Abstract：
Information on the temporal and spatial variations in the sources of ammonium salts (NH4+), a crucial alkaline component in PM2.5, is limited. Here, we simultaneously collected PM2.5 and gaseous ammonia (NH3) samples in both summer and winter from two sites in Tianjin: an urban site (Tianjin University, TJU) and a suburban site (Binhai New-region, BH). NH3 concentrations, the contents of major water-soluble inorganic ions in PM2.5, and the compositions of ammonium‑nitrogen isotopes (δ15N-NH4+) were measured. As a result, (NH4)2SO4 and NH4NO3 were the predominant forms of NH4+ in PM2.5 during summer and winter, respectively. However, the NH4NO3 concentrations were notably greater at TJU (6.2 ± 7.3 μg m-3) than at BH (3.8 ± 4.7 μg m-3) in summer, with no regional differences observed in winter. Both sites displayed almost half the contribution of c-NH3 (combustion-related NH3) to NH4+, differing from the finding of previous isotope-based studies. This discrepancy could be attributed to the combined effects of NHx isotope fractionation and seasonal δ15N value variations in NH3 sources. The contribution fractions of v-NH3 (volatile NH3) and c-NH3 exhibited similar patterns at both sites seasonally, probably caused by coal combustion for heating in winter and temperature fluctuations. However, the contribution fraction of c-NH3 was lower at BH than at TJU in summer but greater in winter than at TJU. In summer, NH4NO3 was unstable and limited its delivery to TJU from BH, and the high contribution of c-NH3 to NH4+ at TJU could be attributed to local vehicle emissions. In winter, the stable particulate NH4NO3 that formed from the c-NH3 in the upwind area could be transported to the downwind area, increasing the NH4+ concentration at BH. Our study provides valuable insights for devising emission mitigation strategies to alleviate the increasing burden of NH3 in the local atmosphere.
摘要：
有关铵盐（NH4）来源的时间和空间变化的信息，PM2.5中至关重要的碱性成分是有限的。这里,我们在夏季和冬季从天津的两个地点同时收集了PM2.5和气态氨（NH3）样品：一个城市地点（天津大学，TJU）和郊区站点（滨海新区，BH).NH3浓度,测量了PM2.5中主要水溶性无机离子的含量以及铵氮同位素（δ15N-NH4）的组成。因此,（NH4）2SO4和NH4NO3是夏季和冬季PM2.5中NH4的主要形式，分别。然而,夏季，TJU（6.2±7.3μgm-3）的NH4NO3浓度明显高于BH（3.8±4.7μgm-3），冬季没有地区差异。这两个站点显示c-NH3（与燃烧相关的NH3）对NH4的贡献的近一半，与以前基于同位素的研究的发现不同。这种差异可能归因于NHx同位素分馏和NH3源的季节性δ15N值变化的综合影响。v-NH3（挥发性NH3）和c-NH3的贡献分数在两个地点的季节性表现出相似的模式，可能是由于冬季燃煤取暖和温度波动造成的。然而,夏季，BH的c-NH3贡献分数低于TJU，而冬季则大于TJU。在夏天,NH4NO3不稳定，限制了从BH向TJU的输送，TJU中c-NH3对NH4的高贡献可归因于当地车辆排放。在冬天,由顺风区的c-NH3形成的稳定颗粒NH4NO3可以被输送到顺风区，增加BH的NH4+浓度。我们的研究为制定减排策略以减轻当地大气中NH3负担的增加提供了宝贵的见解。
