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Understanding gas-phase reactions in model gas mixtures approximating pre-turbine heavy-duty natural gas engine exhaust compositions containing NO, NH3, NO2, CH4, CO, and C(2)H(4)is extremely relevant for aftertreatment procedure and catalyst design and is thus addressed in this work. In a plug-flow reactor at atmospheric pressure, five different model gas mixtures were investigated in the temperature range of 700-1 200 K, using species analysis with electron ionization molecular-beam mass spectrometry. The mixtures were designed to reveal influences of individual components by adding NO2, CH4, CO, and C(2)H(4)sequentially to a highly argon-diluted NO/NH(3)base mixture. Effects of all components on the reactivity for NO(x)conversion were investigated both experimentally as well as by comparison with three selected kinetic models. Main results show a significantly increased reactivity upon NO(2)and hydrocarbon addition with lowered NO conversion temperatures by up to 200 K. Methane was seen to be of dominant influence in the carbon-containing mixtures regarding interactions between the carbon and nitrogen chemistry as well as formaldehyde formation. The three tested mechanisms were capable to overall represent the reaction behavior satisfactorily. On this basis, it can be stated that significant gas-phase reactivity was observed among typical constituents of pre-turbine natural gas engine exhaust at moderate temperature.