Analysis of conversion mechanisms for NO and N2O in Ar plasma suggests that NO is converted through the reaction Ar+ + NO + e(-) --> Ar + N + O, whereas N2O is converted through the reaction Ar+ + N2O + e(-) --> Ar + N-2 + O. A time-averaged lumped model developed on the basis of this analysis matches the experimental data. CO inhibits N2O conversion but not NO conversion. However, parts-per-million levels of CO affect neither N2O nor NO conversion. Compared to N-2 plasma, which produces a weak streamer glow discharge and a small temperature increase along the reactor, Ar plasma produces a strong streamer discharge and a small temperature decrease along the reactor.