Microanalysis of Active Nitrogen Oxides (RONS) Generation Characteristics during DC Negative Corona Discharge at a Needle-Plate Electrode

Abstract

The DC negative corona of needle-plate electrodes can generate atmospheric pressure low-temperature plasma active particles, which have important effects on biological mutagenesis. The DC negative corona discharge of an air needle-plate electrode with effective consideration of NO_x particles was simulated and the Trichel pulse current was obtained, focusing on the development of particles and the distribution of active nitrogen oxides (RONS) at four moments in the pulse process. The simulation results indicate that the positive ions (N₂⁺ and O₂⁺) and negative ions (O⁻ and O₂⁻) were closely related to the current changes, and the negative ions (O⁻ and O₂⁻) presented a typical stratification phenomenon. RONS (H₂O₂, O₃, and NO) were approximately uniformly distributed above the level of the plate electrode at the same instant, with H₂O₂ and O₃ except for the area below the needle tip. They trended to a cumulative increase in concentration with time. This study provides a theoretical basis for corona discharge plasma seed treatment technology.

1. Introduction

Corona discharge is a partial self-sustained discharge phenomenon held by an extremely inhomogeneous electric field, which usually occurs in the vicinity of the tip electrode. Compared to other forms of discharge, corona discharge devices are characterized by their simple structure, low cost, and ability to be utilized on a large scale [1]. It is owing to these promising properties that corona discharge has been widely applied in the fields of electrostatic dust removal [2,3], pollutant treatment [4], surface treatment [5], and bio-mutagenesis [6]. In addition, the high-voltage corona electrical field seed treatment technology is a new physical mutagenesis technology, characterized by a simple device, obvious biological effects, and environmental friendliness [7]. It has been revealed that a large number of plasma active particles active oxygen (ROS), active nitrogen (RNS) as well as electric-field distribution are generated during the discharge process, and these physicochemical elements act complexly on the seed thereby activating the seed vitality [8].

The corona characteristics of different polarities differ greatly and can be divided into negative and positive corona according to polarity. Liu M et al. conducted DC corona discharge experiments on a needle-plate corona discharge model under different DC voltage conditions. The results showed that the positive corona gradually went through several different discharge stages, including initial streaming discharge, periodic streaming discharge, glow discharge, and spark discharge, as the voltage was increased. The negative corona gradually went through the early and middle stages of Trichel discharge, sawtooth discharge, and glow discharge. Compared to the positive corona, the negative corona has a smaller pulse amplitude and time interval, but it has a better uniformity and higher repetition rate [9].

E. Moreau et al. studied the positive and negative corona discharge of needle-plate structure in air and found that the positive corona discharge produced a faster ion wind speed but the effective radius was relatively small [10]; the ion distribution of the negative corona discharge obviously has a larger range [11], and it is obvious that the negative corona electric field can be more widely and stably applied to plant seed mutagenesis. However, the negative corona current is composed of a series of regular pulsed currents known as Trichel pulses [12]. Numerical simulation, as an effective tool complementing experiments, can provide valuable insights into the experiments and even play a decisive role in the overall study. In the study of gas discharge mechanisms, numerical simulation has become an effective tool due to the limitation of experimental methods in space plasma. In the literature [11,13], the DC negative corona of needle-plate electrodes in air was simulated, and the development of the Trichel pulse of the negative corona was explained in detail; however, the hydrodynamic simulation model used in the simulation could only macroscopically analyze the distribution characteristics of positive ions, negative ions, and electrons. In the literature [14], a hybrid numerical model was used to analyze the physical development of air discharges at low pressure, and 27 plasma chemical reactions that best reflect the microphysical processes of particles in air discharges were summarized. In the literature [15], the negative corona discharge model for rod-plate electrodes was improved on the basis of the hydrodynamic model by combining the collision reaction model, and the temporal and spatial development of the electric-field strength, the plasma chemical reaction rate, the space charge density, and the electron density distribution in the Trichel pulse process were analyzed.

In the literature [16,17], a hybrid numerical model was improved for the hydrodynamic model, and a large number of plasma chemical reactions were included in the model to explain the development of the Trichel pulse in terms of specific particle distributions. However, the composition of air is complex, and the discharge process is inevitably accompanied by the generation of activated nitrogen oxides (RONS), so it is necessary to consider a more comprehensive plasma chemical reaction system and analyze the biological pattern of RONS as a refinement of the negative corona–Trichel pulse mechanism in the simulation process.

