# Surface dielectric barrier discharge - From O3 mode to NOx mode as function of dielectric temperature

Zh Aliyeva, C Muja, Ph. Guillot, T Maho

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Zh Aliyeva, C Muja, Ph. Guillot, T Maho. Surface dielectric barrier discharge - From O3 mode to NOx mode as function of dielectric temperature. 2025. hal-05376724

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# Surface dielectric barrier discharge – From O₃ mode to NOₓ mode as function of dielectric temperature

Zh. Aliyeva, C. Muja, Ph. Guillot and T. Maho

Univ Toulouse, INUC, DPHE, Albi, France

**Corresponding author:** thomas.maho@univ-jfc.fr

**Keywords:** surface dielectric barrier discharge, ozone production, nitrogen oxides, humidity effects, RONS control

## Abstract

Surface dielectric barrier discharges (SDBDs) operating in atmospheric air are efficient sources of reactive oxygen and nitrogen species (RONS), making them promising candidates for food decontamination applications. However, their chemical output strongly depends on operating conditions and environmental humidity that can affect treatment efficacy and reproducibility. In this work, we characterize the transition between ozone-dominant (O₃ mode) and nitrogen-oxides-dominant (NOₓ mode) operating regimes by combining ICCD imaging, optical emission spectroscopy, and time-resolved absorption measurements of O₃ and NO₂. We demonstrate that an increase of the applied voltage and the dielectric temperature accelerates the shift from net ozone production to rapid O₃ destruction driven by NOₓ catalytic cycles, while water vapor further suppresses ozone formation through enhanced OH- and H-driven reactions. Spatiotemporal ICCD measurements reveal that humidity simultaneously modifies streamer morphology and enhances overall plasma intensity. These findings establish quantitative operating windows to control RONS production for applications requiring either oxidative (O₃-rich) or nitrosative (NOₓ-rich) biocidal environments, providing essential knowledge for optimizing plasma-based food safety technologies.

## 1. Introduction

Atmospheric-pressure air plasmas generated by surface dielectric barrier discharges (SDBDs) are widely used for diverse applications including air purification, water treatment, surface decontamination, and agricultural product decontamination[1–4]. Their effectiveness is based on the *in-situ* generation of reactive oxygen and nitrogen species (RONS) – including ozone (O₃), atomic oxygen (O), hydroxyl radicals (OH), nitric oxide (NO), and nitrogen dioxide (NO₂) – whose synergistic action enables oxidation and inactivation of organic contaminants, pathogens, and antibiotic-resistant microorganisms[5,6]. In the context of sustainable agriculture, plasma-based technologies seem promising to address emerging food safety challenges associated with water reuse practices[7–9].

The agrifood sector faces mounting pressure to adopt water-efficient irrigation strategies, including the reuse of treated municipal wastewater (REUSE concept) to address global water scarcity[10]. However, reclaimed irrigation water, despite conventional treatment, can contain antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) that contaminate fresh products during cultivation[11]. This contamination pathway involves serious public health risks, as consumption of contaminated fruits and vegetables facilitates ARB/ARG transmission to humans, contributing to the global antimicrobial resistance crisis[12,13]. Cold atmospheric pressure plasmas offer a chemical-free, environmentally sustainable alternative to conventional sanitizers for post-harvest decontamination, with potential to address contamination at both cellular (bacterial inactivation) and genetic (ARG degradation) levels[14–17].
A critical yet often neglected aspect of SDBD operation is that RONS composition is not static but can undergo regime transitions during extended treatments. Two dominant chemical regimes have been documented. An ozone-dominant mode ($O_3$ mode), characterized by high ozone concentrations with minimal nitrogen oxides, and a nitrogen-oxides-dominant mode ($NO_x$ mode), in which ozone is depleted to trace levels while $NO_x$ accumulates to hundreds of ppm[18–21]. These regimes exhibit fundamentally different biocidal chemistries which may exhibit differential efficacy against specific pathogens and ARGs [22–28]. Understanding and controlling these regime transitions is therefore essential for optimizing plasma treatments targeting ARB contamination in REUSE agricultural contexts [16,29,30].

