Ozone, preferably produced in an ambient air stream passed through a non-thermal hyperplasma reactor, is added to the hot exhaust gas stream from a lean-burn (oxygen-rich) combustion source. The ozone converts much of the NO in the NO.sub.x containing exhaust stream to NO.sub.2. The resulting NO.sub.2/NO.sub.x ratio can be controlled by the input energy density to the plasma reactor. Ammonia and/or urea is added to the ozone treated exhaust in proportion to the NO.sub.x content, and the stream passed into contact with a reduction catalyst, such as a base metal-exchanged zeolite, to convert the NO.sub.x to nitrogen.