The role of agricultural versus vehicle emissions in urban atmospheric ammonia (NH) remains unclear. The lockdown due to the outbreak of COVID-19 provided an opportunity to assess the role of source emissions on urban NH. Concentrations and δN of aerosol ammonium (NH) were measured before (autumn in 2017) and during the lockdown (summer, autumn, and winter in 2020), and source contributions were quantified using SIAR. Despite the insignificant decrease in NH concentrations, significantly lower δN-NH was found in 2020 (0.6 ± 1.0‰ in PM and 1.4 ± 2.1‰ in PM) than in 2017 (15.2 ± 6.7‰ in PM), which indicates the NH from vehicle emissions has decreased by∼50% during the lockdown while other source emissions are less affected. Moreover, a reversed seasonal pattern of δN-NH during the lockdown in Changsha has been revealed compared to previous urban studies, which can be explained by the dominant effect of non-fossil fuel emissions due to the reductions of vehicle emissions during the lockdown period. Our results highlight the effects of lockdown on aerosol δN-NH and the importance of vehicle emissions to urban atmospheric NH, providing conclusive evidence that reducing vehicle NH emissions could be an effective strategy to reduce PM in Chinese megacities.
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