Long-term clean-air actions have reshaped fine-particle pollution in North China, but it remains unclear how the source structure of PM_2.5 has changed at the regional background scale and which chemical components are amplified. We combine long-term observations from Tuoji Island (2012–2023), a coastal background site, with chemical analysis, positive matrix factorization (PMF), ion balance, and thermodynamic diagnostics. Although PM_2.5 mass did not decrease monotonically, its composition and source structure changed substantially. Elemental carbon decreased from 1.46 to 0.81 μg m^-3, and the biomass/coal combustion contribution decreased from 43.37 % to 13.74 %, indicating weakened primary-combustion signals. SO₄^2- decreased from 9.86 to 4.10 μg m^-3, while the NO₃-/SO₄^2- ratio increased from 0.64 to 2.08, showing a shift from sulfate dominance toward greater nitrate importance. After mineral-dust outliers were removed, NO₃- accounted for 19.19 % of PM_2.5 in the highest 20 % of samples during 2021–2023, and the PMF secondary nitrate contribution reached 27.59 %, making it the dominant amplified source factor. Greater NH₄+ excess and model-diagnosed aerosol liquid water content indicate increased NH₄+ availability after sulfate neutralisation and wetter particles, favouring NH₄NO₃ formation and persistence. These findings show that emission reductions reorganised PM_2.5 pollution toward weaker primary combustion and sulfate influence but stronger nitrate amplification during late-stage high-PM_2.5 conditions.