Method and apparatus for controlling reductant injection into an exhaust gas feedstream from an internal combustion engine

An exhaust aftertreatment system includes first and second selective catalytic reduction devices (SCRs) and a single reductant injection system. A total ammonia storage capacity and an ammonia storage level are determined for the first and second SCRs, and determine a total SCR ammonia storage level for the first and second SCRs based upon the ammonia storage level on the first and second SCRs. A first storage error is determined, and a second storage error is determined based upon an ammonia storage level and an ammonia storage capacity for the second SCR. A second reductant dosing rate is determined based upon the second storage error. The reductant injection system injects reductant into the exhaust gas feedstream based upon the second reductant dosing rate when the second storage error indicates an imbalance between the ammonia storage on the first SCR and the ammonia storage on the second SCR.