An aftertreatment system for treating exhaust gas discharged from a combustion engine may include a particulate filter, a reductant-injection system, an ammonia sensor and a control module. The particulate filter may be configured to filter exhaust gas discharged from the combustion engine. The reductant-injection system may be configured to inject a reductant into a stream of the exhaust gas upstream of the particulate filter. The ammonia sensor may be configured to sense a concentration of ammonia in the stream of exhaust gas downstream of the particulate filter. The control module may be in communication with the ammonia sensor and may determine a soot load of the particulate filter based on data received from the ammonia sensor.