Abstract: Distribution of pollution reducing agents within an effluent at the effective temperature window for effective gas phase reaction is greatly improved. A two-phase mixture having fine droplets of liquid, e.g. a NO.sub.x reducing agent such as aqueous solutions of urea or ammonia, dispersed in a gaseous component, such as air, is injected into the effluent an sonic velocity to achieve a distribution of particles of sizes effective to uniformly reduce NO.sub.x within the zone of effective temperature while not producing a significant level of droplets so large that they survive beyond the temperature zone.