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. PM2.5 haze pollution driven by secondary inorganic NO3- hasbeen a great concern in East Asia. It is, therefore, imperative to identifyits sources and oxidation processes, for which nitrogen and oxygen stableisotopes are powerful tracers. Here, we determined the δ15N(NO3-) and Δ17O (NO3-) of PM2.5 inSeoul during the summer of 2018 and the winter of 2018–2019 and estimatedquantitatively the relative contribution of oxidation pathways forparticulate NO3- and investigated major NOx emission sources.In the range of PM2.5 mass concentration from 7.5 µg m−3 (summer) to 139.0 µg m−3 (winter), the mean δ15N was−0.7 ‰ ± 3.3 ‰ and 3.8 ‰ ± 3.7 ‰, and the mean Δ17O was 23.2 ‰ ± 2.2 ‰ and 27.7 ‰ ± 2.2 ‰ in thesummer and winter, respectively. While OH oxidation was the dominant pathwayfor NO3- during the summer (87 %), nighttime formation viaN2O5 and NO3 was relatively more important (38 %) duringthe winter, when aerosol liquid water content (ALWC) and nitrogen oxidationratio (NOR) were higher. Interestingly, the highest Δ17O wascoupled with the lowest δ15N and highest NOR during therecord-breaking winter PM2.5 episodes, revealing the critical role ofphotochemical oxidation process in severe winter haze development. For NOx sources, atmospheric δ15N (NOx) estimated frommeasured δ15N (NO3-) considering isotopefractionation effects indicates vehicle emissions as the most importantemission source of NOx in Seoul. The contribution from biogenic soiland coal combustion was slightly increased in summer and winter,respectively. Our results built on a multiple-isotope approach provide thefirst explicit evidence for NO3- formation processes and majorNOx emission sources in the Seoul megacity and suggest an effectivemitigation measure to improve PM2.5 pollution.

atmospheric chemistry and physics

Oxidation pathways and emission sources of atmospheric particulate nitrate in Seoul: based on &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N and Δ&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O measurements