Nitrate transboundary heavy pollution over East Asia in winter

Abstract. High PM_2. 5 concentrations of around 100 µg m⁻³ were observed twice during an intensive observation campaign in January 2015 at Fukuoka (33.52° N, 130.47° E) in western Japan. These events were analyzed comprehensively with a regional chemical transport model and synergetic ground-based observations with state-of-the-art measurement systems, which can capture the behavior of secondary inorganic aerosols (SO₄²⁻, NO₃⁻, and NH₄⁺). The first episode of high PM_2. 5 concentration was dominated by NO₃⁻ (type N) and the second episode by SO₄²⁻ (type S). The concentration of NH₄⁺ (the counterion for SO₄²⁻ and NO₃⁻) was high for both types. A sensitivity simulation in the chemical transport model showed that the dominant contribution was from transboundary air pollution for both types. To investigate the differences between these types further, the chemical transport model results were examined, and a backward trajectory analysis was used to provide additional information. During both types of episodes, high concentrations of NO₃⁻ were found above China, and an air mass that originated from northeast China reached Fukuoka. The travel time from the coastline of China to Fukuoka differed between types: it was 18 h for type N and 24 h for type S. The conversion ratio of SO₂ to SO₄²⁻ (F_s) was less than 0.1 for type N, but reached 0.3 for type S as the air mass approached Fukuoka. The higher F_s for type S was related to the higher relative humidity and the concentration of HO₂, which produces H₂O₂, the most effective oxidant for the aqueous-phase production of SO₄²⁻. Analyzing the gas ratio as an indicator of the sensitivity of NO₃⁻ to changes in SO₄²⁻ and NH₄⁺ showed that the air mass over China was NH₃-rich for type N, but almost NH₃-neutral for type S. Thus, although the high concentration of NO₃⁻ above China gradually decreased during transport from China to Fukuoka, higher NO₃⁻ concentrations were maintained during transport owing to the lower SO₄²⁻ for type N. In contrast, for type S, the production of SO₄²⁻ led to the decomposition of NH₄NO₃, and more SO₄²⁻ was transported. Notably, the type N transport pattern was limited to western Japan, especially the island of Kyushu. Transboundary air pollution dominated by SO₄²⁻ (type S) has been recognized as a major pattern of pollution over East Asia. However, our study confirms the importance of transboundary air pollution dominated by NO₃⁻, which will help refine our understanding of transboundary heavy PM_2. 5 pollution in winter over East Asia.