Tracing anthropogenic nitrogen deposition in Southeast Asian megacities: Isotopic evidence from Singapore and Ho Chi Minh City

Rapid urbanization and industrialization in Southeast Asia have substantially increased emissions of reactive nitrogen (N), raising concerns over atmospheric nitrogen deposition and its environmental impacts in the region. Here, we combine nitrogen stable isotopes measurements with precipitation chemistry and flux observations to investigate the magnitude and sources of wet nitrogen deposition in two major cities, Singapore (SG) and Ho Chi Minh City (HCMC).  Both SG and HCMC exhibit high annual wet nitrogen deposition fluxes of 31.3 and 30.4 kg N ha^-1 yr^-1, respectively, with ammonium (NH₄⁺) and nitrate (NO₃^-) as dominant components. Isotope mixing models indicate comparable contributions from fossil and non-fossil fuel sources to NOx emissions in both cities. In SG, combustion-related NH₃ sources account for ~66% of NH₄⁺ deposition, whereas in HCMC, volatilized sources such as agriculture and waste play a more significant role. Dissolved organic nitrogen was primarily attributed to biogenic emissions, including plant debris. Seasonal variations in deposition are associated with monsoon-driven transboundary transport, reflecting the regional coupling of nitrogen emissions and deposition. Our findings demonstrate the strong influence of anthropogenic activities and cross-border pollutant transport on nitrogen deposition in tropical megacities, with implications for targeted emission control strategies in rapidly urbanizing regions.