The composition of RONS generated by SDBDs depends sensitively on the electrical parameters of the discharge (voltage, pulse duration, frequency), the thermal state of the dielectric, and the ambient gas composition[23,31,32]. Elevated voltages and temperatures have been shown to accelerate ozone destruction and promote $NO_x$ formation, while water vapor significantly alters both discharge dynamics and downstream chemistry[19,33,34]. However, critical knowledge gaps remain that prevent rational optimization of plasma-based food decontamination systems. The thresholds of voltage and temperature required to trigger irreversible transitions between $O_3$-rich and $NO_x$-rich regimes remain not clearly defined for industrially relevant durations, such as the exposure times (30-60 minutes) needed for high (≥3-5 log) reductions in microbial load[3]. Most research teams employ short pulses or brief exposures, yielding mechanistic insight of the plasma discharge [21,35]. Nevertheless, this approach fails to provide guidance for process control at a larger scale.

A key challenge in the development of plasma-based food decontamination processes is the comprehension of the complex relationship between discharge characteristics and the actual production of biocidal molecules ($O_3$, $NO_x$, $H_2O_2$, OH). For example, variations in product moisture can profoundly shift the humidity dependence of RONS output, further altering required power, treatment times, and efficacy [36,37]. Although the effects of humidity on discharge composition and spatiotemporal properties are well-documented, the mechanisms by which these parameters govern chemical regimes throughout extended treatment durations have yet to be fully characterized.

This study employs a multiparameter approach to obtain a systematic characterization of an air SDBD operated under controlled humidity (45–80%), applied voltage (4.5–7.0 kV), and dielectric temperature conditions relevant to agri-food decontamination scenarios. ICCD imaging, optical emission spectroscopy, and absorption measurements of $O_3$ and $NO_2$ are used to track the chemical evolution of the discharge. The principal objectives of this study are threefold:

* First, to identify the voltage and temperature thresholds mediating the transition between $O_3$-rich and $NO_x$-rich plasma regimes in SDBD at durations compatible with industrial food safety processes.

* Second, to investigate the underlying physical and chemical mechanisms that dictate these transitional phenomena.

* Third, to establish operational windows and practical guidelines that enable reproducible, application-specific control of plasma reactivity ($O_3$ vs $NO_x$), thereby contributing to the design of advanced plasma-based decontamination systems for diverse food product matrices.
## 2. Materials and methods

### 2.1. Experimental setup

The experimental setup consisted of a cylindrical glass chamber with a 5 L internal volume operated at atmospheric pressure (Figure 1-A). The chamber is equipped with multiple ports to enable optical measurements via spectroscopy, connection of gas monitoring equipment and introduction of water vapor for humidity control.

Relative humidity (RH) within the chamber is regulated using a concentric pneumatic nebulizer (SeaSpray ARG-1-USS2, Glass Expansion). Simultaneously, synthetic air is injected through the second inlet at 800 mL.min⁻¹, regulated by a mass flow controller (Bronkhorst FG-201CV). A digital hygrometer (KIMO HD 110) monitored the RH inside the chamber.

<figure><img src="image_3.png" /><figcaption>Figure 1 – Schematic view of the experimental setup. (A) Overview of the 5 L hermetically sealed glass chamber equipped with different ports for optical diagnostics. The surface dielectric barrier discharge (SDBD) is positioned 10 cm above the chamber base. (B) Top view of the circular grounded electrode pressed against the upper face of the alumina ceramic dielectric disc. (C) Bottom view of the high-voltage electrode pressed against the lower face of the ceramic disc facing downward toward the chamber base.</figcaption></figure>

The plasma source is a surface dielectric barrier discharge (SDBD) positioned 10 cm above the chamber base. The SDBD configuration consisted of two stainless steel electrodes (grade 316L) separated by an alumina (Al₂O₃) ceramic dielectric disc (Figures 1-B and 1-C). The surface temperature of the ceramic was measured at the begin (T°START) and at the end (T°END) of the experiment with infrared thermometer (AMPROBE IR-710-EUR).

The grounded electrode is circular with a surface area of 64 cm² and is pressed against the top face of the alumina disc. The high-voltage electrode with an active surface area of 29 cm² and is pressed
against the bottom face of the disc, facing downward toward the chamber base where samples to be treated would be positioned. This electrode featured a comb-like design with seven parallel lines.

