## 3 Results and Discussion
### Nitrogen Fixation Process
#### 3.2.2 Liquid Phase Analysis  
Based on [PERSON]'s law, the gas phase NO\({}_{\rm x}\) products can penetrate into the aqueous phase, interact with H\({}_{2}\)O, and form NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) as follows:  
Reaction (7):  
\[2{\rm NO}_{2}+{\rm H}_{2}{\rm O}\rightarrow{\rm NO}_{2}{}^{-}+{\rm NO}_{3}{}^{-}+ 2{\rm H}^{+}\]  
Reaction (8):  
\[{\rm NO}+{\rm NO}_{2}+{\rm H}_{2}{\rm O}\rightarrow 2{\rm NO}_{2}{}^{-}+2{\rm H}^{+}\]  
Reaction (9):  
\[2{\rm NO}+{\rm O}_{2}+{\rm H}_{2}{\rm O}\rightarrow 2{\rm NO}_{2}{}^{-}+2{\rm H}^{+}\]  
Reaction (10):  
\[{\rm NO}+{\rm NO}_{2}+{\rm H}_{2}{\rm O}\rightarrow 2{\rm NO}_{2}{}^{-}+2{\rm H}^{+}\]  
Consequently, the combined concentrations of NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) in the liquid phase correspond to the total amount of NO\({}_{\rm x}\) converted into HNO\({}_{\rm x}\) species which remain dissolved in the liquid. The NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) concentrations in the liquid phase have been evaluated in this study and the results are presented in Figure 7 as a function of SEI for different applied gas flow rates (the same profiles for all flow rates under study are shown in the SI, Figure S8). We also acknowledge that, according to recent studies, in both plasma-catalytic and catalyst-free systems, air plasma with added water can generate NH\({}_{3}\)[27, 58, 59]. However, the selectivity to NH\({}_{3}\) generation is dramatically lower than to (H)NO\({}_{\rm x}\) generation, typically leading to very small amounts of NH\({}_{3}\). Interestingly, NH\({}_{3}\) production in a humidified air plasma has recently been reported [27]. However, in our case, plasma was discharging onto liquid water, and, furthermore, we did not observe any NH emission by OES. Hence, we believe that NH\({}_{3}\) may be formed in our Me-L system, but most likely only in negligible amounts. Consequently, we believe that evaluating solely (H)NO\({}_{\rm x}\) species as NF products is sufficient for the discussion in the present work. Figure 6: (**a,c**) NO and (**b,d**) NO\({}_{2}\) concentrations as a function of SEI for different gas flow rates for both the Me–Me and Me–L systems. It is also important to mention that in order to effectively utilize these products (NO\({}_{\rm x}\)) initially produced in the gas phase as fertilizer, they must be fully dissolved in a liquid to form NO\({}_{3}-\) as the final valuable chemical. As indicated by the reactions in Table S1, in the presence of liquid H\({}_{2}\)O, NO\({}_{\rm x}\) species undergo transformation resulting in NO\({}_{2}-\) and NO\({}_{3}-\) generation. In the presence of air and sufficient contact time with O\({}_{2}\), all NO\({}_{2}-\) is converted into nitrate NO\({}_{3}-\), which is already a fertilizer. In addition, this work focuses only on the oxidation pathways of nitrogen fixation; however, literature reports suggest that even in air plasmas with H\({}_{2}\)O, some NH\({}_{4}+\) generation occurs [27]. To increase the fraction of NH\({}_{4}+\), some nitrates accumulated in the liquid can be reduced (e.g., electrochemically) into NH\({}_{3}\). This essentially creates a solution of NH\({}_{4}\)NO\({}_{3}\), the most widely used fertilizer [27; 59]. Thus, following this approach, the fertilizer can be produced directly on-site without transportation expenses and with no requirement to separate the fertilizer from the solution. Identical to NO and NO\({}_{2}\) (Figure 6c,d) in the Me-L system, for each of the gas flow rates under study, the concentrations of nitrite and nitrate increase with increasing SEI. The results also reveal that the Me-L system mainly promotes the production of NO\({}_{2}-\), as for each experimental condition its concentration was almost one order of magnitude higher compared to NO\({}_{3}-\). This can be attributed to the fact that H\({}_{2}\)O\({}_{2}\) (the main oxidant converting NO\({}_{2}-\) to NO\({}_{3}-\)[60]) is only formed in very minor amounts.