Influences of Atmospheric Pollution on the Contributions of Major Oxidation Pathways to PM2.5 Nitrate Formation in Beijing

Nitrate (NO₃^-), which is mainly oxidized from NO₂ by the OH radical (OH·) and O₃ in the atmosphere, is a major component of inorganic aerosols. However, the contributions of the OH· and O₃ pathways to NO₃^- in urban aerosols and the influence of air pollution to both pathways remain unclear. Oxygen isotopes of NO₃^- were measured for PM_2.5 in Beijing in 2014. The Δ¹⁷O-NO₃^- values (17.0-32.8‰) were significantly higher in winter (27.2 ± 3.6‰) than in summer (24.2 ± 1.3‰). By estimating contributions of O₃ to the NO_x cycle, the Δ¹⁷O values of NO₃^- endmembers produced via the NO₂ + OH· (P1), NO₃· + DMS/HC (P2), and N₂O₅ hydrolysis (P3) pathways were calculated for each observation. The contributions of the three pathways (P1: 32 ± 10%, P2: 34 ± 10%, and P3: 34 ± 20% annually) were calculated using the Stable Isotope Analysis in R model. We found that NO₃^- formation was dominated by the O₃ oxidation pathways (P2 + P3; 68 ± 23% annually, 73 ± 21% in spring, 59 ± 23% in summer, 75 ± 20% in autumn, and 69 ± 22% in winter). Moreover, PM_2.5, NO₂, and NO₃^- pollution decreased the importance of the OH· pathway relative to the O₃ pathways for NO₃^- production. However, O₃ pollution increased the importance of the OH· pathway relative to the O₃ pathways for NO₃^- production. These results provided a comprehensive analysis on the oxygen isotope records in particulate NO₃^- for understanding the relative importance of major oxidation pathways of NO₂. Atmospheric pollution substantially influenced the pathways of NO₂ oxidation to NO₃^- in city environments.