Non-agricultural source dominates the ammonium aerosol in the largest city of South China based on the vertical δN measurements.

Ammonia (NH) is the most prevalent alkaline gas in the atmosphere and plays a critical role in air pollution and public health. However, scientific debate remains over whether agricultural emissions (e.g., livestock and fertilizer application) dominate NH in urban atmosphere in China, which is one of the largest NH emitters in the world. In this study, we first simultaneously collected the fine atmospheric particles (PM) at two heights (ground and 488 m) using the atmospheric observatories in Canton Tower, Guangzhou city, China for the measurements of stable nitrogen isotope composition in ammonium (δN-NH). Our results showed that the average δN-NH value at the ground and the 488 m observatory were 16.9 ‰ and 3.8 ‰, respectively, implying that NH aerosols between the two heights probably have different sources. Moreover, we found that the δN-NH value would sharply decrease to -16.7 ‰ when the air masses came from western Guangzhou, where the urbanization is limited compared to other surrounding areas. The Bayesian mixing model indicated that NH aerosol at the ground observatory was mainly derived from non-agricultural activities (76 %, e.g., vehicular exhaust), with the rest from agricultural sources (24 %). As for the 488 m observatory, the contribution of non-agricultural sources was 53 %, which is lower than the ground observatory. This is expected as the lower air receives more impacts from the local urban emission. However, the current "bottom-up" emission inventory illustrates that only ~20 % NH in Guangzhou is associated with non-agricultural emissions, which is significantly lower than our δN-based results. Overall, our findings strongly imply that non-agricultural sources dominate the urban NH in Guangzhou or maybe in adjacent cities of the Pearl River Delta region as well, suggesting that the emission inventory of NH in this region probably is urgently needed to be revisited in future studies.