Inorganic nitrate production is critical in atmospheric chemistry that reflects the oxidation capacity and the acidity of the atmosphere. Here we use the oxygen anomaly of nitrate (Δ¹⁷O(\(\rm{NO}_{3}^{-}\))) in high-time-resolved (3 h) aerosols to explore the chemical mechanisms of nitrate evolution in fine particles during the winter in Nanjing, a megacity of China. The continuous Δ¹⁷O(\(\rm{NO}_{3}^{-}\)) observation suggested the dominance of nocturnal chemistry (NO₃ + HC/H₂O and N₂O₅ + H₂O/Cl⁻) in nitrate formation in the wintertime. Significant diurnal variations of nitrate formation pathways were found. The contribution of nocturnal chemistry increased at night and peaked (72%) at midnight. Particularly, nocturnal pathways became more important for the formation of nitrate in the process of air pollution aggravation. In contrast, the contribution of daytime chemistry (NO₂ + OH/H₂O) increased with the sunrise and showed a highest fraction (48%) around noon. The hydrolysis of N₂O₅ on particle surfaces played an important role in the daytime nitrate production on haze days. In addition, the reaction of NO₂ with OH radicals was found to dominate the nitrate production after nitrate chemistry was reset by the precipitation events. These results suggest the importance of high-time-resolved observations of Δ¹⁷O(\(\rm{NO}_{3}^{-}\)) for exploring dynamic variations in reactive nitrogen chemistry.