## 3 Results and Discussion
### Electrical and Spectral Characteristics
\[\mathrm{e^{-}+Cu}\mathrm{\rightarrow Cu\ I(3d^{10}4p)+e^{-}} \tag{4}\]  
\[\mathrm{Cu\ I(3d^{10}4p)\rightarrow Cu\ I(3d^{10}4s)+hv} \tag{5}\]  
So, the metal atoms are excited to the excitation state through the electron impact excitation and thermal excitation. Abundant excited metal atoms result in a strong emission intensity of Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm), which indicates that the Cu element can be determined by OES. Based on previous research, plasma gas temperature plays an important role in the detection of heavy metal elements, which will influence the emission intensities of metal elements [25]. In general, the gas temperature can be approximately considered as equal to the rotational temperature of N\({}_{2}\) (C\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\)) at atmospheric plasma. Because the energy gap between rotational levels of molecules is very small, equilibrium can be easily achieved between translational motion and rotational motion for the frequent collisions with heavy particles at atmospheric pressure. Furthermore, the rotational temperature of N\({}_{2}\) (C\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\)) is a reliable method for estimating the gas temperature [26], depending on the fast rotational relaxation and the frequent collisions between heavy particles. The rotational temperature of N\({}_{2}\) (C\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\)) can be acquired by _\"Specair\"_ code [27]. Using _\"Specair\"_, when the rotational and vibration temperature are inputted, a simulated spectrum of N\({}_{2}\) (C\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\)-B\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\), \(\Delta\)\(v\) = \(-\)2) is calculated. Then, the simulated spectrum is compared repeatedly with the experiment until the most fitting one is found, as shown in Figure 5. The result is obtained under the conditions of 5.6 kV applied voltage, 1.68 L/min Ar flow rate, pH = 3.59 and 2% (V/V) ethanol. It is clearly shown that the gas temperature is about 2190 K, and the vibrational temperature of N\({}_{2}\) (C\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\)) can also be acquired, which is about 3165 K. In the nebulized discharge, it is obvious that a very high gas temperature is obtained, which can promote the processes of reaction (1-4), thus obtaining a higher emission intensity [28]. To understand the temperature evolution of nebulized discharge, the gas temperature distribution of plasma is obtained under the conditions of an Ar flow rate of 1.68 L/min, a discharge gap of 2 mm, and an applied voltage of 5.6 kV. As shown in Figure 6a, the area in white dash line is the region for measuring plasma gas temperature, and the corresponding gas temperatures at the respective positions are displayed in Figure 6b. At 0 mm, the local maximum of plasma gas temperature is acquired. With the increase of horizontal position,  
Figure 2:0 Experiment and simulation N\({}_{2}\) (C\({}^{3}\)\(\mathrm{\uparrow}\)\(\mathrm{\downarrow}\)) temperature (applied voltage: 5.6 kV, Ar flow rate: 1.68 L/min, pH = 3.59, 2% (V/V) ethanol). the gas temperature of the plasma first decreases to a certain degree and then increases. In addition, at about 1.7 mm, another absolute maximum of plasma gas temperature is acquired. The symmetrical gas temperature distribution is mainly due to needle-needle electrode structure as shown in Figure 2:1 Besides, comparing with Figures 6a,b, it is found that the saddle-shaped gas temperature distribution is approximate to the luminous intensity distribution in the dash line region.