresponding to very high SEI, no ozone feature is present in the spectrum shown in Figure 6. This absence in the spectrum correlates with the vanishing ozone concentration under these working conditions, as shown in Figure 7b. The ${\mathrm{NO}}_2$ spectrum features replace the overlap of the ${\mathrm{N}}_2$O and ${\mathrm{N}}_2{\mathrm{O}}_5$ spectra. The same energy imposed on the much smaller amount of gas results in processing conditions that are much more favorable for ${\mathrm{NO}}_2$ than ozone, ${\mathrm{N}}_2$O, or ${\mathrm{N}}_2{\mathrm{O}}_5$ generation. According to the reactions (7), (9), and (16), respectively, the production of these three molecules requires the short-lived species. In this case, the delay time due to the gas flow is sufficient to ensure gas mixing and to exclude the influence of the gas speed on the measured concentration of ${\mathrm{NO}}_2$.

The complete disappearance of the ${\mathrm{N}}_2{\mathrm{O}}_5$ fingerprint from the spectrum collected for the 0.5 SLM flow (see Figure 6) could be caused by the low heat transfer from the discharge to the gas cell (see Section 3.1.2) at such a low gas flow, and the resulting condensation of ${\mathrm{N}}_2{\mathrm{O}}_5$ on walls that are not sufficiently warmed up. On the other hand, the presence of ${\mathrm{N}}_2{\mathrm{O}}_5$ can be expected in the discharge itself. It is very probably the source of the ${\mathrm{HNO}}_3$ produced following the reaction (18). However, due to the presence of ${\mathrm{NO}}_2$, reaction (19) also cannot be excluded.

3.4.3. Comparison of FTIR and UV Absorption for Ozone

Each FTIR spectrum measurement is accompanied by a UV absorption measurement of ozone concentration under the same HDBD reactor working conditions. The FTIR line intensities related to ozone are compared with the ozone concentration measured by the UV absorption technique to allow the quantitative evaluation of the FTIR results. Figure 7a,b show the ozone values as a function of the duty cycle for the DBD driver input voltages of 12.5 V and 15 V, respectively. The CDA flow is 0.5 and 2.0 SLM. The HDBD reactor works without Peltier cooling. The pulse frequency of the DBD driver power is 70 Hz.

The trends of the UV and FTIR curves collected for a CDA flow of 2.0 SLM are the same. At both voltages, the ozone concentration increases rapidly for low duty cycle values (10% and 30%). It increases more slowly for high duty cycle values (50% and 100%). It slowly increases for 12.5 V but slightly drops for 15.0 V. This difference can be interpreted as a result of crossing the ozone concentration maximum with an increasing SEI for the 15.0 V curves.

According to Figure 7a,b, the UV-measured concentration is much higher for lower gas flow. This is plausible because the produced ozone is diluted in a larger gas amount. At a low gas flow, the ozone concentration drops strongly, with duty cycle values increasing over 30% for both voltages. Under such high-SEI conditions, the chemical reactions shift toward production of nitrogen oxides. The ozone generation decreases rapidly with increasing SEI due to reactions (10) and (11). The ozone concentration is very low at a duty cycle of 100%, driver voltage of 15 V, and CDA flow of 0.5 SLM. These conditions correspond to the very high SEI, strongly exceeding the optimum value for ozone production. The FTIR results confirm the predominant production of nitrogen oxides.

A systematic difference between the UV-measured and FTIR curves is that the ozone concentration decreases with increasing flow, whereas the FTIR intensity shows the opposite tendency. The reason is presumably the longer optical length for IR absorption measurement with increased gas flow. The gas cell volume is much larger than that of the gas introduced within one minute at 0.5 SLM, and the gap between the gas cell tube and the IR ports is not entirely purged. This situation changes at a gas flow of 2.0 SLM. The effective optical absorption path is then longer. The conversion factor, expressed in ppm/intensity unit, defines the dependence between the UV-measured concentration and the FTIR intensity. Due to the optical length variation, the conversion factor depends on gas flow. It is about 210 ± 10 and 45 ± 2 ppm/intensity unit for 0.5 SLM and 2 SLM, respectively. The UV absorption measurement is not affected by the gas flow because the volume of the UV measurement zone is small compared to the amount of gas passing it per minute.