Electrodes are connected to a pulse generator (Nanogen1, RLC Electronic) enabled frequency adjustment from 1 Hz to 100 kHz, pulse durations from 150 ns to 50 µs, and duty cycles from 1% to 30%. Throughout all experiments, several parameters were held constant: the pulse frequency was set at 20 kHz (period of 50 µs), the pulse duration was maintained at 2.5 µs (5% duty cycle), ambient air was used as the plasma-forming gas, and the SDBD was positioned 10 cm above the chamber base. Figure 2 presents typical voltage (A) and current (B) waveforms recorded by means of a high voltage probe (Tektronix P6015A) and a current probe (Pearson 6585) connected to an oscilloscope (Rohde & Schwarz RTE1204).

<figure><img src="image_4.png" /><figcaption>Figure 2 – Typical voltage and current waveforms delivered by the pulsed power supply. (A) Voltage and (B) current delivered by the Nanogen1 pulse generator at U = 6.5 kV, f = 20 kHz and DC = 5%.</figcaption></figure>

The variable parameters investigated in this study are the applied voltage, which ranged from 4.5 to 7.0 kV in 500 V increments, and the relative humidity, which was tested under two conditions: 45% (ambient) and 80% (humid). Each measure presented in this paper has been repeated 3 times and the error bars correspond to the standard deviation of the measurements.

## 2.2. Optical Emission Spectroscopy

Optical emission spectroscopy was performed to identify reactive species present in the plasma and characterize its composition. An optical fiber (OceanOptics QP6000-2-SR, 600 µm diameter, 200-1100 nm spectral range) was positioned beneath the glass chamber, perpendicular to the ceramic disc and directly facing the high-voltage electrode. This configuration enabled collection of light emitted by the plasma filaments formed along the electrode lines. The collected light was transmitted to a SpectraPro HRS-750 spectrometer equipped with a 300 grooves/mm diffraction grating. Dispersed light was detected by an emICCD camera (PI-MAX4:1024EMB-UV, Princeton Instruments). All spectral acquisitions were controlled using LightField software.

## 2.3. Optical Absorption Spectroscopy

Optical absorption spectroscopy was employed to quantify concentrations of ozone ($O_3$) and nitrogen dioxide ($NO_2$) produced by the plasma. This technique relies on the Beer-Lambert law, which describes how light is attenuated when passing through an absorbing medium[38]. When radiation
crosses the chamber, molecules selectively absorb photons at characteristic wavelengths, reducing the transmitted intensity ($I_1$) compared to the reference intensity ($I_0$) measured without plasma. The concentration of each target species can be calculated using the following equation[39]:

$$C (ppm) = - \frac{\ln\left(\frac{I_1}{I_0}\right)}{\sigma \cdot l} \times \frac{10^6 \cdot R \cdot T}{N_A \cdot P}$$

Where C is the concentration of the target species (ppm), $I_0$ is the transmitted intensity without the plasma (reference), $I_1$ is the transmitted intensity with the plasma, $\sigma$ is the wavelength-dependent absorption cross-section (m².molecule⁻¹), l is the optical path length through the chamber (0.25 m), $N_A$ is Avogadro's constant (6.022 × 10²³ molecules.mol⁻¹), R is the ideal gas constant (8.206 × 10⁻⁵ m³.atm.mol⁻¹.K⁻¹), T is the gas temperature (K), and P is the pressure (atm).

Figure 3-A compares the absorption spectrum of O₃ (green line) and NO₂ (blue line) with the emission spectrum of a low-pressure mercury lamp (black line, model Philips). The 253.65 nm mercury line coincides with strong O₃ absorption in the Hartley band, with a reported cross-section of $\sigma = 1.135 \times 10^{-21} \text{ m}^2\text{.molecule}^{-1}$ at 295 K (Figure 2B) whereas the 404.65 nm mercury line fits with high NO₂ absorption, with a cross section of $\sigma = 5.694 \times 10^{-23} \text{ m}^2\text{.molecule}^{-1}$. These data from "The MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest" correspond to measurements performed at 295 K, which is consistent with our experimental conditions at room temperature [40].