Plasma can promote seed awakening and seed germination and growth, and kill fungal bacteria on the seed-coat surface [18]. Wang Min et al. studied the effects of different voltage intensity atmospheric pressure plasma treatments on lettuce seed development and yield situation. The results showed that the effects of different degrees of voltage and atmospheric pressure varied significantly. Among them, the appropriate voltage treatment obviously promoted the seed vigor and germination, but the treatment with too high a dose did not promote, as well as even inhibit, the germination of seeds [19]. Reaction oxygen species (ROS) are by-products of aerobic metabolism in plants. ROS are continuously produced in plants, and when their levels exceed the threshold, they cause damage to plant-cell membranes. Therefore, the enzymatic scavenging system formed in the plant cell regulates it in a state of dynamic equilibrium. Hydrogen peroxide (H₂O₂) accumulated during seed suckering is essential for breaking seed dormancy [20]. Nitric oxide (NO) is the most abundant reactive nitrogen in plants, and since plants cannot synthesize it themselves, they can only accumulate and metabolize atmospheric NO [21]. In its interaction with plant hormones, NO has different accumulation levels for different cells, so the role of NO in plant-hormone responses remains a complex issue. In conclusion, small molecules have a regulatory role in seed germination.

ROS and RNS have a wide range of regulatory functions in various plant growth and development processes, including plant germination, physiological and biochemical metabolism, and signaling [22]. Exogenous hydrogen peroxide (H₂O₂), ozone (O₃), and nitric oxide (NO) as typical ROS and RNS are able to promote seed germination and growth as well as resistance properties [23]. Sodium nitroprusside (SNP) as a form of exogenous NO can break seed dormancy, as in Arabidopsis and barley [24]. The dormancy of seeds can also be broken, and germination promoted using exogenous ROS [25,26]. DC negative corona under needle-plate electrodes, as one of the common ways of plasma generation, produces a variety of RONS in air discharges that can affect the growth of plants [27,28]. Plasma-activated water (PAW) produced by gas–liquid discharges is rich in active nitrogen oxides (NO₂⁻, NO₃⁻, O₃, as well as H₂O₂), and studies have demonstrated that PAW could promote the germination growth of plant seeds in close correlation with RONS [29,30]. However, the active nitrogen oxides in PAW inevitably derive from gas discharges. Therefore, it is valuable to carry out the microscopic generation as well as the distribution pattern of RONS in DC negative corona air discharges of needle-plate electrodes.

In this paper, a two-dimensional hydrodynamic and chemical model of the DC negative corona of needle-plate electrode with complex air at atmospheric pressure is established, which has 14 species and 34 kinds of chemical reactions, and the validity of the plasma chemical reaction system is verified by simulation and the Trichel pulse current of negative corona discharge is obtained. The development of particles at four moments during the pulse and the distribution of RONS above the lower plate are emphasized. In this study, the needle-plate electrode DC negative corona discharge in air with effective consideration of NO_x particles was simulated, while providing a theoretical basis for corona discharge plasma seed treatment technology.

2. Numerical Modeling

2.1. Discharge Structure and Boundary Conditions

As shown in Figure 1, a two-dimensional plasma hydrodynamic and chemical model of needle-plate electrode for atmospheric pressure air is investigated in this paper. Since the needle-plate model has symmetry, two-dimensional axisymmetric modeling was used for the analysis. The radius of curvature of the needle was 0.12 mm, the lower pole plate was grounded, the gap distance was 3 mm, and the applied voltage was −2 kV. In the model, the setup of the boundary conditions was of great importance, where the electrons will be lost in the walls as well as surface reactions due to the secondary electron emission effect.

The electron fluxes are set at the cathode as well as the anode boundary conditions:

$$n\cdot {\mathit{\Gamma }}_e=\frac{1}{2}{v}_{e,th}{n}_e-{\displaystyle \sum _p{\gamma }_p(}{\mathit{\Gamma }}_p\cdot n)$$

(1)

Heavy matter ions were lost on the wall by surface reactions and the electric field wass directed toward the wall:

$$n\cdot j_k=M_k{R}_k+M_k{c}_k{Z}_k{\mu }_k({\rm E}\cdot n)[Z_k{\mu }_k({\rm E}\cdot n)>0]$$

(2)

where ${\mathit{\Gamma }}_e$, $v_{e,th}$, ${\gamma }_p$, $n_e$ and $n$ are the electron flux vector, the electron thermal velocity, the secondary electron emission coefficient, the electron density, and the outward normal, respectively; $M_k$, $R_k$, $c_k$, $Z_k$, ${\mu }_k$ and ${\rm E}$ denotes the molar mass of species k, the surface reaction rate of species k, the mass fraction of heavy species k, the charge number for species k, the mobility of species k, and the electric field, respectively. The p subscript means the p-type positive ion in the reaction system in the sum.