3.4.4. Influence of Power on ${\mathrm{NO}}_{\mathrm{x}}$ Production

The net power coupled into the HDBD is controlled either by the duty cycle of the PWM or by the DBD driver voltage. Figure 8 shows the influence of the duty cycle on the intensity of the ${\mathrm{N}}_2{\mathrm{O}}_5$, ${\mathrm{N}}_2$O, and ${\mathrm{HNO}}_3$ lines. All three intensities follow a similar trend and increase with the duty cycle. The nitric acid concentration increases by a factor of 30. The increase for nitrous oxide and dinitrogen pentoxide is much smaller. They increase by factors of 10 and 8, respectively. The much faster increase in the nitric acid line intensity depicts a strong rise in the acidic component in the plasma gas with the increasing duty cycle (net power) compared with the oxidic one.

A comparison of Figure 8a,b demonstrates the influence of DBD driver voltage on FTIR intensities. For all three lines, the increase in the DBD drive voltage shifts the intensity lines to higher values. However, this shift differs for varied species. The nitric acid intensity increases by a factor of 2. The intensities of nitrous oxide and dinitrogen pentoxide rise only by about 50%. The stronger increase in the nitric acid intensity confirms an enhancement of the acidic component in the plasma gas with SEI.

3.5. FTIR Characterization for Humidified Air

3.5.1. FTIR Spectra for Humidified Air

Figure 9a compares the spectra of humidified air and CDA under the same conditions: air flow 2.0 SLM; DBD driver voltage 15 V; PWM aspect ratio 100 %; and Peltier module voltage 0 V. The most apparent difference is the lack of ozone features at 1055 and $3055{\mathrm{cm}}^{-1}$ in the humidified spectrum. Two reactions potentially responsible for the ozone losses for high humidity are described by Equations (12) and (13).

A strong spectral feature of humidified air but absent in CDA remote plasma is the double peak of ${\mathrm{NO}}_2$ for wave numbers around 1601 and $1628{\mathrm{cm}}^{-1}$. Also recognizable is the small ${\mathrm{NO}}_2$ peak at $2905{\mathrm{cm}}^{-1}$. Still present is the double peak at $2228{\mathrm{cm}}^{-1}$ assigned to ${\mathrm{N}}_2$O.

A further important observation is that the ${\mathrm{N}}_2{\mathrm{O}}_5$ peak at $743{\mathrm{cm}}^{-1}$, the ${\mathrm{HNO}}_3$ peaks at 891, 1324, and $3554{\mathrm{cm}}^{-1}$, and the common major peak of ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$ at $1750{\mathrm{cm}}^{-1}$ are not present in the humidified air spectrum. Other researchers [140,141] observed this effect previously. ${\mathrm{N}}_2{\mathrm{O}}_5$ is not detected in the FTIR spectra regardless of the DBD power and humidified air flow rate. Cimerman et al. [26] explained this absence as being due to the complete consumption of ${\mathrm{N}}_2{\mathrm{O}}_5$ by the reaction with ${\mathrm{H}}_2$O, which leads to the formation of ${\mathrm{HNO}}_3$ according to Equation (18). However, in our results for humidified air, ${\mathrm{HNO}}_3$ is also missing. The calculated relative humidities at the HDBD reactor output for CDA and humidified air are shown.

Figure 9b demonstrates the influence of Peltier cooling on the humidified air plasma. The spectra for the Peltier cooling DC voltages of 0 and 6 V are compared. The ${\mathrm{NO}}_2$ maximum becomes lower and, at the same time, the ozone peak grows with cooling. The known reason [3,142] is the chemistry shift towards ozone production due to the reduced discharge temperature. The consequence is a lower concentration of nitrogen oxides. The ${\mathrm{N}}_2$O feature remains very visible.