<figure><img src="image_5.png" /><figcaption>Figure 3 – Spectroscopic data supporting wavelength selection for O₃ and NO₂ absorption measurements. (A) Overlaid spectra showing ozone O₃ (green line) and nitrogen dioxide NO₂ (blue line) absorption cross-section versus emission spectrum of mercury lamp (black line). (B) Table of literature values for O₃ absorption cross-section at 253.65 nm and 295 K. (C) Table of literature values for NO₂ absorption cross-section at 404.65 nm and 295 K. Cross-section data compiled from the MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest[40].</figcaption></figure>

O₃ concentration measurements were performed using a commercial UV absorption monitor (Ozone Monitor 103-M) connected to the chamber via a dedicated sampling port. This instrument operates on the Beer-Lambert principle at 253.65 nm and provides direct concentration readouts based on its internal calibration and algorithm.

NO₂ concentrations were determined using a custom-built absorption spectroscopy setup. The low-pressure mercury lamp was mounted at one port of the glass chamber to generate the 404.65 nm probe beam. On the opposite side of the chamber, an optical fiber (OceanOptics QP6000-2-SR, 600 µm diameter) collected the transmitted light toward the spectrometer PI-HRS-750. Between the lamp and the collection fiber, the optical path incorporated several components: two plano-concave
lenses to collimate the divergent beam from the lamp (Thorlabs LA1805, LA1708), an iris diaphragm
to control the beam diameter and reduce stray light, and a bandpass filter centered at 404.65 nm to
isolate the mercury line of interest (Thorlabs FB400-10). Spectra were acquired with the plasma off
(I₀) and plasma on (I₁), and the NO₂ concentration was calculated using the previous equation.

2.4. Camera imaging

Spatial and temporal imaging of the plasma discharge was performed using an intensified charge-coupled device (ICCD) camera (PI-MAX-2K-RB, Princeton Instruments) with a resolution of 1024 × 1204 pixels. Time-resolved sequential acquisitions with a 3 ns gate width were performed to capture the spatiotemporal evolution of the discharge from breakdown initiation through decay during one pulse. For these measurements, the ICCD camera was externally synchronized with the pulsed power supply using a trigger signal derived from the voltage pulse.

3. Results and discussion

3.1. From O₃ mode to NOₓ mode

Figure 4 presents measurements of ozone concentration as a function of the applied voltage, ceramic temperature and relative humidity. All experiments were conducted at fixed frequency (20 kHz) and duty cycle (5%).

Figure 4-A illustrates the temporal evolution of ozone concentration over 30 minutes for applied voltages ranging from 4.5 to 7.0 kV under dry conditions (RH 45%). Between each measurement, the ceramic temperature was fixed at T°START = 25°C. At low applied voltages (4.5-5.0 kV), ozone concentration increased monotonically throughout the 30 min treatment, reaching almost 900 ppm. Above 5.5 kV, O₃ concentration reached a maximum before declining, indicating a shift from ozone production to destruction. The transition time shortened with increasing voltage: 25 min at 5.5 kV, 15 min at 6.0 kV, and 10 min at 6.5-7.0 kV.

Figure 4-B shows that this transition correlates strongly with the temperature of the alumina dielectric. After 30 min of treatment, the ceramic temperature remained below 50 °C for voltages ≤ 5.0 kV but increased sharply to 60-85 °C for ≥ 5.5 kV. The ozone production rate (red circles), calculated as the total accumulated ozone divided by treatment time, exhibited a clear maximum of 600 ppm.min⁻¹ at 5.5 kV, followed by a decline to 450-550 ppm.min⁻¹ at higher voltages. This behaviour indicates that increased energy input simultaneously enhances ozone formation mechanisms while accelerating competing destruction pathways, with the latter dominating at elevated voltages and temperatures [25,41,42].

To isolate thermal effects from electrical parameters, the ozone production was measured at fixed voltage (U = 5.5 kV) and relative humidity (RH = 45%) as a function of initial ceramic temperature (figure 4-C). Pre-heating the ceramic from 25°C to 64°C, 67°C or 85°C prior to plasma ignition resulted in dramatic suppression of ozone production. At the reference temperature (25°C), ozone accumulated progressively to 810 ppm over 30 minutes. However, at 64°C, production was limited to 300 ppm with saturation occurring within 15 minutes. Further temperature increases to 85°C reduced final concentrations to 80 ppm, with saturation times decreasing to 5 minutes. According to literature, the transition from ozone production to decomposition at higher voltages (Figure 4-A) and its temperature-dependent acceleration cannot be explained solely by thermal ozone decomposition but also involve coupled plasma – chemical mechanisms[19,34,43,44].
<figure><img src="image_6.png" /><figcaption>Figure 4 – (A) Temporal evolution of ozone concentration over 30 minutes for applied voltages ranging from 4.5 to 7.0 kV under dry conditions (RH 45%). (B) Temperature of the alumina dielectric over 30 minutes. (C) Temporal evolution of ozone concentration over 30 minutes for T°START ceramic fixed at 25°C, 64°C and 85°C under dry conditions (RH 45%) and fixed voltage (5.5 kV). (D) Temporal evolution of ozone concentration over 30 minutes for relative humidity ranging from 10% to 90% with fixed voltage (5.5 kV).</figcaption></figure>

