Continental outflow interacts with marine sulfur and halogen emissions over China’s marginal seas, yet their effects on nitrate formation remain poorly understood. Here, WRF–CMAQ sensitivity simulations, evaluated against shipborne observations and nitrate isotope evidence, were used to investigate wintertime nitrate formation and the roles of dimethyl sulfide (DMS) and halogen chemistry. Nitrate formation exhibited pronounced regional differences. Heterogeneous N₂O₅ hydrolysis dominated over the Bohai and Yellow Seas (59.8 % and 59.3 %, respectively), but contributed only 38.1 % over the East China Sea, where OH + NO₂ oxidation accounted for 32.1 %. The DMS + NO₃ oxidation pathway contributed up to 4.8 % of nitrate production, while DMS emission changes produced modest particulate nitrate enhancements (0.01–0.02 μg m⁻³). In contrast, halogen perturbations induced stronger, spatially heterogeneous and nonmonotonic responses, with local HNO₃ increases of 0.4–0.8 μg m⁻³. The combined effects of DMS and halogen chemistry were largely additive, with substantial negative interactions occurring only when both emissions were enhanced. These findings highlight the regional diversity of nitrate formation pathways and the importance of marine sulfur and halogen chemistry in regulating coastal nitrate pollution.