Nitrogen and oxygen isotope evidence for the formation and sources of nitrate aerosols in eastern China

Nitrate (NO₃^-) is a critical constituent of ambient fine particulate matter (PM_2.5), impacting air quality and climate change, yet its sources and transformation mechanism remain largely elusive. Here, the dual isotope signatures of nitrate (δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^-) in PM_2.5 were investigated in Ma'anshan, eastern China, from 2023 November to 2024 June. The elevated NO₃^- concentrations were observed during the cold season. The δ¹⁵N-NO₃^-and δ¹⁸O-NO₃^- values varied from -2.5 ‰ to +11.1 ‰ and from +61.2 ‰ to +102.3 ‰, respectively. Seasonal trends of isotope ratios can be ascribed to differences in chemical oxidation pathways and emissions to nitrate. Nitrate formation was primarily driven by heterogeneous N₂O₅ hydrolysis in the cold season, whereas the NO₂ oxidation with hydroxyl radical (OH) predominated during the warm season. Elevated NO₂ conditions prevailed during the cold season, coupled with low ambient temperature enhancing the relative contribution of N₂O₅ hydrolysis process to NO₃^- formation. Using the Bayesian isotope mixing model combined with nitrogen isotope fractionation, our findings revealed that vehicle emission was the dominant contributor in the cold season. In contrast, biomass burning was the primary source during the warm season. Industrial processes were an important steady emission source. The decreased contribution in coal combustion suggested the changes of NO_x emission structure affecting the urban environment in eastern China. Our results provide valuable insight into elucidating the NO_x-NO₃^- relationship and primary NO_x emission sources in an industrial city and suggest effective mitigation measures to improve PM_2.5 pollution.