Fossil fuel-related emissions were the major source of NH pollution in urban cities of northern China in the autumn of 2017.

As the most important gas-phase alkaline species, atmospheric ammonia (NH) contributes considerably to the formation and development of fine-mode particles (PM), which affect air quality and environmental health. Recent satellite-based observations suggest that the North China Plain is the largest agricultural NH emission source in China. However, our isotopic approach shows that the surface NH in the intraregional urban environment of Beijing-Tianjin-Shijiazhuang is contributed primarily by combustion-related processes (i.e., coal combustion, NH slip, and vehicle exhaust). Specifically, the Batch fractionation model was used to describe the partitioning of gaseous NH into particles and to trace the near-ground atmospheric NH sources. With the development of haze pollution, the dynamics of δN-NH were generally consistent with the fractionation model. The simulated initial δN-NH values ranged from -22.6‰ to -2.1‰, suggesting the dominance of combustion-related sources for urban NH. These emission sources contributed significantly (92% on hazy days and 67% on clean days) to the total ambient NH in urban cities, as indicated by a Bayesian mixing model. Based on the Batch fractionation model, we concluded the following: 1) δN-NH can be used to model the evolution of fine-mode aerosols and 2) combustion-related sources dominate the near-ground atmospheric NH in urban cities. These findings highlight the need for regulatory controls on gaseous NH emissions transported from local and surrounding industrial sources.