The generation of ozone in atmospheric pressure air plasmas proceeds via a two-step mechanism (R1, R2)[45,46]. Elevated temperatures reduce the rate constant for the three-body ozone formation reaction (R2) while exponentially increasing the thermal decomposition rate (R3) according to Arrhenius kinetics[47–49]. In parallel, higher gas temperatures modify the electron energy distribution function, decreasing the efficiency of O₂ dissociation by energetic electrons[50]. Consequently, the balance between ozone formation and destruction shifts rapidly toward net loss as temperature rises. This finding is consistent with prior reports highlighting the necessity of active cooling in industrial ozone generators[46,51,52].

$$e^{-} + O_{2} \rightarrow O + O + e^{-} \qquad (R1)$$

$$O + O_{2} + M \rightarrow O_{3} + M \qquad (R2)$$

$$O_{3} + M \rightarrow O + O_{2} + M \qquad (R3)$$

(with M a third particle)

In addition to thermal decomposition, a key additional mechanism is catalytic ozone destruction by nitrogen oxides (NOx), commonly known as discharge poisoning in corona and DBD systems[45,53–
55]. In air plasma, atomic nitrogen is generated via electron-impact dissociation of N₂ (R4) and through quenching of electronically excited nitrogen molecules (R5)[56,57]. Atomic nitrogen rapidly oxidizes to form nitric oxide (R6, R7), which is subsequently converted to nitrogen dioxide (R8, R9) [49,58]. Once NO₂ accumulates to sufficient levels, it can further react with atomic oxygen and ozone to form higher nitrogen oxides (NO₃ (R11), N₂O₅ (R12)), creating a complex chemical network that competes with ozone formation[59].

$$e^{-} + N_{2} \rightarrow N + N + e^{-} \qquad (R4)$$

$$N_{2}^{*} + O \rightarrow NO + N \qquad (R5)$$

$$N + O_{2} \rightarrow NO + O \qquad (R6)$$

$$N + O_{3} \rightarrow NO + O_{2} \qquad (R7)$$

$$NO + O_{3} \rightarrow NO_{2} + O_{2} \qquad (R8)$$

$$NO + O + M \rightarrow NO_{2} + M \qquad (R9)$$

$$NO_{2} + O \rightarrow NO + O_{2} \qquad (R10)$$

$$NO_{2} + O + M \rightarrow NO_{3} + M \qquad (R11)$$

$$NO_{2} + NO_{3} + M \rightarrow N_{2}O_{5} + M \qquad (R12)$$

$$N_{2}O_{5} + M \rightarrow NO_{2} + NO_{3} + M \qquad (R13)$$

The reaction between NO and ozone (R8) proceeds extremely rapidly on millisecond-to-second timescales[58,60]. Critically, NO₂ can regenerate NO via reaction with atomic oxygen (R10), creating a catalytic cycle that continuously consumes ozone. In our condition, at low voltages, NOₓ concentrations remain below the threshold needed to influence ozone chemistry significantly. However, as voltage and temperature increase, enhanced energy deposition accelerates the cumulative build-up of NOₓ and shifts the plasma chemistry irreversibly toward a NOₓ-dominant regime[24].

Figure 5-A provides direct spectroscopic evidence for this transition. The intensity of the 404.65 nm mercury line, used as a probe for NO₂ absorption, remained constant for 40 min, indicating negligible NO₂ concentration (Region 1: O₃-dominant regime). After 40 min, a sharp decrease in transmitted intensity revealed the onset of NO₂ formation (Region 2: transition). Between 70 and 90 min, the signal stabilized, signifying steady-state NO₂ levels (Region 3: NOₓ-dominant regime).

