Winter haze in eastern China is increasingly dominated by particulate nitrate, but its compositions and source contributions remain poorly constrained. We conducted intensive winter observations at a regional site in suburban Shanghai and developed a semi-quantitative single-particle framework combining SPAMS with factor analysis to attribute nitrate to specific particle types and sources in near real time. Nitrate occurred mainly in the accumulation mode (~0.5–0.7 μm), implying high regional persistence. Four nitrate-containing particle classes (NO₃Lv1–Lv4) were identified. A highly aged class (NO₃Lv4) accounted for the largest fraction of both particle number and nitrate mass loadings. This class showed extensive internal mixing with elemental carbon, ammonium, potassium-rich combustion markers, and chloride-depleted sea salt, indicating coupling between secondary inorganic nitrate, combustion emissions, and processed marine aerosol. Episode analysis and factorization further revealed that severe nitrate build-up reflects the co-occurrence of stagnant boundary layers, humid nocturnal heterogeneous formation of both ammonium and non-ammonium nitrate, long-range transport from northern industrial and urban corridors, and marine influence. These results indicate that winter nitrate haze in the Yangtze River Delta is governed by joint NO_x–NH₃ chemistry, primary combustion, and regional transport, requiring coordinated multipollutant control.