From the fact that ${\mathrm{N}}_2{\mathrm{O}}_5$ or ${\mathrm{HNO}}_3$ are absent in the gas phase, it cannot be concluded that these species are not produced at all. The assumed reason for their absence is the “washing out” of the nitric species with good solubility from the gas phase into the liquid-phase water. The condensation processes in the gas phase can deliver nano-droplets and/or large water molecule clusters containing RONS. Due to the high absorbance of IR radiation in water, no signal of these molecules contributes to the spectra measured in the gas phase. A growing amount of liquid-phase water in the gas cell can be expected when increasing the Peltier cooling voltage. The suppression of the strong ${\mathrm{CO}}_2$ peaks at 2350 and $667{\mathrm{cm}}^{-1}$ with increasing cooling can be explained by the dissolution of carbon dioxide in the larger amount of liquid-phase water.

3.5.2. ${\mathrm{NO}}_2$ Concentration

Using the calibration by test gas, the concentration of ${\mathrm{NO}}_2$ in the plasma gas can be determined. Figure 10 shows the influence of SEI and the Peltier cooling on ${\mathrm{NO}}_2$ concentration, given in molar ppm. Figure 10a shows results for CDA without humidifying. The only curve for the maximal SEI (15 V and 0.5 SLM) is presented because, for all conditions related to lower SEI, the ${\mathrm{NO}}_2$ is not detectable. The cooling causes a drastic reduction in the ${\mathrm{NO}}_2$ concentration because the main reactions responsible for ${\mathrm{NO}}_2$ generation, given by Equations (7) and (10), speed up with temperature. The ${\mathrm{NO}}_2$ concentration reaches a maximum of 840 ppm for operation with switched-off Peltier cooling and drops down to 70 ppm for Peltier module voltages of 4 and 6 V.

Figure 10b shows the results for humidified air. The general tendency for all SEI conditions is, analogous to the CDA results, a reduction in the ${\mathrm{NO}}_2$ concentration with increasing Peltier module voltage. It correlates with the increase in ozone concentration with temperature decrease, weakening the loss mechanism described by Equation (10). It can be concluded that the increase in the SEI, either by an increase in the DBD driver voltage or by a decrease in the gas flow, results in an increase in the ${\mathrm{NO}}_2$ concentration. For the minimum SEI (12.5 V and 2.0 SLM), a very low ${\mathrm{NO}}_2$ concentration below 20 ppm is observed. These conditions correspond to high ozone production. On the other hand, the highest value of ${\mathrm{NO}}_2$ concentration, 450 ppm, is reached for the maximum SEI (15.0 V and 0.5 SLM). These results agree with the results obtained in other studies conducted with DBD and humid air.

The maximum ${\mathrm{NO}}_2$ concentration reached in CDA is higher than the maximum value achieved under the same conditions in humidified air. The ${\mathrm{NO}}_2$ loss mechanism, valid only in the presence of humidity and described by Equation (19), is responsible. This is also in agreement with the results obtained by Janda et al. [143], showing a much lower ${\mathrm{NO}}_2$ concentration in humid air than in dry air.

3.5.3. FTIR Spectrum of ${\mathrm{H}}_2{\mathrm{O}}_2$

The reaction (21), possible in DBD in humid air under low-SEI conditions, produces hydrogen peroxide (${\mathrm{H}}_2{\mathrm{O}}_2$) in the gas phase. ${\mathrm{H}}_2{\mathrm{O}}_2$ has a characteristic absorption band at $877{\mathrm{cm}}^{-1}$ associated with the O-O stretching vibration. The band at $1255{\mathrm{cm}}^{-1}$ [144] is attributed to a combination of vibrational modes, including O-H bending and O-O stretching. The additional bands at $3590{\mathrm{cm}}^{-1}$ and $2630{\mathrm{cm}}^{-1}$ are associated with O-H stretching vibrations.

In our spectra, in the case of a high SEI (see Figure 6), no significant contribution of ${\mathrm{H}}_2{\mathrm{O}}_2$ can be recognized, and, in the case of a low SEI (see Figure 5), a strong overlap with ${\mathrm{HNO}}_3$ and ${\mathrm{N}}_2{\mathrm{O}}_5$ does not allow a clear assignment to ${\mathrm{H}}_2{\mathrm{O}}_2$. Only a small amount of ${\mathrm{H}}_2{\mathrm{O}}_2$ measured in the plasma gas-produced PAW (see Section 3.6.2) suggests a small concentration in the gas phase.

