Plasma diagnostics in air plasmas containing water droplets

Atmospheric pressure plasmas are emerging as an effective means to produce reactive nitrogen species (RNS) in water with potential applications in irrigation with plasma-treated water for enhanced agriculture. Indeed, roles of RNS, mainly nitric oxide (NO), nitrite (NO2 ) and nitrate (NO3 ) in plant biology have been well studied [1]. The acidity and RNS concentration in treated water have been shown to be correlated. NO2 conversion to NO3 through the disproportionation/degradation of HNO2 under acidic conditions in a liquid [2] provides a way to quantify RNS concentration via simple pH measurement of the treated water. We conducted experiments on air plasmas containing water droplets produced by a liquid atomizer. Species in the plasma were identified and quantified using optical emission spectroscopy while RNS concentrations in the treated water were determined using pH measurements. Large surface area of the droplets enhances surface reactions, and the plasma treated droplets capture RNS inside. Thus, a tandem atomizer-plasma arrangement can help deliver water with high RNS concentration to targets such as plants and soil. The reactor configuration is shown in Figure 1. A liquid atomizer made of stainless steel acts as the cathode and is positioned at the top of the reactor. The atomizer nozzle is made of Teflon. The anode is a tungsten ring embedded in the Teflon. The air plasma is generated between the two electrodes by powering the anode with a high voltage AC power supply. Air at high velocities (20 slm through a 3 mm nozzle) is fed from the top and merges with deionized water creating water droplets ranging 20-60 m. These water droplets are injected into and interact with the plasma. The evolution of pH as a function of treatment time is shown in Fig. 2. The pH strongly depends on input power with pH values as low as 3.2 being produced. Based on these measurements we estimate that the plasma treated water contains as high as 2 mM of NO2 or 10 mM of NO3 . These RNS concentrations are large enough to have useful effects in plant growth [3].