Moisture inhibits nonthermal-plasma NOx conversion, for example, NO and N2O conversion in N2. OH radicals produced by the electron impact reaction of H2O play a key role; the existence of OH radicals is observed in the emission spectrum in the humid gas mixtures. N(2D) is found to be responsible for the formation of HNO3 and nitrogen oxides, such as NO, NO2, N2O, and N2O5. The concentration of N2O5 and HNO3 in the plasma reactor is related to NO2 concentration. The charge voltage depends on the feed composition because the electric resistance for the reactor in an OFF state has different values for different components. More than 98% energy stored in the capacitors is delivered into the reactor during electrical discharge. The electron collision reactions with water can be neglected when its concentration is below 1,333 ppm, but must be taken into account when its concentration is above 5,000 ppm.