Oxidation pathways and emission sources of atmospheric particulate nitrate in Seoul: based on delta N-15 and Delta O-17 measurements

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 delta N-15 (NO3-) and Delta O-17 (NO3-) 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 NO3- and investigated major NOx emission sources. In the range of PM2.5 mass concentration from 7.5 mu g m(-3) (summer) to 139.0 mu g m(-3) (winter), the mean delta N-15 was -0.7 parts per thousand +/- 3.3 parts per thousand and 3.8 parts per thousand +/- 3.7 parts per thousand, and the mean Delta O-17 was 23.2 parts per thousand +/- 2.2 parts per thousand and 27.7 parts per thousand +/- 2.2 parts per thousand 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 relatively more important (38 %) during the winter, when aerosol liquid water content (ALWC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Delta O-17 was coupled with the lowest delta N-15 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 delta N-15 (NOx) estimated from measured delta N-15 (NO3-) 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 NO3- formation processes and major NOx emission sources in the Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.