Strong Impacts of Regional Atmospheric Transport on the Vertical Distribution of Aerosol Ammonium over Beijing

Ammonium (NH₄⁺) is a significant component of fine aerosol particles (PM_2.5), and its behavior in the atmosphere is crucial to air pollution. We present a novel study that analyzes the vertical distribution and temporal trends of NH₄⁺ in the urban boundary layer of Beijing, tracking hourly concentrations throughout a complete haze episode. Our results unveil a surprising single-peak profile of NH₄⁺ at heights of 300–700 m in the urban boundary layer with its hourly concentration reaching ∼50 μg m^–3, which is 3 times higher than that at the ground level, in contrast to the conventional patterns of decreasing concentrations with height. The vertical structure is closely related to the observed escape of ammonia (NH₃) or NH₄⁺ from upwind industrial sources via elevated chimneys. The NH₄⁺ plumes emitted through these sources are prone to transport at an altitude of 270–750 m for approximately 6 h, covering >250 km to Beijing. This study reveals that non-agricultural point emissions of NH₄⁺ impact the vertical patterns of aerosol NH₄⁺ in the urban boundary layer, demonstrating potential opportunities for limiting such emission sources to curb PM_2.5 pollution in the North China Plain.