<p>A novel plasma-assisted electrospray device has been proposed, enabling the generation of charged nanodroplets and the incorporation of chemically active species, e.g., hydroxyl radicals (OH radicals, OH), produced during plasma–liquid interactions at the droplet surface. We particularly investigated the influence of background gas conditions (N₂/O₂ ratio) on the droplet number concentration, droplet size distribution, and the estimation of OH production in droplets by a chemical probe method using disodium terephthalate (NaTA) as the treatment solution. Furthermore, we analyzed changes in the plasma emission spectrum at the Taylor cone tip. The plasma emission and the shape of the Taylor cone varied with O₂ concentration, which significantly affected the droplet number concentration. These changes were evaluated using two stabilization indices that consider the inhibition of electrostatic spraying by discharge and the spray duration, thereby revealing the macroscopic droplet spraying process. When droplets were exposed to the treatment solution, the detected OH amount increased with increasing O₂ concentration. The detected OH is likely contained within the droplets, and despite the droplet transport time being much longer than the OH lifetime, OH was detected, suggesting that OH may have been regenerated within the droplets during their transport. It is likely that OH radicals are mainly generated through the peroxone process, which involves the reaction between O₃ and hydrogen peroxide (H₂O₂). This can be explained by the fact that the O₂ concentration dependence of the gas-phase O₃ concentration shows a similar trend to that of the HTA concentration.</p>