3.6. Plasma-Activated Water Production

The production of PAW confirms the presence of the RONS expected in plasma gas generation in an HDPE reactor operated with humid air. Critical for the PAW acidity is the ${\mathrm{HNO}}_3$ present in the remote plasma. It can be produced mainly following the two reactions described by Equations (18) and (19), requiring the presence of ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{NO}}_2$, respectively. The high values of the SEI promote the second of these reactions.

3.6.1. NOx Production

Figure 11a shows the decrease in the pH value with time of HDBD reactor operation, with air circulating in the closed loop (see the setup shown in Figure 2). The pH value decreases with the duration of water treatment at 0.12 per minute. The 4.5 reached in this experiment is sufficient to disable microorganisms such as S. aureus [145]. The pH value drops, which can be interpreted as an increase in the ${\mathrm{HNO}}_3$ concentration. From this concentration, the equivalent NOx concentration in the gaseous phase can be estimated.

As a first approximation, the total NOx concentration can be calculated using the following assumptions:

• 

Only ${\mathrm{N}}_1{\mathrm{O}}_x$ species are considered.

• 

All ${\mathrm{N}}_1{\mathrm{O}}_x$ species dissolve as a strong acid.

• 

The water volume is constant at 12.5 mL, and it does not change due to subsequent reactions.

• 

The amount of circulating air $V_{\mathrm{tot}}$ is constant, equal to 1.1 L.

• 

Ideal gas behavior at standard pressure is assumed.

• 

The acidity of PAW is caused by nitric acid.

Since ${\mathrm{HNO}}_3$ is a strong acid, its concentration in the PAW, expressed in mol/L, is given as a function of pH value:

$$c_{\mathrm{HNO}}={10}^{-pH}$$

(23)

The total molar amount of ${\mathrm{HNO}}_3$ accumulated in PAW is

$$N_{\mathrm{HNO}}=c_{\mathrm{HNO}}{V}_{\mathrm{PAW}}$$

(24)

Each nitric acid molecule originates from the nitrogen oxide or acid molecule from the gas phase. Consequently, the same molar amount of $N_{\mathrm{nox}}$ of the equivalent ${\mathrm{N}}_1{\mathrm{O}}_x$ in the gas phase can be estimated. Taking the molar volume of air $V_{\mathrm{A}}$, the equivalent mean NOx concentration $c_{\mathrm{nox}}$ in the constant volume of air $V_{\mathrm{air}}$ in the circulation system is as follows:

$$c_{\mathrm{nox}}={\displaystyle \frac{N_{\mathrm{nox}}}{N_{\mathrm{tot}}}}=N_{\mathrm{nox}}{\displaystyle \frac{V_{\mathrm{A}}}{V_{\mathrm{air}}}}$$

(25)

The increase in equivalent NOx concentration with treatment time determined in this way is shown in Figure 11b.

3.6.2. ${\mathrm{H}}_2{\mathrm{O}}_2$ Production

It is known that ${\mathrm{H}}_2{\mathrm{O}}_2$ is produced in a gaseous discharge containing water vapor [146]. ${\mathrm{H}}_2{\mathrm{O}}_2$ is also present in the PAW in our experiment. The variation of its concentration during the PAW treatment is shown in Figure 11b. However, the maximal concentration is low, at max. 3 ppm, since the high-SEI conditions used for PAW production are not favorable for ${\mathrm{H}}_2{\mathrm{O}}_2$ generation. Also in other studies, the ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration was found to be very low [147]. The ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration reaches a maximum value after a short measurement period, and then the concentration drops. Such behavior was observed in other reported experiments. The ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration in PAW generated in a fixed volume of water in ambient air by Lu et al. [73] increased with time at the beginning, but then a decrease in saturation was observed. The same behavior was shown by the ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration in POW generated by bubbling the plasma gases from DBD measured by Royintarat et al. [96].

