Drivers for the poor air quality conditions in North China Plain during the COVID-19 outbreak

China's lockdown to control COVID-19 brought significant declines in air pollutant emissions, but haze was still a serious problem in North China Plain (NCP) during late-January to mid-February of 2020. We seek the potential causes for the poor air quality in NCP combining satellite data, ground measurements and model analyses. Efforts to constrain COVID-19 result in a drop-off of primary gaseous pollutants, e.g., -42.4% for surface nitrogen dioxide (NO₂) and -38.9% for tropospheric NO₂ column, but fine particulate matter (PM₂₅) still remains high and ozone (O₃) even increases sharply (+84.1%). Stagnant weather during COVID-19 outbreak, e.g., persistent low wind speed, frequent temperature inversion and wind convergence, is one of the major drivers for the poor air quality in NCP. The surface PM_2.5 levels vary between -12.9~+15.1% in NCP driven by the varying climate conditions between the years 2000 and 2020. Besides, the persistent PM_2.5 pollution might be maintained by the still intensive industrial and residential emissions (primary PM_2.5), and increased atmospheric oxidants (+26.1% for ozone and +29.4% for hydroxyl radical) in response to the NO₂ decline (secondary PM_2.5). Further understanding the nonlinear response between atmospheric secondary aerosols and NO_x emissions is meaningful to cope with the emerging air pollution problems in China.