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 4 + ), a crucial alkaline component in PM 2.5, is limited. Here, we simultaneously collected PM 2.5 and gaseous ammonia (NH 3 ) 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 3 concentrations, the contents of major water-soluble inorganic ions in PM 2.5, and the compositions of ammonium‑nitrogen isotopes (δ 15 N-NH 4 + ) were measured. As a result, (NH 4 ) 2 SO 4 and NH 4 NO 3 were the predominant forms of NH 4 + in PM 2.5 during summer and winter, respectively. However, the NH 4 NO 3 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-NH 3 (combustion-related NH 3 ) to NH 4 +, differing from the finding of previous isotope-based studies. This discrepancy could be attributed to the combined effects of NH x isotope fractionation and seasonal δ 15 N value variations in NH 3 sources. The contribution fractions of v-NH 3 (volatile NH 3 ) and c-NH 3 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 3 was lower at BH than at TJU in summer but greater in winter than at TJU. In summer, NH 4 NO 3 was unstable and limited its delivery to TJU from BH, and the high contribution of c-NH 3 to NH 4 + at TJU could be attributed to local vehicle emissions. In winter, the stable particulate NH 4 NO 3 that formed from the c-NH 3 in the upwind area could be transported to the downwind area, increasing the NH 4 + concentration at BH. Our study provides valuable insights for devising emission mitigation strategies to alleviate the increasing burden of NH 3 in the local atmosphere.