Non-stop industries were the main source of air pollution during the 2020 coronavirus lockdown in the North China Plain.

Despite large decreases of emissions of air pollution during the coronavirus disease 2019 (COVID-19) lockdown in 2020, an unexpected regional severe haze has still occurred over the North China Plain. To clarify the origin of this pollution, we studied air concentrations of fine particulate matter (PM2.5), NO2, O3, PM10, SO2, and CO in Beijing, Hengshui and Baoding during the lockdown period from January 24 to 29, 2020. Variations of PM2.5 composition in inorganic ions, elemental carbon and organic matter were also investigated. The HYSPLIT model was used to calculate backward trajectories and concentration weighted trajectories. Results of the cluster trajectory analysis and model simulations show that the severe haze was caused mainly by the emissions of northeastern non-stopping industries located in Inner Mongolia, Liaoning, Hebei, and Tianjin. In Beijing, Hengshui and Baoding, the mixing layer heights were about 30% lower and the maximum relative humidity was 83% higher than the annual averages, and the average wind speeds were lower than 1.5 m s-1. The concentrations of NO3 -, SO4 2-, NH4 +, organics and K+ were the main components of PM2.5 in Beijing and Hengshui, while organics, K+, NO3 -, SO4 2-, and NH4 + were the main components of PM2.5 in Baoding. Contrary to previous reports suggesting a southerly transport of air pollution, we found that northeast transport caused the haze formation. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10311-021-01314-8.