氨气（NH3）对大气二次颗粒物的形成起着至关重要的作用。但不同排放源对大气中NH3的定量贡献仍不确定。大气NH3和细颗粒物（PM2.5）中铵根（NH4+） 的稳定氮同位素组成（即δ15N(NH3)和δ15N(NH4+)）可以提供有关其来源和气粒转化过程的有用信息。因此，研究大气中NH3和NH4+的氮同位素组成和来源，对于了解二次颗粒物的形成和制定大气污染控制政策具有重要意义。本研究测定了典型农业源（牲畜养殖和化肥施用）和非农业源（城市交通和燃煤电厂的NH3逃逸）的δ15N(NH3)源特征。在北京城区不同季节平行采集和分析了大气NH4+和NH3的氮同位素组成，研究了氨气固转化过程中的氮同位素分馏、NH4+分配比和气溶胶酸度与δ15N(NH4+)的关系。基于城市环境中NH4+和NH3的氮同位素组成、NH3的稳定同位素源谱、利用同位素混合模式和化学与传输模型（CTM）全面解析了城市环境中NH4+和NH3的来源。本研究发现，农业和化石燃料燃烧NH3排放的氮同位素源特征明显，可以支持基于氮同位素的NH4+和NH3来源解析。然而，在进行源解析之前，应对被动采样法测定的δ15N(NH3)进行校正，以减少被动采样器的偏差对非农业源贡献的影响。本研究表明，与清洁日（PM2.5<35μg m-3）相比，夏季NH4+分配比(f)为0.2-0.4和冬季轻度污染日（35μg m-3

Ambient ammonia (NH3) plays a vital role in secondary particle formation in the atmosphere. Quantitative contributions of different emission sources to ambient NH3 concentrations remain uncertain. Stable nitrogen isotopic composition (δ15N) of NH3 and aerosol ammonium (NH4+) ((δ15N(NH3) and δ15N(NH4+), respectively) in PM2.5 (particulate matter with an aerodynamic equivalent diameter below 2.5 μm) can yield valuable information about its sources and the gas-to-particle conversion processes. Therefore, it is of great significance to study the nitrogen isotopic composition and source contribution of NH3 and NH4+ in the atmosphere for understanding the formation of secondary particles and formulating effective air pollution control policies.In this study, δ15N(NH3) source signature for typical agricultural sources (livestock waste and fertilizer use) and non-agricultural fossil fuel-related sources (such as urban traffic and NH3 slip from coal-fired power plants) were measured. NH3 and NH4+ were collected in parallel during different seasons in an urban site of Beijing to analyze their nitrogen isotopic composition. The measured δ15N(NH4+) values were investigated in relation to nitrogen isotopic fractionation during NH3 gas-to-particle conversion, NH4+ partitioning ratio and aerosol acidity. Then a comprehensive source apportionment of NH4+ and NH3 was conducted by integrating the nitrogen isotopic composition of NH4+ and ambient NH3, NH3 source signatures, isotope mixing model, and chemistry and transport model (CTM).This study found distinct nitrogen isotope source signatures of agricultural and fossil-fuel combustation related sources collected in this study, indicating that source signatures characterized in this study can support isotope-based source apportionment of NH4+ and NH3. However, this study highlights the need for correction of nitrogen isotopic compositions characterized with passive samplers before performing isotope-based source apportionment of NH3 and NH4+ to reduce the effect of bias in passive samplers in estimated non-agricultural source contributions. This study also found that overall nitrogen isotopic compositions of particulate NH4+ in PM2.5 samples were enriched during slightly polluted days (35 μg m-3 < PM2.5 < 75 μg m-3) in summer and winter compared to clean days (PM2.5 < 35 μg m-3), which was explained by the difference in NH4+ partitioning, particle pH, and isotopic fractionation. Decreasing particle acidity due to thermodynamic equilibrium likely explains the enriched nitrogen isotopic compositions in NH4+ due to overall lower isotopic fractionation when NH4+ partitioning was higher during slightly polluted days in summer. In contrast, the increasing particle acidity due to thermodynamic equilibrium likely explains the enriched nitrogen isotopic compositions in NH4+ due to increasing isotopic fractionation when NH4+ partitioning was dynamic during slightly polluted days in winter.During summer, non-agricultural sources contributed 57.2% to ambient NH4+ and 64.8% to ambient NH3 that were derived from source apportionment of weighted δ15N(NH4+) values and from δ15N(NH3) (corrected) values, respectively. During winter, non-agricultural sources contributed 70.3% to NH4+ and 72.6% to NH3 that were derived from weighted δ15N(NH4+) values and from δ15N(NH3) (corrected) values, respectively. CTM simulations suggested that non-agricultural sources contributed on average 68% and 68.2% to ambient NH4+ and NH3 during summer, respectively. While in winter, CTM simulations suggested that non-agricultural sources contributed on average 66.2% and 66.4% to ambient NH4+ and NH3, respectively. Both the isotope-based receptor model and CTM suggest that non-agricultural sources are main contributors to atmospheric NH3 and particulate NH4+ in urban Beijing. However, the contribution of agricultural emissions to atmospheric NH3 and particulate NH4+ cannot be ignored.