Revealing the changes in chemical compositions and sources of PM_2.5 is important for understanding aerosol chemistry and emission control strategies. High time-resolved characterization of water-soluble inorganic ions, elements, organic carbon (OC), and elemental carbon (EC) in PM_2.5 was conducted in a coastal city of southeast China during the COVID-19 pandemic. The results showed that the average concentration of PM_2.5 during the city lockdown (CLD) decreased from 46.2 μg m^-3 to 24.4 μg m^-3, lower than the same period in 2019 (PM_2.5: 37.1 μg m^-3). Concentrations of other air pollutants, such as SO₂, NO₂, PM10, OC, EC, and BC, were also decreased by 27.3%–67.8% during the CLD, whereas O₃ increased by 28.1%. Although SO₂ decreased from 4.94 μg m^-3 to 1.59 μg m^-3 during the CLD, the concentration of SO₄^2- (6.63 μg m^-3) was comparable to that (5.47 μg m^-3) during the non-lockdown period, which were attributed to the increase (16.0%) of sulfate oxidation rate (SOR). O_x (O₃+NO₂) was positively correlated with SO₄^2-, suggesting the impacts of photochemical oxidation. A good correlation (R₂ = 0.557) of SO₄^2- and Fe and Mn was found, indicating the transition-metal ion catalyzed oxidation. Based on positive matrix factorization (PMF) analysis, the contribution of secondary formation to PM_2.5 increased during the epidemic period, consisting with the increase of secondary organic carbon (SOC), while other primary sources including traffic, dust, and industry significantly decreased by 9%, 8.5%, and 8%, respectively. This study highlighted the comprehensive and nonlinear response of chemical compositions and formation mechanisms of PM_2.5 to anthropogenic emissions control under relatively clean conditions.