An exhaust aftertreatment system having a NOX absorber-catalyst and an ammonia-SCR reactor. During regenerations, the NOX absorber-catalyst generates ammonia. The ammonia-SCR reactor captures this ammonia and later uses it to reduce NOX. A Venturi is provided in an exhaust conduit connecting the NOX absorber-catalyst to the ammonia-SCR reactor. The Venturi draws sufficient air to keep the ammonia-SCR reactor under lean conditions throughout the NOX absorber-catalyst regenerations. Maintaining lean conditions for the ammonia-SCR reactor in this manner mitigates poisoning of the ammonia-SCR reactor by hydrocarbons slipping from the NOX absorber-catalyst during the regenerations. Mitigating this poisoning improves the performance of the exhaust aftertreatment system and reduce the loss of useful ammonia to oxidation.