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 (NO(2)) and −38.9% for tropospheric NO(2) column, but fine particulate matter (PM(25)) still remains high and ozone (O(3)) 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 NO(2) 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.