Seasonal adjustment of particulate matter pollution in coastal East Asia during the 2020 COVID lockdown

Abstract

Seasonally, the East Asian particulate matter (PM) level is higher in the winter–spring period than in summer, at which time the level rapidly decreases due to the summer monsoon migration. Attempting to attribute East Asian PM pollution to a source without considering such natural factors is challenging. However, to what degree the effect of season on an attribution bias remains controversial; the bias may even be implicated in PM-related health effects. This study examined seasonal dynamics including the unusual precipitation evolution during 2020—a year in which coronavirus-related lockdowns occurred frequently worldwide—and suggested a large-scale effect from the removal of PM pollutants from most of the coastal cities in East Asia. In winter–spring 2020, compared with that of previous years, a deeper and farther southward intrusion of the East Asian coastal trough and a stronger surface monsoon flow acted jointly to transport air pollutants over the Korea–Japan region. In summer 2020, the strength and migration of the western North Pacific (WNP) high increased precipitation and removed air pollutants in mid-latitude East Asia, whereas it reduced precipitation in the subtropical WNP. Consequently, the reduced PM level in the subtropical region (including Taiwan) may be irrelevant to the anomalous seasonal pattern. Although an artificial effect is conceivable and may be primarily responsible for the marked decrease in 2020 East Asian PM pollutants in some subtropical cities, the modulation of a large-scale and precipitating effect also deserves consideration.