A method of controlling ammonia and nitric oxides production for selective catalytic reduction systems is provided. The method may include producing an ammonia-containing gas stream having an ammonia concentration. The method may further include operating an engine to continuously produce a NOx-containing gas stream having a NOx concentration. The ammonia-containing gas stream may be supplied to a first exhaust system location upstream of a selective catalytic reduction catalyst, and the ammonia concentration may be determined at a first time. The NOx-containing gas stream may be supplied to a second exhaust system location upstream of the selective catalytic reduction catalyst, and the NOx concentration may be evaluated at a second time which is later than the first time. The NOx concentration may be adjusted based on the ammonia concentration at the first time.