Returning long-range PM2.5 transport into the leeward of East Asia in 2021 after Chinese economic recovery from the COVID-19 pandemic

Changes in the aerosol composition of sulfate (SO42-) and nitrate (NO3-) from 2012 to 2019 have been captured as a paradigm shift in the region downwind of China. Specifically, SO42- dramatically decreased and NO3- dramatically increased over downwind locations such as western Japan due to the faster reduction of SO2 emissions than NOx emissions and the almost constant trend of NH3 emissions from China. Emissions from China sharply decreased during COVID-19 lockdowns in February-March 2020, after which China's economic situation seemed to recover going into 2021. Given this substantial change in Chinese emissions, it is necessary to clarify the impact of long-range PM2.5 transport into the leeward of East Asia. In this study, ground-based aerosol compositions observed at three sites in western Japan were analysed. The concentrations of PM2.5, SO42- and NO3- decreased in 2020 (during COVID-19) compared with 2018-2019 (before COVID-19). In 2021 (after COVID-19), PM2.5 and NO3- increased and SO42- was unchanged. This suggests the returning long-range PM2.5 transport in 2021. From numerical simulations, the status of Chinese emissions during COVID-19 did not explain this returning impact in 2021. This study shows that the status of Chinese emissions in 2021 recovered to that before COVID-19.