## 3 Results
### Optical Emission Spectroscopy  
Optical emission spectrometry was used for active species determination during the fruit surface treatment and for the characterization of the main plasma parameters. All experiments were done at the applied power of 9 W at an argon flow of 7 L/min with an air cooling flow of 4 L/min. The free plasma torch and the torch with the fruit surface placed, as in previous experiments, at its tip 9 mm away from the end of the quartz capillary, were measured with an axial resolution of about 0.5 mm. The intensity profiles of selected active species are shown in Figure 6. The presented intensities were not corrected to the device spectral sensitivity, so all values are relative. The NO radiation increases along the torch axis up to its end (i.e., the highest intensity is observed at the treated surface). Intensities are higher during the fruit treatment. We suppose that this is due to the more turbulent flow along the torch because of flow reflection by the fruit and mainly by the Petri dish used as the fruit holder. Thus, better mixing of the plasma argon flow with the surrounding gas is obtained. This effect is higher during the treatment of smaller fruits because of the shorter distance between the discharge and the Petri dish. The OH radical intensity follows the profile of argon lines intensity (see recently published data in [32]), which is a result of the slow quenching of the surface electromagnetic wave propagating along the plasma torch (for the detailed theory of the surface wave sustained MW discharge, see [34] and the Supplementary Materials of [32]). The enhancement of intensities during the fruit treatment with respect to the free jet can be explained by the combination of two effects. The first is the change in the gas flow (more turbulent, especially during the treatment of small fruits) and, additionally, by the humidity increase due to the presence of fruit and water evaporation. The intensities of the nitrogen molecular ion (and also excited molecular nitrogen, which is not presented graphically here) and atomic oxygen show an increase at distances closer to the quartz capillary end and later follow the general intensity decrease of all other species (except NO). The stronger increase in nitrogen molecular ion intensities during the fruit treatment is rather surprising and its explanation will be a subject of further experimental as well as theoretical studies. The atomic oxygen intensity decrease during the fruit treatment is a result of atomic oxygen consumption by the OH radical formation. A small O intensity enhancement under the fruit treatment point is due to the reflection of the torch radiation by  
Figure 5: The maximal fruit surface temperature during the plasma treatment and after it. Each point is the average value of 10 pieces’ treatment. the fruit surface that is more efficient in the near-infrared spectral region than in the ultraviolet and visible regions. Based on the selected band intensities, the plasma parameters were calculated according to the Boltzmann plot methods (for details of calculation, see [32]). The excitation temperature of argon atoms was about (4400 \(\pm\) 200) K regardless of the fruit treated. The vibrational temperature calculated from the nitrogen second positive system was (3200 \(\pm\) 100) K at the end of the quartz capillary and decreased linearly to (2800 \(\pm\) 100) K at the treated surface regardless of the fruit type. The rotational temperature obtained from the nitrogen second positive system was (2400 \(\pm\) 150) K at the end of the quartz capillary and decreased linearly to (1800 \(\pm\) 150) K at the treated surface. The rotational temperature obtained from the OH radical was (650 \(\pm\) 50) K at the end of the quartz capillary and decreased linearly to (600 \(\pm\) 50) K at the treated surface. Also, the rotational temperature was independent of the type of fruit treated.