A method for improving NOx conversion efficiency of a lean exhaust gas aftertreatment device by determining an accurate amount of reductant required is presented. The method includes calculating a base reductant injection amount based on a steady state amount of NOx in the engine feedgas and adjusting the base amount to compensate for transient NOx emissions. The method further teaches using an air assist heated reductant delivery system to inject the adjusted base reductant amount into the device, thus further improving NOx conversion efficiency of the device.