## 3 Results and Discussion  
### Electrical and Spectral Characteristics  
The waveforms of applied voltage and discharge current that explain the time evolution characteristics of nebulized discharge are shown in Figure 2. The result is obtained under the conditions of a 2.1 L/min Ar flow rate, a 2 mm discharge gap, and a 6.2 kV applied voltage. In the condition of atmospheric pressure Ar, when the high voltage applied on the positive electrode exceeds the breakdown voltage, the gas between the electrodes is broken down and stable discharge plasma is produced in the AC power. When the discharge current is present, it shows that the gas is broken down and the conductivity is increased in the discharge gap, so the application voltage drops significantly from about 3.26 kV to 1.87 kV. When applied voltage is 6.2 kV, the current duration of nebulized discharge is almost 27 us, and with the existence of 3 k\(\Omega\) ballast resistor, and the current reaches a peak of 0.1 A. The image of the nebulization plasma reactor is shown in Figure 3a. A larger view of the discharge region in Figure 3a is shown in Figure 3b. In Figure 3b, the image of nebulized discharge is captured using a digital camera (a Cannon 5D mark IV) with an Ar flow rate of 1.68 L/min, a discharge gap of 2 mm, and an applied voltage of 5.6 kV. It is obvious that the shape of the discharge region is fan-shaped. Because of the action of Ar airflow, the discharge does not occur between the axial directions of the electrodes, but forms a fan-shaped discharge. In addition, the maximum discharge luminescence intensity appears above the plasma on either side and has uniform distribution of luminescence intensity. In nebulized discharge, the formed fan-shaped discharge region has a larger particle contact area than the normal needle discharge, and the aerosol stays in the discharge region for a longer time, which makes it easier to detect heavy metal. Figure 2: Waveforms of applied voltage and discharge current. (Ar flow rate: 2.1 L/min, discharge gap: 2 mm, applied voltage: 6.2 kV). The OES in the range of 300-420 nm containing 1 mg/L Cu\({}^{2+}\) is shown in Figure 4, which is mainly composed of the atomic spectral lines of Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm), the bands of OH (A\({}^{2}\)\(\sum\)-X\({}^{2}\)T) and N\({}_{2}\) (C\({}^{3}\)T\({}_{\rm{lu}}\)-B\({}^{3}\)T\({}_{\rm{lg}}\)). Under the action of the high-temperature and high-energy plasma, the atoms and molecules of the discharge region are collided, excited, and ionized to generate excited atoms, excited molecules, and free radicals, etc., and, eventually, OES is generated [24]. Among them, the generation of Cu atoms includes two main processes which are the atomization process of positive Cu ions and the excitation of Cu atoms [13]. The Cu element mainly exists in the solution composed of positive Cu ions and there are amounts of positive Cu ions in the discharge region. Atomization of Cu is mainly through the recombination reaction of electrons and positive Cu ions under a high gas temperature, as the following process. \[2\rm{e}^{-}+Cu^{2+}\rightarrow Cu \tag{1}\]  
In addition, the inelastic collision occurring between positive Cu ions and hydrogen atoms is also an important pathway for the formation of Cu atoms, as the following processes. \[2\rm{H}+Cu^{2+}\rightarrow Cu+2H^{+} \tag{2}\]  
Figure 4: Typical spectrum of 1 mg/L Cu\({}^{2+}\) in the range of 300–420 nm (applied voltage: 6.2 kV, Ar flow rate: 2.1 L/min, no organic matter and nitric acid added). Figure 3: (**a**) Nebulization plasma reactor; (**b**) image of nebulized discharge. (Ar flow rate: 1.68 L/min, discharge gap: 2 mm, applied voltage: 5.6 kV). In the nebulized discharge, H atoms are released, because vaporized H\({}_{2}\)O molecules are dissociated by electron impact. \[\mathrm{H_{2}O+e^{-}\to OH+H+e^{-}} \tag{3}\]  
On the other hand, the main processes of copper atoms' excitation are shown as the following processes.