Particulate nitrate (inline-formula
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) not only influences regional climates but also contributes to the acidification of terrestrial and aquatic ecosystems. In 2016 and 2017, four intensive online measurements of water-soluble ions in PMinline-formula2.5 were conducted in Nanjing City in order to investigate the potential formation mechanisms of particulate nitrate. During the sampling periods, inline-formula
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was the predominant species, accounting approximately for 35 % of the total water-soluble inorganic ions, followed by inline-formula
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(33 %) and inline-formula
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(24 %). Significant enhancements of nitrate aerosols in terms of both absolute concentrations and relative abundances suggested that inline-formula
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was a major contributing species to high-PMinline-formula2.5 events (hourly PMinline-formula2.5≥150 inline-formulaµg minline-formula−3). High inline-formula
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concentrations mainly occurred under inline-formula
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-rich conditions, implying that the formation of nitrate aerosols in Nanjing involved inline-formulaNH3. During the high-PMinline-formula2.5 events, the nitrogen conversion ratios (inline-formulaFn) were positively correlated with the aerosol liquid water content (ALWC; inline-formulaR>0.72 and inline-formulap<0.05). Meanwhile, increasing inline-formula
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concentrations regularly coincided with increasing ALWC and decreasing inline-formulaOx (inline-formula
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). These results suggested that the heterogeneous reaction was probably a major mechanism of nitrate formation during the high-PMinline-formula2.5 events. Moreover, the average production rate of inline-formula
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by heterogeneous processes was estimated to be 12.6 % hinline-formula−1 (4.1 inline-formulaµg minline-formula−3 hinline-formula−1), which was much higher than that (2.5 % hinline-formula−1; 0.8 inline-formulaµg minline-formula−3 hinline-formula−1) of gas-phase reactions. This can also explain the abrupt increases in nitrate concentrations during the high-PMinline-formula2.5 events. Utilizing the ISORROPIA II model, we found that nitrate aerosol formation in Nanjing during the high-PMinline-formula2.5 events was inline-formulaHNO3 limited. This indicated that control of inline-formulaNOx emissions will be able to efficiently reduce airborne particulate nitrate concentrations and improve the air quality in this industrial city.