A plasma reactor for automotive exhaust gas applications which efficiently promotes diffusion, mass transfer and chemical reaction processes of atoms, ions and radicals, in that the ground (outer) electrode has an axially discrete pattern which provides alternating regions of active and passive electric field along the axial direction of the plasma reactor. As the exhaust gas passes axially along the plasma reactor, each active region produces plasma atoms, ions and radicals, which then have time to react with the NO.sub.x over the course of the adjacent passive region. In this manner, successive active regions produce copious atoms, radicals and ions, and the adjacent passive regions provide time for these radicals and ions to react with the NO.sub.x and hydrocarbons before the next active region is encountered by the moving stream of exhaust gas, thereby enhancing the performance of the plasma reactor.