Quantitative measurements of O₃ and NO₂ concentrations under identical conditions (20 kHz, 5% duty cycle, 6.5 kV, RH 45%) confirmed a strong inverse correlation as reported in literature[19,31]. Ozone rose rapidly to 1000 ppm within 15 min and then declined to near zero after 40 min, while NO₂ increased from less than 20 ppm to 800 ppm between 40 and 90 min. These results illustrate the mutual exclusivity of O₃ and NOₓ at high concentrations due to reaction R8, one of the fastest gas-phase reactions known [49].
<figure><img src="image_7.png" /><figcaption>Figure 5 – (A) Temporal evolution of the 404.65 nm mercury absorption line intensity over 90 minutes. (B) Quantitative concentrations of ozone (red circles) and nitrogen dioxide (black squares) measured simultaneously via absorption spectroscopy. Conditions: U = 6.5 kV, f = 20 kHz, DC = 5%, RH = 45%.</figcaption></figure>

Relative humidity (RH) further modulates the chemical balance between production and destruction of ozone (Figure 4-D). At fixed voltage (5.5 kV), increasing RH from 10% to 90% reduced final ozone concentration by almost 50%. Water vapor influences plasma chemistry through multiple concurrent mechanisms. Water vapor is readily dissociated by electron impact and reactions with excited species, producing hydroxyl radicals (OH) and hydrogen atoms (R14). These species directly destroy ozone via R15 and R16, both of which proceed rapidly at room temperature[49,56]. OH radicals also consume atomic oxygen (R17), the immediate precursor to ozone, thereby reducing ozone formation rates[46]. Electron-impact dissociation and ionization of H₂O divert energetic electrons from O₂ dissociation, reducing the mean electron energy and decreasing atomic oxygen production[61,62]. Water molecules quench electronically excited oxygen species that contribute to alternative ozone formation pathways[63]. Collectively, these processes account for the strong humidity dependence observed in O₃ yield.

$$e^{-} + H_{2}O \rightarrow OH + H + e^{-} \qquad (R14)$$

$$OH + O_{3} \rightarrow HO_{2} + O_{2} \qquad (R15)$$

$$H + O_{3} \rightarrow OH + O_{2} \qquad (R16)$$

$$OH + O \rightarrow H + O_{2} \qquad (R17)$$

Based on the combined O₃ and NO₃ measurements, we selected an applied voltage of 6.5 kV as the optimal operating condition for subsequent plasma treatment experiments. This voltage represents a compromise condition that ensures either a sustained O₃ or NOₓ mode for treatment durations of 30-45 min, depending on the initial thermal state, while avoiding intermediate or transient modes. A voltage of 6.5 kV finally maintains the ceramic temperature within a range that supports controlled chemistry (60-80 °C) without excessive thermal stress.

The chemical transition between the O₃ and NOₓ-dominant regimes highlights the strong coupling between plasma electrical parameters, thermal conditions and the surrounding gas composition [24]. However, absorption spectroscopy alone provides only integrated, time-averaged information on the chemical species produced during discharge operation. To further elucidate the plasma dynamics and understand how environmental parameters – particularly humidity – affect the discharge behaviour, the following section presents a description of the spatiotemporal dynamics observed by ICCD
imaging and the emission spectra of the SDBD operated at 6.5 kV under dry and humid conditions, in $O_3$-mode.

## 3.2. Spatio-temporal evolution

Figure 6-A shows the synchronized evolution of discharge current and optical emission under 45% (red) and 80% (black) relative humidity. The current waveform exhibits two distinct peaks, at the rising and falling edges of the voltage pulse, typical of capacitive DBD behaviour[64]. Correspondingly, plasma emission occurring at both edges confirms that light emission and current peaks are temporally correlated. Furthermore, different discharge characteristics appear between humidity conditions. During the voltage rise, the first discharge event is more intense at RH 80%, whereas the second event is stronger at RH 45%. This inversion aligns with previous reports showing that humidity modifies streamer creation and propagation by altering the local breakdown field, space-charge dynamics, and charge accumulation on the dielectric surface[2,65,66].

