Changes in Air Quality and Drivers for the Heavy PM(2.5) Pollution on the North China Plain Pre- to Post-COVID-19

Under the clean air action plans and the lockdown to constrain the coronavirus disease 2019 (COVID-19), the air quality improved significantly. However, fine particulate matter (PM(2.5)) pollution still occurred on the North China Plain (NCP). This study analyzed the variations of PM(2.5), nitrogen dioxide (NO(2)), sulfur dioxide (SO(2)), carbon monoxide (CO), and ozone (O(3)) during 2017–2021 on the northern (Beijing) and southern (Henan) edges of the NCP. Furthermore, the drivers for the PM(2.5) pollution episodes pre- to post-COVID-19 in Beijing and Henan were explored by combining air pollutant and meteorological datasets and the weighted potential source contribution function. Results showed air quality generally improved during 2017–2021, except for a slight rebound (3.6%) in NO(2) concentration in 2021 in Beijing. Notably, the O(3) concentration began to decrease significantly in 2020. The COVID-19 lockdown resulted in a sharp drop in the concentrations of PM(2.5), NO(2), SO(2), and CO in February of 2020, but PM(2.5) and CO in Beijing exhibited a delayed decrease in March. For Beijing, the PM(2.5) pollution was driven by the initial regional transport and later secondary formation under adverse meteorology. For Henan, the PM(2.5) pollution was driven by the primary emissions under the persistent high humidity and stable atmospheric conditions, superimposing small-scale regional transport. Low wind speed, shallow boundary layer, and high humidity are major drivers of heavy PM(2.5) pollution. These results provide an important reference for setting mitigation measures not only for the NCP but for the entire world.