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

It was previously believed that ammonia (NH) 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 NH episode in urban Beijing when fertilizer was intensively applied on the North China Plain, with the highest hourly concentrations of 66.9 μg m throughout the year. The stable nitrogen isotopic composition of NH (δN-NH) during this episode (-37.0 to -20.0‰) fell in the range of endmembers of fertilizer and livestock, suggesting the long-range transport of NH from agricultural to urban regions. Based on a Bayesian isotope mixing model, the contribution of agriculture (fertilization) to urban NH 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 NH and ammonium was considered. In contrast to the limited contribution of agricultural sources, we found that nonagricultural emissions, particularly vehicles, dominate the source of NH in urban Beijing, even during the fertilization period. This finding indicated that nonagricultural sources should be considered when designing a control strategy for NH to reduce haze pollution in the urban atmosphere.