A method of adjusting reductant injection for a selective catalyst reduction device with a soot filter (SCRF) includes calculating, through a processor, an amount of soot in the SCRF, determining, through the processor, a shrinking core model of the SCRF based on the amount of soot, calculating, with the processor using the shrinking core model, an amount of reductant to inject into the SCRF, and injecting the amount of reductant into the SCRF.