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Nitrate (NO3−), one of the most important inorganic aerosols in the atmosphere, is mainly formed by oxidation of NOx by the hydroxyl radical (OH) and ozone (O3) in urban atmospheres. However, the fractional contributions of its various oxidation pathways remain unclear. Here, we collected particulate matter with aerodynamic diameter less than 2.5 μm (PM2.5) samples in a second‐tier city in southeast China from 1 September to 31 December 2017 and measured the NO3− and nitrate isotopic compositions (δ15N and δ18O). The average concentration of NO3−, δ15N, and δ18O values were 14.7 ± 11.6 μg/m3, (+4.3 ± 4.3)‰, and (+71.8 ± 14.7)‰ with the ranges from 0.8 to 57.7 μg/m3, −10.5‰ to +12.5‰ and +34.5‰ to +91.9‰, respectively. All three species were significantly higher in winter than in summer. Based on a Bayesian mixing model with a dual isotope array for NO3−, contributions of (37.1 ± 33.4)%, (60.3 ± 32.2)%, and (2.6 ± 2.7)% to NO3− could be attributed to OH oxidation, N2O5 hydrolysis, and NO3 + hydrocarbon (HC) pathways, respectively. Higher OH radical concentrations with higher ratios of OH to O3 led to lower NO3− concentrations, while lower OH radical concentrations with higher ratios of O3 to OH led to higher contributions of N2O5 hydrolysis, forming higher NO3− concentrations in winter. Under low OH, an increased O3 to NOx ratio increased the contribution of the NO3 + HC pathway. The comprehensive analysis of the isotopic compositions of nitrate helped identify the importance of major oxidation pathways of NOx in this city.

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