Non-thermal plasmas can be used for water treatment system because they allow free radical generation without heating. Advanced oxidation processes using non-thermal plasmas have been developed to decompose persistent organic pollutants. In this study, a water treatment method that involves spraying a solution into a coaxial dielectric barrier discharge (DBD) tube using an air, argon or oxygen carrier gas was investigated. The alumina DBD tube had an inner high-voltage electrode and an outer ground electrode. Acetic acid was used as the decomposition target because acetic acid is a known persistent organic material. An acetic acid solution, diluted 10,000 times in purified water, was atomized by an ultrasonic atomizer unit and introduced into the DBD tube. The residence time of the droplets sprayed into the discharge area was almost 7 ms. Acetic acid was effectively decomposed and the decomposition ratio reached almost 80 % when Ar was employed as the carrier gas. This is due to the very fine droplets in the sprayed mist having a large specific surface area and OH radicals being able to react directly in solution. Furthermore, the results suggest that the chemical processes involved in acetic acid decomposition can be controlled by varying the carrier gas composition.