The systems and methods described herein beneficially provide an exit gas having a third, relatively low, NO/NO2 ratio that closely approximates the NO/NO2 ratio found in the exhaust of various internal combustion engines. The systems and methods described herein receive a feed gas having a first, relatively high NO/NO2 ratio. The feed gas is apportioned into a first feed gas portion that is passed through a non-thermal plasma generation system to provide an intermediate gas having a second, relatively very low, NO/NO2 ratio and a second feed gas portion having the first NO/NO2 ratio. The intermediate gas and the second feed gas portion are combined to provide the exist gas having the third, relatively low, NO/NO2 ratio. The systems and methods described herein beneficially provide an exit gas having a variable NO/NO2 ratio to simulate exhaust from a variety of internal combustion engines.