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The COVID-19 outbreak in China led to dramatic changes in human activities resulting from the sudden infection prevention and control measures. Here, we use ground-level observations and model simulations to examine the nationwide spatial–temporal variations of six air pollutants before and after the initiation of First-Level Public Health Emergency Response. The level of ambient NO₂ declined significantly, and in most cities, the decline was dominated by reduced emissions. Meanwhile, the level of O₃ increased significantly during this period, and the nonmeteorological factors explained the increase. For the other air pollutants (PM_2.5, SO₂, and CO), the observed declines on the national scale were obviously affected by the meteorological conditions. In Wuhan, significant declines were found for air pollutants except O₃ and emissions dominated the changes, while in Beijing during the same period, only the …