## 1 Introduction  
Nitrogen (N\({}_{2}\)), a major constituent (78%) of the Earth's atmosphere [1], is an essential compound for the growth of plants and living organisms [2]. However, these organisms are not able to directly use nitrogen present in air, due to the high stability of the N\(\equiv\)N electronic configuration, which has a high dissociation energy of 9.79 eV. Within this context, nitrogen fixation (NF) is a process that provides chemical pathways to make nitrogen accessible for living organisms by converting N\({}_{2}\) molecules into species consumable by plants, such as NH\({}_{3}\) and (H)NO\({}_{3}\)[3, 4]. The beginning of the 20 th century was marked by a rapid increase in nitrogen-containing fertilizers production via the Haber-Bosch (H-B) process for NH\({}_{3}\) synthesis [5]. The synthesis is carried out at high pressure and temperature in the presence of a catalyst, supplying nitrogen from the ambient air, together with hydrogen and energy from natural gas (CH\({}_{4}\)) steam reforming. Such harsh operational conditions and extreme dependence on CH\({}_{4}\) bring a number of significant disadvantages, namely, greenhouse gases emission (although part of the emitted CO\({}_{2}\) is reacted with the produced NH\({}_{3}\) to yield a fertilizer consisting of a solution of urea and ammonium nitrate in water, also known as UAN) and utilization of 3-5% of the total natural gas output [6]. Finally, H-B is feasible only on a large industrial scale. Considering the firm reliance of the H-B process on CH\({}_{4}\) and the consequent disadvantages, the need to develop and integrate a new alternative approach is of paramount importance [7]. Recently, various methods have been considered to replace (or at least to compete with) the H-B process, such as metal-complex NF, electrocatalysis, photocatalysis, thermal catalysis, and nonthermal plasma (NTP) [8; 9]. Plasma is an ionized gas, consisting of ions, electrons, and noncharged species (photons, molecules, atoms, and radicals) in excited and ground states. Specifically, NTP generates an intensified plasma chemistry due to the enhanced electron temperature (T\({}_{\rm e}\)), while due to its nonequilibrium nature (i.e., the electron temperature and the vibrational temperature are much higher than the rotational temperature), the background gas remains closer to ambient conditions. Although electrical power is required to generate plasma, it can be supplied from renewable energy sources, providing multiple advantages of this method, namely, its compact size, making it ideal for decentralized production, its quick start-up, and its rapid response to changes in the composition of the working gas [7; 10; 11]. Despite the good performance of some low-pressure plasma sources [12; 13], their demand to utilize vacuum systems is considered an evident disadvantage preventing industrial implementation. In contrast, considerable work is focused on developing high-pressure plasma-assisted NF processes (>1 atm) [14]. Due to its simplicity and portability, most attention is, however, paid to NF using various NTP sources operating at atmospheric pressure, including gliding arcs [15; 16], corona discharges [17], plasma jets [18; 19], and dielectric barrier discharges (DBDs) [20]. It must be noted that the H-B process produces NH\({}_{3}\), which on its own is not a fertilizer, but a fertilizer building block, and fertilizers such as UAN and NH\({}_{4}\)NO\({}_{3}\) are most commonly used. Industrially, nitrates are produced via the oxidation of NH\({}_{3}\) via the Ostwald process [21; 22]. In contrast, plasma NF largely focuses on (H)NO\({}_{\rm x}\) production, because it shows the most promising results in terms of energy cost and production rates. The plasma-produced (H)NO\({}_{\rm x}\) can be subsequently (partially) reduced to NH\({}_{3}\) to produce aqueous solutions of NH\({}_{4}\)NO\({}_{3}\)[23; 24].