2.2. Control Equations

The model was calculated by solving the coupled electron transport equation, the heavy matter transport equation, and the Poisson equation, represented as follows:

The electron continuum equation is:

$$\frac{\partial }{\partial \mathrm{t}}(n_e)+\nabla \cdot [-({\mu }_e\cdot E)n_e-D_e\cdot \nabla n_e]=R_e$$

(3)

The heavy substance transport equation is:

$$\rho \frac{\partial }{\partial \mathrm{t}}({\varpi }_k)+\rho (u\cdot \nabla ){\varpi }_k=\nabla \cdot j_k+R_k$$

(4)

$$R_k={\displaystyle {\int }_0^{\infty }{\epsilon }^{1/2}}f(\epsilon )\sigma (\epsilon )d(\epsilon )$$

(5)

$$D_e={\mu }_e{T}_e$$

(6)

where $u$, ${\mu }_e$, $D_e$ and $T_e$ are the electron mobility, the electron diffusion coefficient, and the electron temperature, respectively; $R_e$ is the electron source term; $\rho$, ${\varpi }_k$, $j_k$, $u$ and $R_k$ denote the mixture density, the mass fraction of heavy particles k, the diffusion flux vector, the mean fluid velocity vector, and the rate expression for species k, respectively. $f(\epsilon )$ is the electron energy distribution function and $\epsilon$ is the average electron energy [31]. $\sigma (\epsilon )$ is the collision intercept of the reaction [32].

The Poisson equation is:

$$\nabla \cdot {\epsilon }_0{\epsilon }_{\mathrm{r}}E=\rho$$

(7)

$$E=-\nabla V$$

(8)

where $V$ is the space potential, ${\epsilon }_0$ and ${\epsilon }_r$ are the vacuum permittivity and relative permittivity, respectively, and $\rho$ is the space charge density.

2.3. Type of Crash Response

The discharge model in this paper used a simplified air–chemical reaction system, which was N₂:O₂ = 4:1, including 34 chemical reactions and 14 species: e, N₂, O₂, N₂⁺, O₂⁺, O₂⁻, O, O⁻, N, O₃, NO, NO₂, N₂O, and NO₃. The chemical reactions of the plasma are shown in

Table 1. Major particle chemical reactions.

Number	Reaction	Rate Coefficient
R1	e + N2 => e + N2	f(ɛ)
R2	e + N2 => N2+ + 2e	f(ɛ)
R3	e + O2 => e + O2	f(ɛ)
R4	e + O2 => O2−	f(ɛ)
R5	e + O2 => O + O−	f(ɛ)
R6	e + O2 => O2++ 2e	f(ɛ)
R7	e + 2O2 => O2 + O2−	5.17 × 10−43Te−1
R8	e + N2 + N2+ => 2N2	6.07 × 10−34Te−2.5
R9	2e + N2+ => N2 + e	7.18 × 10−39Te−4.5
R10	O2+ + e => 2O	6.25 × 10−15Te−1
R11	N2+ + e => 2N	2.8 × 10−13(300/Te)0.5
R12	e + O3 => O2 + O−	1 × 10−17
R13	e + O2+ => O2	4 × 10−18
R14	2e + O2+ => O2 + e	7.18 × 10−39Te−4.5
R15	e + N2+ => N2	7.72 × 10−14Te−0.5
R16	O2− + O2+ => 2O2	2 × 10−13
R17	O2 + N2+ => N2 + O2+	1.04 × 10−15Tg−0.5
R18	O2+ + O2 + O2− => 2O2 + O2	2 × 10−37
R19	O2+ + N2 + O2− => 2O2 + N2	2 × 10−37
R20	O + O2 + N2 => O3 + N2	2.5 × 10−46
R21	O + O2 + O2 => O3 + O2	2.5 × 10−46
R22	O− + O2+ => O + O2	3.46 × 10−12Tg−0.5
R23	O3 + N => NO + O2	6 × 10−19
R24	O + NO2 => NO + O2	2.63 × 10−17exp(−13,790/Tg)
R25	O2 + N => NO + O	1.0 × 10−31(300/Tg)2.5
R26	O3 + NO2 => NO + 2O2	1.0 × 10−31(300/Tg)2.5
R27	O3 + NO => NO2 + O2	3.6 × 10−18exp(−1560/Tg)
R28	O + N2O => 2NO	1.66 × 10−16exp(−14,090/(Te)
R29	3NO => NO2 + N2O	2.95 × 10−50exp(−13,490/Te)
R30	NO + N2O => NO2 + N2	4.17 × 10−16exp(−25,160/Te)
R31	2NO2 => NO3 + NO	5.37 × 10−18exp(−12,880/Te)
R32	NO + NO3 => 2NO2	1.6 × 10−17exp(150/Tg)
R33	O3 + NO2 => NO3 + O2	1.2 × 10−19exp(−2450/Tg)
R34	NO + NO3 => 2NO + O2	2.71 × 10−17Tg−0.23exp(−947/Tg)

