Substantial Changes in Nitrogen Dioxide and Ozone after Excluding Meteorological Impacts during the COVID-19 Outbreak in Mainland China

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 O-3 increased significantly during this period, and the nonmeteorological factors explained the increase. For the other air pollutants (PM2.5, 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 O-3 and emissions dominated the changes, while in Beijing during the same period, only the level of NO2 significantly declined. This study clearly shows that the meteorological changes contributed substantially to the observed changes in most air pollutants, and this must be considered in evaluating the impacts of pollutant source changes on air quality during the specific event and in assessing source-oriented risks.