Abstract

During the COVID-19 pandemic period, anthropogenic activities were severely restricted, leading to a substantial reduction in emissions. The effects of the emission reduction on air quality have been reported worldwide, but the focus has been over urban areas. Tropospheric ozone (O₃) increased due to the weakening of the NO titration effect caused by the large decrease in NO_x emissions. In this study, we considered the time lag in the emission decreases due to COVID-19 over East Asia and evaluated O₃ changes in April and May, 2020 over the whole of Japan, from urban to remote areas, with different timescales. The decreases in O₃ concentration were evaluated according to both ground-based observation and air quality modeling in April and May, with a greater change observed in May. The air quality modeling result demonstrated that meteorological changes, particularly solar radiation and relative humidity, could explain around 50% of the O₃ change in April and 70–80% in May. The domestic anthropogenic emission changes during COVID-19 led to a decrease in O₃ over the whole of Japan; however, the monthly mean O₃ concentration increased and the maximum O₃ concentration peaks decreased over urban areas. The effects of emission changes were smaller than the effects of meteorology in April and May. To understand the worsening of O₃ pollution in East Asia, we clarified the need to evaluate O₃ abatement strategies, focusing on the characteristics of different areas (urban and remote) and timescales (mean and maximum) based on the lessons from the COVID-19 pandemic.