Nitrogen isotopic characteristics of aerosol ammonium in a Chinese megacity indicate the reduction from vehicle emissions during the lockdown period

The role of agricultural versus vehicle emissions in urban atmospheric ammonia (NH3) remains unclear. The lockdown due to the outbreak of COVID-19 provided an opportunity to assess the role of source emissions on urban NH3. Concentrations and delta N-15 of aerosol ammonium (NH4+) 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 NH4+ concentrations, significantly lower delta N-15-NH4+ was found in 2020 (0.6 +/- 1.0 parts per thousand in PM2.5 and 1.4 +/- 2.1 parts per thousand in PM10) than in 2017 (15.2 +/- 6.7 parts per thousand in PM2.5), which indicates the NH3 from vehicle emissions has decreased by similar to 50% during the lockdown while other source emissions are less affected. Moreover, a reversed seasonal pattern of delta N-15-NH4+ 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 delta N-15-NH4+ and the importance of vehicle emissions to urban atmospheric NH3, providing conclusive evidence that reducing vehicle NH3 emissions could be an effective strategy to reduce PM2.5 in Chinese megacities.