Various embodiments of an apparatus, system, and method are disclosed for reducing NOx emissions using ammonia storage on an SCR catalyst. For example, according to one representative embodiment, an apparatus for reducing NOx emissions in an engine exhaust includes a NOx reduction target module that is configured to determine a NOx reduction requirement. The apparatus also includes an ammonia storage module that is configured to determine an ammonia storage modifier. An ammonia target module of the apparatus is configured to determine an ammonia addition requirement. The ammonia target module is communicable in data receiving communication with the ammonia storage module to receive the ammonia storage modifier, which is added to the ammonia addition requirement. The apparatus also includes a reductant target module that is configured to determine a reductant injection requirement that includes an amount of reductant added to the exhaust gas stream to achieve the ammonia addition requirement.