Factors affecting recent PM concentrations in China and South Korea from 2016 to 2020.

This study used observational data and a chemical transport model to investigate the contributions of several factors to the recent change in air quality in China and South Korea from 2016 to 2020. We focused on observational data analysis, which could reflect the annual trend of emission reduction and adjust existing emission amounts to apply it into a chemical transport model. The observation data showed that the particulate matter (PM) concentrations during winter 2020 decreased by -23.4 % (-14.68 μg/m) and - 19.5 % (-5.73 μg/m) in China and South Korea respectively, compared with that during winter 2016. Meteorological changes, the existing national plan for a long-term emission reduction target, and unexpected events (i.e., Coronavirus disease 2019 (COVID-19) in China and South Korea and the newly introduced special winter countermeasures in South Korea from 2020) are considered major factors that may affect the recent change in air quality. The impact of different meteorological conditions on PM concentrations was assessed by conducting model simulations by fixing the emission amounts; the results indicated changes of +7.6 % (+4.77 μg/m) and + 9.7 % (+2.87 μg/m) in China and South Korea, respectively, during winter 2020 compared to that during winter 2016. Due to the existing and pre-defined long-term emission control policies implemented in both countries, PM concentration significantly decreased from winter 2016-2020 in China (-26.0 %; -16.32 μg/m) and South Korea (-9.1 %; -2.69 μg/m). The unexpected COVID-19 outbreak caused the PM concentrations in China to decrease during winter 2020 by another -5.0 % (-3.13 μg/m). In South Korea, the winter season special reduction policy, which was introduced and implemented in winter 2020, and the COVID-19 pandemic may have contributed to -19.5 % (-5.92 μg/m) decrease in PM concentrations.