Note: The reaction rate constants in Table 1 are in m³/s for two-body collisions, m⁶/s for three-body collisions, and K for T_g and T_e.

[33,34].

3. Results and Discussion

This paper was based on the plasma module of COMSOL Multiphysics for modeling and analysis, with an ambient pressure of 1 atm, a temperature of 300 K, and a total of 273,021 cells in the computational grid, to study the process of Trichel pulsed-current discharge and the generation and distribution pattern of RONS.

3.1. Validation of the Validity of Chemical Reaction Systems

To ensure that the chemical reaction system had the property of predicting the generation pattern of the reaction products, this paper imported the chemical reaction system into the Dielectric Barrier Discharge (DBD) model under the same experimental conditions (V_am = 0.6 kV, f = 15 kHz, air gas gap pressure of 500 Pa). Comparing the experimental and simulated voltage and current waveforms, as shown in Figure 2, the current waveform morphology was similar between the two, and the discharge breakdown phase was basically the same [35]. Therefore, the chemical system used in this model was capable of qualitatively analyzing the DC negative corona discharge products of the needle-plate electrode.

3.2. Discharge Current of Negative Corona

Figure 3 shows the current waveform of the Trichel pulse with a negative corona under the needle-plate gap, the pulse has the characteristics of rapid rise and slow fall, and as shown in Figure 4, a multilayered distribution of negative ions appeared with the Trichel pulse, the Trichel pulse appears to have a multilayered negative ion distribution, which indicates that the simulation results have credibility [11]. In this paper, the microphysical changes of particles were specifically analyzed with four moments of the 6th pulse (t₁: 2.05 μs, t₂: 2.10 μs, t₃: 2.20 μs, t₄: 2.30 μs). The blue symbols represent the four selected moment points, and the red lines represent the current pulse variations, while the dashed lines represent the local plots within this range.

3.3. Ionic Behavior during Discharge

The major positive ions considered in the model were N₂⁺ and O₂⁺, and the negative ions were both O⁻ and O₂⁻. As shown in Figure 4a, the positive ions in space at the moment t₁ were at the end of the previous pulse as well as at the beginning of the next pulse, and were present only near the cathode, where the number densities of N₂⁺ and O₂⁺ reached 2.08 × 10¹⁸ m⁻³. With the rapid rise of current, the number density of positive ions grew significantly and compressed toward the cathode surface. The cathodic-sheath layer was fully formed, and by the moment t₂ of the peak current, the number density of positive ions reached 1.31 × 10²⁰ m⁻³. At moments t₃ to t₄, the number densities of N₂⁺ and O₂⁺ continued to decrease, but the positive ion density at the cathode remained at 1.07 × 10¹⁸ m⁻³. This indicates that the rising and falling processes of the Trichel pulse corresponded to the development of rising and falling cathodic-sheath positive ions.

