Oxidation pathways and emission sources of atmospheric particulate nitrate in Seoul: based on δ15N and Δ 17O of PM2.5

Abstract

Abstract. PM2.5 haze pollution driven by secondary inorganic NO3− 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 (NO3−) and Δ17O (NO3−) of PM2.5 in Seoul from 2018 to 2019, and estimated quantitatively the relative contribution of oxidation pathways for particulate NO3− and major NOx emission sources. In the range of PM2.5 mass concentration from 7.5\u2009g\u2009m−3 (summer) to 139.0\u2009g\u2009m−3 (winter), the mean δ15N was −0.7\u2009±\u20093.3\u2009‰ and 3.8\u2009±\u20093.7\u2009‰, and the mean Δ17O was 23.2\u2009±\u20092.2\u2009‰ and 27.7\u2009±\u20092.2\u2009‰ in the summer and winter, respectively. While OH oxidation was the dominant pathway for NO3− during the summer (87\u2009%), nighttime formation via N2O5 and NO3 was more important (38\u2009%) during the winter, when aerosol liquid water content (AWLC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Δ17O was coupled with the lowest δ 15N and highest NOR in record-breaking winter PM2.5 episodes, revealing the critical role of photochemical oxidation process in severe winter haze development. For NOx sources, vehicle emissions were confirmed as a main contributor, followed by biomass combustion from various activities. The contribution from biogenic soil and coal combustion was slightly increased in summer and winter, respectively. Our results built on multiple-isotope approach provide the first explicit evidence for NO3− formation processes and major NOx emission sources in Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.\n