Sulfate (SO₄^2-), plays a key role in the formation, growth, and chemical properties of aerosol particles and cloud droplets in marine boundary layer, with profound implications for climate and air quality. Despite this, the formation pathways and emission sources of particulate sulfate under a mixed marine–continental environment remain highly unconstrained. Here, year-round sulfur isotopic composition of PM_2.5 sulfate was measured over, together with that of SO₂ in summer and winter. During the observation period, sulfate accounted for 20–70% of the total water-soluble inorganic ions in PM_2.5, highlighting its substantial contribution to coastal particulate pollution. The δ³⁴S–SO₄^2- values were found higher in spring and summer (9.0±2.3‰) than in autumn and winter (6.6±2.2‰), together with the higher δ³⁴S–SO₂ values observed in summer than that in winter. The corresponding sulfur isotope fractionation between gas-phase SO₂ and particulate SO₄^2- were further determined for the pathway resolution. Results suggest photochemically driven oxidation by H₂O₂/O₃ and OH collectively dominated secondary sulfate formation, contributing approximately 50% in all four seasons. By contrast, the contributions of transition-metal-ion (TMI) catalyzed oxidation and HOCl oxidation exhibited pronounced seasonal variability, with lower contributions in spring and summer than in autumn and winter. This pattern suggests an enhanced influence of polluted continental air masses from inland northern China during the colder seasons. Source apportionment results derived from the isotopic composition of sulfur emission showed that vehicle emissions (36.5–46.9%) and coal combustion (33.5–40.8%) were the principal sources of ambient sulfur across the four seasons. Ship emissions contributed 7.5–13.0%, whereas marine biogenic dimethyl sulfide (DMS) made a comparatively small contribution, reaching a maximum of 9.8% in summer and a minimum of 3.6% in winter. Thus, despite the frequent influence of marine air masses, particulate sulfate in this coastal city was predominantly derived from local and regionally transported anthropogenic emissions, while marine sulfur sources played a relatively limited role. These findings demonstrate the importance of controlling vehicle- and coal-related emissions for mitigating particulate sulfate pollution in coastal regions in eastern China.

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