A general feature in the diurnal cycle of atmospheric ammonia (NH₃) concentrations is a morning spike that typically occurs around 07:00 to 10:00 (LST). Current hypotheses to explain this morning’s NH₃ increase remain elusive, and there is still no consensus whether traffic emissions are among the major sources of urban NH₃. Here, we confirmed that the NH₃ morning pulse in urban Beijing is a universal feature, with an annual occurrence frequency of 73.0% and a rapid growth rate (>20%) in winter. The stable nitrogen isotopic composition of NH₃ (δ¹⁵N−NH₃) in winter also exhibited a significant diurnal variation with an obvious morning peak at 07:00 to 10:00 (−18.6‰, mass-weighted mean), higher than other times of the day (−26.3‰). This diurnal pattern suggests that a large fraction of NH₃ in the morning originated from nonagricultural sources, for example, power plants, vehicles, and coal combustion that tend to have higher δ¹⁵N−NH₃ emission signatures relative to agricultural emissions. In particular, the contribution from vehicular emissions increased from 18% (00:00 to 07:00) to 40% (07:00 to 10:00), while the contribution of fertilizer sources to NH₃ was reduced from 15.8% at 00:00 to 07:00 to 5.2% at 07:00 to 10:00. We concluded that NH₃ concentrations in winter mornings in urban Beijing were indeed enhanced by vehicle emissions, which should be considered in air pollution regulations.