Nitrogen isotope characteristics and source apportionment of atmospheric ammonium in urban cities during a haze event in Northern China Plain

Ammonium (NH₄⁺) is a well-known alkaline species for haze formation and possesses potential impacts on human health and biological diversity. To date, regional source apportionments of NH₄⁺ have been poorly understood in Northern China Plain (NCP). In this study, daily PM_2.5 samples were synchronously collected in eight cities over the NCP region. In addition to water-soluble ions, the nitrogen isotopic composition of NH₄⁺ (δ¹⁵N–NH₄⁺) was also analyzed. The average concentrations of NH₄⁺, a targeted species, varied from 0.9 μg/m³ to 42.5 μg/m³. Note that the significant enhancements of NH₄⁺ coincided with increasing PM_2.5 masses, suggesting that NH₄⁺ was one of the contributing species for haze formation. The δ¹⁵N–NH₄⁺averaged 12.4 ± 6.9‰ and the average δ¹⁵N–NH₃ were further calculated to be −9.0 ± 6.7‰ over the NCP region. Both δ¹⁵N–NH₄⁺ and δ¹⁵N–NH₃ showed negative correlations with PM_2.5 mass and there were no obvious differences among those 8 cities. Combining air-mass backward trajectories and quantifying contributions of potential sources, we pointed out that non-agricultural sources (fossil fuel related and NH₃ slip) were dominant sources (totally contributed 42%–85%) not only on extreme haze phase but also on haze accumulation/dissipation phase in all 8 cities of NCP region in winter, the heating season in North China. However, the contribution of fossil fuel related source reduced to only 20%–35% while livestock breeding source rose to about 15%–32% and NH₃ slip source kept contributing about 30% on the extreme haze phase. Thus, we considered that volatilization-related agricultural sources (especially livestock breeding) could discharge considerable NH₃ to form SIA-led haze event in urban cities of the NCP region, even though non-agricultural sources were the foremost sources.