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

• Published in: Journal of geophysical research. Atmospheres 2019-04, Vol.124 (7), p.4174-4185
• Main Authors: Wang, Yan‐Li, Song, Wei, Yang, Wen, Sun, Xin‐Chao, Tong, Yin‐Dong, Wang, Xue‐Mei, Liu, Cong‐Qiang, Bai, Zhi‐Peng, Liu, Xue‐Yan
• Format: Article
• Language: English
• Subjects: Aerosols; Air pollution; Atmospheric particulates; Atmospheric pollution deposition; Environmental monitoring; Geophysics; Haze; Hydrolysis; Isotopes; nitrate; Nitrates; Nitrogen compounds; Nitrogen dioxide; Organic chemistry; Oxidation; oxidation pathway; Oxides; Oxygen; Oxygen isotopes; Particulate matter; Particulates; PM2.5; Pollution; Pollution control; Public health; Records; Stable isotopes; Summer; Urban environments; Winter
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2019JD030284

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 PM2.5 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 NOx 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, PM2.5, 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. Plain Language Summary Air pollution is a major environmental and public health issue; to strengthen studies on haze formation mechanisms is a national environment demand. Although monitoring works have been widely conducted on atmospheric particulates, it has long been difficult to quantify production mechanisms of key chemical components. Combined oxygen isotopes (18O, 17O) of nitrate with the Stable Isotope Analysis in R model, we evaluated the fractional contributions of three major oxidation pathways (NO2 + OH· [P1], NO3· + DMS/HC [P2], and N2O5 hydrolysis [P3]) to the NO3− of PM2.5. We found that annually 68 ± 23% of NO3− in PM2.5 at Beijing was produced by the O3 oxidation pathways (P2 + P3). Moreover, PM2.5, NO2, and NO3− pollution decreased the importance of the OH· pathway relative to the O3 pathways for NO3− production. Our results provided a comprehensive analysis on the oxygen isotope records in particulate NO3−