As shown in Figure 4a, at the moment t₁, the field strength near the needle tip reached 77.6 kV/cm, which was much larger than the air breakdown electric-field strength of 30 kV/cm, thus making the air ionize rapidly. Positive ions produced secondary electrons on the cathode surface to form an electron collapse, and the electrons formed negative ions O⁻ and O₂⁻ due to the oxygen-attachment reaction, with a number density of 1.79 × 10¹⁷ m⁻³. As the current pulse rose to the peak moment t₂, the electric-field strength continued to increase by 250 kV/cm, and the particles near the needle tip reacted violently, the electron density increased significantly, and the corresponding negative ion density also increased. The negative ions and electrons moved away from the cathode under the force of the electric field, and the number densities of the positive ions, negative ions, and electrons reached a maximum of 1.31 × 10²⁰ m⁻³, 1.82 × 10¹⁸ m⁻³, and 1.98 × 10¹⁹ m⁻³, respectively. At the moments from t₂ to t₄, electrons and negative ions continued diffusing to the anode, the number density of the electrons decreased, and the adsorption capacity decreased due to the migration of the electric field to the anode as well as the weakening, while the number density of the negative ions decreased continuously. As the negative ions continued to strengthen away from the cathode, the electric field strength near the needle tip was re-centralized (it can be considered that the electric field strength recovered from 71.4 kV/cm to 77.6 kV/cm from the moment t₄ to the moment t₁ of the next pulse), a process that ensured the continuity of the pulse.

As shown in Figure 4b, by analyzing and comparing the maximum number densities of N₂⁺ ions, it can be seen that the maximum number densities of N₂⁺ ions were 3.1 × 10¹⁷ m⁻³, 8.15 × 10¹⁹ m⁻³, 5.41 × 10¹⁷ m⁻³, and 1.1 × 10¹⁷ m⁻³ at moments t₁ to t₄, respectively. With time, the number density of N₂⁺ ions reached a maximum at the peak current moment t₂ then gradually decreased from t₃ to t₄. By a similar analysis, the maximum number densities of the O₂⁺ ions were 1.87 × 10¹⁸ m⁻³, 5.0 × 10¹⁹ m⁻³, 3.48 × 10¹⁸ m⁻³, and 9.85 × 10¹⁷ m⁻³ at moments t₁ to t₄, respectively. With time, the number density of the O₂⁺ ions reached a maximum at the peak current moment t₂ then gradually decreased from t₃ to t₄. By analyzing the pattern of change of the two positive ions, it can be concluded that the rising and falling processes of the Trichel pulse still corresponded to the development of cathodic-sheath positive ions. The maximum number densities of the O₂⁺ ions were higher than the maximum number densities of the N₂⁺ ions by nearly an order of magnitude at moments t₁, t₃, and t₄. At moments t₁ to t₄, the minimum number density of the O₂⁺ ions (3.09 × 10⁷ m⁻³, 3.13 × 10⁷ m⁻³, 3.1 × 10⁷ m⁻³, and 3.51 × 10⁷ m⁻³) was similarly higher than the minimum number density of N₂⁺ ions (2.78 × 10⁷ m⁻³, 2.78 × 10⁷ m⁻³, 2.76 × 10⁷ m⁻³, and 2.76 × 10⁷ m⁻³), respectively. Although the maximum number densities of the O₂⁺ ions and N₂⁺ ions were in the same order of magnitude at moment t₂, the overall analysis shows that O₂⁺ ions were the more numerous positive ions.

By analyzing and comparing the maximum number densities of the O₂⁻ ions, it can be seen that the maximum number densities of O₂⁻ ions were 1.79 × 10¹⁷ m⁻³, 1.82 × 10¹⁸ m⁻³, 4.93 × 10¹⁷ m⁻³, and 2.26 × 10¹⁷ m⁻³ at moments t₁ to t₄, respectively. With time, the number density of the O₂⁻ ions reached a maximum at the peak current moment t₂ then gradually decreased from t₃ to t₄. However, the maximum number density of the O⁻ ions remained almost unchanged. At each moment, the maximum number density of O₂⁻ ions was much higher than that of the O⁻ ions, while the cloud-graph distribution of O₂⁻ ions was more obvious. Therefore, O₂⁻ ions were the main negative ions.

3.4. Generation Pattern of ROS

3.4.1. Generation Pattern of Ozone (O₃)

Ozone (O₃) has strong oxidizing properties, and the use of ozone to treat seeds before planting crops can improve seed germination and kill pathogenic microorganisms [26]. The production of ozone in air discharge is mainly provided by three-body collision reactions (O + O₂ + O₂ => O₃ + O₂, O + O₂ + N₂ => O₃ + N₂), while the production of O plays a major effect on the production of ozone in air, and O is mainly provided by the reactions (e + O₂ => O + O⁻, e + O₂⁺ => 2O, O⁻ + O₂⁺ => O + O₂, O₂ + N => NO + O). The leftmost side of the complete needle-plate electrode boundary was used as the 0 start point, and since most of the biological objects treated based on the needle-plate structure were placed above the plate electrode, the distribution of particles was viewed in this paper at a level of 1 mm from the grounded electrode plate. As shown in Figure 5, the density of the O on the 1 mm surface of the plate electrode from t₁ to t₄ was gradually decreasing, while the number density of the O₃ was the opposite. At the same time, the number density of the O₃ above the plate electrode can be approximated as uniformly distributed at the same instant except for the area below the needle tip. Figure 6 shows a graph of the average molar concentration of the O₃ over time in the entire discharge area, which indicates that the concentration of O₃ showed a cumulative upward trend as time passed.

