Physicochemical properties of particulate matter are strongly influenced by the interplay between marine and terrestrial sources in the coastal atmosphere. In this study, we investigated the chemical composition and mixing state of individual particles by a single particle technique in a typical southeastern coastal city in China, Xiamen (XM) in autumn of 2021 and 2023. The particles were dominated by organic-rich (CNOS), sulfate, and aged sea salt types, with minor fractions of dust–organic mixtures (Org-Dust), and salt-bearing organics (Org-Salt). CNOS was more abundant in 2021, whereas sulfate was higher in 2023. Under clean conditions, fresh sea salt was significantly elevated in 2021, indicating stronger primary marine emissions. In 2023, increased sulfate and Org-Salt fractions is attributed to secondary formation. The mixing state index in 2021 ranged from 55% to 80% reflecting an internally mixed aerosol consistent with extensive aging, whereas it was generally below 50% in 2023 indicating a more externally mixed population. Backward trajectory analysis showed that air masses in 2021 originated mainly from the western Pacific, whereas in 2023 they shifted to continental regions and marine pathways with substantially terrestrial input. The chemical composition, mixing state, and sources of particulate matter in XM underwent a systematic shift between these two years, likely driven by changes in air mass origins and secondary formation intensity.