Revealing underestimated ship contributions to atmospheric nitrate over the Pearl River Delta: Based on dual isotopes

Nitrate (NO₃^-), as a major inorganic component of fine particulate matter (PM_2.5) in coastal cities, poses serious threats to human health, and its deposition also exerts important impacts on marine ecosystems. In port cities, ship emissions are a dominant factor of atmospheric NO₃^- pollution, yet in the Pearl River Delta (PRD), one of the world's busiest ports, their contribution remains poorly quantified despite its critical implications for coastal air quality management. In this study, we present year-round dual isotopes (δ¹⁵N and δ¹⁸O) of NO₃^- in Guangzhou, a coastal megacity over the PRD. Bayesian isotope mixing model (MixSIAR) reveals that ship emissions contributed 45.0 ± 1.5 % of atmospheric NO₃^- annually in the PRD, closely matching traffic sources (46.6 ± 1.3 %) and far exceeding coal combustion (8.4 ± 0.2 %). The results were further validated by satellite data and emission inventory, confirming the sustained contribution of ship emissions.