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Stable nitrogen isotope composition (<i>δ</i>¹⁵N) offers new opportunities to address the long-standing and ongoing controversy regarding the origins of ambient ammonia (NH₃), a vital precursor of PM_2.5 (particulate matters with aerodynamic diameter equal or less than 2.5 µm) inorganic components, in the urban atmosphere. In this study, the <i>δ</i>¹⁵N values of NH₃ samples collected from various sources were constrained using a novel and robust chemical method coupled with standard elemental analysis procedures. Independent of the wide variation in mass concentrations (ranging from 33 (vehicle) to over 6000 (human excreta) µg m⁻³), different NH₃ sources have generally different <i>δ</i>¹⁵N values (ranging from −52.0 to −9.6 ‰). Significantly high <i>δ</i>¹⁵N values are seen as a characteristic feature of all vehicle-derived NH₃ samples (−14.2 ± 2.8 ‰), which can be distinguished from other sources emitted at environmental temperature (−29.1 ± 1.7, −37.8 ± 3.6, and −50.0 ± 1.8 ‰ for livestock, waste, and fertilizer, respectively). <br><br> The isotope <i>δ</i>¹⁵N signatures for a range of NH₃ emission sources were used to evaluate the contributions of the different sources within measured ambient NH₃ in Beijing, using an isotope mixing model (IsoSource). The method was used to quantify the sources of ambient NH₃ before, during and after the 2014 Asia-Pacific Economic Cooperation (APEC) summit, when a set of stringent air quality control measures were implemented. Results show that the average NH₃ concentrations (the overall contributions of traffic, waste, livestock, and fertilizer) during the three periods were 9.1 (20.3, 28.3, 23.6, and 27.7 %), 7.3 (8.8, 24.9, 14.3, and 52.0 %), and 12.7 (29.4, 23.6, 31.7, and 15.4 %) µg m⁻³, respectively, representing a 20.0 % decrease first and then a 74.5 % increase in overall NH₃ mass concentrations. During (after) the summit, the contributions of traffic, waste, livestock, and fertilizer decreased (increased) by 56.7 (234.2), 12.0 (−5.0), 39.4 (120.8), and −87.7 % (−70.5 %) when compared with periods before (during) the summit, respectively, signifying that future NH₃ control efforts in megacities like Beijing should prioritize traffic sector as well as livestock breeding. The results show that isotope ratio measurements of NH₃ to be a valuable tool to quantify the atmospheric sources of NH₃ in urban atmospheres.