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 g m−3 (summer) to 139.0 g 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 the summer and winter, respectively. While OH oxidation was the dominant pathway for NO3− during the summer (87 %), nighttime formation via N2O5 and NO3 was more important (38 %) 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.

Highlights

  • In Northeast Asia, air pollution characterized by high PM2.5 and ozone concentrations is a cause of public concern due to its serious effects on human health (Lelieveld et al, 2015; Xie et al, 2019)

  • A clear seasonal difference in PM2.5 concentration and its composition were observed with significantly higher concentrations of mass and inorganic constituent in the winter than in the summer (Table 1 and Figure S2)

  • Our stable isotope-based study confirms that vehicle emissions are the main source of NO3- in PM2.5 in

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Summary

Introduction

In Northeast Asia, air pollution characterized by high PM2.5 (particulate matters with aerodynamic diameter smaller than 2.5 m) and ozone concentrations is a cause of public concern due to its serious effects on human health (Lelieveld et al, 2015; Xie et al, 2019). A large increase in SIA is associated with efficient formation of NO3- in cold months (most frequently in Dec. to Mar.), leading to a rapid increase in PM2.5 concentration and developing a persistent haze pollution on a regional scale (Li et al, 2018; Xu et al, 2019). Recent studies suggest that the regional occurrence of the PM2.5 haze events derived by NO3- across Asian continent has been associated with long-range transport of air pollutants promoted by cold fronts and their intrusion in downward regions by the development of the atmospheric boundary layer (Kang et al, 2019; Lee et al, 2019). Scientific understanding is still limited because the rapid increase of PM2.5 NO3- is the result of complex interplay of oxidation and transformation mechanisms producing

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