Severe level of photochemical oxidants (Ox) over the western coast of Japan during autumn after typhoon passing

Abstract

Photochemical oxidants (O_x; mainly O₃) are a concern in East Asia. Because of the prevailing westerly wind in the midlatitudes, O₃ concentration generally shows a high in spring over Kyushu Island, western Japan, and O_x warnings have been issued in spring. However, the record from 2000 to 2021 of O_x warning days in Kyushu Island contains one warning case in autumn 2020. Interestingly, a typhoon had passed the day before this O_x warning. To relate these events, a modelling simulation was conducted and it showed the transboundary O₃ transport from the Asian continent to the western coast of Japan due to the strong wind field determined by the location of Typhoon Haishen (2020). The sensitivity simulations for changing Chinese anthropogenic sources suggested that both nitrogen oxides (NO_x) and volatile organic compound (VOC) emission regulations in China could decrease high O₃ over the downwind region of Japan. Furthermore, VOC emission regulation in China led to an overall O₃ decrease in East Asia, whereas NO_x emission regulation in China had complex effects of decreasing (increasing) O₃ during the daytime (nighttime) over China. The association between air quality and meteorology related to typhoons should be considered along with global warming in the future.