We presents results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular we have addressed the effect of pulsed operation mode of a planar dielectric barrier discharge. As it was conjectured, the large difference in the time scales involved in the fast dissociation of molecules in plasmas and their subsequent reactions to produce stable chemical species, makes the presence of a continuously repeated plasma production stage unnecessary and a waste of electrical power and efficiency. Results on NOx remediation, ozone production, water vapor and ammonia dissociation are discussed. A few comparisons with experimental findings in a dielectric barrier discharge reactor already used for applications is briefly addressed too. Our results clearly indicate a pattern for the optimization of the discharge using a carefully designed repetition rate and duty-cycle.