Combustion Sources Dominated Ammonia Concentrations During Severe Air Pollution Episodes During "Two Sessions" 2025 in Beijing.

Abstract

This study conducted online observations of atmospheric ammonia (NH₃) concentrations from 20 February to 5 March, 2025, during which the "Two Sessions" were held in Beijing and emission control measures were typically implemented in surrounding regions. An unexpected heavy pollution episode occurred during this period, with NH₃ concentrations reaching 59.7 ppb, significantly higher than those observed on clean days (17.8 ± 4.5 ppb). During the development of the pollution episode, NH₃ concentrations exhibited synchronous variations with PM_2.5, indicating that NH₃ played a crucial role in the formation of secondary inorganic aerosols. Additionally, NH₃ concentrations displayed a distinct morning peak, primarily attributed to temperature-driven natural emissions, traffic emissions during morning rush hour, and re-mixing of residual layers. NH₃ was significantly correlated with SO₂ and CO, suggesting contributions from fossil-fuel combustion. Furthermore, the overlap of concentration-weighted trajectory analysis and MODIS fire-point data indicated that biomass burning in south-eastern Beijing likely contributed additional NH₃ emissions. Overall, this study highlights the importance of reducing combustion-related NH₃ sources to mitigate heavy air pollution and improve air quality through cross-regional collaborative control measures.