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To study the combined effects of the East Asian monsoon and ocean emissions on wintertime and summertime PM 2.5 in island cities, filter samples were collected simultaneously at four different functional sites. Based on the chemical compositions of PM 2.5 , and positive matrix factorization (PMF) model analysis, the pollution characteristics and sources were determined. Insignificant differences, and some correlation in PM 2.5 reconstructed compositions were found among the sites (P t -test > 0.05, P >.05), while significant differences, and no correlation, were found between winter and summer (P ANOVA < 0.05, P >.05). There was more serious chloride depletion in summer (0.88 ± 0.05), caused by both significant Cl− depletion and Na+ enrichment, than in winter (0.18 ± 0.10). The concentrations of non-sea-salt-SO 4 2− in winter were close to those in summer, but the sulfate oxidation ratio (SOR) in winter was much lower than that in summer. The results could be explained by the fact that sea-salt-SO 4 2− has an important contribution to secondary inorganic aerosol on island cities. From both air trajectory clustering and PMF analysis, it was found that there was significant aerosol aging and regional transport in the island city during the East Asian monsoon, and that continental air masses control the variation of air pollution in winter, while sea breezes dominate the characteristics of PM 2.5 in summer. This study helps to understand the characteristics and source mechanisms of PM 2.5 pollution under complex meteorological conditions in island cities. • The combined effects of the East Asian monsoon and ocean emissions on PM 2.5 were investigated in an island city. • Obvious Cl-depletion was found in summer, and sea-salt-SO 4 2− has an important contribution to secondary inorganic aerosol. • Northeastward winds brought aged aerosols in winter, while oxidation conditions and sea salt enhanced the formation of aged aerosols in summer. [ABSTRACT FROM AUTHOR]