Transboundary air pollution of nitrate in winter over East Asia: Important role of ammonia to decide the long-range transport type

High PM_2.5 concentrations reached around 100 μg/m³ were observed twice during an intensive observation campaign in January 2015 at Fukuoka, western Japan (33.52°N, 130.47°E). This two times events were analyzed by 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 (SO₄^2-, NO₃^-, and NH₄⁺) and related gas species (HNO₃ and NH₃). The first episode was dominated by NO₃^- (type N), whereas the second episode was dominated by SO₄^2- (type S). The concentration of NH₄⁺, which is the counterion for SO₄^2- and NO₃^-, was high for both types. The sensitivity simulation of the chemical transport model showed that the dominant contribution was from transboundary air pollution for both types. To investigate the differences between these types of transboundary heavy pollution further, the chemical transport model results were examined in combination with the backward trajectory analysis and examination of the adjusted gas ratio (adjGR), which indicates the sensitivity of NO₃^- to changes in SO₄^2- and NH₄⁺. For both types, high concentrations of NO₃^- above China were found, and the air mass that originated from northeast China reached Fukuoka, but the traveling time from the coastline of China to Fukuoka was 18 h for type N and 24 h for type S. The conversion ratio of SO₂ to SO₄^2- (Fs) was less than 0.1 for type N, but reached 0.3 for type S as the air mass approached Fukuoka. The higher Fs for type S was related to the higher relative humidity and concentration of HO₂, which produces the most effective oxidant, H₂O₂, for the aqueous-phase production of SO₄^2-. The adjGR showed that the air mass over China was super NH₃-rich for type N, but was 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 SO₄^2- for type N, whereas the production of SO₄^2- led to decomposition of NH₄NO₃ and more SO₄^2- was transported for type S. The transboundary air pollution dominated by SO₄^2- (type S) has been recognized as major pattern over East Asia. However, our study confirms the importance of the transboundary air pollution dominated by NO₃^- (type N), which will help refine our understanding of the transboundary heavy PM_2.5 pollution in winter over East Asia.