## 3 Results and Discussion
### Generation of the Discharge
With the increase of the conductivity of treated DIwater, occasional pulsed arc discharges appeared and finally, after 17-20 s of treatment, the discharge changed from repeating sparks to the stable AALCA. Figure 2: I & V waveforms (**a**) and photo (**b**) of corona discharge, I and V waveforms (**c**) and photo (**d**) of spark discharges at 9.5 W of input power, I and V waveforms (**e**) and photo (**f**) of spark discharges at 23 W of input power, and I & V waveforms (**g**) and photo (**h**) of ambient air low-current arc (AALCA) at 25 W of input power. Figure 3 shows the conductivity of 5 mL of DI water as a function of the duration of the treatment by AALCA. The conductivity of the water sample increased from about 0.1 \(\upmu\)S/cm for pristine DI water to 190 \(\upmu\)S/cm after treatment for 20 s by spark discharges, which enabled the generation of a low-current arc that was transferred to the surface of the DI water, which initially had low conductivity. Moreover, the pH value gradually decreased from 6.76 for pristine DI water to 2.89 after treatment for 2 min, as shown in Figure 4, which is a considerably low value as compared with the PTW reported elsewhere [18; 24; 31; 43]. The low pH value suggests a high concentration of RONS and possibly some ions (delivered from the electrodes by erosion) in the treated liquid, which could explain the increase of conductivity after the plasma treatment. The presented approach allows generation of a stable AALCA discharge that is transferred to the surface of a liquid, even if it has initially low electrical conductivity (such as DI water), without preparation and modification of the liquid. Variation of conductivity affected only the duration of the state of the repeating spark discharges (17-19 s for DI water with an initial conductivity of about 0.1 \(\upmu\)S/cm and 0-2 s for tap water with an initial conductivity in the range of 170-250 \(\upmu\)S/cm). Figure 4: pH value of DI and tap water as a function of the duration of plasma treatment. Figure 3: Conductivity of deionized water (DI) water as a function of the duration of plasma treatment. The main drawback of using AALCA is the heating and evaporation of the treated liquid, which is used as the cathode and interacts with high-energy ions and other charged species. Thermographic observations showed that the temperature of the 5 mL DI water sample treated by AALCA did not exceed 45 \({}^{\circ}\)C (see Figure 5a) after 1 min of treatment and 70 \({}^{\circ}\)C (see Figure 5b) after 5 min of treatment. The relatively low temperature of the small volume of liquid treated with AALCA could be explained by local evaporation of the liquid. During treatment, the liquid was heated to the boiling temperature locally and evaporated when a bulk liquid was heated at a lower rate and remained below the boiling temperature. The higher temperature of the liquid in the area of contact with the plasma can be observed from thermographic images (Figure 5a,b). It was confirmed that 1.2 mL of water was evaporated from the original 5 mL sample after 5 min of treatment. To ensure stable generation of AALCA even after partial evaporation of the liquid and change in the length of the discharge gap, 28 W of input power was used in all the experiments described below.