Is fertilization the dominant source of ammonia in the urban atmosphere?

It was previously believed that ammonia (NH₃) has a short residence time in the atmosphere and cannot be transported far from its sources. In late March, however, this study observed a severe NH₃ episode in urban Beijing when fertilizer was intensively applied on the North China Plain, with the highest hourly concentrations of 66.9 μg m^-3 throughout the year. The stable nitrogen isotopic composition of NH₃ (δ¹⁵N-NH₃) during this episode (-37.0 to -20.0‰) fell in the range of endmembers of fertilizer and livestock, suggesting the long-range transport of NH₃ from agricultural to urban regions. Based on a Bayesian isotope mixing model, the contribution of agriculture (fertilization) to urban NH₃ concentrations was apportioned as 43.5% (26.0%) on polluted days. However, these contributions were reduced to 29.1% (12.8%) when nitrogen isotope fractionation between NH₃ and ammonium was considered. In contrast to the limited contribution of agricultural sources, we found that nonagricultural emissions, particularly vehicles, dominate the source of NH₃ in urban Beijing, even during the fertilization period. This finding indicated that nonagricultural sources should be considered when designing a control strategy for NH₃ to reduce haze pollution in the urban atmosphere.