## 1 Introduction  
Recently, treatment of liquids using various atmospheric pressure plasmas has attracted much attention owing to its wide range of possible applications, such as synthesis of nanocarbons and nanoparticles, sterilization of microorganisms, growth enhancement of plants, plasma farming, cancer therapy, and water purification [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Typically, low-temperature nonequilibrium atmospheric pressure plasma jets (NEAPPJs) or dielectric barrier discharges (DBDs) are used for activation of liquids by plasma [5; 12; 13; 14; 15; 16]. However, in both the case of atmospheric pressure plasma jet (APPI) and DBD, the production rate of reactive oxygen and nitrogen species (RONS), which play key roles in bactericidal effect and plant growth promotion, is low and irradiation of the liquid takes a long time (from several minutes to hours) due to the low density of the plasma [5; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Moreover, in most cases, plasma is generated using pure gases (e.g., argon, helium, or oxygen) at a high voltage, so a substantial amount of electrical energy is required to sustain the discharge and only a limited volume (typically several milliliters) of medium can be treated at one time [4; 7; 19; 20; 21; 22]. The low efficiency, the requirement of pure gases, and the limited volume of liquid that can be treated make plasma-treated water (PTW) expensive and its use in medicine and agriculture impossible. The most promising method for effective and low-cost production of PTW is treating liquids with high-density plasma generated by using air as a process gas [3; 18; 23; 24; 25; 26]. Furthermore, it may be possible to scale up production and reduce the cost of PTW by applying plasmas generated in ambient air to treat a continuous flow of liquid. Several works have reported on treating a liquid flow using air as the process gas, however, the major drawback of these studies is the considerably low production rate of RONS, which limits the wide application of the produced PTW [13; 15; 18; 27; 28].  
Typically, cold nonequilibrium plasmas are used for biomedical and agricultural applications to avoid thermal damage of the treated target [12; 13; 29; 30]. However, in the case of using liquids treated by plasma, a plasma-treated medium (PTM) could be stored after treatment to reduce the temperature and for use after storage considering the presence of the long-living RONS [31; 32]. For that reason, thermal and warm plasmas could be used for irradiation of medium following the cooling of PTM and use in biomedical applications. Considering the high efficiency of arc or gliding arc discharges in the decomposition of CO\({}_{2}\) and other gases and the effective production of RONS, this type of discharge could also be employed for the effective generation and fast delivery of RONS to the target liquid [3; 25; 33; 34]. There are many works on the generation of arc and discharges and other types of plasmas inside or on the surface of highly conductive liquids using various configurations and experimental conditions, however, generation of high-density plasma transferred to the surface of deionized water (DI) or other liquids with low electrical conductivity in ambient air is still challenging owing to the relatively high electric currents required for the stable generation of the discharge [10; 11; 18; 23; 35; 36; 37; 38; 39]. It appears that it is possible to generate a low-current arc in ambient air that can be transferred to the surface of a liquid that has low electrical conductivity by using a high-voltage pin electrode as the anode and the target liquid as the cathode. In this work, we developed a simple and compact generator for direct treatment of liquids using an ambient air low-current arc (AALCA) and analyzed the discharge process and production rate of RONS.