The critical role of volatile organic compound emissions 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 (NOx=NO+NO2), plays an important role in atmospheric chemistry and air quality, yet its formation mechanism remains poorly constrained in the plateau region. In this study, we report for the first time the yearly variation in the signatures of the stable oxygen isotope anomaly (Δ17O=δ17O-0.52×δ18O) in NO3- collected in the urban Lhasa (3650 meters above sea level (m a.s.l.)), on the Tibetan Plateau, China. Our results show that NO2+OH was the largest contributor to NO3- formation (46±26 %), followed by NO3+volatile organic compound (VOC) (26±18 %) and N2O5+H2O (28±11 %) using the Bayesian Isotope Mixture Model. Notably, there were significant differences in the NO2+OH, NO3+VOC, and N2O5+H2O pathways between spring and the other three seasons (T test, p<0.05). Using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) dispersion model, we highlighted the influence of VOC emissions from regions such as Afghanistan and northern India, which enhanced NO3- concentrations in Lhasa during spring. Furthermore, the diurnal distribution of NO3- oxidation pathways varied distinctly across seasons, suggesting that these differences in NO3- pathways were attributed to aerosol liquid water content (ALWC), VOC concentrations, and the atmospheric lifetime of NO3-.