<figure><img src="image_8.png" /><figcaption>Figure 6 - (A) Full 3 µs window showing synchronized evolution of discharge current (green trace, right axis) and spatially integrated optical emission intensity under dry (RH 45%, red trace) and humid (RH 80%, black trace) conditions. High-resolution temporal zooms of (B) the rising edge discharge (0-0.15 µs) and (C) the falling edge discharge (2.5-2.65 µs). Conditions: U = 6.5 kV, f = 20 kHz, DC = 5%, $O_3$ mode.</figcaption></figure>

Figures 6-B and 6-C present zooms of the discharge evolution during the first 150 ns (rising front) and during the 2.5-2.65 µs interval (falling front). The dashed lines mark the selected time points shown in figure 7, where ICCD images visualize the discharge morphology under both dry (7-A) and humid (7-B) conditions. To allow a proper comparison, the intensity scale is the same for each image. At t = 0 ns, the discharge initiates at the tips of the high-voltage electrode line, regardless of RH, consistent with studies on serrated and comb-shaped electrodes showing that streamers preferentially initiate at electrode edge and propagate perpendicularly[66,67]. Their subsequent interactions are then governed by geometric field enhancement and mutual electrostatic coupling. At t = 12 ns, under RH 45%, the plasma propagates along the outer contours of the electrode, while at RH 80%, the propagation extends simultaneously along the inner edges, indicating that water vapor facilitates surface charge spreading[51]. At t = 21 ns, the discharge reaches its maximum intensity in both cases, with visibly higher brightness at RH 80%. Beyond t = 30 ns, streamers spread over the dielectric surface, and the overall emission gradually decays until extinction at 45 ns.
<figure><img src="image_9.png" /><figcaption>Figure 7 - ICCD images captured at selected time steps (t = 0, 12, 21, 30, 45 ns) during the rising edge discharge in (A) dry conditions (RH 45%) and (B) humid condition (RH 80%). Conditions: U = 6.5 kV, f = 20 kHz, DC = 5%, gate width = 3 ns, accumulation = 15000 frames, intensifier gain = 200, O₃ mode. The intensity scale is the same for each image.</figcaption></figure>

To more effectively visualize and interpret streamer dynamics, regions corresponding to electrode lines 1 and 2 were magnified and tracked over time using nanosecond ICCD imaging (Figure 8). To enable a proper comparison, the intensity scale is the same for each image. The results align well with prior studies where streamers propagating from the outer edge of an isolated tooth extend freely outward into ambient air, resulting in longer, brighter, and more spatially extensive plasma filaments[67]. In contrast, when two adjacent lines face each other (1→2 and 2→1), streamers propagating toward each other tend to interact via space-charge and field shielding mechanisms, leading to mutual quenching and early extinction of both filaments[68,69]. At early times (t = 15 ns), streamers trace the electrode contours, with both dry and humid conditions initiating along similar trajectories. By t = 24-36 ns, perpendicular propagation toward the dielectric dominates. The clearest distinction emerges under humid conditions, where streamers become more luminous, broader, and propagate further. Humidity-enhanced ionization and improved charge mobility drive this broader and more intense discharge, while also promoting a greater degree of filament multiplication across the surface.

<figure><img src="image_10.png" /><figcaption>Figure 8 - High-resolution ICCD imaging focused on electrode lines 1 and 2 at selected time points (t = 6, 15, 24, 36, 45 ns) during the rising edge in (A) dry conditions (RH 45%) and (B) humid condition (RH 80%). Conditions: U = 6.5 kV, f = 20 kHz, DC = 5%, gate width = 3 ns, accumulation = 15000 frames, intensifier gain = 200, O₃ mode. The intensity scale is the same for each image.</figcaption></figure>
### 3.3. Optical emission spectrum

Figure 9 shows the optical emission spectra of the SDBD plasma operated in O₃-mode at 6.5 kV under two relative humidity conditions: RH 45% (dry, black) and RH 80% (humid, red). The spectra cover the 200-500 nm range and exhibit the characteristic features of atmospheric-pressure air plasmas.

