Chemical characterization and dual isotope-based nitrate source identification of PM2.5 over the Chinese marginal seas.

Despite the growing anthropogenic influence on marine atmospheric composition, aerosol characterization over marginal seas lags behind urban research due to logistical challenges. This study presents a ship-based characterization of springtime PM2.5 over the Yellow Sea and the East China Sea, integrating water-soluble ions, trace metals, iron speciation, and nitrate dual isotopes (δ15N and δ18O). Secondary inorganic ions dominated aerosol composition (85.87%), while over 60% of heavy metals showed anthropogenic enrichment. A moderate positive correlation between water-soluble iron (WS-Fe) and NO3- (R2 = 0.54) indicated that nitrate-related atmospheric processes may facilitate iron dissolution, or that NO3- and WS-Fe are co-transported within the same air masses. Bayesian isotope modeling identified vehicle emissions (31.0 ± 15.8%; 95% CI: 3.0%-62.5%) and ship emissions (18.2 ± 11.1%; 95% CI: 1.2%-41.8%) as dominant NOX sources. Nighttime pathways accounted for 52.5% of nitrate formation, nearly equal to daytime hydroxyl radical (·OH) oxidation (47.5%). This work extended compound-specific isotope analysis beyond traditional source apportionment by linking isotope-constrained NOX sources to aerosol iron solubility, thereby providing a uniquely integrated dataset for China's marginal seas. The results underscored the critical role of anthropogenic nitrogen emissions in modulating marine aerosol composition and acidity, carrying profound implications for iron bioavailability and toxic metal deposition in marginal sea ecosystems.