Aerosol nitrate is a key terminal product of reactive nitrogen in the marine atmosphere, which is controlled by NOₓ emission sources and multiphase oxidation processes. However, conventional δ¹⁸O-based isotopic approaches are unable to fully resolve competing formation pathways, and the interference of sea-salt halogen chemistry further perturbs isotopic fractionation signals, leading to persistent uncertainties in both nitrate formation mechanisms and NOₓ source apportionment. To address these limitations, this study establishes a dual-isotope (δ¹⁵N–δ¹⁸O) quadrilateral constraint framework based on three ship-based campaigns conducted over the East China Sea during autumn, winter, and spring (2021–2023), enabling quantitative attribution of nitrate formation pathways and NOₓ sources. Results reveal pronounced multi-pathway competition in nitrate formation, dominated by OH oxidation (P1, 48 ± 16%), followed by NO₃ radical oxidation (P3, 28 ± 11%) and N₂O₅–Cl⁻ heterogeneous reactions (P4, 20 ± 6%), while N₂O₅ hydrolysis remains consistently minor (P2, <10%). Seasonally, P1 peaks in spring (57%), reflecting enhanced photochemical OH oxidation under higher temperature conditions. In contrast, winter and autumn are characterized by increased contributions from nighttime oxidation pathways (P3: 29–34%; P4: 21–24%), closely associated with elevated levels of OC, PAHs, and Cl⁻, indicating enhanced organic activity and sea-salt halogen activation under polluted and low-temperature conditions. Spatially, a clear offshore transition is observed, shifting from OH-dominated chemistry near the coast (P1, 50%→42%) to a mixed regime governed by NO₃ radical oxidation and halogen chemistry offshore (P3, 26%→34% and P4, 19%→22%), driven by combined influences of marine biogenic emissions and sea-salt chloride inputs. Meanwhile, NO_x source apportionment exhibits strong seasonal variability, transitioning from ship emissions dominance in autumn (35%) to biomass burning in winter (36%), with mixed contributions from biomass burning (32%) and mobile sources (26%) in spring. Offshore, ship emissions increase from 22% to 30%, highlighting enhanced marine anthropogenic influence in remote regions. This study establishes a robust dual-isotope framework for disentangling marine aerosol nitrate formation pathways and NOx sources, providing new quantitative constraints on reactive nitrogen cycling and multiphase atmospheric chemistry over the East China Sea.