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 center dot) and O-3 in the atmosphere, is a major component of inorganic aerosols. However, the contributions of the OH center dot 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 Delta O-17-NO3- values (17.0-32.8 parts per thousand) were significantly higher in winter (27.2 +/- 3.6 parts per thousand) than in summer (24.2 +/- 1.3 parts per thousand). By estimating contributions of O-3 to the NOx cycle, the Delta O-17 values of NO3- endmembers produced via the NO2 + OH center dot (P1), NO3 center dot + 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 center dot pathway relative to the O3 pathways for NO3- production. However, O3 pollution increased the importance of the OH center dot 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 (O-18, O-17) of nitrate with the Stable Isotope Analysis in R model, we evaluated the fractional contributions of three major oxidation pathways (NO2 + OH center dot [P1], NO3 center dot + 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 O-3 oxidation pathways (P2 + P3). Moreover, PM2.5, NO2, and NO3- pollution decreased the importance of the OH center dot pathway relative to the O3 pathways for NO3- production. Our 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.