3.4.2. Generation Pattern of Hydrogen Peroxide (H₂O₂)

Exogenous active oxygen ROS (H₂O₂) can act as a signaling molecule to influence biological effects. In this paper, a simplified dry-air model was used; however, H₂O₂ is mainly generated by the combination of OH with OH, which is considered to be directly decomposed mainly by the electron bombardment of H₂O [36]. Therefore, considering the presence of water molecules in the actual air, this paper characterizes the H₂O₂ generation pattern in terms of the electron generation pattern.

As shown in Figure 4, the electron density of the cathode-sheath layer near the needle tip was nearly zero at the moments from t₁ to t₄, due to the repulsion of the cathode. The electron collapse started at the cathode of the needle tip and continued to diffuse and migrate to the anode plate, and consequently, H₂O₂ was also continuously produced. As shown in Figure 7, the dashed lines represent the local plots within this range. The variation pattern of the electron density was the same as that of the current pulse, and the electron density was maximum at the moment of the rising edge t₂, which gradually decreased with the development of the current pulse, but the electron density was greater than 0 throughout the period from t₁ to t₄. Thus, the concentration of H₂O₂ shows the same trend of cumulative increase with time and can be approximated as a uniform distribution of H₂O₂ above the plate electrode at the same instant except for the area below the needle tip.

3.5. Generation Pattern of RNS

Exogenous active nitrogen RNS (NO) can break seed dormancy and promote germination and growth of biological seeds [24,28]. Figure 8 shows the graph of the average molar concentration of NO over time in the whole discharge area, which reveals that the concentration of NO tended to increase cumulatively with time. As shown in

Table 2. Density of horizontal NO and N.

Time	NO (1/m3)	N (1/m3)
t1	2.4514 × 1010	2.4514 × 1010
t2	2.4515 × 1010	2.4515 × 1010
t3	2.4517 × 1010	2.4517 × 1010
t4	2.4520 × 1010	2.4520 × 1010

, NO was mainly produced by the reactions (O₃ + N => NO + O₂, O₂ + N => NO + O) and was approximately uniformly distributed horizontally at the same moment. The N played a primary part in the production of NO [37]. Under negative DC corona discharge, N was mainly supplied by the reaction (e + N₂⁺ => 2N); meanwhile, it accumulated with time and was uniformly distributed above the plate electrode.

4. Conclusions

In this paper, a qualitatively validated air–chemical reaction system was used to simulate the DC negative corona discharge characteristics of needle-plate electrodes as well as the generation and distribution of active nitrogen oxides (RONS) capable of biologically promoting plant seeds.

The conclusions are as follows:

(1)

During a Trichel pulse period, the rising and falling edges of the current correspond to the development of positive ions (N₂⁺ and O₂⁺⁾ in the cathode-sheath layer, respectively. Negative ions (O⁻ and O₂⁻) move away from the cathode and diffuse continuously with the development of the electron collapse and the electric-field force, showing a typical layering phenomenon.

(2)

Active oxygen ROS (O₃) is horizontally above the plate electrode except for the area below the needle tip, where its density is approximately uniformly distributed at the same instant, and the concentration of O₃ tends to increase cumulatively with time.

(3)

The variation pattern of active oxygen ROS (H₂O₂) is the same as that of the current pulse, with the maximum density at the peak moment along the rise of the current pulse and a decrease with the development of the current pulse. The number density of H₂O₂ is greater than 0 throughout the pulse period. Thus, the concentration of H₂O₂ shows the same trend of cumulative increase with time and can be approximated as a uniform distribution of H₂O₂ above the grounding electrode plate at the same moment except for the area below the needle tip.

(4)

The concentration of active nitrogen RNS (NO) increases cumulatively with time throughout the pulse period and is approximately uniformly distributed at the same moment in the levels.