Oxidation pathways and emission sources of atmospheric particulate nitrate in Seoul: Based on Δ¹⁵N and Δ¹⁷O measurements

PM2.5 haze pollution driven by secondary inorganic NO-3 has been a great concern in East Asia. It is, therefore, imperative to identify its sources and oxidation processes, for which nitrogen and oxygen stable isotopes are powerful tracers. Here, we determined the Δ15N (NO-3 ) and Δ117O (NO-3 ) of PM2.5 in Seoul during the summer of 2018 and the winter of 2018-2019 and estimated quantitatively the relative contribution of oxidation pathways for particulate NO-3 and investigated major NOx emission sources. In the range of PM2.5 mass concentration from 7.5 μgm-3 (summer) to 139.0 μgm-3 (winter), the mean Δ15N was -0.7‰±3.3‰ and 3:8‰±3.7 ‰, and the mean Δ117O was 23:2‰±2.2‰ and 27:7‰±2.2‰ in the summer and winter, respectively. While OH oxidation was the dominant pathway for NO-3 during the summer (87 %), nighttime formation via N2O5 and NO3 was relatively more important (38 %) during the winter, when aerosol liquid water content (ALWC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Δ117O was coupled with the lowest Δ15N and highest NOR during the record-breaking winter PM2.5 episodes, revealing the critical role of photochemical oxidation process in severe winter haze development. For NOx sources, atmospheric Δ15N (NOx ) estimated from measured Δ15N (NO-3 ) considering isotope fractionation effects indicates vehicle emissions as the most important emission source of NOx in Seoul. The contribution from biogenic soil and coal combustion was slightly increased in summer and winter, respectively. Our results built on a multiple-isotope approach provide the first explicit evidence for NO-3 formation processes and major NOx emission sources in the Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.