Gaseous ammonia (NH₃) is a key precursor in the formation of PM_2.5 in the atmosphere.To enhance the precision of identifying NH₃ sources through isotopic techniques and alleviate urban air PM_2.5 pollution,this paper reviews recent researches on the following aspects:methods for sampling and quantifying gaseous NH₃,techniques for determining or estimating its nitrogen isotope ratios,the nitrogen isotope compositions and fractionations of NH₃ in air and emitted from diverse sources,the isotopic compositions of NH₄⁺ in atmospheric particulate matter and rainfall,as well as the source apportionment of atmospheric NH₃.Several recommendations for future research are accordingly proposed.The minimum amount of samples should be determined before collecting air samples with passive method considering nitrogen isotope fractionation of NH₃.It is necessary to clarify the nitrogen isotope compositions of ammonia from biomass combustion,natural soils,oceans,sewage plants,vegetation,and other potential sources.The nitrogen isotopic variation mechanism for ammonia from diverse sources needs to be further investigated.Both mass concentrations and isotopic compositions of gaseous NH₃ and particulate NH₄⁺ should be measured simultaneously with higher temporal resolution to explore the mechanism of nitrogen isotope fractionation during the formation of particulate NH₄⁺.Additionally,exploring the isotopic fractionation of gaseous NH₃ under diverse meteorological conditions and air pollution status is recommended.