The explanation for such time dependence can be found in the dynamic character of the experiment. At the beginning of circulation, the discharge zone is colder, and the relative humidity of the air is low. These are conditions promoting the generation of oxygen chemistry, especially ozone, which results in the generation of OH radicals in water and, in the next step, the ${\mathrm{H}}_2{\mathrm{O}}_2$. Up to now, consensus on the chemical mechanisms of ${\mathrm{H}}_2{\mathrm{O}}_2$ formation at the border between plasma and water has not been reached. The theoretical and experimental analysis using the direct discharge plasma indicates that the recombination of dissolved OH radicals is the dominant process for the ${\mathrm{H}}_2{\mathrm{O}}_2$ formation in liquid [148]. Continuing the circulation of the processed air, the concentration of soluble RONS increases and they accumulate in the water. The acids produced there out of RONS have an oxidative character, while ${\mathrm{H}}_2{\mathrm{O}}_2$ has a reducing effect. Both can therefore be considered as opposing factors. This can reduce the ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration with a simultaneous significant increase in acidity.

4. Conclusions

The focus of the investigation was the RONS produced in a novel, compact HDBD reactor. Application of the special electrode and careful selection of other materials allow for durable resistance against an aggressive species produced in the humid-air discharge. The fine adjustment of the DBD driver voltage, the duty cycle of the PWM, and the gas flow allows for the control of the SEI and, consequently, a selective production of chemical species. A way to influence the chemistry further is by controlling the discharge temperature using Peltier cooling of the dielectric barrier. The thermal conditions of the HDBD reactor were investigated by measurement of the temperatures at the gas input and output, and at the heat sink of the Peltier module. The experiments were conducted with air at 7%, 92%, and 97% humidity. FTIR absorption spectroscopy was used to determine the chemical composition of the gases produced by the HDBD reactor. In addition to the FTIR analysis, the plasma-activated water (PAW) was investigated. PAW was produced by bubbling the HDBD plasma gas through 12.5 mL of distilled water in closed-loop circulation at high power. The pH value decreased with the duration of water treatment at 0.12 per minute.

In the FTIR spectra collected in the CDA plasma, the spectral features of ${\mathrm{O}}_3$, ${\mathrm{N}}_2$O, ${\mathrm{NO}}_2$, ${\mathrm{N}}_2{\mathrm{O}}_5$, and ${\mathrm{HNO}}_3$ were identified. For a low relative humidity of 7%, the spectral intensities of ${\mathrm{N}}_2$O, ${\mathrm{N}}_2{\mathrm{O}}_5$, and ${\mathrm{HNO}}_3$ increased with the increasing SEI. The increase in ${\mathrm{HNO}}_3$ was much faster than for the other species, which implicates the influence of the SEI in the chemistry in DBD.

For a high relative humidity (>90%), only the molecules ${\mathrm{O}}_3$, ${\mathrm{N}}_2$O, and ${\mathrm{NO}}_2$ were identified in the FTIR spectrum. It is possible to control the ${\mathrm{NO}}_2$ concentration not only via SEI but also by Peltier cooling. The main reason for the absence of ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$ in the FTIR spectra could be their absence in the gaseous phase of the measurement cell. However, they can still be present, dissolved in the condensed water in small droplets or large clusters, absorbing the FTIR radiation. Their presence in the plasma products is indicated by the increase in the acidity of the plasma-activated water (PAW) produced by the HDBD reactor. Further research is needed to confirm the specific mechanism of ${\mathrm{N}}_2{\mathrm{O}}_5$ and ${\mathrm{HNO}}_3$ detection in the plasma products in high-humidity air. No hydrogen peroxide was detected in the FTIR spectra. This fact correlates with the low concentration, below 3 ppm, of ${\mathrm{H}}_2{\mathrm{O}}_2$ in the PAW produced in the HDBD reactor at a high SEI.

The obtained results are interesting and important for CAPP applications requiring a high level of balance between nitrogen and oxygen chemistry control.