<p>Nitrate (NO₃^–) levels in air pollution have shown a sustained increase across eastern China. However, the key drivers behind rising surface NO₃^– concentrations remain unclear, posing challenges for targeted pollution control strategies. PM_2.5 samples were collected from September 2015 to August 2016 at both urban and suburban sites in Nanjing, a megacity in the Yangtze River Delta (YRD), for compositional analysis and source apportionment. The measured annual mean PM_2.5 concentration was 96.8 ± 46.0&#xa0;µg m^–3. The positive matrix factorization model identified four primary PM_2.5 sources in Nanjing: secondary nitrate (19.4%), secondary sulfate (36.8%), coal and biomass burning (40.6%), and industrial emissions (3.2%). Water-soluble secondary inorganic aerosols (NO₃^–, SO₄^2–, NH₄⁺) dominated PM_2.5 composition, accounting for 92.7% of ionic components and 37.0% of total mass. NO₃^– concentrations exhibited significant increases in both absolute and relative terms as PM_2.5 pollution levels rose, suggesting its important role in PM_2.5 pollution. The results indicate that NO₃^– formation is enhanced under ammonia-rich conditions with low temperatures, high humidity, and elevated acidity. Policy-driven reductions in SO₂ and NO_x, without simultaneous NH₃ control, may have contributed to ammonia-rich conditions that facilitated NO₃^– formation, leading to NO₃^–-dominated PM_2.5 pollution in the YRD. Therefore, our results indicate that coordinated control of both nitrogen oxides and ammonia emissions may be necessary to mitigate NO₃^–-driven PM_2.5pollution.</p> Graphical abstract <p></p>