Monday, 8 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
High PM2.5 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 (SO42-, 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 SO42- (type S). The concentration of NH4+, which is the counterion for SO42- 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 SO42- 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 SO42- (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 SO42-. 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 SO42- for type N, whereas the production of SO42- led to decomposition of NH4NO3 and more SO42- was transported for type S. The transboundary air pollution dominated by SO42- (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 PM2.5 pollution in winter over East Asia.
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