An exhaust aftertreatment system for purifying an exhaust gas feedstream expelled from an internal combustion engine that is operable at an air/fuel ratio that is lean of stoichiometry is described. The exhaust aftertreatment system includes a plasma reactor disposed upstream of a selective catalytic reactor device. The plasma reactor is electrically connected to a plasma controller. The plasma controller controls the plasma reactor to generate ozone from constituents of the exhaust gas feedstream, and the ozone reacts to oxidize nitrogen oxide contained in the exhaust gas feedstream to form nitrogen dioxide. The nitrogen dioxide reacts with a reductant in the selective catalytic reactor device to form elemental nitrogen and water.