From atmosphere to river: Tracing the origins and deposition of ammonium in both PM2.5 and precipitation in Harbin.

Atmospheric ammonium (NH4+) is a key component of terrestrial nitrogen cycling that substantially impacts regional environmental pollution. However, its origins and deposition patterns remain insufficiently constrained in urban hotspots with intensive anthropogenic emissions. This study employed δ15N isotope tracing to investigate the origins and deposition of atmospheric NH4+ in Harbin, Northeast China. Using PM2.5, precipitation, and surface water samples from the Songhua River, this study quantified the source contributions of the NH4+ precursor (NH3), NH4+-N deposition fluxes, and their deposition contributions to the riverine NH4+-N load. The results indicated that NH4+-N dominated atmospheric inorganic nitrogen deposition across the study area. The wet deposition flux was 10.10 kg N ha-1 yr-1, which was lower than the estimated dry deposition flux of 20.51 kg N ha-1 yr-1. Source apportionment showed that non-agricultural sources (53.00%) dominated the NH4+ precursor in PM2.5, whereas agricultural sources (primarily fertilizer application) prevailed (62.00%) in precipitation. For the Songhua River in the Harbin section, atmospheric deposition was the primary source of NH4+-N, with wet and dry deposition contributing 36.00% and 18.00%, respectively. Furthermore, rainfall events increased riverine NH4+ concentration. Overall, our results show that agricultural sources contributed more to NH4+ in precipitation than in PM2.5 (primarily because of more extensive regional air-mass transport), and highlight that atmospheric deposition, particularly wet deposition, serves as a critical exogenous NH4+-N source in the urban river.