The most intense emission structures correspond to the second positive system of molecular nitrogen, N₂ (C³Π → B³Π), observed between 295 and 500 nm. This system is a well-known signature of non-thermal plasmas in air and indicates efficient electronic excitation of N₂ molecules [64,70]. The high intensity of the N₂ (C-B) bands confirms that nitrogen excitation is the dominant energy transfer pathway in this SDBD configuration [71,72]. The excitation thresholds of the N₂ (B³Π) and N₂ (C³Π) states, 7.35 eV and 11.03 eV respectively, demonstrate the presence of electrons with energies exceeding 11 eV within the discharge. In the near-UV region (370-430 nm), weaker but clearly resolved lines correspond to the first negative system of N₂⁺ (B²Σ⁺u → X²Σ⁺g), visible at 391 and 427 nm. The presence of these ionic transitions confirms that the electric field is locally strong enough to ionize nitrogen, consistent with the filamentary behaviour typical of surface DBDs [73–75]. In the 250-290 nm region, the NOβ-bands (B²Π → X²Π) are detected, resulting from reactions between electronically excited nitrogen and atomic oxygen, which play a crucial role in determining the chemical regime [1,23,72].

<figure><img src="image_11.png" /><figcaption>Figure 9 – Optical emission spectra of the surface dielectric barrier discharge (SDBD) operated under two relative humidity conditions: 45 % RH (black) and 80 % RH (red). Conditions: U = 6.5 kV, f = 20 kHz, DC = 5%, O₃ mode.</figcaption></figure>

Increasing the relative humidity from 45% to 80% increases the intensity of N₂(C-B), N₂⁺(B-X), and NOβ emission bands. This enhancement points to water vapor promoting discharge activity through multiple synergistic mechanisms. Adsorbed water on the dielectric surface increases surface conductivity and modifies the local electric field distribution, which facilitates a broader spatial coverage of microdischarges as seen previously through ICCD imaging without altering the fundamental discharge mode [2,51,75]. Changes in surface conductivity also affect charge
accumulation dynamics on the dielectric, thereby influencing the memory voltage from one pulse to the next and favouring more vigorous microdischarges under repetitive excitation [76].

In the gas phase, water molecules participate in additional ionization pathways. Electron-impact dissociation and ionization of H₂O and subsequent charge-transfer reactions (e.g., H₂O⁺ + N₂ → N₂⁺ + H₂O) increase the production of secondary electrons and ions [5,33,65]. These processes modify the electron energy distribution function, often reducing high-energy electron density locally but simultaneously expanding the active discharge region. As a result, the per-unit-volume excitation of N₂ may decrease while the overall emission increases due to a larger discharge volume [75,77].

Furthermore, dissociation products of water, particularly OH and H, enhance the formation of NO through fast radical reactions such as N + OH → NO + H [78]. This contributes to the observed increase in NOₓ-band emission and modifies the balance of reactive oxygen and nitrogen species in humid air plasmas.

### 4. Conclusion

This study provides a detailed analysis of the transition between ozone-dominant and NOₓ-dominant regimes in an atmospheric-pressure SDBD operated in air. By combining optical and chemical diagnostics, this work demonstrates that the applied voltage, dielectric temperature and humidity jointly determine the reactive species composition. Higher voltages and temperatures accelerate the onset of the NOₓ regime through catalytic O₃ consumption by nitrogen oxides, while elevated humidity enhances O₃ destruction via OH- and H-driven reactions and modifies electron-driven dissociation pathways. ICCD imaging reveals that humidity also influences the morphology and propagation of surface streamers, leading to enhanced optical emission and broader discharge coverage.

These findings clarify the mechanisms controlling RONS production in air SDBDs and define the operating conditions required to maintain stable O₃-rich or NOₓ-rich regimes in our experimental setup. This knowledge is essential for optimizing plasma reactors for environmental treatment, agriculture, and other applications in which specific oxidative or nitrosative chemistries are desired. Future work will focus on completing ICCD and OES diagnostics in NOₓ-mode and coupling plasma-phase chemistry with surface interactions to further precise control of plasma-generated reactive environments.

### 5. Acknowledgment

The authors would like to thank Region Occitanie and Institut National Champollion university for their financial support. We also thank Florent Prouhet from INU Champollion university for his technical help.

**Author contributions:** Z. A.: investigation, data curation, formal analysis, writing – original draft. C. M.: conceptualization, formal analysis, supervision, writing and reviewing. P. G.: conceptualization, supervision and funding acquisition. T.M.: conceptualization, methodology, data curation, formal analysis, investigation, writing – original draft, reviewing and editing.

### 6. Conflict of interest statement

The authors have no conflict to disclose.

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