Strategic control of combustion-induced ammonia emissions: A key initiative for substantial PM 2.5 reduction in Tianjin, North China Plain

Information on the temporal and spatial variations in the sources of ammonium salts (NH ), a crucial alkaline component in PM , is limited. Here, we simultaneously collected PM and gaseous ammonia (NH ) 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). NH concentrations, the contents of major water-soluble inorganic ions in PM , and the compositions of ammonium‑nitrogen isotopes (δ N-NH ) were measured. As a result, (NH ) SO and NH NO were the predominant forms of NH in PM during summer and winter, respectively. However, the NH NO concentrations were notably greater at TJU (6.2 ± 7.3 μg m ) than at BH (3.8 ± 4.7 μg m ) in summer, with no regional differences observed in winter. Both sites displayed almost half the contribution of c-NH (combustion-related NH ) to NH differing from the finding of previous isotope-based studies. This discrepancy could be attributed to the combined effects of NH isotope fractionation and seasonal δ N value variations in NH sources. The contribution fractions of v-NH (volatile NH ) and c-NH 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-NH was lower at BH than at TJU in summer but greater in winter than at TJU. In summer, NH NO was unstable and limited its delivery to TJU from BH, and the high contribution of c-NH to NH at TJU could be attributed to local vehicle emissions. In winter, the stable particulate NH NO that formed from the c-NH in the upwind area could be transported to the downwind area, increasing the NH concentration at BH. Our study provides valuable insights for devising emission mitigation strategies to alleviate the increasing burden of NH in the local atmosphere.