The Critical Role of Volatile Organic Compounds Emission in Nitrate Formation in Lhasa, Tibetan Plateau: Insights from Oxygen Isotope Anomaly Measurements

Abstract. Atmospheric particulate nitrate aerosol (NO₃^-), produced via the oxidation of nitrogen oxides (NO_x = NO + NO₂), plays an important role in atmospheric chemistry and air quality, yet its formation mechanism still poorly constrained the plateau region. In this study, we first reported the yearly variation of the signatures for the stable oxygen isotope anomaly (∆¹⁷O = δ¹⁷O - 0.52 × δ¹⁸O) in NO₃^- collected in the urban region of Lhasa city (3650 m a.s.l), Tibetan Plateau, China. Our results show that NO₂ + OH is the largest contributor to NO₃^- formation (46 %), followed by NO₃ + VOC (26 %), and N₂O₅ + H₂O (28 %) using the Bayesian Isotope Mixture Model. Notably, there are significant differences in the NO₂ + OH, NO₃ + VOC, and N₂O₅ + H₂O pathways between spring and other three seasons (p < 0.05). Our results highlight the influence of VOC emissions from regions such as Afghanistan and northern India, which enhance NO₃^- concentrations in Lhasa during spring. Furthermore, the diurnal distribution of NO₃^- oxidation pathways varied distinctly across seasons, suggesting that these difference in NO₃^- pathways are attributed to ALWC, VOC concentration, and pollution levels.