The top-level emergency response to the COVID-19 pandemic involved exhaustive quarantine measures in China. The impacts of the COVID-19 quarantine on the decline in fine particulate matter (PM.sub.2.5) were quantitatively assessed based on numerical simulations and observations in February. Relative to both February 2017 and the climate mean, anomalous southerlies and moister air occurred in the east of China in February 2020, which caused considerable PM.sub.2.5 anomalies. Thus, it is a must to disentangle the contributions of stable meteorology from the effects of the COVID-19 lockdown. The contributions of routine emission reductions were also quantitatively extrapolated. The top-level emergency response substantially alleviated the level of haze pollution in the east of China. Although climate variability elevated the PM.sub.2.5 by 29 % (relative to 2020 observations), a 59 % decline related to the COVID-19 pandemic and a 20 % decline from the expected pollution regulation dramatically exceeded the former in North China. The COVID-19 quarantine measures decreased the PM.sub.2.5 in the Yangtze River Delta by 72 %. In Hubei Province where most pneumonia cases were confirmed, the impact of total emission reduction (72 %) evidently exceeded the rising percentage of PM.sub.2.5 driven by meteorology (13 %).