Identifying the sources of aerosol ammonium is essential because ammonium can impact the Earth’s radiative balance, as well as human health and biological diversity. The sources of ambient ammonia concentrations can be quantified based on the stable isotopic composition of ammonia for various endmembers. However, isotopic source apportionment of aerosol ammonium is challenging in the urban atmosphere, where there is excess ammonia and nitrogen isotopic fractionation commonly occurs.