Abstract: The nitric oxide (NO) and nitrogen dioxide (NO2) content of lean-burn engine exhaust is beneficially prepared for selective catalytic reduction (SCR) of these oxides to nitrogen by two sidestream additions to the exhaust. An ozone-containing air stream is added to the exhaust to affect oxidation of NO to NO2. And a hydrocarbon(s) fuel constituent is added to a second humidified ozone-containing air stream and that mixture subjected to UV radiation. Strongly oxidizing hydroxyl radicals are formed by interaction of ozone, water, and UV radiation for reaction with the hydrocarbon. The resulting partially oxidized hydrocarbons (Pox) are added to the exhaust, providing effective reduction materials for the SCR of NO2 to nitrogen and water.