Influences of Atmospheric Pollution on the Contributions of Major Oxidation Pathways to PM 2.5 Nitrate Formation in Beijing

Nitrate (NO 3 − ), which is mainly oxidized from NO 2 by the OH radical (OH·) and O 3 in the atmosphere, is a major component of inorganic aerosols. However, the contributions of the OH· and O 3 pathways to NO 3 − in urban aerosols and the influence of air pollution to both pathways remain unclear. Oxygen isotopes of NO 3 − were measured for PM 2.5 in Beijing in 2014. The Δ 17 O‐NO 3 − 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 O 3 to the NO x cycle, the Δ 17 O values of NO 3 − endmembers produced via the NO 2  + OH· (P1), NO 3 · + DMS/HC (P2), and N 2 O 5 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 NO 3 − formation was dominated by the O 3 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 , NO 2 , and NO 3 − pollution decreased the importance of the OH· pathway relative to the O 3 pathways for NO 3 − production. However, O 3 pollution increased the importance of the OH· pathway relative to the O 3 pathways for NO 3 − production. These results provided a comprehensive analysis on the oxygen isotope records in particulate NO 3 − for understanding the relative importance of major oxidation pathways of NO 2 . Atmospheric pollution substantially influenced the pathways of NO 2 oxidation to NO 3 − in city environments.