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It was previously believed that ammonia (NH 3 ) 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 3 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 NH 3 (δ 15 N-NH 3 ) 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 3 from agricultural to urban regions. Based on a Bayesian isotope mixing model, the contribution of agriculture (fertilization) to urban NH 3 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 3 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 3 in urban Beijing, even during the fertilization period. This finding indicated that nonagricultural sources should be considered when designing a control strategy for NH 3 to reduce haze pollution in the urban atmosphere., Competing Interests: Declaration of competing interest The authors declare no competing financial interest., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Ammonia (NH 3) is a vital precursor of secondary aerosols, which play an important role in urban air pollution. The passive observation of NH 3 in the urban atmosphere showed higher concentrations than conventionally believed; however, the NH 3 dynamics remain limited due to the paucity of active measurements with fast response instruments. In this study, intensive online measurements of ambient NH 3 were conducted using cavity ring-down spectroscopy in late spring and early summer 2020 at the megacities of Beijing and Shanghai in northern and southern China. The results show that the average NH 3 concentration in Beijing (23.1 ± 10.3 ppb) was nearly double that in Shanghai (12.0 ± 5.0 ppb). This spatial difference was confirmed by satellite column observations and bottom-up emission patterns. In addition, there was a significant increase in NH 3 concentrations during the morning in our study. This special diurnal profile was a universal feature, with an occurrence frequency exceeding 50% in both cities. These frequent morning increases were mainly caused by motor vehicle emissions, with an additional contribution from the residual layer breakup for one-quarter of the days in the morning increase. On these days, the NH 3 concentration increased by approximately 20% due to the downward mixing of the residual layer rupture. The unique data observed here could be used to validate the chemistry and transport models at a higher time resolution and improve the understanding of the current status of NH 3 pollution in the urban atmosphere. [Display omitted] • 1 Hz NH 3 observations were conducted at megacities of Beijing and Shanghai. • Atmospheric NH 3 concentrations in urban Beijing were double that in Shanghai. • A morning pulse of NH 3 levels was a common feature at Beijing and Shanghai. • Highly frequent morning increases of NH 3 were mainly caused by vehicle emissions. • Boundary layer rupture further enhanced the rapid increase of NH 3 in the morning. [ABSTRACT FROM AUTHOR]