<p>Aerosol particles, particularly fine particulate matters, exert substantial influences on climate and human health, with pronounced effects in rapidly developing urban areas. Nevertheless, the comprehensive long-term records of particle number size distributions (PNSD) trends in coastal urban environments are still scarce. In this study, PNSD in 7–300 nm were measured in Xiamen, a coastal city in Southeastern China, from 2020 to 2024. It was found that particle number concentration (PNC) declined steadily from 2020 to 2022 but began to rebound in 2023. New particle formation (NPF) events happened frequently in Xiamen (36%), with the highest prevalence during summer. GR_7-25 dropped in 2020–2021, rebounded in 2022, and stabilized by 2024. Based on a machine learning (ML)-based de-weathering method, anthropogenic emissions were the dominant driver of PNSD variations, with surface solar radiation and 10-meter meridional wind speed identified as marked meteorological factors. K-means clustering analysis furtherly revealed distinct seasonal source contributions of PNSD, which was shaped by synergistic effect of NPF events, traffic emissions, and meteorological conditions in coastal city with slight particle pollution. Our findings offer a valuable insight into the evolution and source contributions of PNSD in coastal urban environments.</p><p></p>